National Disaster Management Authority approached CSIR- CRRI to prepare the ‘Guidelines for Planning and Construction of Roads in Cyclone Prone Areas’. This task was jointly undertaken by a team from Geotechnical Engg Division and Bridges and Structures Division of CSIR-CRRI.

1. Guidelines for Planning and Construction of
Roads in Cyclone Prone Areas
CRRI Report – July 2013
Sponsored by
National Disaster Management Authority
New Delhi
Geotechnical Engineering Division
Central Road Research Institute is an ISO 9001 Institution

2. DISCLAIMER
All the data and technical information furnished in this report are based on the literature review and
discussions held with expert members/field engineers and site visits undertaken by CSIR-Central
Road Research Institute (CSIR-CRRI) team. The responsibility of CSIR-CRRI is limited to the
technical and scientific matters contained in this report. All the procedural/ legal/ operational
matters would be responsibility of implementing agencies who would be using this report.

3. FOREWORD
India has long coastline of about 7500 km including its island territories. Thriving cities and ports
have been built on our coasts. Road network is very vital for providing connectivity to these
population centres. However, road infrastructure in coastal region faces constant threat due to
tropical cyclones. With the technological advancement, many new products and techniques are
now available for civil engineers to provide protection to road infrastructure against cyclone impact.
Keeping in view these issues, National Disaster Management Authority (NDMA) approached CSIR-
CRRI to prepare the ‘Guidelines for Planning and Construction of Roads in Cyclone Prone Areas’.
This task was jointly undertaken by a team from Geotechnical Engg Division and Bridges and
Structures Division of CSIR-CRRI.
The project team is grateful to NDMA for giving us an opportunity to work on this task. Special
thanks are due to Prof Prem Krishna, Prof D.K.Paul and Dr.S.Arunachalam who reviewed the draft
many times and provided valuable suggestions and comments. Acknowledgements are due to
Hon’ble members of ‘Disaster Management Committee’ and ‘Earthwork, Embankment and Ground
Improvement Committee’ of Indian Roads Congress, New Delhi and also to Prof.M.R.Madhav,
Member, Research Council, CSIR-CRRI for many useful comments/suggestions received from
them. The draft report was presented in three workshops held at Visakhapatnam, Bhubaneswar
and at New Delhi and received suggestions/ comments from various officers and engineers of state
disaster management authorities and Public Works Departments. These reviews/comments/
suggestions immensely helped in improving the draft. CSIR-CRRI Team expresses special thanks
to all of them.
(Dr.S.Gangopadhyay)
Director, CSIR- CRRI

4. Draft Preparation Team at CSIR-CRRI
Dr.S.Gangopadhyay
Director, CSIR-CRRI
Shri Sudhir Mathur
Chief Scientist & Advisor
Shri U.K.Guru Vittal
Head, Geotechnical Engg Division (Project Leader)
Dr. Lakshmy Parameswaran
Chief Scientist
Dr. Rajeev Garg
Head, Bridges and Structures Division
(Technical Assistance: Shri J.Ganesh and Dr.Pankaj Gupta)
Expert Committee for Review
Dr.Prem Krishna, FNAE
Honorary Visiting Professor, Department of Civil Engineering
Indian Institute of Technology, Roorkee – 247667 (Uttarakhand)
Dr.D.K.Paul
Dean of Faculty Affairs & Professor, Department of Earth Quake Engg
& Head, Centre for Excellence in Disaster Mitigation and Management
Indian Institute of Technology, Roorkee – 247667 (Uttrakhand)
Dr.S.Arunachalam
Formerly Advisor (M), SERC, Chennai
Director, Wind Engineering Application Centre
Jaypee University of Engineering & Technology
A.B. Road, P.B. No. 1, Raghogarh, Dist: Guna (M.P.) - 473226

5. CONTENTS
Page No
Chapter – 1 Introduction 1
Chapter – 2 Destructions Caused by Cyclones 4
Chapter – 3 Planning of Road Network in Cyclone Prone Areas 8
Chapter – 4 Construction of Road Embankments 11
Chapter – 5 Sea Erosion Control Techniques & River Bank Protection 19
Chapter – 6 Road Pavements in Cyclone Prone Areas 37
Chapter – 7 Mitigation Measures for Culverts and Bridges 42
Chapter – 8 Road Traffic Operations During Evacuation 55
References 59
Annexure – I (Technical Specifications for Geotextile
Tubes)
61

6. LIST OF FIGURES
Figure No. Title Page No
1.1 Wind and Cyclone Zones in India (Ref: NDMA) 2
1.2 Cyclone Hazard and PMSS Map (Ref: BMPTC) 3
2.1 Effect of Cyclone ‘Aila’ on Embankment 7
2.2 Another View of Eroded Embankment due to Cyclone ‘Aila’ in West Bengal 7
4.1 Rill Erosion in Road Embankment 13
4.2 Deep Cut in Road Embankment Due to Erosion 13
4.3 Severe Erosion of Road Embankment 13
4.4 A Type of Polymeric Vertical Drain (Band Drain) 17
5.1 Provision of Sea Wall and Concrete Tetrapods for Sea Erosion Protection at
Mumbai
21
5.2 Protection Against Sea Erosion by Retaining Wall and Gabions – Puri Konark
Road
21
5.3 Another View of Protection Works – Puri Konark Road 21
5.4 Beach Protection Using Boulder Revetment at Paradip, Odisha 22
5.5 Another View of Boulder Revetment at Paradip, Odisha 22
5.6 Masonry Sea Wall Constructed at Sea Coast at Koteshwar, Kori Creek, Gujarat 22
5.7 Typical Components of a Geotextile Tube 25
5.8 Geotextile Tube Application for Coastal Protection 26
5.9 Applications of Geotextile Tubes 27
5.10 Typical Use of Multiple Geotextile Tubes for Coastal Embankment Construction 27
5.11 Geotextile Tubes with Gabions as Armour Protection layer 27
5.12 View of geotextile Tubes Covered with Armour Protection Layer of Gabions 28
5.13 Shore Reclamation Using Geotextile Bags 29
5.14 Geotextile Bag 29
5.15 Use of Gabions for River Bank Protection 30
6.1 Construction of Roller Concrete Pavement for Rural Roads 38
6.2 Problem of Sand Dunes Encroaching Road Pavement 41
6.3 Close up View of Black Top Pavement Abraded by Sand 41
7.1 Cable Restrainer 49
7.2 Cable Restrainer Installed in Longitudinal Direction 49
7.3 Examples of Connecting the Beam Ends of Adjacent Spans 49
7.4 Connection of Deck to the Substructure 51
7.5 Cable restrainer between superstructure and substructure 51
7.6 Typical Details of a Restrainer 52
7.7 Tying the Restrainer from the Girders Around the Pier 53
7.8 Reaction Block/Stopper 54
7.9 Seat Extension to Accommodate Large Longitudinal Displacements 54

7. LIST OF TABLES
Table
No.
Title Page
No
1.1 Classification of Cyclones 2
2.1 Storm Intensity and Expected Damages 5
4.1 Property Requirements for 3-D Mat 16
5.1 Basic Engineering Parameters for Geotextile Tubes filled with Sand 26
5.2 Geotextile Hydraulic Property Requirements under Different Regimes 31
7.1 Hourly Mean Wind Speed and Pressure at 10m Level for Cyclone Resistant
Design of Bridges Situated Within 60 km off the Coast
44
7.2 Transverse Wind Forces Due to Cyclone Acting on Unit Exposed
Frontal Area of Bridge Deck at 10m Level (Plain Terrain)
45
7.3 Qualitative Damage State Descriptions for Typical Cyclone Induced Bridge
Damage (FEMA, 2003)
47

8. LIST OF ABBREVIATIONS
AOS Apparent Opening Size of Geotextile (Also known as O95)
BIS Bureau of Indian Standards
CBP Concrete Block Pavement
CDO Central Dense Overcast (Area immediately surrounding eye region of cyclone)
CRZ Coastal Regulation Zone
DPR Detailed Project Report
ICBP Interlocking Concrete Block Pavement
JGT Jute Geotextile
MDD Maximum Dry Density
MDR Major District Road
MORD Ministry of Rural Development, Government of India
MoRTH Ministry of Road Transport and Highways, Government of India
NH National Highways
NRRDA National Rural Roads Development Agency
ODR Other District Roads
OH Organic Soil having High Liquid Limit
OI Organic Soil having Medium Liquid Limit
OL Organic Soil having Low Liquid Limit
OMC Optimum Moisture Content
PCMS Portable Changeable Message Signs
PMSS Probable Maximum Storm Surge
Pt Peat
PVD Polymeric Vertical Drain/ Prefabricated Vertical Drain
RCCP Roller Compacted Concrete Pavement
RECP Rolled Erosion Control Product
SH State Highways
TRM Turf Reinforcement Mats
VR Village Roads
WMO World Meteorological Organisation
WPS Wireless Priority Service

9. 1
Chapter – 1
INTRODUCTION
A tropical cyclone is a storm system characterised by a large low pressure centre and numerous
thunderstorms that produce strong winds and flooding rain. Tropical cyclones feed on heat released
when moist air rises, resulting in condensation of water vapour contained in the moist air. The term
‘tropical’ refers to both the geographic origin of these systems, which form almost exclusively in tropical
regions of the globe, and their formation in maritime tropical air masses. The term ‘cyclone’ refers to
such storms’ cyclonic nature, with counter clockwise rotation in Northern Hemisphere and clockwise
rotation in the Southern Hemisphere. Depending on its location and strength, a tropical cyclone is
called by many other names, such as hurricane, typhoon, tropical storm, cyclonic storm, tropical
depression and simply cyclone. While tropical cyclones can produce extremely powerful winds and
torrential rain, they are also able to produce high waves and damaging storm surges. They develop
over large bodies of warm water, and lose their strength if they move over land. This is the reason for
coastal regions receiving a significant damage from a tropical cyclone, while inland regions are
relatively safe from their effect. Heavy rains, however, can produce significant flooding inland, and
storm surges can produce extensive coastal flooding up to 40 kilometres from the coastline. Although
their effects on human populations can be devastating, tropical cyclones can also relieve drought
conditions. They also carry heat and energy away from the tropics and transport it toward temperate
latitudes, which make them an important part of the global atmospheric circulation mechanism. As a
result, tropical cyclones help to maintain equilibrium in the earth’s troposphere, and to maintain a
relatively stable and warm temperature worldwide.
A strong tropical cyclone usually harbours an area of sinking air at the centre of circulation. This area is
called ‘eye of the cyclone’. Weather in the eye is normally calm and free of clouds, although sea may
be extremely violent. The eye is normally circular in shape, and may vary in size from 3 km to 370 km in
diameter. Surrounding the eye is the region called ‘Central Dense Overcast (CDO)’, a concentrated
area of strong thunderstorm activity. Curved bands of clouds and thunderstorms trail away from the eye
in a spiral fashion. These bands are capable of producing heavy bursts of rain and wind, as well as
tornadoes. If one were to travel between the outer edge of a hurricane to its centre, one would normally
progress from light rain and wind, to dry and weak breeze, then back to increasingly heavier rainfall and
stronger wind, over and over again with each period of rainfall and wind being more intense and lasting
longer.
1.1 Classification of Tropical Cyclones
Tropical cyclones with an organised system of clouds and thunderstorms with a defined circulation, and
maximum sustained winds of 61 kmph or less are called ‘tropical depressions’. Once the tropical
cyclone reaches wind speed of more than 61 kmph, they are typically called a ‘tropical storm’ and
assigned a name. When maximum sustained winds reach a speed of 119 kmph, such a cyclone is
called a ‘severe cyclonic storm’. The criteria followed by the Meteorological Department of India to
classify the low pressure systems in the Bay of Bengal and in the Arabian Sea as adopted by the World
Meteorological Organisation (WMO) are given in Table 1.1. Cyclones affect both Bay of Bengal and the
Arabian Sea. The areas affected by cyclone in India are shown in Fig 1.1 and 1.2.
1.2 Scope of These Guidelines
These guidelines cover various aspects related to planning and construction of road infrastructure in
cyclone prone areas, mainly dealing about preparedness in the eventuality of a cyclone disaster.

10. 2
Table – 1.1 Classification of Cyclones
Type of Disturbances Associated Wind Speed in the Circulation
Low Pressure Area Less than 17 knots (< 31 kmph)
Depression 17 to 27 knots (31 to 49 kmph)
Deep Depression 28 to 33 knots (50 to 61 kmph)
Cyclonic Storm 34 to 47 knots (62 to 88 kmph)
Severe Cyclonic Storm 48 to 63 knots (89 to 118 kmph)
Very Severe Cyclonic Storm 64 to 119 knots (119 to 221 kmph)
Super Cyclonic Storm 120 knots and above (222 kmph and above)
Source: India Meteorological Department
Fig – 1.1 Wind and Cyclone Zones in India
(Ref: NDMA)

11. 3
Fig – 1.2 Cyclone Hazard and PMSS Map
(Ref: BMPTC)

12. 4
Chapter – 2
DESTRUCTIONS CAUSED BY CYCLONES
There are three elements associated with a cyclone, which cause destruction. These have been
described below:
2.1 Storm Surge
Cyclones are associated with high-pressure gradients and consequent strong winds. These, in turn,
lead to storm surges. A storm surge can be defined as an abnormal rise of sea level near the coast
caused by a severe tropical cyclone; as a result of which, sea water inundates low lying areas of
coastal regions drowning human beings and live-stock, eroding beaches and embankments, destroying
vegetation and reducing soil fertility. Storm surge is the single major cause of devastation from tropical
storms. Storm surge is formed due to pushing of sea water towards shore by the force of the winds
swirling around the storm. In addition, wind driven waves are superimposed on the storm tide. This
advancing surge may happen to combine with the high tides to create the hurricane storm tide, which
can increase the average water level to 4.5 m or more. The level of surge in a particular area is also
determined by the slope of the continental shelf. Storm surge is inversely proportional to the depth
of sea water. A shallow slope off the coast will allow a greater surge to inundate coastal communities.
Communities with a steeper continental shelf will not see as much surge inundation, although large
breaking waves can still present major problems.
Vulnerability to storm surges is not uniform along Indian coasts. The following segments of Indian coast
are most vulnerable to high surges:
a) North Odisha and West Bengal Coasts
b) Andhra Pradesh coast between Ongole and Machilipatnam
c) Tamilnadu Coast, south of Nagapattinam
The west coast of India is less vulnerable to storm surges than the east coast of India in terms of height
of storm surge as well as frequency of occurrence. However, the following segments of western coast
are vulnerable to significant surges:
a) Maharashtra coast, north of Harnai and adjoining south Gujarat coast and the coastal belt
around the Gulf of Mumbai
b) The coastal belt around the Gulf of Kutch
The world’s highest recorded storm surge was about 12.5m (about 41 ft) and it was associated with the
Backergunj cyclone in 1876 near the Meghna estuary in present-day Bangladesh. The Probable
Maximum Storm Surge (PMSS) is the highest along the West Bengal coast where it ranges from 9 m to
12.5 m. It reduces to about 3.8 m in Khurda district, Orissa, increasing again to about 8.2 m along the
south Andhra Pradesh coast in Krishna, Guntur and Prakasam districts. A small region in south Tamil
Nadu around Nagapattinam coast also has higher PMSS of about 8.4 m. Along the west coast; the
PMSS varies from about 2 m near Thiruvananthapuram to around 5 m near the Gulf of Khambat in the
Saurashtra region of Gujarat. Expected storm surge height in metres along India’s coastline is shown in
Fig 1.2.

13. 5
2.2 Strong Winds/ Squall
Cyclones are known to cause severe damage to infrastructure through high speed winds and gusts.
Very strong winds which accompany a cyclonic storm, damage installations, dwellings, communication
systems, trees, etc., resulting in loss of life and property. A tropical cyclone damages and destroys
structures in two ways. First, many homes are damaged or destroyed when the high speed wind simply
lifts the roof of the dwellings. High speed wind moving over the top of the roof creates lower pressure
on the exposed side of the roof relative to the attic side. The higher pressure in the attic lifts the roof.
Once lifted, the roof acts as a sail and is blown clear of the dwelling. With the roof gone, the walls are
much easier to be blown down by the hurricane wind. The second way that wind destroys buildings can
also be a result of the roof becoming airborne. The wind picks up the debris (i.e. wood, metal siding,
toys, trash cans, tree branches, etc.) and sends them hurling at high speeds into other structures.
Based on observations made during damage investigations, researchers have concluded that much of
the damage in windstorms is caused by flying debris. Brief details about damages caused by winds of
different speed are given in Table 2.1 (Ref: Saffir-Simpson Hurricane Scale – Management of
Cyclones: NDMA Guidelines – http://nidm.gov.in/PDF/guidelines/cyclones.pdf)
Table – 2.1 Storm Intensity and Expected Damages
Scale No
(Category)
Sustained winds
(kmph)
Damage
Storm Surge
in m
1 119 – 153 Minimal: Unanchored mobile homes,
vegetation and signs
1.2 to 1.5
2 154 – 177 Moderate: All mobile homes, roofs, small craft
and flooding
1.6 to 2.4
3 178 – 209 Extensive: Small buildings, low lying roads
cut-off
2.5 to 3.6
4 210 – 249 Extreme: Roofs destroyed, trees down, roads
cut-off, mobile homes destroyed, beach
homes flooded
3.7 to 5.5
5 250 or more Catastrophic: Most buildings destroyed,
vegetation destroyed, major toads cut-off,
homes flooded
More than
5.5
The vertical wind shear in a tropical cyclone environment is also important. Wind shear is defined as the
amount of change in the wind velocity direction or speed with increasing altitude. The damages
produced by winds are extensive and cover areas occasionally greater than the areas of heavy rains
and storm surges which are in general localised in nature. The impact of the passage of the cyclone
eye, directly over a place is quite different from that of a cyclone that does not hit the place directly. The
latter affects the location with relatively unidirectional winds i.e. winds blowing from only one side, and
the lee side is somewhat protected. An eye passage brings with it rapid changes in wind direction,
which imposes torques and can twist the vegetation or even structures. Part of structures that were
loosened or weakened by the winds from one direction are subsequently severely damaged or blown
down when hit upon by the strong winds from the opposite direction. A partial eye passage can also do
considerable damage, but damage would be less than a total eye passage.
2.3 Torrential Rains and Inland Flooding
Torrential rainfall (more than 30 cm/hour) associated with cyclones is another major cause of damage.
It also creates problems in post cyclone relief operation. Unabated rain gives rise to unprecedented

14. 6
floods. Rainwater on the top of the storm surge may add to the fury of the storm. Rain is the serious
problem for the people who become shelterless due to a cyclone. Heavy rainfall resulting from a
cyclone would be usually spread over a wide area. As a result, soil erosion also occurs on a large
scale. Heavy rains inundate the low-lying ground and cause softening of the soil due to soaking. This
contributes to weakening of the embankments, leaning of utility poles or even collapse of pole type
structure. Heavy and prolonged rains due to cyclones cause river floods and submergence of low lying
areas. River floods occur when the runoff from torrential rains, brought on by landfall of cyclones reach
the rivers. Even after the wind has diminished, the flooding potential of cyclonic storms remains for
several days. Most of the fatalities due to flooding occur because people underestimate the power of
moving water and purposely walk or drive into flooding conditions. It is common to think that stronger
the storm the greater the potential for flooding. However, this is not always the case. A weak, slow
moving tropical storm can cause more damage due to flooding than a more powerful fast moving
hurricane. In addition to the storm surge, tropical cyclones usually cause flash flooding. Flash floods are
rapidly occurring events. This type of flood can begin within a few minutes or hours of excessive
rainfall. The rapidly rising water can reach heights of 10 m or more and can roll boulders, rip trees from
the ground, and destroy buildings and bridges. Urban area floods are also rapid events although not
quite as severe as a flash flood. Still, streets can become swift-moving rivers and basements can
become death traps as they fill with water. The primary cause is due to the conversion of fields or
woodlands to roads and paved parking lots.
It may be mentioned that all the three factors mentioned above occur simultaneously and, the rescue
and relief operations for distress mitigation become difficult. So, it is imperative that advance action is to
be initiated for relief measures before commencement of adverse weather conditions due to cyclones.
2.4 Effect of Cyclones on Road Infrastructure
From the above discussion, it becomes apparent that cyclonic storms affect human habitations and
infrastructure in multiple ways. Providing road connectivity in cyclone prone areas emerges as a vital
tool for undertaking rescue and rehabilitation operations. It is also obvious that road infrastructure
created in cyclone prone areas need to be designed and constructed to withstand the onslaught of
cyclonic storms. The first step would be to identify the road stretches which are vulnerable to effect of
cyclonic storm. Principle mode of destruction of road embankments and pavements due to cyclonic
storms would be through erosion caused due to storm surge and flooding. The storm water causes
damage to the road pavement surface, washes away portions of road at many locations, even
breaching portions of embankment. Because of these problems, such road stretches in cyclonic areas
become bottle neck and hamper relief operations.
Embankments and road pavements are not much susceptible to damage due to winds. However wind
forces affect design of bridge structures. Wind forces also affect certain road furniture like sign boards,
electric and telephone poles and trees planted along roadside causing disruptions to traffic flow by
uprooting of trees, falling branches of trees, electric/ telephone poles falling in roadway, etc. Keeping
these points in view, identified vulnerable road stretches in cyclone prone areas would have to be
designed/ constructed to ensure that damage due to storm surge/flooding/winds are minimised or
totally alleviated. Impact of a cyclone on road infrastructure may lead to (a) Damage to the roads due to
storm surge / flooding (b) Hindrance to traffic movement due to deposition of debris left on roadway (c)
Unseating / drifting of bridge superstructure due to storm surge / flooding (d) damages to the bridges
due to debris impact. Impact damages can occur due to barge impact, boats, oil rigs, uprooted trees,
boulders etc. The impact damage is manifested in the form of span misalignment, damage to fascia
girder, fender, pier or pile damage. During cyclone occurrence, bridges may fail due to unseating of
individual span, depending on the connection between the bridge deck and pier. The bridge decks with

15. 7
low elevation are likely to fail as a result of excessive longitudinal or transverse movement of bridge
deck. Under the storm surge, the bridge decks are subjected to buoyant forces and pounding action of
waves. Bearings also suffer damages due to the unseating/drifting of bridge deck. In some bridges,
shifting of span due to lateral wave and wind forces often causes damages to the abutments, pier caps,
or girders. Damage to parapets on bridge decks, scouring of bridge foundations, erosion of abutment,
etc are seen after cyclone disaster. Further details regarding damages caused to bridges due to
cyclone are given in Chapter 7. While designing road structure in cyclone prone areas, the above
mentioned factors are to be considered and suitable remedial measures described in subsequent
chapters are to be provided. Further it is to be noted that usually a package of remedial or protection
works are usually fashioned to suit individual site conditions.
CHAPTER – 3
Fig 2.1 – Effect of Cyclone ‘Aila’ on Embankment
Fig 2.2 – Another View of Eroded Embankment due to Cyclone ‘Aila’ in West Bengal

16. 8
Chapter – 3
PLANNING OF ROAD NETWORK IN CYCLONE PRONE AREAS
As in any other part of the country, in a similar manner, the hierarchy of road infrastructure in cyclone
prone area would comprise of various categories like National Highways (NH), State Highways (SH),
District Roads and Rural Roads. National and State Highways are the arterial roads connecting major
cities, ports, state capitals, industrial centres, etc. These roads should provide uninterrupted road
communication throughout their length. District roads are intended to act as important roads within a
district serving areas of production and markets and connecting with each other or with the main
highways of a district. They are further sub-divided into two categories – Major District Roads (MDR)
and Other District Roads (ODR). Village Roads (VR) provide connectivity between villages or
habitations and District Roads. The term ‘Rural Roads’ is used to denote both ODR and VR. All these
types of roads are very important in the overall road network of cyclone affected areas. Effective road
connectivity ensures fast deployment of men, materials and machinery to cyclone affected areas and
also ensures speedy evacuation of people from vulnerable places to safer areas in the face of an
impending disaster threat. Hence the need for development of a reliable road network in the vulnerable
areas is very vital to ensure coordination of relief and response in the event of a cyclone. Designation of
arterial roads like National Highways and State Highways are based on traffic volume and importance
of cities/ towns/ ports to be connected by them.
Planning a rural road network in a district is carried out in our country based on guidelines provided by
Ministry of Rural Development (MORD) and the Indian Roads Congress (IRC). As per the MORD
Guidelines, all-weather road access is to be provided to all villages/habitations of population greater
than 500 people. The Operations Manual of MORD states that an all weather road is defined as one
which is negotiable during all weathers, with some permitted interruptions. Essentially this means that
at cross-drainage structures, the duration of overflow or interruption at one stretch shall not exceed 12
hours for ODRs and 24 hours for VRs in hilly terrain, and 3 days in the case of roads in plain terrain.
The total period of interruption during the year should not exceed 10 days for ODRs and 15 days for
VRs. As per MORD Guidelines, population criteria for providing all weather connectivity has been kept
equal to 250 in case of hill States (North-Eastern states, Sikkim, Himachal Pradesh, Jammu & Kashmir
and Uttarakhand), desert areas and tribal areas.
The most important issue with the road construction is the alignment of the road. Many issues of
drainage, inundation and breaching of road embankment can be tackled at planning stage by choosing
the best possible alignment. But in many projects it may not be possible to change the alignment of
existing track due to problems like land acquisition, forest area, etc. These are important issues but
such issues need to be taken care of by respective state Governments. Moreover, road projects involve
huge investment. Hence, it is crucial to give adequate attention at the planning and design stage itself
so as to achieve better and economical alignment. For planning road network in cyclone prone areas,
following additional points need to be considered:
a) For planning higher category of roads like NH, SH and MDRs, ‘20 Year Road Development Plan’
and ‘Vision 2020 – Road Development Document’ published by IRC/ MoRTH are to be considered.
While such arterial roads are necessary to connect main cities and towns, considerations of traffic to
be catered and trade/economic importance of cities being connected are also equally essential.
Additionally in case of cyclone prone areas, arterial roads required for evacuation in the event of
cyclone occurrence need to be identified. Upgradation of such arterial evacuation routes to
NH/SH/MDR category depending upon their importance and population being catered by such
routes needs to be considered. Width of the important arterial evacuation routes (SH or MDR)

17. 9
should be preferably Two-Lane or atleast they should be of intermediate lane width. While planning
the alignment of the rural link roads, it is imperative to connect existing and proposed cyclone
shelters in addition to providing connectivity to habitation/ village.
b) Rural link roads in cyclone prone areas are very crucial for evacuation and rescuing of people.
Similar to hilly areas and tribal regions, in areas prone for severe cyclone impact (coastal belt of 25
km from the sea), population criteria for providing all weather connectivity can be kept equal to 250
for a habitation to be connected by an all weather road. All weather roads, as already pointed out,
may experience interruptions to the traffic due to submergence of a bridge for periods extending
from 24 hours to 72 hours. However, roads built in cyclone prone areas need to be designed to
reduce the duration of traffic interruption due to flooding. This duration of disruption for roads
identified for evacuation (belonging to ODR and VR category) should be preferably not more than 3
hours even after highest flooding expected in that region.
c) The geometric design standards for rural roads are to be followed as per IRC SP – 20, ‘Rural Roads
Manual. Roads are always associated with culverts and bridges as the terrain demands, to make
them fit throughout the year. Geometric design of NH and SH are to be carried out as per IRC: 73.
While selecting the bridge site, factors like (i) permanency of the channel, (ii) presence of high and
stable banks (iii) narrowness of the channel and average depth compared to maximum depth,
straight reach of the stream, freedom from islands in both upstream side and downstream side,
possibility of right angled crossings, good approaches, etc., are to be given adequate attention so as
to keep them functional in the event of any disaster.
d) Concerned state agencies who are in charge of the road project should prepare beforehand ‘Hazard
Zonation Maps’ of suitable scale, showing extent of cyclone/ flood hazard expected in that region.
These maps should indicate vulnerable roads and bridges and risk assessment is to be carried out.
In case of failure of bridge/road pavement during a cyclone event, alternate routes should be
identified for evacuation/ rescue and relief on these maps. Missing links/ additional infrastructure
needs should also be marked on these maps so that they can be attended to during planning
process. These state agencies may take assistance from State Disaster Management Authorities
and local bodies for preparation and validation of such maps.
e) Construction of roads is to be taken up in such a manner that roads are atleast 500 m away from
seashores / coastal regulation zones (CRZ). Intensive protective works to prevent erosion towards
seashore side of the road should be planned.
f) The most important consideration for construction of road in the cyclone prone area would be its
alignment avoiding inundation of the road under cyclonic rain. Adequate cross drainage works
should be provided to prevent such occurrence. Therefore, a survey along the most probable route
is needed to ascertain the highest flood level that had occurred during its past history. The free
board allowance for different categories of roads is indicated in section 5.6. Provision of minimum
free board as per section 5.6 will ensure connectivity even after the cyclonic storm.
g) Rigid pavements are preferable over flexible pavements as there is no appreciable variation in
temperature to cause significant thermal stress and resulting distress. Cement concrete pavements
withstand flooding/ waterlogging in a better manner than bituminous pavements. Techno-economics
of adopting cement concrete pavements vis-a-vis flexible pavement needs to be undertaken before
making a final choice.

18. 10
h) Road top level/ alignment are to be decided after taking into account high flood levels and flooding
pattern. While fixing the road top levels, special care will have to be taken to cater for rapid changes
in underground water table and consequent movement of the soil moisture. This can be achieved by
designing and constructing an efficient drainage system. Keeping the road levels above the high
flood levels and highest water table need to be ensured. For provision of storm water drainage for
roads in urban areas, IRC SP: 50, ‘Guidelines on Urban Drainage’, can be referred to.
i) Preparation of a Detailed Project Report (DPR) for each of the proposed road is a pre requisite for
proper evaluation of the project and it ensures timely completion and avoids time and cost over
runs.

19. 11
CHAPTER – 4
CONSTRUCTION OF ROAD EMBANKMENTS
Successful performance of an embankment depends as much on adopting standards of good
compaction in construction as on careful pre investigations leading to selection of appropriate borrow
material and design features of the embankment. Soil is the primary construction material for
embankment and also for road subgrade. So soil and construction material survey forms the basic step
for preparation of DPR for any road project. While carrying out soil survey along proposed road
alignment, representative samples should be collected wherever there is a visible change in soil type.
In case the same type of soil continues, at least three representative samples from each kilometre
length of road alignment should be collected for laboratory testing.
4.1 Material Specifications for Embankment and Subgrade
The material used in embankments, subgrades, earthen shoulders and miscellaneous backfills shall be
soil, moorum, gravel, a mixture of these or any other suitable material approved by the engineer. Such
material shall be free from organic materials like logs, stumps, roots, rubbish or any other ingredient
likely to deteriorate or affect the stability of the embankment/subgrade. The following material shall be
considered unsuitable for embankment:
 Materials from swamps, marshes and bogs, peat, log, stump and perishable material, any soil
classified as OL, OI, OH or Pt in accordance with IS: 1498
 The fill soil to be used should have liquid limit less than 70 and plasticity index less than 45
 Materials having salts which may result in leaching in the embankment
Expansive clay exhibiting marked swell and shrinkage properties (‘free swell index’ exceeding 50 when
tested as per IS: 2720–Part 40) shall not be used as fill material. Where expansive clay with free swell
index less than 50 is used as a fill material, subgrade and top 500 mm portion of the embankment
below subgrade shall be non-expansive in nature. The soil to be used as embankment fill or subgrade
should also meet maximum dry density and other requirements as specified in MoRTH Specifications
(in case of NH/SH works) or MORD Specifications (in case of rural roads).
4.2 Design of Embankments
The cyclone impact occurs in the form of erosion of road embankments. Apart from preventing erosion,
the designer has to ensure stability of road embankments. For details regarding design of road
embankment IRC: 75 can be referred to. Failure of embankments may be due to either inadequate
bearing capacity or due to deep seated shear failure. The objective of the stability analysis is to ensure
that embankment does not face any risk of shear failure. Generally in the slip circle method failure
plane is assumed to be circular. A particular circle gives the minimum factor of safety. Calculation of
factor of safety of different circles until the critical circle is located is a very time consuming process.
Available software may provide quick solutions.
4.3 Important Considerations for Embankment Construction
(a) Fill material should conform to MoRTH/ MORD Specifications depending on road classification.
Borrow pit excavation should be located at a distance atleast 5 m away from the toe of
embankment. Top soil should not be used as fill material. Top soil should be spread back on the
excavated land or used for covering the side slopes of the embankment.

20. 12
(b) After clearing the site, limits of the embankment are to be marked by fixing batter pegs and
marking toe lines on both sides at regular intervals as guides. Where ever feasible, stagnant water,
if any, from the roadway (embankment foundation area) should be removed.
(c) After removing the topsoil / unsuitable material, the ground surface should be loosened upto a
minimum depth of 150 mm by ploughing or scarifying and compacted to the specified density. For
embankment construction over ground not capable of supporting equipment, successive loads of
embankment fill material should be spread in a uniformly distributed layer of adequate thickness to
support equipment and to construct the lower portion of the embankment. In case of soft sub-soil
areas (marine clay sub-soil), ground improvement measures may be necessary to prevent failure
of embankment. Expert advice should be obtained in such cases and specified foundation
treatment should be carried out in a manner and to the depth as specified. Brief details of ground
improvement techniques are given in section 4.5.
(d) The soil should be spread over the entire width of the embankment in layers not exceeding
required loose layer thickness. The moisture content of the fill material spread for compaction
should be within ±2 per cent of the optimum moisture content of the soil. Clayey soils should be
compacted at moisture content slightly higher than OMC (upto 2 per cent above OMC).
(e) Each layer of fill material should be compacted using rollers to meet the specified compaction
requirements. Adequate quality control and field tests as per MoRTH/ MoRD specifications are
needed to ensure this.
(f) The top 50 cm of the embankment (in case of NH and SH) or 30 cm (in case of rural roads) which
forms the subgrade should be built to specification requirements of the subgrade.
For further details/ specifications, reference may please be made to ‘MoRTH Specifications for Road
and Bridge Works’ or ‘MoRD Specifications for Rural Roads’ and ‘IRC 36 – Recommended Practice for
Construction of Earth Embankment and Subgrade for Road Works’.
4.4 Embankment Slope Protection against Soil Erosion
Road embankments experience a high degree of damage due to erosion from torrential rains which
accompany cyclones and hence erosion protection of embankment slopes should receive special
attention in such areas. Soil erosion is the process of detachment and transportation of soil particles by
wind or water. Cohesionless soil particles may get blown away by wind (Aeolian) erosion. However
erosion due to surface run-off would be the principal cause for failure of road embankments in the
aftermath of a cyclone disaster. The kinetic energy of falling raindrops causes detachment of soil
particles which are subsequently carried away by surface run-off. Nature of soil and impact of rain
drops are determinant factors in the erosion process. Silty and sandy types of soils are more
susceptible to erosion than clayey soils. Distress in the form of rills to gullies and finally to erosion
ditches develop when intensity of rainfall is high and the slope is steep. These problems will impair
slope stability if not controlled with proper protective measures. The surface protection of embankment
against action of rain and wind is usually achieved by promoting vegetation growth. When
embankments are constructed using non-cohesive material, cover of 0.3 to 0.6 m thick cohesive
material can be given. In case of high embankments, a system of kerb channel and median drains
coupled with chutes should be provided to drain off the rain water from the road embankments.
Different engineering measures which may be adopted for erosion protection of roads built in cyclone
prone areas are briefly described below. For more details, IRC: 56, ‘Recommended Practices for
Treatment of Embankment and Roadside Slopes for Erosion Control’ can be referred to.

21. 13
Fig 4.1 – Rill Erosion in Road Embankment
Fig 4.2 – Deep Cut in Road Embankment Due to Erosion
Fig 4.3 – Severe Erosion of Road Embankment

22. 14
4.4.1 Slope protection by simple vegetative turfing
Vegetation is ideal for erosion control because it is relatively inexpensive to establish and maintain and
it presents aesthetically appealing look. Vegetation on the embankment side slopes provides adequate
canopy interception to the falling rain drops and saves the soil from splash erosion, while the mass of
litter and Rhizomes act as speed breakers for running water on the slope. Mechanical function of plant
is to reinforce the soil by binding the loose soil particles with its fibrous root system.
However, planting of tree species which grow considerably big/tall should not be permitted alongside
the road in cyclone prone areas. During cyclones, such trees may get uprooted/ braches may snap
which may cause obstruction to movement of traffic and may even lead to accidents. Moreover, roots of
big trees may tend to loosen the structure of the embankment when shaken by wind storm which would
cause cracks in the embankment. Shrubs, thorny bushes and short grass growing on the slope of
embankments provide good protection against erosion and such vegetation should be promoted. Tree
plantation should be carried out in areas beyond road land (Right of way) width. Generally the side
slopes and unpaved shoulders in the top portion of the embankment should be turfed with grass sods
and this turfing should extend beyond the toe on the country-side and the river side by 6.0 meters and
3.0 meters respectively. This is as per existing practices of some cyclone prone states.
Simple vegetative turfing method should be adopted where the soil has enough nutrients and the
environmental conditions are conducive to promote vegetation growth. The density of sowing is of great
importance. In general, while sowing a mixture of grass and legume plants, seed rate would be
normally 15 gm/m2. Prior to sowing, the soil surface should be adequately prepared. On highly erodible
slopes where seeding or sprigging is liable to be washed down before they have had time to take root.
In such circumstances, it is advisable to go for special techniques such as the ones recommended in
the succeeding paragraphs.
4.4.2 Transplantation of readymade turfs of grass
‘Sodding’ technique which involves bodily transplantation of blocks of turfs of grass (with 5-8 cm of soil
covering the grass roots) from the original site to the barren slopes to be treated can be adopted in
locations where ensuring grass growth would require considerable time. The sod to be used for
transplantation should consist of dense, well-rooted growth of permanent and desirable grasses,
indigenous to the locality where it is to be used, and it should be practically free from weeds or other
undesirable matter. Thickness of the sod should be as uniform as possible, with some 50-80 mm or so
of soil covering the grass roots depending on the nature of the sod, so that practically all the dense root
system of the grasses is retained in the sod strip. The completed embankment side slopes should be
scarified to a depth of about 25 mm and application of fertiliser/ manure should be carried out. After the
sods have been laid in position, the surface shall be cleaned of loose sod, a thin layer of top soil shall
be scattered over the surface of top dressing and the area thoroughly moistened by sprinkling with
water. For further details MoRTH Specifications for Road and Bridge Works, Clause 307 and 308 can
be referred to.
4.4.3 Application of mulch
The term ‘mulch’ refers to any loose or soft organic material, e.g. straw with cowdung or wood shavings
mixed with cowdung or saw dust and dung mixture, etc laid down on the slopes to protect the roots of
plants. In the case of embankments which are less than 3 m high, where the severity of the erosion
problem is not of a high order, the mulch application would be very helpful for vegetation growth even in

23. 15
infertile slopes. The approximate thickness of mulch cover should be about 2.5 cm. The organic mulch
covering the soil slopes can be held in place and made resistant to being washed downhill or being
blown away by pegging them down with bamboos, at suitable intervals, in a grid pattern. Cellulose
based fibrous mulches can be hydraulically spray applied with the seed. These ‘spray-on’ mulch
systems (also called Hydro-mulching or Hydro-seeding) are somewhat more resistant to erosion than
dry applied systems but they are relatively costlier also.
4.4.4 Promotion of vegetative turfing by using jute/ coir netting
Growth of appropriate vegetation on exposed soil surface is facilitated by use of natural (agro based)
geotextiles such as open weave jute geotextiles (JGT) or coir netting. Such nettings laid on slopes
provides a cover over exposed soil lessening the probability of soil detachment and at the same time
reduces the velocity of run-off, the main agent of soil erosion. Natural geotextiles bio-degrade within
one to three years. In spite of this, agro based geotextiles facilitate rapid growth of dense vegetation
during its service life. Once dense vegetation develops on the slope, plant cover would prevent erosion
and it would be self sustaining. Hence biodegradability of jute/ coir nettings cannot be considered as a
drawback in areas which experience adequate precipitation to ensure green vegetation cover
throughout the year. For more details and specifications of this technique, IS: 14986 ‘Guidelines for
application of Jute Geotextile for rain water erosion control in road and railway embankments and hill
slopes’, IS: 15869 ‘Open weave coir Bhoovastra-Specification’ and IS 15872 ‘Application of coir
geotextiles (coir woven Bhoovastra) for rain water erosion control in roads, railway embankments and
hill slopes-Guidelines’ may be referred to.
4.4.5 Erosion control using two dimensional (2–D) synthetic geogrids/ Geosynthetic nettings
Geosynthetic nettings/ geogrids can be used for promoting vegetation growth on barren slopes in a
manner similar to biodegradable nettings. Under erratic weather conditions, successful vegetation
growth and its sustenance depends on un-seasonal rainfall and hence longer life of reinforcing material
would be required for ensuring vegetation growth apart from contribution from the mesh towards
reduction in velocity of surface runoff. Agro based nettings may fail to provide erosion prevention in
areas which experience repetitive change in climate, prolonged drought in particular. Use of polymer
geogrid mesh provides a permanent protection as it is not biodegradable, long lasting and has almost
unfailing success rate for vegetation growth, year after year.
4.4.6 Three dimensional erosion control mat / Rolled erosion control products
Relying upon vegetation growth alone may be sometimes very unpredictable and unreliable as it may
be extremely difficult to achieve 100 per cent vegetation coverage, leaving exposed areas vulnerable to
erosion. Furthermore, vegetation may sometimes dry up or become diseased, reducing its erosion
control capability. Reinforced vegetation (or reinforced grass) is a better method that can be adopted
for enhancing slope stability and erosion control. Such erosion control products are usually three
dimensional mats, having multi-filamented materials of specified thickness. Such materials are known
as Rolled Erosion Control Products (RECPs)/ 3-D Mats and also as ‘Turf Reinforcement Mats (TRM).
While mats made using natural fibres last for one to two years, polymeric mats are used in situations
where such products are required to last for a longer time. 3-D mats having a wide ranging variety of
strength are available. The material used for manufacturing these mats also varies. Hence following
general specifications are given (Table 4.1) for guidance. However, field conditions like harsh areas/
high survivability requirements may warrant use of 3-D mats with tensile strength as high as 35 kN/m or
even more.

24. 16
Table – 4.1 Property Requirements for 3-D Mat
3-D Mat Property Specified value* Test Method
Minimum Tensile Strength 2 kN/m ASTM D 5035
UV Stability (Min % tensile strength
retention)
80% ASTM D 4335 (500 hour
exposure)
Minimum thickness 6.5 mm ASTM D 6525
Mass per unit area (Minimum) 250 gm/ m2 ASTM D 3776
* Minimum Average Roll Values, machine direction only for tensile strength test
4.4.7 Preformed polymer geosynthetic cells or webs
Often, embankments are to be constructed in areas where vegetation may be difficult to establish and
erosion problem might be severe due to water bodies. It may also be not possible to mitigate potential
erosive forces that are likely to overcome the strength of the root system. In such cases ‘Geosynthetic
Cells’ can be adopted. However, geosynthetic cells would be relatively more costly than all other
techniques outlined above.
4.5 Ground Improvement Techniques
Often problems like slip failure of road embankment or high degree of unevenness of road pavements
which occur in coastal roads can be traced to inadequate consolidation of clayey sub-soil found in such
locations. Such problems in the coastal and delta areas arise due to low shear strength and high
compressibility of soft clay sub-soils which are commonly referred to as marine clays. In severe cases,
road embankments may even fail or pavement surface may experience unacceptable levels of
settlements stretching over considerable period of time. Improvement of the load response behaviour of
such soft sub-soil becomes necessary if the embankments are to be built economically and
serviceability levels are to be kept high. Accelerating the consolidation process by providing vertical
drains has been widely adopted for road embankment construction in such marine clay areas.
4.5.1 Ground improvement using vertical drains
Vertical drains have been in use for more than half a century to promote rapid consolidation of thick soft
clay deposits like marine clays, where preloading alone will be insufficient. Sand drains were the
earliest type of vertical drains used for consolidation of soft clay layer. Installation of sand drains is
usually done by drilling boreholes in soft clay and back filling the borehole using sand of specified
gradation. The major problem in this case would be formation of cavities due to bulking of sand.
Polymeric vertical drains (PVD) which are also known as ‘Band drains’ have now virtually replaced sand
drains/ sand wick technique for ground improvement.
4.5.2 Band drains (PVD)
Band drains consists of a plastic/polymeric core formed to create channels or paths which are
surrounded by a thin geotextile filter jacket. Typically the size of band drains is 10 cm in width and 3 to
9 mm in thickness. The primary use of band drains is to accelerate consolidation and to greatly
decrease the settlement time of embankments over soft soils. By doing so, band drains also accelerate
the rate of strength gain of the in-situ soils. Band drains are used in consolidation situations where soil
to be treated is a moderate to highly compressible soil with low permeability and fully saturated in its
natural state. The soil should be either normally consolidated or under consolidated prior to loading.
The loading should exceed maximum past consolidation pressure for the band drains to be beneficial.

25. 17
Band drains are generally installed by displacement methods. The mandrels used with band drains are
hollow and normally rectangular or trapezoidal in cross section. The mandrel covers and protects the
band drain material during installation. All installation methods employ some form of anchoring system
(generally using a disposable end shoe) to hold the drain in place when mandrel is withdrawn.
Commonly used methods employ an installation mast (called ‘Stitcher’) which contains the material
reels, mandrel and provision for providing installation force. Added to this is a carrier, which is a crawler
excavator or crawler crane, depending somewhat on the depth of installation. Usually for drain
installation depth upto 20 m, the mast can be mounted on a crawler excavator. Drains requiring depth
greater than 20 m most often require an installation mast mounted to a crane to provide stability. The
most important criteria for method of installation is the size of the installing mandrel. The mandrel
should be kept to a minimum size, usually not greater than 80 cm2 unless larger size is required for
penetrating to greater depth. Although equipment is available to work over slopes, a level granular
surface containing no large obstructions is ideally required for band drain installation. Sufficient head
room is also required for its installation. A thumb rule for head room required would be 3 m longer than
depth of installation. Band drains have been installed upto 60 m depth, by using specialised equipment.
It is essential to recognise that band drains serve no structural function. By providing a shorter drainage
path, it provides a faster release of excess pore pressure, thereby resulting in faster settlement and
quicker strength gain through consolidation. For sites with a stability problem, the soil will initially have
the same strength with or without the band drains installed. Further band drains do not play any role in
secondary consolidation. Therefore in cases where secondary consolidation is expected to be
significant, it is necessary to provide excess surcharge and/or extended waiting periods prior to final
construction. It is not recommended to install band drains where pre-drilling is necessary for installation.
A drainage layer of coarse sand or gravel is provided above the ground to drain off water from band
drains. Generally sand layer is provided for a thickness of 0.5 to 1.0 m.
4.5.3 Stone columns
Stone columns comprise of boreholes of designed diameter made at specified distance apart in the soft
soil, which are then back filled using stone aggregates and compacted. The diameter of stone columns
varies from about 0.4 m to 0.7 m and their spacing varies from 1.5 m to 3.5 m. This method is used in
soft subsurface soils to both accelerate settlement and provide sufficient increase in strength to
Fig – 4.4 A Type of Polymeric Vertical Drain (Band Drain)

26. 18
minimise settlement and prevent deep seated shear failure. However stone column technique would be
comparatively costlier than providing polymeric vertical drains. Hence stone column technique is
selectively adopted to support structures which are sensitive to large amount of settlement or in cases
where it is also required to increase the bearing capacity of the sub-soil. At locations where undisturbed
shear strength of clayey soil (Su) is lower than 15 kPa, providing stone columns may result in
considerable wastage of stone aggregates and Geosynthetic encased stone columns may be adopted
in such places. IS 15284 (Part 1) provides guidelines for design and construction of stone columns.
4.5.4 Instrumentation and monitoring
Field instrumentation such as piezometers, settlement platform, settlement gauges and inclinometers
are used to monitor performance of band drains and possibly control the rate of embankment
construction and/or surcharge. It is important that both the designer and the instrumentation personnel
have a full appreciation of the instrumentation being installed. Generally settlement measuring devices
of different types like settlement platforms, deep settlement points or horizontal deflection devices are
used to measure only the rate and total amount of consolidation. An inclinometer is used to measure
horizontal deflection with depth and as a warning device against potential failure. The pore pressure
devices (piezometers) are used for both calculation of achieved consolidation rate and excessive build
up of pore pressure which are an indication of potential failure. Proper selection of instrumentation
devices and the frequency of monitoring a project are important. For simple projects where stability is of
no concern, and time is not the critical factor, only surface settlement platforms, which are relatively
easy to install, are needed. In situations where stability is critical, pore pressure measurements and
measurements of horizontal deformations (using inclinometer) are also necessary. The monitoring can
be done daily or once in two/three days during loading period depending on rate of loading. The
periodicity of taking the readings from the instruments can then be reduced to once a week or ten days
gradually after loading is over. The design of PVD system and monitoring of consolidation using
instrumentation are a specialised job and hence advice of geotechnical consultants is to be obtained in
these tasks.
4.6 Embankment Construction in Waterlogged Areas
When embankment construction is to be undertaken through an existing pond, dewatering and slush
removal should be taken up before placing the embankment fill. In case dewatering is not considered to
be feasible and embankment is to be constructed under water, only acceptable granular material shall
be used. Acceptable granular material should consist of well graded, hard durable particles with
maximum particle size not exceeding 75 mm. The material should be non plastic having coefficient of
uniformity not less than 10. The material placed in standing water shall be deposited by end tipping
without compaction.
Other methods which can be adopted in water logged areas include – Depressing the water table by
using geotextile wrapped aggregate drains (also known as trench drains), raising the embankment
height and providing a capillary cut off. Custom made synthetic drains made of polymeric materials are
also available which can be used in place of aggregate trench drains. For more details regarding
embankment construction in waterlogged/ salt infested areas or in areas where ground water table is
very high, IRC: 34, ‘Recommendations for road construction in areas affected by waterlogging, flooding
and/or salts infestation’ may be referred.

27. 19
Chapter – 5
SEA EROSION CONTROL TECHNIQUES & RIVER BANK PROTECTION
Coastal beach erosion occurs in various forms around the world. This phenomenon gets more acute
during cyclones and in-turn causes damage to infrastructure facilities including roads. This is due to
severity of waves and storm surge which result in coastal erosion. The basic approach to mitigate
coastal erosion related problems is to provide suitable cover to the soil. The measures to control
coastal erosion can be categorised as structural and soft/ non-structural. These can be taken up
together or separately also. Structural measures used for arresting coastal erosion are sea wall,
revetment (rock armour, gabion mattress or precast concrete block revetment systems), offshore
breakwater, groynes, etc. Soft measures generally adopted to prevent coastal erosion are artificial
nourishment of beaches, vegetative cover such as mangrove plantation, etc. Instead of providing rock
armour layer, latest and environmental friendly technologies which make use of geosynthetics for
construction of armour protection layer can also be adopted.
5.1 Wave Generation in Sea
Waves are caused by a disturbance of the water surface. Such disturbances become more prominent
during cyclones because of wave surge and high speed winds. Most waves are generated by wind.
After waves are formed, they can propagate across the surface of the sea for thousands of miles. When
waves break on a shoreline or coastal structure, they have fluid velocities and accelerations that can
impart significant forces. The wave period of individual waves remains constant through the
transformations until breaking but the direction of propagation and the wave height can change
significantly. As a wave moves into shallower water the wavelength decreases and the wave height
increases. Waves break at two general limits:
 In deepwater, waves can become too steep and break when the wave steepness defined as, H/L,
approaches 1/7 (where H = Height of the wave i.e., distance between crest of the wave and water
surface, L = Wave length defined as distance between two successive wave crests).
 In shallow water, waves break when they reach a limiting depth (d) of water.
This depth-limited breaking is important in the design of coastal revetments protecting highways. For an
individual wave, the limiting depth is roughly equal to the wave height and lies in the range given below:
0.8 <
Maxd
H






> 1.2 ........... Equation 5.1
A practical value of wave height which can be considered when there is mild sandy slope offshore is:
Maxd
H






≈ 0.8 ........... Equation 5.2
5.2 Systems for Protection of Coastline Against Sea Erosion
The systems adopted for protection against water erosion comprise of two different parts – the outer
revetment or armour layer to absorb the hydraulic energy of velocity of water flow and/or the wave
energy; and the inner part of filter layer. Revetment systems in the form of rip-rap blocks, prefabricated
concrete elements or gabion mattresses or RCC/stone masonry walls are most commonly used as

28. 20
armour layer. The function of inner filter layer is to prevent soil particles from being eroded and to allow
free escape of internal water simultaneously. Conventionally several layers of granular material with
well designed grain size distribution and thicknesses are used for this purpose. Geotextiles can be
successfully adopted to replace such granular filter material. They are now being increasingly adopted
owing to various technical advantages, cost benefits, ease of installation, faster completion of the
project and superior long term performance of the system. Fig 5.1 to 5.6 show photos of protection
measures adopted at various locations in India for protection of sea coast.
5.2.1 Bulkheads and revetments
The distinction between revetments, seawalls, and bulkheads is one of functional purpose. Revetments
are layers of protection on the top of a sloped surface to protect the underlying soil. Seawalls are
designed to protect beach against large wave forces. Bulkheads are designed primarily to retain the soil
behind a vertical wall in locations with less wave action. Bulkheads are mostly adopted where wave
heights are very small. Seawalls are more common where wave heights are quite large. Revetments
are often common in intermediate situations such as on bay or lake shorelines. Seawalls can be rigid
structures or rubble-mound structures specifically designed to withstand large waves. Vertical sheet pile
seawalls with concrete caps are common but require extensive marine structural design. A more
common seawall design type is a rubble-mound that looks very much like a revetment with larger
stones to withstand the design wave height. Thus, the two terms, seawalls and revetments, can be
used interchangeably with the former typically used for the larger wave environments.
5.2.2 Seawall
Seawall is useful in case of protection of specific area from erosion due to waves and storm surges.
Seawalls are constructed along the coast adopting stone masonry technique or using reinforced
cement concrete. Seawall can be constructed using gabions also when wave heights are low, typically
less than about 1.0 m. Seawalls constructed using gabions are permeable and flexible; thereby they
would be able to withstand differential settlement without loss of its structural integrity. Provision of filter
layer behind the seawall is essential to prevent piping of sand and subsequent destabilisation of
structure. Sometimes a combination of sea wall constructed using masonry or reinforced cement
concrete is further protected on sea side using gabions or concrete blocks/ tetrapods. Design of the
masonry or gabion seawalls is to be carried out in a manner similar to design of retaining walls, to
ensure stability against overturning, sliding, excessive foundation pressure (bearing capacity failure)
and water uplift. Additionally ‘Wave flume studies’ may also have to be adopted to arrive at satisfactory
design of stone, rock and/or concrete armour units.
5.2.3 Breakwater
Breakwaters are coastal structures constructed to protect an area from the effects of waves.
Breakwaters are adopted to protect a ship berthing area, to train and prevent silting of the entrance of
river mouths or to prevent erosion of coastlines. However, adverse effects are observed on down drift
side and it should be avoided unless their main purpose is to protect a specific area at the cost of
adjoining areas. An off-shore breakwater may be constructed to prevent beaches or coastlines from
erosion by wave activity. The off-shore breakwaters are submerged structures located at certain
distance offshore in order to dissipate wave energy before they reach shoreline. The broken waves
would not be having the energy to erode the beach or coastline and the coastline may even increase in
extent as a result of accretion. It is an expensive option and needs regular maintenance.

29. 21
Fig – 5.1 Provision of Sea Wall and Concrete Tetrapods for Sea Erosion Protection at Mumbai
Fig – 5.2 Protection Against Sea Erosion by Retaining Wall and Gabions – Puri Konark Road
Fig – 5.3 Another View of Protection Works – Puri Konark Road

30. 22
Fig – 5.4 Beach Protection Using Boulder Revetment at Paradip, Odisha
Fig – 5.5 Another View of Boulder Revetment at Paradip, Odisha
Fig – 5.6 Masonry Sea Wall Constructed at Sea Coast at Koteshwar, Kori Creek, Gujarat

31. 23
5.2.4 Soft Structural/ Non-structural measures – Artificial nourishment of beaches
Beach nourishment may be adopted for protection and beach development. Combination of
nourishment of beaches with seawall/ groynes will create beach in front of protected area and eliminate
leeside erosion.
5.2.5 Vegetation cover
Plantation of mangroves and palm trees can be taken up for beach protection. Vegetation cover can
restrict sand movement and erosion.
5.2.6 Artificial reef balls
A reef ball is a designed artificial reef used to restore ailing coral reefs and to create new fishing and
scuba diving sites. Reef balls are the only type of artificial reefs that can be floated and towed behind a
boat. Reef balls are made of a special grade marine environment resistant concrete and are designed
to mimic natural reef systems. They are also used widely to create habitats for fish and other marine
and fresh water species. Reef balls are made in many sizes to best match the natural reef system
which is being mimicked.
Out of these measures, depending on the techno-economic viability, any suitable measure can be
adopted, while a combination of these measures usually gives optimum results.
5.3 Design of Coastal Rip-rap Against Wave Attack
Rip-rap can be used for protection from four different types of hydraulic situations: direct rainfall
impacts, overland flow, stream or river currents, and waves. This section addresses only wave attack.
IRC:89 provides procedures for the design of riprap revetments for channel bank protection on larger
streams and rivers where the active force of the flowing water exceeds the bank material’s ability to
resist movement. Brief details of the same are provided in section 5.6. Flow in a stream or river is
unidirectional and typically aligned parallel to the banks. Waves produce oscillatory velocities and
accelerations that can be in almost any direction on a revetment. In such situations, it is recommended
that ‘Hudson’s equation’ be used to estimate stone size for revetments subject to wave action. This
involves determining the design wave height (as per equation 5.1) and using Hudson’s equation to size
the stones to be used for rip-rap. This approach can lead to designs with larger stones and narrower
stone gradations than designs for non-wave situations. The difference is due to the higher forces
caused by waves. Situations where riverine and wave flows are also significant, the design engineer
should consider both design approaches and develop a conservative design. A simplified version of
Hudson’s equation for calculating required median weight for the outer, or armour layer, stones is:
θcot
280H
W
3
50  ........... Equation 5.3
Where, W50 = Median weight of armour stone in kgs
H = Design wave height in m
Θ = Slope
The range of recommended slopes for revetments is up to 2:1 (horizontal:vertical) or flatter. Hence cotΘ
would be equal to 2 for a 2:1 slope and cotΘ=3 for a 3:1 slope. Apart from armour stone, either graded
aggregate filter layers or preferably geotextile needs to be placed below armour to prevent piping

32. 24
failure. Selection of geotextile can be carried out as per IRC SP:59. A typical rip-rap gradation for
coastal revetment with a median weight W50 = 350 kgs, will have 50 per cent of stones weighing
between 100 kgs to 350 kgs, 30 per cent weighing between 350 kgs to 700 kgs, and 20 per cent
weighing between 700 kgs to 1350 kgs. Thus, the recommended coastal revetment gradation
precludes the smaller stones and allows for some larger stones as compared to gradation adopted for
river bank protection. These smaller stones are typically not included in coastal revetments because of
their tendency to move in response to wave action. Further it may be noted that the construction of a
revetment, while it protects the upland, does not address the underlying cause of erosion. The depths
at the toe of the revetment will likely increase if the erosion process continues. The presence of a
revetment or seawall can increase the vertical erosion at its base. A common practice to overcome toe
erosion is to extend revetment beyond the slope inside water and provide toe protection. A commonly
proposed alternative to rubble mound revetments is a concrete block revetment, which are also known
as 'Tetrapods'. Some of these have physical interlocking between individual blocks also. Many such
concrete blocks which have a patented shape are also available. These are essentially unreinforced
concrete objects designed to resist the wave action. If the intensity of wave action is severe, then
additional layers of armour protection would be required. In such cases, ‘Tetrapods’ can be placed over
stone blocks.
5.4 Use of Geosynthetic Products as Revetment
Coastal and waterways protection applications comprised the earliest use of geosynthetics. Over the
last 40 years, there have been numerous coastal and waterway protection projects that have utilised
geosynthetics. Geosynthetics can be used as components of coastal and waterway protection
measures in two different ways – they can be used as filters within coastal and waterway protection
structures and they can also be used to create revetment systems (containers) to act as mass-gravity
protection works. During the construction of structural measures to control sea erosion, problem
generally faced is the non availability of construction materials like big size boulders, sand, etc., within
reasonable and cost effective distance. This problem can be sorted to a great extent by using
geosynthetic revetment systems. The most universal and widely used geotextile containers are well-
known, ubiquitous sand bags which are seen world over for shoring up flood defences in times of
natural calamity. The dominant geosynthetic material used for making revetment systems is geotextiles,
which are robust and permeable materials. Three types of geoetextile revetment systems differentiated
by geometrical shape are available. They are geotextile tubes, geotextile containers and geotextile
bags. Geotextile tubes are tubular containers that are filled in-situ on land or in water. Geotextile
containers are large volume containers that are filled above water and then deposited into the
submarine environment. Geotextile bags are small volume containers that are filled on land or above
water and then pattern-placed either near water or below water level. The geotextile revetment systems
have the following advantages:
1. They are resistant to chemical attacks occurring in usage, especially to alkalies and acids.
2. Geotextiles are quite durable when exposed to elements of nature like – Sun, precipitation, etc.
However, ultraviolet radiation reduces their strength in long term. Hence they need to be treated to
enhance their ultraviolet resistance if they are going to be exposed to sun during their service life.
3. They are resistant to organic attacks like bacteria and fungus and are not attractive to rats or
termites.
Geotextile containers behave as mass-gravity elements that can resist hydraulic forces. For these
applications, the geotextile skin should have specific mechanical, hydraulic and durability requirements.
Distinction must be made between those applications where the geotextile containment is required for
only temporary use and those applications that require long term performance. For temporary works the
requirements of the geotextile container itself is fairly basic as it only has a short life over which it has to

33. 25
perform, however for long term applications, the performance requirements of the geotextile container
are more severe. With regard to long term performance, distinction must also be made according to
type of hydraulic environment acting on the geotextile container. For example, the action of still water,
or intermittent water flows, will have a different effect on the geotextile container than the action of
breaking waves.
5.4.1 Geotextile tubes
Geotextile tubes are large cylindrical structures made using high strength woven geotextile material
which are then filled with dredged material in-situ. Geotextile tubes may be used for a range of coastal
and waterway protection applications where barrier type, mass-gravity, structures are required. The
dredged material is usually pumped in a slurry form from nearby area and consists of a mixture of
sandy soil and water. The geotextile tube, being permeable, enables the excess water to pass the
geotextile skin while the fill attains a compacted, stable mass within the tube. For coastal and waterway
applications the type of fill used is sand, or a significant percentage would be sand. The reasons being
– sand can be compacted to a good density by hydraulic means, sand has good internal shear strength
which gets further improved by the presence of confining geotextile tube skin, and this type of fill once
compacted, will not undergo settlement, which would change the shape of the filled up geotextile tube
The tube is filled by direct coupling to a hydraulic pumping system conveying dredged material.
Designed with appropriately sized openings called ‘Filling Ports’, the geosynthetic tubes retains fill
material while allowing water to permeate through tube wall. After dewatering typically very little
consolidation will occur in case of pure sands while it may be as much as 70 per cent in case of tube
that has been filled with fine grained organic material. Openings called, fill ports are provided in
geotextile tubes at a spacing of about 8 to 10 m for filling dredged material. Special high strength
seaming techniques are adopted in their manufacturing process to resist pressure during pumping
action. Geotextile tubes permanently trap granular material in both dry and underground construction.
Geotextile tubes are generally about 1 m to 3 m in diameter, though they can be custom made to any
size depending on their application. Geotextile tubes ranging in diameters from 1.5 m to 5.0 m are
available for coastal and waterway protection applications. Stacking of geotextile tubes one over other
can also be made to construct structures of higher heights. Geotextile tubes may be used for a range of
coastal and waterway protection applications where barrier type, mass gravity structures are required.
Geotextile tubes can be used for construction of groynes, off-shore breakwater, etc. When geotextile
tubes are used as off-shore breakwater structures, the dimensions of geotextile tubes are to be chosen
in such a way that waves break over the geotextile tubes.
Geotextile tubes are normally described in terms of either a theoretical diameter, D or a circumference,
C. While these two properties represent the fundamental characteristics of geotextile tubes they are not
of direct interest when it comes to engineering parameters for coastal and waterway protection
Fig – 5.7 Typical Components of a Geotextile Tube

34. 26
applications where the geotextile tube in its filled condition is of prime importance. When the geotextile
tube has been filled with sand, it assumes an oval shape. The width of the oval tube and its height are
of importance from engineering performance point of view. Table 5.1 lists relationships between the
fundamental geotextile tube characteristics and engineering parameters. The relationships are
applicable to geotextile tubes that have a maximum strain of about 15 per cent, low unconfined creep,
and are filled to maximum capacity with sand. It is also assumed that the foundation beneath the tube is
a flat, solid surface.
Geotextile tubes are used for revetments where their contained fill is used to provide stability. They
have been used for both submerged as well as exposed revetments (Fig 5.8). For submerged
revetments, the geotextile tube is covered by local soil and is only required to provide protection when
the soil cover has been eroded during the periods of intermittent storm activity. Once the storm is over,
the revetment is covered by soil again either naturally or by maintenance filling. For exposed
revetments, the geotextile tube is exposed throughout its required design life. To prevent erosion of the
foundation soil in its vicinity, and undermining of geotextile tube revetment, it is common practice to
install a scour apron. This scour apron usually consists of a geotextile filter layer that passes beneath
the geotextile tube and is anchored at the extremity by a smaller sized geotextile tube, called anchor
tube.
Table – 5.1 Basic Engineering Parameters for Geotextile Tubes filled with Sand
Engineering parameter In terms of theoretical diameter, D In terms of tube circumference, C
Maximum filled height, H H ≈ 0.6 D H ≈ 0.19 C
Filled width, W W ≈ 1.4 D W ≈ 0.45 C
Base contact width, b b ≈ 0.9 D b ≈ 0.29 C
Cross sectional area, A A ≈ 0.65 D2 A ≈ 0.07 C2
Average vertical stress
at base, σ
σv ≈ 0.72 γ D
(γ = Density of the fill)
σv ≈ 0.24 γ C
(γ = Density of the fill)
Revetments using multiple-height geotextile tubes are also constructed. Here the geotextile tubes are
staggered horizontally to achieve the required stability. Considerable care should be exercised during
construction of these types of revetments to ensure the water emanating from hydraulic filling of upper
geotextile tubes does not erode the soil and undermine the lower geotextile tubes in the multiple-height
revetment structure. In a similar manner, geotextile tubes can be used for constructing offshore
breakwaters, protection dykes, containment dykes and groynes as shown in Fig 5.9.
Fig 5.8 – Geotextile Tube Application for Coastal Protection

35. 27
Fig 5.9 – Applications of Geotextile Tubes
Fig 5.10 – Typical Use of Multiple Geotextile Tubes for Coastal Embankment Construction
Fig 5.11 – Geotextile Tubes with Gabions as Armour Protection layer

36. 28
5.4.2 Geotextile containers
Geotextile containers, as their name may imply, are large volume containers that are filled above water
and then positioned and placed at reasonable water depth. Geotextile containers are made from high
strength woven geotextiles or a combination of woven and non woven geotextiles (depending on the fill
characteristics) which are filled with sand/ dredged material. The volumes of these containers more
commonly range from 100 m3 to 700 m3, although containers as large as 1000 m3 have also been
installed. To facilitate the installation of geotextile containers, an efficient and practical installation
system is required. To date, this has been achieved by using split bottom barges. This entails, the filling
of the geotextile container in a split bottom barge. The container is then sealed and the barge is
positioned at the correct dumping location. The split bottom of the barge is then opened and the
container is deposited on the seabed. Geotextile containers are used as mass-gravity structural
components in coastal and waterway protection applications such as offshore breakwaters,
containment dykes, artificial reefs, slope buttressing, etc in a manner similar to geotextile tubes.
5.4.3 Geotextile bags
Geotextile bags are made from high strength woven and nonwoven geotextiles or a combination of
these can be used (depending on the fill characteristics) which are filled with sand/ dredged material.
Geotextile bags are used at sea shores or bunds adjacent to rivers which are to be protected from
erosion, especially during emergency situations. Geotextile bags have also been used as revetments,
breakwaters, etc to build structural erosion protection measures during normal periods also (Fig 5.13).
Geotextile bags provide stability and prevent erosion. Geotextile bags are filled off-site and then
installed to the geometry required in a similar manner to geotextile containers. For best performance,
they have to be filled to maximum volume and density with sand in an identical manner to geotextile
tubes. However, geotextile bags have two major differences to other geotextile containment techniques
– they can be manufactured in a range of shapes, and they are installed in a pattern-placed
arrangement that greatly improves their overall stability and performance. Geotextile bags ranging in
volume from 0.05 m3 to about 5 m3, which are pillow shaped or box shaped or mattress shaped are
available, depending on the required application. Filling geotextile bags with dry sand becomes more
difficult as the volume of the bag increases, but filling task can be efficiently done by using sand+water
mixture (hydraulically filling the sand into a bag). Filled density and volume are important from the view
point of maximising the stability, but it is also important from the view point of minimising the effects of
Fig 5.12 – View of geotextile Tubes Covered with Armour Protection Layer of Gabions

37. 29
fill liquefaction and loss of shape of the geotextile bags. To ensure that the contained fill is maintained
in its dense state, the geotextile skin should have adequate tensile strength.
One major advantage of geotextile bags is that these small volume units can be used to construct
hydraulic and marine structures that require adherence to designed geometrical shape accurately. This
makes them preferable to large volume units such as geotextile containers when specific slope and
height tolerances are to be attained. Another advantage of small volume units of geotextile bags is that
maintenance and remedial works can be carried out easily by replacing the failed bags. This is much
simpler than carrying out remedial works on large volume containment units.
5.4.4 Gabions
Gabions, which are mesh like structures filled with relatively small size stones, are an attractive
alternative to large boulder stones for various erosion control and scour protection applications.
Gabions by holding the small stones together, function like large boulders but at the same time
facilitates easy construction and offer a flexible structure. Thereby gabions provide a technically
Fig 5.14 – Geotextile Bag
Fig 5.13 – Shore Reclamation Using Geotextile Bags

38. 30
satisfactory and cost effective solution. Gabions can be made from either polymeric material or double
twisted steel wires having zinc+polymer coating. Gabions are generally available in a prefabricated
collapsible form with the bottom and four sides held together by appropriate binding and with a flip open
top lid. Filled with stones, the gabion becomes a large, flexible and permeable building block using
which a broad range of structures can be built. Because of their inherent flexibility, gabion structure can
yield to earth movement and retain their full efficiency while remaining structurally sound. They are
quite unlike rigid or semi-rigid structures, which may suffer complete failure when even slight changes
occur in their foundation. Besides the above, gabions can be easily lifted by cranes, they are suitable
for underwater construction and several gabions can be tied together to create continuous, integral
structures. The pervious structure of gabions gradually absorbs the heavy wave impact than an
impervious structure. IS 16014 provides specifications for zinc+polymer coated steel wire gabions.
Compared to steel wire gabions, polymeric gabions have advantages like superior corrosion resistance,
ability to withstand acidic and alkaline environment, excellent durability, excellent flexibility to take the
shape of ground contour, etc. However, these gabions due to their very high flexibility, may not be as
much amenable to construction of retaining structures as compared to steel wire rope gabions.
5.5 Geotextiles as Filters
Below the revetments (either stone/rock or concrete armour units), filters are invariably required to
prevent soil washout. Traditional granular filters usually consist of several layers of stone aggregates. If
the water forces are strong enough and the soil to be protected is fine grained, then upto four layers of
granular materials may be required to satisfy the hydraulic design requirements. Hence, this kind of
relatively complex structures can be expensive and difficult to construct. Furthermore, granular/
aggregate filters are difficult to place on steep slopes, cannot always be installed in tidal zones and
laying process demands reliable and expert supervision. Geotextiles can be used as substitutes for one
or more granular under layer materials below revetments. Geotextiles offer many advantages over
granular filter materials:
 They enable design flexibility with regard to the choice of the size of the granular material in the
layer immediately adjacent to the geotextile filter.
 They are easier to install to specific geometrical configurations than granular materials – in
many cases below water level.
 In general, in-situ quality control test requirements for geotextiles are nominal.
Where geotextiles are used as filters for coastal and waterway protection, their primary function is to
prevent the erosion of soil through the protection structure and thus prevent instability. In case of
geotextile filters, hydraulic characteristics like apparent opening size and permittivity are most
Fig 5.15 – Use of Gabions for River Bank Protection

39. 31
important. The selection of filter fabric with correct opening size depends on the percentage of finer
material available in bed material. In order to fulfil its function, the geotextile material has to be robust
enough to resist mechanical stresses applied to it during installation. Secondly, the geotextile material
must have required hydraulic properties in order to perform as a filter material. Thirdly, the geotextile
must have adequate durability to maintain its mechanical and hydraulic properties throughout the
design life of revetment. The criteria for selection of filter fabric can be based on IRC SP: 59. As the
weight of the stones/ drop height increases, thicker geotextile having greater mass per unit area would
be required. Another important property would be trapezoidal tear strength. Normally, geotextiles
having trapezoidal tear strength varying from 200 to 600 N (ASTM D 4533) are used in coastal works.
When determining the appropriate hydraulic properties for the geotextile revetment filter consideration
needs to be given to the critical hydraulic regime that will act on the revetment structure over its design
life. Table 5.2 list the geotextile filter hydraulic properties requirements according to the type of
hydraulic regime. When several different hydraulic regimes occur at the same location then the most
critical hydraulic regime (1 being the least critical and 3 being the most critical in Table 5.2) should be
chosen for design. While installing geotextile filter, it is to be first laid out on the soil surface prior to
placing stones and rocks. For good long term performance, the geotextile filter should be covered with
an adequate thickness of granular material to ensure that it remains protected from the effects of long
term exposure to ultra-voilet (UV) rays. The minimum thickness of stone coverage above the geotextile
filter to protect against UV radiation should be atleast two times the maximum stone size in the rock
armour layer above the geotextile filter. During installation, it may be inevitable that the geotextile filter
would be exposed to UV rays and this condition may extend, depending upon pace of construction. To
cover such eventualities, the UV stability of requirement of geotextile to be used should meet the
specification requirements as per IRC SP:59. The geotextile filter coverage beneath the revetment
armour layer should extend beyond the zone of erosion. This would ensure that revetment structure will
remain stable throughout the life of the structure.
Table – 5.2 Geotextile Hydraulic Property Requirements under Different Regimes
1 Water current flows parallel to revetment face
Non-dispersive soil O95 ≤ 0.35 mm
Dispersive soil d15 ≤ O95 ≤ d85
2 Gradual reversing water flows d15 ≤ O95 ≤ d85
3 Impacting wave activity d15 ≤ O95 ≤ d50
1. d15, d50, and d85 are percentile particle size fractions to be protected
2. O95 is apparent opening size (AOS) of the geotextile filter (ASTM D 4751)
5.5.1 Geotextile filters for breakwaters
For rubble mound and caisson wall breakwaters geotextile filters are placed on top of the sea bed prior
to construction of the breakwater. In this location, the primary role of geotextile filter is to prevent
erosion of sea bed and the undermining of the breakwater. To facilitate installation on the sea bed, the
geotextile filter is usually prefabricated onsite into a fascine mattress structure. This technique involves
the fabrication of geotextile filter into large sheets on land and attaching an interconnecting grid of
fascines, bamboo or timber. The resulting mattress is then pulled into the water and floated into place
and sunk on the sea bed. This technique has proved to be an efficient and cost effective means of
installing geotextile filters on the sea bed. The tensile stresses imposed on the geotextile filter during
fascine mattress installation procedure are relatively high. Consequently, woven geotextiles with wide-
width tensile strengths ranging from 80 kN/m to 200 kN/m are normally used for this type of application.

40. 32
Offshore breakwaters also may be constructed to protect beaches or coastlines from erosion by wave
activity. In such cases, the breakwaters would be submerged structures that force the waves to break
when passing thus, expending much of their wave energy. The broken waves would not have the
energy to erode the beach or coastline and the coastline may even extend outwards into the sea as a
result.
5.5.2 Geotextile filters for containment dykes
To reclaim land from sea, it is common to first construct a containment dyke around the extremity of the
reclamation area. Soil or sand fill is then dry dumped or hydraulically pumped into the containment area
to form dry land. The function of the containment dyke is to prevent loss of the placed soil or sand fill
into the surrounding water. The nature of the containment dyke is slightly different depending on
whether the reclamation occurs in relatively deep water or in shallow water. Where land reclamation
occurs in relatively deep water, the size of the containment dyke is fairly large and may require two or
more stages to complete the structure. Commonly, the dyke consists of a rubble mound of dumped
rock with a geotextile filter placed across the base of the dyke. The role of the geotextile filter is to
prevent the loss of reclamation fill through the rubble mound dyke and the erosion of the sea bed
beneath the rubble mound. The geotextile filter across the base of the dyke can also prevent the loss of
the rubble mound material into the sea bed if the foundation is soft. For permanent protection, a rock
armour layer may be placed on the outside of the rubble mound depending on the water forces acting
on the structure.
Where land reclamation occurs in relatively shallow water, the containment dyke is normally
constructed in a single stage. Commonly, the bund consists of a rubble mound with geotextile filter
placed across the base of the dyke. Again for permanent protection, a rock armour layer may be placed
on the outside of the rubble mound depending on the water forces acting on the structure.
It is not uncommon for the base geotextile filter beneath the containment dyke to have different
properties on different faces. Normally, the base geotextile filter is installed in a manner similar to the
breakwater structure which may require a fascine mattress approach to installation. This imparts
relatively high tensile stresses on the geotextile filter during installation, and consequently woven
geotextile filters with wide width tensile strengths between 80 kN/m and 200 kN/m are usually used for
this purpose.
5.6 Mangrove Cultivation
Among soft/ non-structural measures for coastal protection, mangrove cultivation is one of the most
effective techniques. Mangrove is a group of typical tropical and specialised trees growing in the saline
and brackish water system. The mangrove trees are highly productive, economical and most
importantly they protect the shoreline from erosion and cyclonic impact. The mangroves are
angiosperms, with about 45 species found in India. They have special characters like viviparous
germination, pneumatophores, prop or knee roots and salt glands. These trees form a thick forest belt
on the deltas, along major estuaries, and fringe the estuarine banks, as well as backwaters. This
unique tree resource is useful for tannin extraction, paper and pulp, firewood, timber, charcoal, fodder
and several other by-products. The mangrove swamps are rich in the larvae of many economically
important fishes, prawns, crabs and bivalves. These are the most suitable area for feeding, breeding
and nursery grounds of these marine organisms and hence important for aquaculture
purposes. Afforestation of coastal areas suitable for mangrove cultivation would go a long way for
preventing soil erosion. Mangrove trees generally prefer soft, clay mud for their growth. These species
show different salinity tolerance limits. The expanse of mangrove forest depends on the intertidal

41. 33
expanse, substratum and salinity of soil as well as water. Out of 45 mangrove species occurring in
India, some are true mangrove while others are considered as 'associated' flora. The most dominant
mangrove species found along the east and west coast of India are listed below:
Rhizophora mucronata
R. apiculata
Bruguiera gymnorrhiza
B. parviflora
Sonneratia alba
S. caseolaris
Cariops tagal
Heretiera littoralis
Xylocarpus granatum
X. molluscensis
Excoecaria agallocha
Lumnitzera racemosa
Avicennia officinalis
A. marina
The species mentioned above are available easily and their seedlings (propagules) or seeds are also
available in considerable quantity in mangrove forest. Mangrove seeds (fruits and seedlings) are
always available in small quantity throughout the year. The main fruiting or seedling season, however,
start from June to September, when plenty of seedlings of all the Rhizophoraceae, Avicennia and other
types can be collected. Only mature seedlings of these mangrove species should be collected for
afforestation or nursery purpose. The seedlings of rhizophoracious trees have a podlike structure with
tapering end of varying sizes and with typical morphological characters. Avicennia fruits are triangular in
shape while Sonneratia is globular. It is however, always advisable to store these seedlings partially
immersed (pointed end in water) in seawater. There are two ways of planting the mangrove seedlings
 Direct planting in the swamp
 Raising seedling in the nursery
Seedlings which are healthy, non-infected and fully matured should only be used for planting. Any
intertidal area (between the high tide and low tide) where mangroves are absent and the substratum is
of soft clay or mud and is inundated by regular tidal waters every day, are suitable for direct mangrove
planting. Along the Gujarat coast and West Bengal, where intertidal expanse is very large with highest
tidal amplitude of 6 to 8 m, the upper limit of 1 m tidal water level has to be selected. After selecting the
area to be planted, planting of seedlings may be undertaken according to the length of the propagules.
Rhizophora mucronata or Rhizophora apiculata whose seedlings are the longest should always be
planted towards the waterfront, these can be followed by Kandelia, Ceriops, Bruguiera, Avicennia,
Lumnitzera, etc. Species with smallest seeds like Sonneratia should come to the landward side of the
intertidal expanse, followed by species of grasses. Direct planting method has to be used in open
areas. Nursery technique method is useful where the mangrove species are not available in plenty.
This also has advantages like selected species can be grown in large numbers. Mangrove nurseries
can be developed in the upper part of the intertidal region where seedlings can be grown in
polyethylene bags supported with bamboos. The mangrove nursery may be located near the estuary or
sea where seawater or estuarine water is available. The nursery may be on the open ground or in the
low lying protected areas where seawater reaches. The collected and selected seedlings are inserted in
the polyethylene bags filled with mangrove soil. If the nursery is on the raised ground then the
perforations in the bags are not needed, but the nurseries in the low lying area need the perforations in
the polyethylene bags. Care should be taken to cut open the polythene bags at the base before