2. I. INTRODUCTION
• Provision of adequate drainage is an essential part of
pavement design.
– Protection of pavement structure
– Improves road safety
• Can be categorically studied in three parts:
1. Surface Drainage
• Drainage on the adjoining land and roadway
surface
• Side Drainage and Cross Drainage
2. Sub-surface Drainage
3. I. INTRODUCTION
• Effects of water on the pavement
structure
• Presence of moisture causes:
o reduction in the stability of the soil mass.
o considerable variation in volume of subgrade in clayey
soils.
o Waves and corrugations failure in flexible pavements.
o Stripping failure in flexible pavements.
o Mud pumping failure in rigid pavements.
4. II.DESIGN OFSURFACE DRAINAGE SYSTEMS
• Can be divided into three phases:
i. Estimation of the quantity of water that can
iii.
reach any element of the system.
ii. Hydraulic design of each element of the
system.
Comparison of alternative systems and
materials
• Criteria-Lowest annual cost alternative
5. II.DESIGN OFSURFACE DRAINAGE SYSTEMS
1. Rainfall Intensity
• Runoff is obtained by considering expected sever
storm.
Return period of 5, 10, 20, 25, 50, and 1 0 0
years
• Quantity of runoff depends on intensity and duration.
• Duration= Time of Concentration
• The time required for water from the remotest place
to reach a specific point on the drainage system.
• =T1 +T2
• T1 = over land flow time
• T2 = time of flow in the longitudinal drain
7. II. DESIGN OF SURFACE
DRAINAGE
SYSTEMS
Source: ERA Manual, 2002
8. II.DESIGN OFSURFACE DRAINAGE SYSTEMS
2. Computation of Runoff
• Rain water expelled from the road surface
i. Infiltration
ii. Runoff
iii. Evaporation- insignificant
• Infiltration depends on:
• Type and gradation of soil
• Soil covers, moisture content of the soil
• Presence of impervious layers near the surface.
9. II.DESIGN OFSURFACE DRAINAGE SYSTEMS
• Infiltration contd.
• Rate of infiltration on bare soil is less than on a
turfed soil.
constant
• Frozen soil is impervious
• Rate of infiltration is assumed to be
during any specific design storm.
• Runoff depends on:
• Nature of the ground, degree of saturation, and
slope of the surface
• Rate of runoff greater on smooth surfaces.
10. II.DESIGN OFSURFACE DRAINAGE SYSTEMS
• Rational Formula- accurate way of estimating
runoff up to areas of 0.5 km2
Q 0.00278 CIA
C 1 A1 C 2 A 2
C
A1 A 2
• If the water shade
is made up of
different surfaces
• = runoff (m3/sec)
• C=coefficient, representing ratio of runoff to rainfall
• I = intensity of rainfall (mm/hr)for a duration equal to the time
of concentration
• A = catchment area tributary to the design location, ha
14. III.DESIGN OFSIDE DITCHESAND OPEN
CHANNELS
• THE MANNING’S FORMULA
• Once the quantity of runoff is known, the design of
ditches and similar structures is based on the
principles of open channel flow.
• Mannings’s formula assumes steady flow in a
uniform channel.
V
1
R 2 / 3
S1 / 2
n
Q V A
Where:
• V = mean velocity (m/sec)
• R = hydraulic radius (m)= Area/wetted perimeter
• S=slope of the channel (m/m)
• n=Manning’s roughness coefficient
17. III.DESIGN OFSIDE DITCHESAND OPEN
CHANNELS
• Examples:
1. The maximum quantity of water expected in one of the
open longitudinal drains on clayey soil is 0.9
m3/sec. Design the cross section and longitudinal
slope of trapezoidal drain assuming the bottom
width of the trapezoidal section to be 1.0 m and
cross slopes to be 1V:1.5H. The allowable velocity
of flow in the drain is 1.2 m/sec and Manning’s
roughness coefficient is 0.02.
18. III.DESIGN OFSIDE DITCHESAND OPEN
CHANNELS
• Examples:
2. The surface water from road side is drained to the
longitudinal side drain from across one half a bituminous
pavement surface of total width 7.0 m, shoulder and adjoining
land of width 8.0 m one side of the drain. On the other side of
the longitudinal drain, water flows across from reserved land
with grass and 2% cross slope towards the side drain, the
width of this strip of land being 2 5 m. The run off coefficients
of the pavement, shoulder and reserve land with grass surface
are 0.8, 0.25, and 0 3 5 respectively. The length of the
stretch of land parallel to the road from where water is
expected to flow to the side drain is about 4 0 0 m. Estimate
the quantity of run-off flowing in the drain assuming 2 5 years
period of frequency.
19. IV.SUBSURFACE DRAINAGE
1. Lowering of Water Table
• Highest level of water table should be below the
subgrade.
• Practically 1.0 to 1.2 m below subgrade
• Relatively permeable soil-
• Longitudinal drains are mainly used
• Impermeable soils-
• Transverse drains may be necessary inaddition
to longitudinal drains
20. IV.SUBSURFACE DRAINAGE
1. Lowering of Water Table
Fig. Symmetrical longitudinal drains used to lower the
groundwater table and to collect water infiltrating the
pavement.
21. • Lowering of Water Table
IV. SUBSURFACE DRAINAGE
Longitudinal
Drain
Transverse
Drains
Fig. Lowering of water table using
Transverse Drains (Plan View)
22. IV.SUBSURFACE DRAINAGE
2. Seepage Control
• If seepage zone is at a depth less than
0.6 to 0.9 m below subgrade level,
• Use longitudinal pipe drain in
trench with filter material to
intercept the seepage flow.
• This phenomenon can be
explained using figures.
23. IV.SUBSURFACE DRAINAGE
2. Seepage Control
Fig. Longitudinal interceptor drain used to cut off
seepage and lower the groundwater table.
24. IV.SUBSURFACE DRAINAGE
2. Seepage Control
Groundwater seeps through the slope where the
water table intersects the land slope, and
Groundwater flows beneath the pavement while
also entering the pavement foundation materials.
Fig. (A) Illustration of ground water flow along a sloping
impervious layer toward a roadway.
26. IV.SUBSURFACE DRAINAGE
2. Seepage Control
Fig. Longitudinal collector drain used to remove
water seeping into the pavement structural
section.
27. IV.SUBSURFACE DRAINAGE
3. Control of Capillary Rise
• Capillary rise can be controlled by
Using a layer of granular material of suitable thickness.
Using a layer of impermeable capillary cutoff.
Capillary water should not rise above the thickness of the
granular layer
Granular material
Capillary rise
Highest water
Fig. Gtraabnleularcapillary
cutoff
28. IV.SUBSURFACE DRAINAGE
3. Control of Capillary Rise
• Bituminous layer or other geo-textiles can be used
as an impermeable layer.
Impermeable layer
Capillary rise
Highest water table
Fig. Impermeable layer capillary cutoff
29. IV.SUBSURFACE DRAINAGE
4. Design of Filter Material
• Proper filter material should be used for:
Subsurface drainage system and backfilling the
drainage trenches and
Criteria:
Permeability and Piping
5 Permeability criteria
D15 offilter
D15 of foundation
5
D15
of filter
of foundationD85
Piping criteria
30. IV.SUBSURFACE DRAINAGE
1 0
4. Design of Filter Material
DP=size of perforation in drain pipe
D85 Filter = 2DP
0
8 0
2DP○
6 0
4 0
Foundation
soil
Filter
Material
The area between
the two red curves
represents the
filler material.
Percent
passing
1.0
0.
0
2 0
0.0
5
○ ○5D1
85D
5
0.
Particle siz1e (mm), l1og scale
0 .00
1
1.
0
Fig. Design of Filter Material
31. • REFERENCES:
1. Highway Engineering, 7th Ed. Paul H. Wright and Karen
K. Dixon. Wiley (2004)
2. Highway Engineering, 8th Ed. S.K. Khanna and C.E.G.
Justo. (2001)
