1. Chapter I
INTRODUCTION
For the purpose of understanding soil and
Specific Objectives characterize it soils are classified. There exist
To get a broad out line of various classifications of soil based on
topics of study in soil Mechanics –I. particular purpose. Agricultural scientists
To understand importance of classified soil based on fertility; road engineers
soil mechanics in Civil Engineering. have classified soil on criteria of suitability of
To identify typical problems soil for pavement construction and so on. From
encountered in soil mechanics. engineering point of view soil have been
To know the development of classified on the basis of plasticity
soil mechanics over ages. characteristics and grain size distribution.
To acknowledge the It has been very well said that most of the
importance of learning from failures in problematic behaviour of soil are not on its
soil mechanics. own, but due to water present in it. In soil
To pay honour to those mechanics, we study behaviour of soil with
worked for the development of the various moisture content, movement of water
subject. through soil, its hydraulic conductivity /
permeability, seepage forces on soil masses,
The subject / course Soil mechanics piping failure, quick conditions etc.
discuss over the properties of soil, formation of Stresses on soil mass due to its own
soil, its behaviour under loads, movement of weight and due to externally applied load are
water through it, pressure it exerts on interesting. Due to presence of water and low
structures retaining it, its compressibility, its permeability of some soils these stresses are
stability in its natural forms etc. quite different from stresses in homogeneous
To understand soil and its properties and material. The particulate nature of soil adds
causes of its behaviour it is necessary to know some complexity to the problem. These
how a particular soil is formed, a soil deposit is concepts are deliberated in soil mechanics.
formed. Whether a soil deposit was formed due A special characteristic of soil is to undergo
to weathering of Igneous rocks or sedimentary large compression deformation without
rocks or metamorphic rocks, whether it was undergoing failure and simultaneous increase
formed by a process of mechanical weathering in its strength will be learnt in soil mechanics.
or chemical weathering, whether it was And another unique phenomenon of gradual
deposited at a place by water or wind or by transfer of load from entrapped water to solid
gravity does have a bearing on the behaviour particles of soil, consolidation, reveals itself in
of soil. soil mechanics.
To learn and anticipate behaviour of soil Soil being particulate mass derives its
from the point of view of engineering / resistance to external load / pressure etc from
technology number of properties have been its shear strength. We will study basic
defined, by knowing them for a particular soil properties contributing to its shear strength.
its behaviour can be anticipated. These We also study the effect of water in voids and
properties are index properties, consistency disturbance from its natural state on shear
indices and grain size distribution. Soil strength.
comprises of solid particles, water and air and It also enriches our knowledge as to the
as such a three phase model has been magnitude of forces a soil exerts on its
suggested for it. These simple properties of soil retaining structures, excavated cliffs etc. It
enable us to understand it from engineering also deals with stability of slopes of soil under
perspective. various conditions.

2. HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT
It deals with load carrying capacity of soil • Are natural or proposed earth
or bearing capacity of soils, when it is slopes stable? If not what must we do
homogeneous, layered, at edge of slope etc. to stabilize them?
and when subjected to axial load along with • What type of foundations is
moment and inclined load etc. necessary to support structures; how
Apart from the topics listed so far it also we can design them?
addresses various fields of knowledge • If project requires retaining
associated with soil and necessary for walls, what kind would be best and
engineering community like behaviour of soil how should we design them?
under dynamic loads along with • How will the site respond to
reinforcement, soil improvement etc. potential earthquakes?
It is a well developed subject, with theories A safe, economical and satisfactory
and concepts well defined, addressing host of solution to above issues can be obtained with
situations, for civil engineers, in their projects / the help of soil mechanics. As we saw earlier
construction. nearly all Civil engineering projects must be
supported by ground and thus require at least
Role of Soil Mechanics some Geotechnical Engineering.
Soil is more or less taken for granted by an Soil engineers/ geotechnical engineers
average person. It makes up the ground on usually begin by assessing the underground
which we live, it is for growing crops and it conditions and the engineering properties of
makes us dirty. Most people are not overly the various strata; process called site
concerned with soil beyond these observations. exploration and Characterization.
However a Civil Engineer, Geologist, The next step is to perform engineering
Constructors, Hydrologists, Agronomists are analysis based on the information gained from
deeply concerned over soil. site exploration. The analytical tools we use to
“Virtually every structure is supported by perform these analyses are collectively known
soil or rock. Those that aren’t either fly, float or as soil mechanics and rock mechanics. Thus
fall over” said Richard L Handy. Most Soil Mechanics and Rock Mechanics are to
structures of all types rest either directly or Geotechnical Engineering what Structural
indirectly upon soil, and proper analysis of the Mechanics is to Structural Engineering. In both
soil and design of structure’s foundation is fields; “mechanics” refers to the analytical
necessary to ensure a safe structure free of tools, while “engineering” is a broader term that
undue settling and/or collapse. A also includes the rest of the design and
comprehensive knowledge of soil in a specific construction process.
location is also important in many other
contexts. Thus, study of soils should be an Some Typical Soil Problems
important component in the education of Civil In design and construction of various Civil
engineers. engineering projects we come across different
For a Civil engineer Soil mechanics/ Soil problems related to soil. Some of them are
engineering/ Ground engineering/ listed and diagnosed briefly below to
Geotechnical engineering addresses host of underscore the role of Soil Mechanics for Civil
issues few of which are listed below. Engineers.
• Can the soils and rocks A typical problem we come across
beneath a construction site safely frequently is foundation problem. One will
support the proposed project? speculate as to a particular soil sustaining the
• What will be the impact of any load coming from a column, wall or machine
planned excavation, grading or filling? foundation. We want to know whether the soil
in site able to withstand a particular footing
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3. HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT
pressure without failure and excessive
settlement.
When a vertical surface is required or a
steep slope of soil, at which it can not stay
without failure, is required we support the soil
by a retaining wall or sheet pile wall or bracing.
To design them safe and economically we
need to know the soil pressure on them. What
happens to its magnitude when rain water
enters in to soil in the back fill?
When an earthen dam is being designed
we need to select slopes which are safe
against failure ( sliding) also does not allow any
considerable amount of water to pass through
or seep through its body or through foundation
soil. We need to examine water tightness and
erodability of soil.
For a roadway or railway we need to cut
soil to a particular slope so that it won’t slide or
fail. We speculate about angle of slope to
ensure no failure occurs or methods of
avoiding failure with required slope i.e.
stabilizing measures.
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4. HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT
While designing a foundation for machine ground gets in to motion”. These design
we need to have idea about behaviour of methods were usually satisfactory so long as
foundation soil under cyclic loads. Also the the construction projects were modest in
settlement under these loads is of interest for scope, similar to previous projects (and thus
the designer. tied to experience) and built away from
The above soil problems are very typical obviously poor sites. Using these methods, the
and we come across them very frequently in ancient builders sometimes accomplished
Civil Engineering projects. Apart from these amazing feats of construction, some of which
there are numerous other issues related to soil still exist. Some dams in India were in service
such as differential settlement, time bound for more than 2000 years. Pyramids built in
settlement of saturated fine grained soils, pharos time (300BC) still stand tall resisting
heave of soil on availability of moisture, stability natural forces.
of excavation trenches, dewatering of site for During middle ages, builders built larger
construction, underground contamination, uplift and more sophisticated structures such as
pressures, seepage through crevices etc. cathedrals etc. These projects pressed builders
beyond the limits of experience, so the old
Historical development rules of thumb did not always apply, and
Even though we are interested in concepts, unfortunate failures occurred.
logic reasoning and application of them for the The Leaning tower of Pisa is the most
betterment of man kind through safe and famous example of soil related problems from
economical engineering achievement, historical this era. Its construction began in AD 1173 and
vignettes are necessary to gain an appreciation continued off-and-on nearly 200 years. The
for our heritage as engineers. It also helps us tower began to tilt during construction, so the
understand how technical advances have builders attempted to compensate by providing
occurred in the past, because this guides us in a slight taper to the upper stories. The
developing future advancements. movement continued after construction, and by
People have been building structures, 1982 the top of the 58.4m (192 ft) tall structure
dams, roadways, aqua ducts and other projects was 5.6m (18.4 ft) off plumb.
for thousands of years. However until recently Investigations of the sub surface conditions
these projects did not include any rational of leaning tower of Pisa revealed a weak clay
engineering assessment of the underlying soil stratum about 11m below the ground surface.
or rock. Early construction was based on The clay is very compressible, and has settled
common sense, experience, intuition and rules- under the concentrated weight of the tower.
of thumb, and builders passed this collective South side has settled more than the north,
wisdom orally from generation to generation which has caused the tower to tilt.
through trade guilds. Early scientists were Leonardo da Vinci (1452 – 1519), the
concerned with more lofty matters and artist/ scientist; was one of the few to briefly
generally considered the study of soil and rock study the behaviour of soils. He observed the
beneath their dignity. angle of repose in sands, proposed test
Builders used crude methods / tests to methods to determine the bearing capacity of
assess the soil conditions. Italian architect soils and speculated on the processes of
Palladio (1508 – 1580) wrote that firm ground granular hydrology.
could be confirmed “if the ground does not Engineers and scientists began to address
resound or tremble if something heavy is the engineering behaviour of soil more
dropped. In order to ascertain this, one can seriously during the 17th and 18th centuries.
observe whether some drum skins placed on Most of this work focused on the analyses and
the ground vibrate and give off a weak sound design of retaining walls. It was generally
or whether the water in a vessel placed on the dictated by military needs and mostly done by
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5. HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT
individuals associated with army, especially in One particularly disastrous failure occurred
France. Henry Gautier, B.F.Belidor, Charles in 1913 when 185m of track slipped in to Lake
Augustin Coulomb and others developed Aspin. This event prompted the formation of
methods of predicting forces imparted by soil Geotekniska kommission (geotechnical
on to retaining walls. Coulomb’s work, which he Commission) of the Swedish state Railways to
published in 1776, is often considered the first study the problem and develop solutions.
example of rational soil mechanics. Wolmer Fellinius (1876 – 1957) who had
Unfortunately, much of this work extended become familiar with soil problems when he
well beyond the 18th century abilities to served as the port engineer in Göteburg was
measure relevant engineering properties in made chair of the commission.
soil, and thus was difficult to apply to practical They developed lab test equipment,
problems. developed methods of obtaining undisturbed
The last half of nineteenth century was a samples of soils, studied the behaviour of soils
period of rapid industrialization, which and produced new methods of analysis and
produced tremendous growth in both the design. They investigated more than 300 sites,
amount and scope of construction projects. collected 20,000 soil samples. Their soil
These projects drove advances in structural mechanics laboratory established in 1914
engineering, Hydraulic engineering and other appears to have been the first of its kind in the
fields. Some of the advancements become world.
useful to Geotechnical Engineers. Henry The commission’s final report completed in
Darcy’s research on flow through sand filters 1922 was the world’s first comprehensive
helped in analysis of ground water flow. Otto Geotechnical report. However the rest of the
Mohr’s research on stresses helped in world had little or no knowledge of this work
analyzing shear strength of soils. until much later. The task of promoting
When working with problematic soils some Geotechnical Engineering on a widespread
engineers drove steel rods in to soil to roughly international level required more people to
assess soil conditions. These tests were called spread the message, and one of them became
soundings. recognized as a leader in this effort: Karl
The increasing size of projects, especially Terzaghi.
after 1880, raised more concerns about the Karl Terzaghi (1883 – 1965) has often
consequences of failure. The time was ripe for been called “father of soil mechanics”. His
Geotechnical Engineering to emerge as clearly contribution to soil mechanics is immense even
defined discipline with in Civil engineering for though several others have contributed for its
inventing better techniques of assessing soil shaping.
and rock conditions, and for developing sound Terzaghi born in Prague, then part of
methods of integrating them into Civil Austria earned an undergraduate degree in
Engineering. mechanical engineering. However he found
The first large scale attempts at that it was not his liking and so started his
Geotechnical Engineering occurred in Sweden career with a civil engineering firm in Vienna
during early decades of 20th century. They that specialized in RCC. He later earned a
introduced the word “Geotechnical” (in Swedish doctorate based on his work in RCC design.
‘Geotekniska’). During this period, Terzaghi became
“Necessity is the mother of all inventions”. interested in the ignorance of Civil Engineers in
Extremely poor soil conditions underlie much of matters relating to earth work and foundation
Sweden. Soft, weak clays are present beneath design. He felt this topic needed scientific
the most populated areas, and they are the approach, and decided to focus his attention on
source of many problems, including excessive developing rational design methods.
settlement and catastrophic land slides.
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6. HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT
In 1916 he joined at Imperial School of previously been a maze of incoherent facts and
Engineers at Istanbul. Later he moved to observations.
Robert College in Istanbul. He researched in to Other prominent individuals who made
the behaviour of soils, including studies of contributions to the fledging profession of
piping failures in sands beneath and settlement geotechnical engineering are
in clays. The work on clays eventually led to his • Willium Housel - University
theory of consolidation. It has been considered of Michigen - Soil sampling, analysis
one of the most significant mile stones in Civil and design. Taught first university soil
Engineering. mechanics course at Michigen in 1927.
• Gregory Tschebotorioff --
German - served at Princeton
University – worked on earth retaining
structures.
• Arthur Casagrande –
Harvard University – disciple of
Terzaghi – analysis of soft clays, soil
composition and classification,
seepage, earth dams etc.
• Fred Converse – professor at
California Institute of technology.
Cofounder of one of first geotechnical
consultancy firm.
• Daniel Moran – Foundation’s
engineer and builder – constructed
major bridges across country.
• R R Proctor - assessment of
compacted fills during construction.
In 1936 first international Conference on
If we wish to define a certain time as the Soil Mechanics and Foundation Engineering
‘birth’ of Geotechnical Engineering as a widely met in Cambridge, Massachusetts.
recognized discipline, it would be the year In spite of these advancements, Terzaghi
1925, in which year Terzaghi published the first was presenting lectures with titles like “Soil
comprehensive book on subject titled Mechanics – a new chapter in Engineering
“Erdbaumechanik auf Bodenphysikalischer Science” as late as 1939. Fifties and sixties
Grundlage” ( germane for The mechanics of finally established Geotechnical Engineering as
earth construction based on soil physics) was a routine part of nearly all significant Civil
published in Vienna. In 1925 he started to give engineering projects.
lectures at MIT USA as visiting lecturer. In Modern geotechnical engineering has
1929 he returned to Vienna to hold a developed a matured ‘personality’ that is
Professor’s post at Technical University and slightly different from other civil engineering
continued research, speaking and consulting. disciplines.
In 1939 he returned to USA accepting We work with soil and rock, which are
professorship at Harvard University and it natural materials. As such their properties are
became his home for rest of his life. more complex and difficult to characterize than
Terzaghi had a remarkable ability to those of manufactured materials such as steel.
develop rational and practical solutions to real For site exploration and characterization
engineering problems from jumble of what had geotechnical engineers and their staff spend a
great deal of time in field and laboratory.
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7. HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT
Practical economic constraints limit As city grew and water demands increased
number of exploratory borings. As a result we many wells were installed through this clay in
have direct knowledge of very small portion of to deeper water bearing sand layers. Pumping
soil or rock beneath a project site. This of water increased effective stress and caused
introduces many potential sources of error. large settlements. Between 1898 and 1966
Because of potentially large errors in our parts of the city settled 6m to 7m. At times rate
site characterization programme, we use large of settlement has been as great as 1mm/ day.
measure of ‘engineering judgement’ when Fortunately Mexican Geotechnical engineers
using lab and field data in to our analysis. As a (Dr. Nabor Curillo) recognized the association
result we use large factors of safety and more of settlement with ground water extraction and
conservative designs. convinced government to prohibit pumping in
We rely heavily on engineering judgement central city area.
– combination of experience, subjectivity, Apart from widespread settlement, local
reliance on precedent etc. settlements also occurred beneath heavy
We have more extensive involvement structures and monuments. Palacio de las
during construction, and frequently revise our Bellas Artes ( palace of fine arts) built between
design recommendations when conditions 1904 and 1934 experienced large settlements
encountered during construction are different even before completion. By 1950 the palace
from those anticipated. and immediately surrounding ground were 3m
Accuracy in geotechnical engineering lower than adjacent streets. Stair cases were
analysis is as good as accuracy of data. One of built from streets down to building area.
the most common mistakes among students Settlement problems are not limited to
studying Geotechnical engineering and even buildings. For a highway bridge underlain by
among practicing engineers, is to overestimate soft clay deposit pile foundations were used
the accuracy of geotechnical analysis. This through the clay into harder soils below and the
often leads to overconfidence and ultimately bridge was built on the piles. These
may result in construction failures. foundations protect it from large settlements.
It also was necessary to place fill adjacent
Soil failures to the bridge abutments so the roadway could
Some of most dramatic examples of soil reach the bridge deck. These fills are very
settlement are found in Mexico city. Parts of heavy and their weight increased effective
the city are underlain by one of the most stress causing underlying clay to settle. After
troublesome soils in any urban area of the twelve years fill had settled 1m in comparison
world, a very soft lacustrine clay that was to bridge deck causing great inconvenience to
deposited in the former lake Texcoco. road traffic.
It was an extraordinary soil with water Individual slope failures can be very
content varying from an average of 281% to disastrous and expensive. For example Thistle
max 500; liquid limit from average of 289% to debris slide near Thistle, Utah in 1983 created
max 500% and plastic limit from average of a huge dam across a canyon, forming a new
85% to max 150% and void ratio of average lake. It caused $ 200 million in direct damages,
6.9. including the destruction of two major highways
and main line of the Denver and Rio Grande
western Rail road. This new lake had no out
let, so it would have eventually overtopped the
side and quickly eroded it causing a massive
flood. Therefore lake was drained, first with
pumps and later with a permanent tunnel.
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8. HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT
In 1786 in China’s Sichuan province similar
land slide happened. It dammed a river and
river soon overtopped the dam and rapidly
eroded it causing extensive floods
downstream, suddenly, drowning 1,00,000
people.
The soils below apartment buildings in
Niigata, Japan liquefied during 1964
earthquake, which produced bearing capacity
failures. The failure reportedly occurred very
slowly and buildings were very strong and rigid,
so they remained virtually intact. There was
very little damage to interior, doors and
windows functioned after failure. Inhabitants
Teton dam was a 93m tall earth dam on
were able to evacuate by walking down the
the Teton River in South eastern Idaho. It failed exterior wall.
on June5, 1976; only eight months after the
first filling of reservoir began. The failure began
as a cloudy seep on the face of the dam, and
rapidly progressed to a piping failure through
the embankment.
These failures teach us about the failures
of concepts and /or unforeseen actions and we
can learn a lot from these failures.
***Hand outs are not meant to be exhaustive; students are
advised to refer various books on the subject.
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