Soil Mechanics for Civil Engineers

1. Importance of the soil mechanics for the Civil Engineers
In soil mechanics we study about the various properties of the soil to be used for various engineering
construction works. There are various reasons that as a civil Engineer one must study this new branch of the
Engineering science.
(1) Foundations: All the civil Engineering structures, ultimately rest on the soil. They transfer their whole load
to the soil, so we have to construct the foundations to retain these structures. In case of the hard soil/ having
sufficient strength we can provide the shallow foundations. If we know the strength of the soil then we can
decide which type of foundation is to be used. If the soil is weak in strength then we have to provide the deep
foundations like pile foundation, well foundation etc. It is important to know the method to calculate the method
to know the strength of the soil.
(2) Earthen Dams: There are so many earthen dams constructed to retain the water. The soil to be used for
the construction of these earthen dams must be suitable enough to use it in its construction. Various properties
of the soil, like it permeability, strength, and density are checked on regular basis to know if the soil compacted
to required density or not. The earthen dams are costly structure and also they have a high risk of getting
failed, so they must be constructed with great care, so it is very important to study the properties of the soil.
(3) Embankments: There are embankments constructed to raise the levels of the highways on the plains
because there are chances of the floods etc, and also it is required to keep the foundation of the pavement
above the water table. The embankments are generally constructed of the soil, which is tested for its various
properties. There is need to design a economical embankment which is only possible by studying the various
soil properties.
(4) Canals or other retaining and under ground structures: The canals also are formed by the soil which
are to be constructed to be impermeable and of enough strength. The retaining structure like the retaining
walls, are constructed to retain the earth. The earth properties are important to know about. The properties like
the earth pressure, shear strength etc gives us the idea to design the retaining structure. The soil strata is
constantly investigated by the geologist to give the idea of the type of construction to be carried further in case
of the tunneling.
(5) There are various techniques to be used to improve the soil properties, which can help with the
economy of the construction works dealing with the soil.
Factors affecting stability of slope (Geotechnical Engg.)
Hi,
 Stability of Slopes A slope is an inclined boundary surface between air and the body of the earthwork e.g. high ways cut or fill,
railway cu or fill, earth dams, levees and river training works.
 Stability of slopes can be affected by various reasons.Some of them may result in the increased stress and some may cause
reduction in the strength.
 Factors causing increased stress:
1. Increased unit weight of soil by wetting.
2. Added external load (moving loads, buildings etc.)

2. 3. Steep end slopes either by excavation or by erosion.
4. Shock loads
 Factors causing reduction in strength:
1. Vibrations and earthquakes
2. increase in moisture content
3. increase in pore water pressure.
4. loss of cementing material.
Now, to analyse the stability of the slopes, there are few assumptions:
1. The problem is two dimensional.
2. Coulomb's theory can be used to compute shear strength and shear strength is assumed as uniform along the slip
surface.
3. The flownet, in case of seepage, can be drawn and seepage force can be evaluated.
Methods of Stability analysis of slope(Geotech)
Hi here is a brief introduction to the stability analysis of the slopes.
There are following methods of stability analysis:
 (A)Slip circle method or Swedish circle method or Method of Slices:
This method assumes the surface of sliding is an arc of a circle. Soil is either purely cohesive or it will be a cohesive and frictional soil. So, analysis can be
done separately as below:
 Analysis of purely cohesive soils:
An arc of a circle is assumed as the failure plane and the weight of the circular arc wedge provides a disturbing moment. This disturbing moment is
stabilized by the stabilizing moment developed due to the presence of the cohesion along the arc surface.
These two moments are equated to get the limiting values. The ratio of the resisting moment to the stabilizing moment gives us the Factor of safety.
 Cohesive and frictional soils:
In such soil again a circular arc is assumed to be a failure wedge and the weight of the wedge is resolved into tangential and normal directions. The
tangential weight will provide us with the disturbing moment about the center of rotation and the normal force will provide us the frictional resistance
which along with the cohesion force will produce the stabilizing moment.
 (B) Friction circle method
This method is applicable to cohesive as well as frictional soils and assumes the failure
surface as an arc of a circle. There is a small circle known as friction circle.
Stability of Earth Dam (slope)
Hello,
Earth dams must be safe against slope and foundation failure for all operating conditions. There are three generally recognized critical
stages based on pore pressure condition for which the stability of the embankment should be ascertained. These three situations are
 End of construction
 Steady state seepage
 rapid state seepage.
Usually construction pore pressure reach their maximum value when the embankment reaches maximum height. After the reservoir has
been filled for a long time, pore pressures are determined by steady-stage seepage conditions and may be estimated by the construction of
flow net.
Rapid lowering of the reservoir produces the third critical situation, particularly for low permeable soils. Upstream slope stability may be
critical for the construction or rapid draw-down condition. The downstream slope should be checked for the construction and steady-seepage
condition.
GATE, PSUs preparation- Soil Engineering ( notes)- Part 11
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Welcome to the part 11 of the one liner notes useful for the preparation of GATE and other similar examinations.

3. 1. For compaction of cohesion-less soils vibration techniques, flooding the soil and heavy weights dropping from a height
are most suitable methods.
2. Standard split spoon sampler is most suitable soil sampler for saturated sands and other soft and wet soils.
3. Raft are used when structural load is uniform and soil is soft clay, made up of marshy land.
4. Piles are used when structural load is heavy and/or soil is having low bearing capacity for a considerable depth.
5. Footings are used when soil is having good bearing capacity at shallow depth and structural load is within permissible
limit.
6. Well or pier is used when structural load of bridge is to be transferred through sandy soil to bed rock.
7. At critical void ratio, the void ratio does not vary with shear strain.
8. Vane shear test is performed on soft clay.
9. Standard penetration test is performed on sandy deposits.
10. Static cone penetration test is useful for end bearing and skin friction resistance determination.
11. Pressure meter test is useful for In-situ stress strain characteristics.
12. Differential settlement of foundation is hazardous because it leads to damage to superstructure.
13. Lowering of ground water table can cause settlement of foundation.
14. Consolidation and unconfined compression tests require undisturbed samples.
15. Clays which exhibit high activity contains montmorillonite and have a low plasticity index.
16. Size ranges of voids in soil also effect the permeability.
17. During seepage through an earth mass, the direction of seepage is perpendicular to the equipotential lines.
18. Limitation of direct shear test are plane of failure is predetermined, no control over drainage and there is non-uniform
distribution of stresses.
19. Coulomb's theory assumes that failure occurs on a plane surface and failure wedge is a rigid body.
GATE, PSUs preparation- Soil Engineering ( notes)- Part 10
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Welcome to the part 10 of the one liner notes useful for the preparation of GATE and other similar examinations.
 Allowable bearing pressure is the net loading intensity at which neither soil fails in shear nor is there any excessive settlement.
 Ultimate bearing capacity is the minimum gross pressure intensity at the base of foundation at which soil fails in shear.
 Net safe bearing capacity is defined as the net ultimate bearing capacity divided by factor of safety.
 Safe bearing capacity is the maximum pressure which soil can carry safely without risk of shear failure.
 The concept of useful width is used to determine ultimate bearing capacity of an eccentrically loaded square footing.
 Pressure distribution in pure clayey soil subjected to a uniformly distributed load(udl) through a rigid footing is parabolic with
minimum at the center and maximum at the outer edges, but it becomes maximum at the center and minimum at the edges when udl is
transmitted through rigid footing placed on the surface of a cohesion less soil.
 The ultimate bearing capacity of a pile is given by (9.Cu.Ab + a.Cu1.As), where 'Cu' is the given cohesion, 'Cu1' is the average
coheasion and 'a' is adhesion factor given by Cu1/2.
 The upstream slope of an earth dam under steady seepage condition is equipotential line.
 In tri-axial test intermediate and minor principal stresses are equal, volume changes can be measured and field conditions can be
stimulated.
 Active earth pressure occurs when wall moves away from backfill, passive earth pressure occurs when wall moves towards the
back-fill and earth pressure at rest occurs when the wall is at rest.
GATE, PSUs preparation- Soil Engineering ( notes)- Part 9
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Welcome to the part 9 of the one liner notes useful for the preparation of GATE and other similar examinations.
 A shallow foundation is defined as a foundation which has a depth of embedment less than its width.
 For sand of uniform spherical particles, the ratio of void ratios in the loosest and the densest states is 2.6.
 The description of 'sandy silt clay' signifies that the soil contains unequal proportions of the three constituents such that
clay>silt>sand.
 A soil having particles of nearly the same size is known as uniformly graded.
 The soils most susceptible to liquefaction are saturated fine and medium sands of uniform particle size.

4.  The value of bearing capacity factor for cohesion Nc, for piles as per Meyerhof is taken as 9.0.
 The slope of the e - log p curve for a soil mass gives compression index, Cc.
 Degree of freedom of a block type machine foundation is 6.
 Given that damping ratio = 0.10 and damping coefficient = 225 kN sec/m. The the critical damping coefficient in kN sec/m will be
2250.
 The natural frequency of a vibrating foundation system increases as the square root of the spring stiffness and decreases with the
square root of the mass of the body.
 Terzaghi's equation of ultimate bearing capacity for a strip footing may be used for square footing resting on pure clay soil with the
correction factor 1.3.
GATE, PSUs preparation- Soil Engineering ( notes)- Part 8
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Welcome to the part 8 of the one liner notes useful for the preparation of GATE and other similar examinations.
 According to Terzaghi's theory, the ultimate bearing capacity at ground surface for a purely cohesive soil and for a smooth base of
a strip footing is 5.14.C where, C = unit cohesion of soil.
 The net ultimate bearing capacity of a purely cohesive soil is independent of both depth and width of footing.
 The rise of water table below the foundation influences the bearing capacity of soil mainly by reducing cohesion and effective unit
weight of soil.
 Terzaghi's general bearing capacity formula for a strip footing gives us the ultimate bearing capacity of soil. Formula uses three
bearing capacity factor.
 Terzaghi's bearing capacity factors are the functions of angle of internal friction only.
 In the plate load test for determining the bearing capacity of soil, the size of square bearing plate should be between 300 mm and
750 mm.
 Bearing capacity of soil depends upon type of soil, shape and size of footing and is independent of rate of loading.
 Rise of water table up to ground surface reduces the net ultimate bearing capacity of soil approximately by 50%.
 Contact pressure beneath a rigid footing resting on cohesive soil is more at edges compared to middle.
 According to IS specifications, the minimum depths of foundation in sand and clay should be respectively 800 mm and 900 mm.
 The maximum differential settlement in isolated footings on clayey soils should be limited to 40 mm.
 A combined footing is generally used when number of columns is two and they are spaced close to each other.
 Negative skin friction on a pile acts downwards and reduces the load carrying capacity of the pile.
 Generally the bearing capacity of a pile group is equal to the sum of bearing capacities of individual piles in case of end bearing
piles.
 The settlement of a group of friction piles as compared to that of a single pile is more.
 Negative skin friction is caused by relative settlement of soil and skin frictional resistance is caused by relative settlement of pile.
 Static formulas are suitable for friction piles driven through cohesive soils.
 Dynamic formulas are suitable for friction piles driven through cohesion-less soils.
 Dynamic formulas do not take into account the reduced bearing capacity of a pile in a group.
 Mechanical stabilization of soil is done with the help of proper grading.
 Lime stabilization is very effective in treating plastic clayey soils.
 Undisturbed samples are obtained by thin-walled samplers.
 Stationary piston sampler and rotary sampler are both thin-walled sampler.
 Greater skin friction retards the sinking of the well.
 If the bearing capacity of a footing on a saturated clay is 120 kN/m2 , the bearing capacity of a circular footing(diameter=width) will
be more than 120 kN/m2.
 A plate load test is useful to estimate both bearing capacity and settlement of foundation.