CE 240 — SOIL MECHANICS

PART A — (10 ? 2 = 20 marks)
1. The shrinkage limit of a soil is 12%. The soil is dried from its initial water content of 8% to the dry state. Calculate the percentage of volume change in this process.
2. Classify the soil having the following properties as per BIS :
Fraction passing 75 micron sieve : 60%
Liquid limit : 19%
Plastic limit : 14%
3. Why is falling head permeability test preferred to constant head method for finding coefficient of permeability of fine grained soils?
4. The void ratio of a soil is 1.0 and the superficial velocity through the soil is
1 ? 10–5 cm/s. Find the seepage velocity.
5. State the assumptions made in Boussinesq stress distribution theory.
6. A soil sample consolidating in a ring of diameter 60 mm has a thickness of
18.9 mm at a particular stage. Find the void ratio at this stage if the mass of dry soil is 78 g and specific gravity of solids is 2.65.
7. Can saturated sand exhibit ? If so, under what circumstances?
8. Draw the Mohr circle at failure and strength envelope corresponding to unconfined compression test.
9. Find the factor of safety of an infinite slope of Cohesionless soil of angle of internal friction 36?, if the slope angle is 30?.
10. State any two ways by which a finite slope may fail.
PART B — (5 ? 16 = 80 marks)
11. (i) A saturated specimen of undisturbed clay has a volume of 22.5 cm3 and
mass of 35 g. After oven drying, the mass reduces to 20 g. Find its
moisture content, specific gravity of solids, void ratio and dry density.
(10)
(ii) Explain how moisture content and compactive energy influence the compaction of soils. (6)
12. (a) (i) In a falling head permeameter, the sample was 18 cm long and
having a cross sectional area of 22 cm2. Calculate the time required
for the drop of head from 25 cm to 10 cm, if the cross sectional area
of the stand pipe was 2 cm2. The sample of soil was heterogeneous
having coefficient of permeability of 3 ? 10–4 cm/s for the first 6 cm,
4 ? 10–4 cm/s for the second 6 cm and 6 ? 10–4 cm/s for the last 6 cm
thickness. Assume the flow taking place perpendicular to the
bedding planes. (8)
(ii) State the properties of flownet. (8)
Or
(b) (i) For a field pumping test, a well was sunk through a horizontal
stratum of sand 14.5 m thick and underlain by a clay stratum.
Two observation wells were sunk at horizontal distances
of 16 m and 34 m respectively from the pumping well. The
initial position of water table was 2.2 m below ground level. At
a steady pumping rate of 1 m3/min., the drawdowns in the
observation wells were found to be 2.45 m and 1.2 m
respectively. Calculate the coefficient of permeability of the
sand. Derive the equation used, if any. (12)
(ii) What are the conditions under which quick sand phenomenon
occurs? (4)
13. (a) (i) A sandy soil of average void ratio of 0.7 and specific gravity of solids
2.7 extends for a large depth from the ground level. Find the
effective stress at a depth of 5 m when the soil is (1) Saturated
(2) Submerged. (6)
(ii) A saturated soil has a compression index of 0.25 and coefficient of
permeability of 3.4 ? 10–7 mm/s. If its void ratio at an effective
pressure of 100 kPa is 2.02, find the void ratio when the pressure is
increased to 190 kPa. Also, find the settlement of the stratum, if it
is 5 m thick and coefficient of consolidation for this pressure range.
(10)
Or
(b) (i) An elevated structure is supported on a tower with four legs. The
legs rest on piers located at the corners of a square of side 7 m. If
the value of vertical stress increment due to this loading
(considering 4 equal concentrated loads) is 25 kPa at a point 8 m
beneath the centre of the structure, what will be the stress
increment at 10 m below one of the legs? (10)
(ii) Define or give concise description :
(1) Normally consolidated soil
(2) Over consolidated soil
(3) Compression index. (6)
14. (a) (i) What are the limitations of direct shear test? (6)
(ii) A sample of cohesionless soil was subjected to a drained shear test under a cell pressure of 100 kPa. When an additional vertical load of 220 N was applied, the sample failed. Find the angle of internal friction of the soil, if the total quantity of water collected in the burette was 9 cm3. The initial length and diameter of the sample were 80 mm and 38 mm respectively. A change in length of 5 mm was observed in the sample. (10)
Or
(b) (i) Using Mohr's diagram, prove that the angle made by failure plane
with the major principal plane is (45 + ??2)??where ? is the angle of
internal friction. (5)
(ii) In a direct shear test conducted on a cohesionless soil, the sample failed when a shear stress of 600 kPa was applied under a normal stress of 800 kPa. Locate the principal planes and find the principal stresses. (11)
15. (a) A 10 m high embankment of infinite slope is made of a soil having cohesion, angle of internal friction and unit weight of 30 kN/m2, 20? and 17 kN/m3 respectively. Find the factor of safety with respect to height if the slope angle is 30?. Derive from the first principles the equation used, if any. (16)
Or
(b) (i) By slip circle method, derive the expression for factor of safety of a
finite slope of a pure cohesive soil for a trial slip circle of radius ‘R’.
(8)
(ii) A 15 m high embankment is inclined at 30? to the horizontal. If the
cohesion, angle of internal friction and unit weight of the soil are
15 kPa, 15? and 17.5 kN/m3 respectively. Find the factor of safety
with respect to cohesion. Take Taylor's stability number as 0.05. (3)
(iii) Say True or False : The side slopes of a canal are more critical when
there is a sudden drawdown rather than when the canal is running
full. Justify your answer. (5)