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# [Day-2] GATE 2021 | Civil Engineering solved questions – Soil Mechanics part – 2

Sep 20, 2020

Here below you can find the GATE Civil Engineering solved question with explanations in the topic of Soil Mechanics – Effective stress principle. Which is very useful for the upcoming GATE 2021 examination.

Question No.1: An excavation was made in a clay stratum having = 2 T/m3. When the depth was 7.5 m, the bottom of the excavation cracked and the pit was filled by a mixture of sand and water. The thickness of the clay layer was 10.5 m, and below it was a layer of pervious water-bearing sand. How much was the artesian pressure in the sand layer? Solution:

When the depth of excavation was 7.5 m, at the interface of the CLAY and SAND layers, the effective stress was equal to zero.
Downward pressure due to weight of clay = Upward pressure due to artesian pressure
(10.5 – 7.5) = , where h = artesian pressure head
3 x 2 = 1 x h h = 6 m = 0.6 kg/cm2 or 6 T/m2 artesian pressure

Question No. 2: The specific gravity and insitu void ratio of a soil deposit are 2.71 and 0.85 respectively. The value of the critical hydraulic gradient is
(A) 0.82 (B) 0.85
(C) 0.92 (D) 0.95

Explanation: Question No.3: For the soil strata shown in figure, the water table is lowered by drainage by 2m and if the top 2m thick silty sand stratum remains saturated by capillary action even after lowering of water table, the increase in effective vertical pressure in kPa at mid-height of clay layer will be
(a) 0.2 (b) 2
(c) 20 (d) 200

Explanation: Question No.4: Assuming that a river bed level does not change and the depth of water in river was 10 m, 15 m and 8 m during months of February, July and December respectively of a particular year. The average bulk density of the soil is 20 kN/m3 . The density of water is 110 kN/m3 . The effective stress at a depth of 10 m below the river bed during these months would be
(a) 300 kN/m2 in February, 350 kN/m2 July and 320 kN/m2 in December
(b) 100 kN/m2 in February, 100 kN/m2 July and 100 kN/m2 in December
(c) 200 kN/m2  in February, 250 kN/m2 July and 180 kN/m2 in December.
(d) 300 kN/m2 in February, 350 kN/m2 July and 280 kN/m2 in December.

Ans: (b)

Explanation: Effective stress does not change if water level changes

Question No.5: For the soil deposit shown below, draw the total stress, pore water pressure and effective stress diagrams.The water table is at ground level. Solution: Total stress
At – 4m, = 1.92 x 4 = 7.68 At -11m, = 7.68 + 2.1 x 7 = 22.38 Pore water pressure
At – 4 m, u = 1 x 4 = 4 At -11 m, u = 1 x 11 = 11 Effective stress
At – 4 m , = 7.68 – 4 = 3.68 At -11m , = 22.38 – 11 = 11.38 Question No.6: The range of void ratio between which quick sand conditions occurs in cohesion less granular soil deposits is
(a) 0.4-0.5 (b) 0.6-0.7
(c) 0.8-0.9 (d) 1.0-1.1

Explanation: Ic= (G-1)/(1+e), Ic=1 (as per the problem)

(2.65-1)/(1+e)=1

by solving above eqn, e=0.65

Question No.7: Which of the following statement is NOT true in the context of capillary pressure in soils?
(a) Water is under tension in capillary zone
(b) Pore water pressure is negative in capillary zone
(c) Effective stress increases due to capillary pressure
(d) Capillary pressure is more in coarse grained soils
Ans:(d) Capillary pressure is more in coarse grained soils

Question No.8: A sand layer found at sea floor under 20 m water depth is characterized with relative density = 40 %, maximum void ratio = 1.0, minimum void ratio = 0.5, and specific gravity of soil solids =
2.67. Assume the specific gravity of sea water to be 1.03 and the unit weight of fresh water to be 9.81 KN/m3

What would be the effective stress (rounded off to the nearest integer value of kPa) at 30 m depth into the sand layer?
(A) 77 kPa (B) 273 kPa
(C) 268 kPa (D) 281 kPa

Explanation: Question No.9: Assertion (A): The possibility of quicksand condition occurring is more on the downstream of a weir on a permeable foundation. Reason (R): Seepage lines are directed upwards at the downstream of such a weir. Codes:
a) Both A and R are true and R is the correct explanation of a
b) Both A and R are true but R is not a correct explanation of A
c) A is true but R is false.
d) A is false but R is true.
Ans: a) Both A and R are true and R is the correct explanation of a

Question No.10: The ground conditions at a site are as shown in the figure. The water table at the site which was initially at a depth of 5m below the ground level got permanently lowered to a depth of 15m below the ground level due to pumping of water over a few years. Assume the following data
i. unit weight of water = 10kN/m3
ii. Unit weight of sand above water table = 18kN/m3
iii. Unit weight of sand and clay below the water table = 20kN/m3
iv. Coefficient of volume compressibility = 0.25 /MN
What is the compression of the clay layer in mm due to the lowering of the water table?
(A) 125 (B) 100 (C) 25 (D) 0  