Safe Depth of Excavation

Q. A 1.25 m layer of soil (n = 0.35, G = 2.65) is subjected to an upward seepage head of 1.85 m. What depth of coarse sand would be required above the existing soil to provide a factor of safety of 2 against piping? Assume that coarse sand has the same porosity and specific gravity as the soil and that there is negligible head loss in sand Take ${\gamma }_w=9.81kN/m^3$.

1. 2.2

Q. A layer of clay of thickness 12.5 m is underlain by sand. The ${\gamma }_{sat}$ of the clay is 18.5 $kN/m^3$. When the depth of an open trench excavated in the clay reached 8 m, the bottom cracked and the water started entering the trench from below. The height to which water would have risen from the top of sand in a bore hole if it was drilled into sand prior to the excavation is ($Take{\gamma }_w=10kN/m^3$).

1. 8.3 m
2. 10.2 m
3. 3.83 m
4. 6.3 m

Q. In a 6 m thick stratum of fine sand having submerged density of $11kN/m^3$ , quicksand condition occurred at a depth of 4.2 m of excavation. What is the depth of lowering of groundwater table required for making an excavation 5 m deep? Take density of water as $10kN/m^3$.

1. 3.85 m
2. 1.68m
3. 1.1 m
4. 0.897 m

Q.
To make certain that the backfill material is more pervious than the soil to be drained, the relationship used is

1. $(D_{15}{)}_{filter}\le 5(D_{85}{)}_{protectedsoil}$
2. $(D_{15}{)}_{filter}\ge 5(D_{85}{)}_{protectedsoil}$
3. $(D_{15}{)}_{filter}\le 5(D_{15}{)}_{protectedsoil}$
4. $(D_{15}{)}_{filter}\ge 5(D_{15}{)}_{protectedsoil}$

Q. At a construction site, a contractor plans to make an excavation as shown in the figure.






The water level in the adjacent river is at an elevation of + 20.0 m. Unit weight of water is 10 KN/$m^3$. The factor of safety (upto decimal places) against sand boiling for the proposed excavation is

1. 1

Q. Match List-I (Unit) with List-II (Purpose) and select the correct answer using the codes given below the lists:
List-I
A. Graded filter
B. Lime treatment
C. Impervious clay core
D. Curtain grouting
List-II
1. To reduce seepage of water through body of earth dam
2. To reduce water seepage through foundation below dam
3. To stabilize black cotton soils
4. To drain water without losing fines from the soil
Codes:
A B C D

1. 4 3 1 2
2. 1 2 4 3
3. 4 2 1 3
4. 1 3 4 2

Q. Which one of the following is the purpose of providing the downstream sheet pile in a dam?

1. To stop failure due to uplift pressure
2. To stop failure due to sliding
3. To control failure due to piping by reducing value of exit gradient
4. To control failure due to scour