DHARMSEEPAGE AND FLOW NETS 201
6.13 An excavation is to be performed in a stratum of clay, 9 m thick, underlain by a bed of sand. In a
trial bore hole, the ground-water is observed to rise up to an elevation 3 m below ground surface.
Find the depth of which the excavation can be safely carried out without the bottom becoming
unstable under uplift pressure of ground-water. The specific gravity of clay particles is 2.70 and
the void ratio is 0.70. If the excavation is to be safely carried to a depth of 7 m, how much should
the water table be lowered in the vicinity of the trench?
6.14 There is an upward flow of 0.06 ml/s through a sand sample with a coefficient of permeability
3 × 10–2 mm/s. The thickness of the sample is 150 mm and the cross-sectional area is 4500 mm^2.
Determine the effective stress at the bottom and middle of the sample, if the saturated unit
weight of the sample, is 18.9 kN/m^3.
6.15 A deposit of cohesionless soil with a permeability of 0.3 mm/s has a depth of 12 m with impervi-
ous ledge below. A sheet pile wall is driven into this deposit to a depth of 8 m. The wall extends
above the surface of the soil, and a 3 m depth of water acts on one side. Sketch the flow net and
determine the seepage quantity.
6.16 A homogeneous earth dam has a top width of 6 m and a height of 42 metres with side slopes of 3
to 1 and 4 to 1 on the upstream side and downstream side respectively. The free board is 2 m.
There is a horizontal filter at the base on the downstream side extending for a length of 60 m
from the toe. If the coefficient of permeability of the soil is 9 × 10–2 mm/s, find the quantity of
seepage per day for 100 metre length of the dam.
6.17 A double wall sheet pile coffer dam retains a height of water 9 m on one side. A flow net con-
structed for this structure, driven into a pervious deposit overlain by an impervious ledge, con-
sists of five flow channels and fifteen potential drops. The length of the coffer dam is 90 metres.
If the coefficient of permeability of the pervious deposit is 10–2 mm/s determine the seepage in
cubic metres per day.
6.18 A concrete dam retains water to a height of 9 m. It has rows of sheet piling at both heel and toe
which extend half way down to an impervious stratum. From a flow net sketched on a trans-
formed section, it is found that there are four flow channels and sixteen head drops. The average
horizontal and vertical permeabilities of the soil are 6 × 10–3 mm/s and 2 × 10–3 mm/s, respec-
tively. What is the seepage per day, if the length of the dam is 150 metres?