A site was investigated for a surface development several years ago and was know

Question

A site was investigated for a surface development several years ago and was known to be underlain by 2m of silty clay, 2m of fine to coarse sand, 5m of clay and then a very deep layer of sandy gravel as shown in Figure 2. Although the original development did not proceed, a new development was proposed which would require a basement to be constructed to a depth of 4m below ground level.
It was suspected that the sandy gravel was subjected to artesian water pressures which could mean seepage problems at the base of the proposed excavation. In order to investigate this, a borehole was drilled into the sandy gravel and a piezometer, comprising an open ended 50mm PVC pipe, was installed so that it terminated in the sandy gravel. After installation, water in the piezometer rose to a level which was 1m above ground level. A second piezometer was installed in the sand layer and it was found that the ground water level was 1m above the upper surface of the clay layer.
Estimate:
a) The water pressure at the top, middle and base of the clay layer. (12 marks)
b) If a similar piezometer were to have its lower open end located at the middle of the clay layer, the level below the ground surface to which the water in the pipe would eventually rise. (3 marks)
c) The rate of seepage from the sandy gravel to the sand expressed in litres per square metre per day, if the permeability of the clay is 1 x 10-7 m/sec. (5 marks)

Water level in piezometer piezometer Water level in sand lm Silty clay 2m 2m Sand 1m Clay k 1x 107 m/sec Im Sandy gravel Figure 2 (not to scale)

Explanation / Answer

a)

Here, clearly the water pressure at the base of clay layer is 10 m.

At top of the clay layer, water pressure is 1 m (because water level is 1 m beloe in sand layer)

Total head loss in 5 m clay layer = pressure at bottom - pressure at top = 10-1= 9 m

hence pressure at middle of clay layer= 10 - 9/5*2.5= 5.5 m

b)

water level will rese 5.5 m from middle of clay layer means

(2+2+2.5 - 5.5) = 1m below ground surface

c)

q= kiA

q/A= ki

i= head loss / layer theckness = 9/5= 1.8

q/A= 1.8*1*1-7 = 1.8* 10-7 m3/s/m2 = 1.8*10-7*3600*24= 0.01555 m3/day/m2

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