Permeability and Flow of Water in Soils - Elements of Soil Mechanics - page 71

Fig. 2.32 Flow net for seepage through two soils of different permeabilities.

Now q 1 = q 2 ,

k
k

tan
tan

α
α

1
2

1
2

⇒ =

A flow net which illustrates the effect is illustrated in Fig. 2.32.

Exercises

Exercise 2.1

In a falling head permeameter test on a fine sand the sample had a diameter of
76 mm and a length of 152 mm with a stand pipe of 12.7 mm diameter. A stopwatch
was started when h was 508 mm and read 19.6 s when h was 254 mm; the test was
repeated for a drop from 254 mm to 127 mm and the time was 19.4 s.
Determine an average value for k in m/s.

Answer 1.5 × 10 − 4 m/s

Exercise 2.2

A sample of coarse sand 150 mm high and 55 mm in diameter was tested in a con-
stant head permeameter. Water percolated through the soil under a head of 400 mm
for 6.0 s and the discharge water had a mass of 400 g.
Determine k in m/s.

Answer 1.05 × 10 − 2 m/s

Exercise 2.3

In order to determine the average permeability of a bed of sand 12.5 m thick overlying
an impermeable stratum, a well was sunk through the sand and a pumping test carried
out. After some time the discharge was 850 kg/min and the drawdowns in observation
wells 15.2 m and 30.4 m from the pump were 1.625 m and 1.360 m respectively. If the
original water table was at a depth of 1.95 m below ground level, find the permeability
of the sand (in m/s) and an approximate value for the effective grain size.

Answer k = 6.7 × 10 − 4 m/s, D 10 ≈ 0.26 mm