Example 2: 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