**Homework Problem 5**

Homework Problem 5 asks you to calculate total
stress, effective stress and pore water pressure in confined aquifers for
various conditions.

1) A confined sand layer of thickness 5
meters is overlain by 6 meters of clay with a mass density of 1.8 metric
tons/meter^{3} . The hydraulic head in the sand is +5 meters above
the sand's upper surface. The clay layer is excavated to a depth of X
meters below the land surface.

**Determine **the maximum value of X that
will not produce "quick" conditions.

2) In order to prevent seawater
intrusion into a confined coastal aquifer , the potentiometric surface in the
aquifer has to be raised by 8 meters over an area of 2 km by 20 km along the
coast. The aquifer's storativity is S= 5 X 10 ^{-5}.

**Determine** the volume of water that
should be injected for this purpose through an array of wells parallel to the
coast.

3) A horizontal aquifer is overlain by
40 ft of saturated clay. The dry bulk density of the clay is 127 lbs/ft^{3}.
The porosity of the clay is 39%.

a) **Calculate** the
total stress acting on the top of the aquifer.

b) If the pressure head at
a point in the aquifer is 87 feet, **calculate** the effective stress in the
aquifer at that point.

c) If the aquifer is
pumped and the total head is reduced by 27 feet, what will be the resulting
changes in **pressure head, the fluid pressure, the effective stress, and the
total stress. **

d) If the compressibility
of the aquifer is 1 x 10^{-6} ft^{2}/lb., and the aquifer
thickness is 47 feet, **how much **will the thickness of the aquifer change
during the head reduction of part c above?