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/meter³ .  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/ft³.  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 ft²/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?