Civil Engineering Reference
In-Depth Information
Figure 6.6
Settlements due to changing effective stresses.
A simple experiment which demonstrates the action of effective stresses is illustrated
in Fig. 6.7. This shows the influence of pore pressure on the capacity of deep and
shallow foundations. The soil should be fine to medium sand; if it is too coarse it will
become unsaturated when the water table is lowered and if it is too fine pore pressures
may not equalize in a reasonable time. Place the gravel and sand in water to ensure
they are saturated and then open the valve to lower the water table to the gravel. Place
a heavy foundation (a steel cylinder about 40 mm in diameter and 80 mm long works
very well) and an eccentrically loaded pile as illustrated. Close the valve and raise
the water table by pouring water into the standpipe; if the sand and gravel remained
saturated it will only be necessary to fill the standpipe. As the water table and the pore
pressures rise, effective stresses will fall and both foundations will fail.
If you then open the valve and drain the gravel pore pressures in the sand decrease
and the sand will regain its strength.
Figure 6.7
Rising groundwater experiment.
