CONCEPT OF PORE PRESSURE AS APPLIED TO EARTH DAMS

Abstract

(This dissertation aims at a systuatic presentation and discussion of relevant published aatter from various sour-1 ceo. These sources are acknowledged at the end. The author does not slain to have added any new findings as no original research is carried out). The earth dam is most commonly designed for safety against sliding by the slip circle analysis by the Standard Method of Shoal. The slide failure Is a shear failure and the unit shear strength of the soil at various points along the sliding circle is not of a constant value but is a function of the weight of the fill above each point as is apparent from the equation S - C' + I (N-U) teen 0', where N the nornab comp. orient of the weight of the slice is different for each point and U the pore pressure on the base of the slice acting against N is also different from slice to slice. This pore pressure has its seat in the water and/or air in the pores of the soil skeleton. N is called he total normal stress and N-U is the effective normal stress for each slice. The critical stages in the life time of an earth dam during which the stability of its slopes is likely to be the least are (i) the rapid draw down state for the upstream slope, (ii) the full reservoir state for the down stream slope and (iii) the during-construction state for both the slopes. In the first two states the soil below the top seepage line is in the saturated condition and the pore pressures at different points in the fill can be found out from the flow net. Appro-ximate methods are however available for easier application. In the third state the soil is in the unsaturated condition, and herein, the concept of pore-pressure is not very clear even to this day. Estimation of pore pressures in this case may be done by theoretical methods making use of Boyle's and Henry's Laws or by making a rough approximation based on observed pore pressures in laboratory test-samples by simulating field conditions or based on pore pressures observed in other dams in comparable instances. Different methods of controlling pore pressures by providing drainage, by controlling construction moisture, etc., are available.....