PLASTIC ANALYSIS OF STRESSES AND STRAINS IN EARTH DAM BY FINITE ELEMENT METHOD

Abstract

Oldest earth dams, as high as 70 ft, were constructed in India even before the Christian Erat It was not until the begin ning of the twentieth century that earth dams upto 125 ft height could be built successfully(195)+! Increasing importance of earth dams may be seen from the fact that more than 55* of about 4000 dams built after World War II are earth dams. Despite their great econo mic importance, structural design of earth dams did not receive adequate attention of research workers. Their design has been con sidered as mainly a problem of stability of slopes. It has now be come necessary to examine (i) Whether the conventional stability analysis ensures complete structural safety? (ii) Does the tendency for progressive failure warrant a revision in the conventional ana lysis? (iii) What are the changes due to excessive deformation against which the stability analysis does not safeguard the earth dam? and (iv) Is theoretical analysis of displacements and stresses in the body of earth dam possible? The present concept of stability analysis is based on Coulomb's (1776) earth pressure theory which considers the soil on both sides of the rupture surface to behave like rigid bodies according to Amonton's law of friction (1699) rather than dilatant bodies (Roscoe, 157). In early 20th century, the analysis for plane rupture surface (148) was extended for circular surfaces (141, 177). Subsequently, method of slices was evolved to take into acco unt non-homogeneity and pore water pressure*54, 58, 116, 191, 192, 202). +~An~account of oldest earth dams and modern high earth dams is given by Narain (130) ++Number in the parenthesis denotes the serial number of reference given in the bibliography. In the case of materials which lose most of their strength beyond peak failure, a slide could begin when the plastic limit is reached and proceed as progressive failure. Jurgenson (87), Middlebrooks (123) and Patrick (139) evolved approximate methods to estimate the factor of safety aqainst overstressing, in which the embankment was replaced by eauivalent surcharge. The discre- pancy due to residual shear stresses at the slopes can be ignored for slopes flatter than 3:1 (Benett, 8 and Bishop, 173). There are a number of well studied case histories (Table 1) which disgualify the use of conventional stability analysis. Further, the stress analysis is imperative when the excavations are made in a overconsqlidated clay strata having high residual stresses (Skempton, 181 and Mencl et al, 121).