EFFECT OF ROCK INTRUSION INFOUNDATION MEDIA ON THE PERFORMANCE OF A BARRAGE

Abstract

The analysis for the design of the barrage, along conventional lines, is done by Hetenyi's approach wherein the foundation media is assumed to be uniform. However field conditions with regard to foundation media do not present a foundation media having uniform characteristics throughout its extent. It becomes important to examine the situation when the foundation media is not homogeneous by numerical techniques such as the finite element method. In order to investigate the effect of non-uniform foundation conditions a typical barrage has been selected, located at the site where the subsoil is partly river bound material (RBM) and partly rock. The barrage consists of five bays constructed on three blocks. The first block consists of bays 1 & 2, the second block bays 3 &4 and the third block consists of bay 5. Bays 1 & 2 and bays 3 & 4 are totally on RBM with a layer of compacted soil matrix (CSM) extending from the base of the raft to the bottom of the cut-off. Bay 5 is completely on rock; however the foundation beyond the down stream of this bay is RBM. Further the rock mass extents to a significant height along the abutment. Solid45 3-D elements (8 noded brick element) have been used for modeling of the barrage with relevant amount of surrounding soil. Intrusion of the rock has been considered from the bottom of the models vertically as well as laterally. Gravity, Hydrostatic and Seismic analysis has been performed to find out the variation of moments and stresses at the three sections of the raft namely (i) up stream section (ii) ogee section (iii) down stream section. Free vibration characteristics are obtained considering the stiffness of the soil and mass and stiffness of the structure. The study concludes that rock intrusion affect significantly on the moments and stresses and hence on the design of the raft on the basis of static analysis. The structure experiences larges seismic excitation with increasing rock intrusion in foundation media and hence is more vulnerable to seismic forces on the basis of dynamic analysis.