SOIL STRUCTURE INTERACTION AROUND BURIED CONDUITS

Abstract

A large number of cases have been studied for the soil - structure interaction between a horizontally buried circular structure and its surrounding soil, by using finite element method. A computer programme based on the above technique using numerically integrated isoparametric elements with nonlinear material properties and simulation of placement of layers as placed in the field (sequential construction) has been developed. The parameters included for projection conditions are variation of height of fill/pipe diameter ratio, backfill properties, bedding conditions, interfacial boundary conditions and effect of surcharge. Further, for trench conduits the variation of backfill properties alongwith trench sides and ground, diameter thickness ratio, bedding conditions and width of the trench have been analysed. Lastly the analysis of a rigid concrete conduit of varying thickness section, placed in V trench on a lean concrete bedding with various parameters affecting the load is included. Few cases have been analysed with sequential placement of backfill in layers, with linear and nonlinear material properties for both projection as well as trench conditions and the results are compared with single lift construction. The nonlinear material constants of the fill have been determined for locally available sand. The variation of contact pressure distribution (normal as well as tangential), horizontal and vertical pressure distribution and deformations are plotted for all the cases analysed. The effort necessary to produce these plots from the numerical solution is excessive without automated graphical devices. Hov/ever, a major part of the information can be presented by these plots. The pressure distribution diagrams are particularly important, as these reflect essential information, about the transfer of forces from the surrounding medium to the body of the conduit. Further, if these distribution are once known the bending and axial stresses in the conduit section can be evaluated by using any standard method of analysis of conduit. A critical examination of Marston-Spangler load theory as applied to conduits laid in projection, trench or in V trench condition has been made. The limitations of this widely used theory is discussed in detail. It is summarised that the finite element method provides an excellent numerical modelling of interaction problems of buried conduits. It can account for material nonlinearity, sequential placement of backfill and any interfacial boundary condition between conduit and the surrounding medium and any interfacial behaviour between fill and the conduit. As a departure from conventional theory the rational normal and tangential soil pressures on the conduit have been presented in the form of an equation involving various parameters such as geometry of the system and properties of backfill with respect to ground. For possible design modifications a comparative study of soil pressures for linear and nonlinear sequential analyses have been presented. The effects of deformation of the conduit and the effect of bedding angle is found to be negligible within the range studied. Lastly, the loads obtained by Marston's load theory are compared with finite element solutions for each case and it is observed that the former gives higher loads, specially for conduits in trench and V-trench conditions. Further the distribution of vertical contact pressure at top and bottom of the conduit is found to be non-uniform as against uniform pressure distribution assumed by Spangler. A comparison of the finite element solution with the field measurements has been attempted for a typical case.