BEARING CAPACITY OF SOIL UNDER STATIC AND TRANSIENT LOAD

Abstract

The study presents a general nonlinear anisotropic constitutive relations of strain rate dependent material. Simplification of this general relation shows thai, when the soil is assumed to behave as linear, visco-elastic isotropic material, two elastic constants and two viscous resistance constants are needed to fully represent the behaviour. Based on principle of continuum mechanics, it is shown that the number of independent elastic constants of an anisotropic elastic material is fifteen instead of twenty one as understood so far. It is further shown, that if the directions of principal stresses are coincident with those of principal strains, then the number of elastic constants reduces to six. Testing procedures adoptM in laboratory .-aheo*. fcasfc* are briefly described and their merits and demerits for determination of the constitutive relations of soil are discussed. Available information is summarised and discussed regarding various factors influencing stress-strain relat ion and strength of soil as well as the mathematical models proposed by various investigators for representation of the stress-strain relation. The finite element technique is used herein for loadsettlement analysis. The stress and stiffness matrices of an element are infinite, if the material of the element is incompressible( Poisson's ratio v=0.5). In fact, the Poisson's ratio -iiof saturated clay and sand at critical void ratio is equal to 0.5. A finite element procedure suitable for such material has beer presented. Computer programmes needed for the analysis are prepared and checked for their reliability. The effect of size of elements and depth and lateral distance of assumed cig.id boundary are investi gated by considering a problem of unit uniform normal load acting over a semi-infinite linear elastic and isotropic body. The computed results are compared with Boussinesq's solution. The stress strain characteristics of a partially saturated clay and a dense sand as obtained from triaxial shear test are represented by equation of hyperbola. The variation of initial tangent modulus of sand with confin ing pressure is represented by relation given by Janbu(l963). Using the above relations and finite element technique, the load settlement characteristic and ultimate bearing capacity of rigid square footings resting on surface of a clay and a sand medium are computed and compared with plate load test results.For the first time,the effect of Poisson's ratio and stress dependent anisotrophy on load settlement characteristics and ultimate bearing capacity of soil is scientifically and logically discussed. The study shows that for given strength parameters'C' and VJ the ultimate bearing capacity of soil depends on its Poisson's ratio 1v' and stress dependent anisotrophy. An increase in value -111- of Poisson's ratio increases the ultimate bearing capacity. Based on finite element—technique procedure, a relation for computation of load-settlement character istics of a clay from that of another clay has been suggested. Similarly, a relation for extrapolation of load-settlement characteristics of soil from one size to other size of foundation is presented. The effect of depth of rough rigid boundary and non-linear isotropic property of a clay on fundamental natural period of soil foundation system is analysed and discussed. The study shows, if the equivalent mass of the static load acting on the footing is negligible, then the fundamental natural period of soil foundation system is influenced to a considerable extent by the depth of rough rigid boundary. On the other hand, if the equivalent mass of static load lumped at foundation is considerable, then the effect of rigid boundary on fundamental natural period is negligible provided the rigid boundary is at depth greater than four times the diameter of footing. Equivalent effective soil mass increases with increase in size of foundation irrespective of deformational properties of soil. On the other hand, it decreases with increase in intensity of static load, if the stress strain relation of clay is of nonlinear stress soften ing type. -IVGoverning matrix differential equation of motion for linear visco-elastic material subjected to dynamic load has been .derived. Importance of determining viscoelastic properties for proper evaluation of dynamic response of footing is pointed out. For axi-symmetric problem, equations for distribution of total mass of an element to the nodes of the element is derived. Recurr ence relation of governing matrix differential equation of motion is formulated, according to G-alerkin's weighted residual process, load settlement characteristic of a partially saturated clay subjected to transient load is obtained using nonlinear stress strain relation of the clay considered and compared with plate load test results. The procedure adopted in this study is found to be- suit able for obtaining acceptable results for such a complic ated problem. This study reveals that, even if there is an increase in shear strength of soil under transient load, the bearing capacity may be less than the static value, if the period of the applied transient load is close to the natural period of soil foundation system.