Modelling The Behaviour Of Sand Under Strain-Controlled Loading BY The Finite Element Method

Abstract

Strain-controlled tests are conventional in soil mechanics laboratories. It isintended in this paper to simulate both triaxial and simple shear teststheoretically by using the finite element method. The solution of the nonlinearequations is obtained by several iterations. The Newton-Raphson withtangent stiffness method in which the stiffness matrices are tangents isadopted. The model used in this paper is the ALTERNAT model whichforms the major component of a double hardening model for the mechanicalbehaviour of sand under alternating loading.The finite element method is used in simulating the behaviour of rounduniform quartz sand under monotonic drained loading with constant meanstress and cyclic constant volume loading (undrained). The monotonic testwas conducted with constant mean stress, where the specimen wascompressed in one direction and extended in other directions while the meanstress (the average of the principal stresses) is kept constant and equal to 137kPa. It is noticed that the peak stress is occurring at very small strain(0.122). The stress-strain behaviour may be attributed to the particleroundness and grain size uniformity.In the cyclic tests, the specimen is sheared by cycling the shear strain whilethe volume was kept constant. By doing this, an undrained strain-controlledcyclic test similar to that typically done in many laboratories is numericallysimulated.It was found that the mean stress during shearing is higher than the initialconsolidation pressure. This implies that only negative pore pressures occurin the first two cycles. A careful study shows that there exists an effectivestress ratio line or zero-dilatancy line in both compression and extensionregions, beyond which the specimen dilates.