Abstract

Modified Cam-Clay model is often used for simulation of soft geomaterials such as soil or sand. In this model, Young’s modulus continuously changes via the bulk modulus. Poisson’s ratio can change or be kept constant. If the Poisson’s ratio is kept constant, shear modulus will change with time. If the shear modulus is kept constant, Poisson’s ratio is allowed to change with time. In general, the effective stress tensor at time t+1 can be computed when all state variables at time t are known. The material can be either in elastic or plastic mode depending on the stress state. In the case of plasticity, there are many steps involved to obtain a correct stress so that the equilibrium force balance could be satisfied within a tolerance. In the case of plastic deformation when the stress is out of a yield surface at time t+1, the stress must be brought back to the yield surface. The stress computation procedure during loading and unloading has an important effect on the results of deformation and rebounding after a plastic deformation. In this study a new approach for stress computation during loading and plastic unloading path is introduced. In the new proposed method, the relationship between mean effective stress change and volumetric strain change is used as a parameter to determine the amount of plastic deformation during unloading. The new approach will be appropriate for any case with large residual strain and a proposed exponential parameter (n) will determine based on the value of residual strain. A sensitivity analysis was carried out for different values of the exponential parameter which showed the displacement to be gradually less sensitive when the value of the parameter “n” is 1.003 or higher during the unloading stage. This method was then implemented and successfully tested for a study on matured offshore Malaysian field.

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