Constitutive modeling and finite element analysis of slowly moving landslides using hierarchical viscoplastic material model.

Abstract

The prediction of motion of slowly moving landslides, also referred to as creeping slopes, is important for the reduction of landslide hazards. Such continuous and slowly moving landslides do not represent the usual stability problems of geotechnical analysis because these slopes are neither still nor ruptured but they move. For proper modeling of the motion of landslides, it is essential to develop improved techniques that integrate appropriate modeling of geological materials involved, laboratory and field tests, and verifications using computational methods. This dissertation focusses attention on the development of such an appropriate model for the time-dependent behavior of creeping landslides. Based on field observations it is proposed that the phenomenon of creeping landslides can be considered as involving the motion of a large mass of soil over a parent (fixed) mass with pronounced shear deformations occuring in a thin layer between the moving mass and the parent mass. The thin layer is refered to as interface zone while the overlying mass is refered to as solid body. The generalized Hierarchical Single Surface (HiSS) series of plasticity models are adopted to characterize the solid body. The interface zone is modeled using the specialization of the HiSS models for conditions occuring in the thin layer. Time dependency is introduced in constitutive models by adopting Perzyna's elastoviscoplastic formulation. The parameters for the HiSS and interface models are determined from laboratory tests on soils obtained from an actual slowly moving landslide at Villarbeney in Switzerland. Triaxial tests along various stress paths and oedemeter tests are conducted for the solid body. New analytical solutions are derived for prediction of oedometer tests. A general procedure for determination of viscous parameters is developed and techniques to process raw creep test data are proposed. Novel and representative simple shear interface tests are conducted to find parameters for the interface model. Special techniques for experimental analysis have been developed. A modified interface model to simulate the observed phenomenon of only compaction under shear is proposed. The parameters for the constitutive models are verified by numerically backpredicting experimental tests. An existing finite element code has been modified to incorporate various aspects of the small strain elastoviscoplastic formulation. Field measurements in the form of inclinometer profiles at various borehole locations on Villarbeney landslide are available. These inclinometer profiles are predicted using the proposed model. A comparison of the field measurements and the results from finite element analysis shows that such a model can be successfully used for predicting the behavior of slowly moving landslides.