A Numerical Investigation of the Effects of Loading Conditions on Soil Response

Abstract

All three principal stresses play a part in the stress-strain-strength response and volumetric behavior of solids and granular materials. In geotechnical engineering, conventional triaxial compression (CTC), plane strain (PS), and direct shear (DS) are the three most commonly used laboratory tests to simulate the field conditions. It is natural to assume that specimens subjected to different loading conditions will show different responses and behaviors. In reality, many soil problems involving shear strength approximate to PS loading conditions in the field (e.g., earth dam, embankment, and retaining wall). However, CTC or DS test is typically used to measure the stress-strain-strength parameters for design because of their simplicity and versatility compared with the complexity and difficulty of the PS test, even though they might not closely mimic the field condition. The current research focuses on the numerical analysis of effects of different loading conditions (e.g., CTC, PS, and DS) on the macro- and micro-behaviors of granular materials using discrete element method (DEM). Analytical, statistical, and stereological approaches are employed. It is the first work to compare the results under the three most common loading conditions (PS, CTC, and DS) in DEM modeling. Models of the CTC, PS, and DS tests are developed. A new method to simulate the membrane behavior is proposed. Parametric analyses to qualitatively assess the effects of the specific parameter on the macroscale response of the specimen are performed. Macroscale responses of sets of simulations of assemblies under PS, CTC, and DS loading conditions are studied. Small-strain responses, shear strengths, and volumetric behaviors of the assemblies under different loading conditions are investigated. Microscale analyses on the assembly behaviors (e.g., void ratio and coordination number) and particle behaviors (e.g., particle rotation and displacement) are conducted. Particle orientation and contact properties (e.g., contact normal and contact force) are investigated using statistical analysis method. An algorithm to generate numerical slicing images which is to simulate the way in laboratory experiments is proposed. The local void ratio distribution analysis and particle orientation distribution analysis are performed using stereological method. Integrating macro-, micro-, and stereological methods, some issues such as strain localization, critical state, and principal stress direction rotation of DS test are investigated.