Particle-scale mechanisms in undrained triaxial compression of biocemented sands: Insights from 3D DEM simulations with flexible boundary

A three-dimensional (3D) discrete element method (DEM)-based numerical model is used to simulate the macromechanical response of sand strengthened using microbially induced carbonate precipitation (MICP) under undrained triaxial compression and inform the particle-scale mechanisms responsible for the behavior. The constant volume method is used to simulate saturated media. Although simulations using rigid boundaries are capable of representing the response of uncemented sands, virtual undrained triaxial tests on cemented sands require the use of flexible boundaries. Flexible membrane boundaries are created using particle facets (PFacets) as the building blocks. A methodology to implement virtual undrained triaxial compression using PFacet-based membrane boundaries is developed. The macroscale response of sands with varying degrees of cementation is adequately captured by this model. A cohesive bond strength, used to express the degree of cementation, is found to be well related to the shear-wave velocity through the soil sample. The model correctly predicts the occurrence of strain localization in cemented media, and the expected trends in shear band formation. The evolution of normal contact force distributions and coordination numbers as functions of both the cementation level and axial strain are also predicted.