Geomechanical Modeling of Gas Hydrate Bearing Sediments and Other Complex Soils

Abstract

The research presented in this dissertation is aimed at advancing the current understanding of the mechanical behavior of three distinct complex soil systems, as follows: gas hydrate bearing sediment, partially saturated clay silt and microbially induced calcite precipitation treated sands. Particular emphasis is placed on the mechanical constitutive modeling of these different soil systems. Gas hydrate bearing sediments (GHBS) are considered a potential future energy resource. The existence of the ice-like hydrates in the pore space and the associated phase change during hydrate dissociation make the modeling of GHBS very challenging. This thesis presents two novel constitutive models for GHBS that incorporate a number of improvements that allow simulating features of sediments behavior that were not captured by previous approaches. First, a simpler model was developed based on the critical state soil mechanics theory for strain hardening materials which was enhanced and validated with experimental tests involving shearing at constant hydrate saturation. This basic model was then upgraded using strain-partition concepts with the aim to achieve a better description of GHBS behavior. This model allows tracking the evolution of the mechanical contribution from the sediment and hydrate during shearing and dissociation. This is a novel aspect that was not considered in previous constitutive models and that greatly assists to gain a better understanding about the geomechanical response of this complex multiphase material. The progresses and developments made in the constitutive modeling of GHBS were adapted and extended to model other two geomaterials of great interest nowadays, as follows: unsaturated soils and treated soils by microbially induced calcite precipitation (MICP). The study of unsaturated soils is very relevant as they are often encountered in engineering applications. Furthermore, the mechanical behavior of partially saturated soils can be very different compared to that of fully saturated ones. The most popular framework to study the behavior of unsaturated soils is the so-called Barcelona Basic Model (BBM). This is an excellent model able to capture the main features of unsaturated soils, however it has some limitations to properly model materials exhibiting dilatancy during shearing. This model has been enhanced in this thesis. A critical comparison between the performances of these two models is carried out. It is observed that the enhanced model is able to satisfactorily capture the complex behavior observed in the lab and improve the response of the BBM for this type of soils. Finally, the focus is on the study of MICP treated soils. Microbially induced calcite precipitation (MICP) is a promising soil improvement technique for improving the performance of soft/loose soils. Sand is often the selected host soil in the lab to investigate this type of treatment. The mechanical behavior of MICP treated sand is carefully reviewed and an elastoplastic constitutive model is proposed for first time. The proposed model is widely validated against a number of laboratory experiments under different conditions. Also in this case the results are very satisfactory showing that the proposed model is capable of dealing with this type of treated soils.