Fluid injection is one of the major triggering factors of infrastructure collapse and geo-hazards, among which some of the most recurrent ones are rainfall-induced landslides. Such events exhibit complicated spatiotemporal patterns, spanning from slow deformation to extremely rapid movements influencing large regions. To prevent the occurrence of catastrophic failures and limit the potential damage, robust diagnostic tools are required to analyze the stability condition of a landslide-prone area and provide engineering guidance to take effective mitigation measures. Such methodologies usually involve multiple length scales and various physical processes, including soil deformation, transient fluid flow, and regional-scale growth of ground instability hotspots. This thesis intends to advance the state of the art on geomaterials stability with the goal to explain the initiation of runaway failures driven by soil saturation. For this purpose, it aims to formulate new analytical and numerical tools to examine the impact of soil instability on the hydro-mechanics of variably saturated deformable porous systems. For this purpose, the controllability theory based on the second-order work principle is further developed to capture the elastoplastic instability under various loading paths for both saturated and unsaturated soil systems. Since the accuracy of stability analyses highly relies on the performance of the selected constitutive model, an elastoplastic model with mixed isotropic-rotational hydro-mechanical hardening is proposed to capture the permanent deformations caused by wetting paths. Special emphasis is given to detect crucial material properties responsible for the transition from stable to unstable conditions upon wetting. Furthermore, the governing equations imposing momentum and fluid mass balance are inspected from an analytical standpoint to clarify the connection between their mathematical characteristics and the coupled hydro-mechanical instabilities embedded in constitutive laws. Distinct scenarios are considered in terms of material behaviors, distinguishing elastoplastic from viscoplastic materials. To test the validity of the analytical derivations, a coupled hydro-mechanical finite element solver is implemented. Wetting tests on soil columns are carried out under both compression and simple shear conditions. The results show that plastic failure due to fluid injection leads to an ill-conditioned global stiffness matrix, but the incorporation of slight amounts of viscosity restores numerical stability and suppresses the ill-posedness of the underlying field equations. Finally, the framework of viscoplasticity with strain-hardening is used to explore new directions to explain the rheology of clay systems with widely varying water content and propose a possible unification of solid-like and fluid-like clay rheology. The analysis points out the existence of mathematical analogies between the well-known geomechanical concept of tertiary creep and the notion of viscosity bifurcation often used in fluid rheology. Such findings have been used to propose a preliminary framework to unify the rheological modeling of solid-like and fluid-like fine-grained materials, based on the decomposition of stress components into intra-aggregate and inter-aggregate contributions.

Creator

• chen, yanni

DOI

• https://doi.org/10.21985/n2-ytef-tq33

Subject

• Civil engineering

Language

• en

Alternate Identifier

• http://dissertations.umi.com/northwestern:15183
• etdadmin_upload_754611

Keyword

• instability criteria
• viscoplasticity
• constitutive model
• wetting-induced failure
• unsaturated soil
• solid-fluid transition

Date created

• 2020-01-01

Resource type

• Dissertation

Rights statement

• In Copyright