Morphological Perspectives of Sand Liquefaction and its Mitigation

Abstract

Liquefaction of soils under seismic conditions has profound implications for infrastructure resilience in earthquake-prone regions. Saturated loose cohesionless granular soils have the tendency to readily lose grain contacts and exhibit fluid-like rheology during seismic shaking conditions. This change of state, referred to as liquefaction, causes complete loss of shear strength and damage to infrastructure. Particle morphology, which encompasses the grain size and shape features, highly influences the intergranular interactions at a micro level and compressibility, drainage, and strength at a macro level, thereby governing the liquefaction response of sands. The complex interplay of the micro-level particle morphology and macro-level liquefaction response of sands remains elusive to date. Also, reinforcement inclusions in the form of layers, columns, and fibers used as liquefaction mitigation measures, interact with sand particles in multiple ways while reducing their tendency to separate. These interfacial interactions are also primarily governed by particle morphology. In this context, the primary focus of this thesis is to bring out the morphological perspectives of sand liquefaction by examining various fundamental mechanisms involved in the shearing behaviour of unreinforced and reinforced sands before, during, and after liquefaction.

A series of laboratory monotonic and cyclic constant volume simple shear tests are performed on granular ensembles with distinct morphological features representing fine, medium, and coarse sands with rounded, subrounded, and angular particles. Digital image analysis is used to quantify the morphological characteristics of granular materials like sphericity, roundness, angularity, and roughness, to understand the grain-scale kinematic interactions. Fundamental mechanisms of soil liquefaction, which include the mounting up of pore pressures, accumulation of shear strains, stress path traversed, stress-strain response, cyclic stiffness degradation, and strain energy dissipation, are critically examined in the light of particle morphology and individual contributions of grain size and shape on these processes are quantified. Results showed that both particle size and shape highly influence the pre-liquefaction, liquefaction, and post-liquefaction shearing behaviour of granular ensembles. An increase in the grain size or angularity of the particles resulted in an increase in the liquefaction resistance and shear strength before and after liquefaction, due to the rise in the tendency to dilate and interlock at the particle scale.

The performance of various reinforcement alternatives, including nonwoven geotextile layers, geotextile encased granular columns, geofoam discs, and discrete natural coir fibers in improving the liquefaction resistance of sands is studied, and the relative efficacy of these techniques is analyzed. In the case of geotextile-reinforced granular ensembles, grain shape is found to predominantly influence the liquefaction response, whereas the effects of grain size are found to be minimal. Since the constriction sizes of the geotextile are much smaller than the grain sizes, the particle scale interactions and their effects on interfacial shear strength and the overall shear response are limited. On the other hand, angular particles can plough the geotextile and can mobilize the maximum interfacial shear strength compared to rounded and subrounded particles, thus influencing the shear behaviour and liquefaction response significantly. Different configurations of various inclusions, like multiple layers of geotextile, geotextile-encased granular columns in single and group configurations, geofoam buffers in different densities and layer thicknesses, and coir fibers in different percentages, are investigated. Liquefaction resistance is found to improve with the increase in the number of geotextile layers, high area replacement ratio and group configuration of granular columns, lighter and thicker geofoam discs, and higher fiber content. The order of the best inclusion choice for liquefaction mitigation is geofoam > encased granular columns > coir fibers > geotextile. The excellent energy absorption characteristics of geofoam make it an excellent choice for improving liquefaction resistance. However, these benefits come with an adverse effect of reduction in pre- and post-liquefaction monotonic shear strength of sand, due to the high compressibility of geofoam. Natural coir fibers could improve the liquefaction resistance better than polymeric geotextile layers, and they are also effective in improving the pre- and post-liquefaction shear strength. Hence, this thesis emphasizes and recommends the usage of coarse sand with angular particles as base material and eco-friendly materials like coir fibers as reinforcement for constructions in high seismic zones, to derive maximum resistance against liquefaction.