Shear Behaviour of GCL-Sand Interrfaces under Static and Dynamic Conditions

Abstract

Geosynthetic clay liners (GCL) are unique geocomposites that combine the beneficial properties of bentonite clay and geosynthetics in providing effective hydraulic barriers in landfill systems. In landfills, GCLs form interfaces with soils and geosynthetics, resulting in inhomogeneity of the system. Inadequate shear strength mobilization at the interfaces results in translational failures in Geosynthetic Clay Liner (GCL). Interface shear strength of GCLs with the sand particles is predominantly influenced by the surface characteristics of the GCL, size and shape of the sand particles and their interaction mechanisms. These mechanisms change drastically with the hydration of GCLs and under repeated and dynamic loading conditions. This thesis examines the GCL-sand interactions and quantifies the interface shear strength under static and dynamic conditions with dry and hydrated conditions in sand. Illegal sandmining has resulted in the depletion of natural river sand and its scarcity for various constructional activities. To combat this issue, this study proposes the use of Manufactured sand (Msand) as a suitable subgrade or cover soil in landfills and evaluates its performance as an interfacing material with GCLs and compares it with the performance of river sand. Since the particle shapes of natural river sand and Msand are significantly different, a part of this thesis is focused on quantifying the shape parameters of the sands and investigating the effects of particle shape on the interaction mechanisms and shear strength of different GCL-sand interfaces. This thesis presents three different types of interface shear tests – modified direct shear tests, inclined plane tests and shaking table tests on GCL-sand interfaces with a natural sand and a manufactured sand under dry and hydrated conditions. Gradation of natural sand and Msand was kept identical to eliminate the particle size effects. The static shear strength of GCL-sand interfaces was evaluated through modified direct shear tests at higher normal stresses and inclined plane tests at lower normal stresses. The dynamic frictional properties were estimated using shaking table tests conducted at different g-levels, normal stresses, and excitation frequencies under dry and saturated conditions. Results from the interface shear tests were analysed in the light of shape analyses of sand particle and digital image analysis of sheared GCL surfaces. Particle shape parameters were obtained using computational algorithms applied to digital images of particles in MATLAB. To investigate the performance of GCLs under repeated shear conditions, modified direct shear tests were carried out for eight cycles of shearing in dry and hydrated conditions. The GCL used in the current study has a nonwoven geotextile as the carrier layer and a woven geotextile as the cover layer. Interface shear studies were carried out on both woven and nonwoven geotextiles interfacing with natural and manufactured sands under dry and hydrated conditions. Manufactured sand particles are less spherical and less rounded compared to river sand particles and their roughness is about twice to that of the roughness of the river sand particles. Results from the experimental and image studies showed that manufactured sand provides better particle-fibre interlocking compared to river sand under all test conditions, due to the favourable shape of its grains. Hence the natural sand interfacing with GCL in liners and capping components of landfills can be replaced with manufactured sand, with added benefits. Digital image analysis of GCL specimens exhumed after the shear tests provided important clues to the microscopic interactions that govern the overall shear strength of the interfaces. Surface changes to woven and nonwoven geotextiles due to shearing against natural and manufactured sand are compared in terms of percentage area of sand particle entrapment, extent of bentonite extrusion at different normal stresses under hydrated conditions and damage to the geosynthetic fibres during repeated shear. Shaking table studies showed that the dynamic friction angle of GCL-sand interfaces is only one third of the static friction angle, indicating the need for choosing materials that provide higher interface resistance for the construction of landfills in locations prone to earthquakes.