Moisture Uptake and Loss of Geosynthetic Clay Liners from a Silty Sand Subgrade Under Isothermal Hydration and Thermal Cycles

Abstract

Geosynthetic clay liner (GCL) performance is said to rely on adequate hydration from the subgrade which is controlled by factors such as the properties of the GCL, subgrade and permeant, and their exposure to solar radiation. Much research has been conducted on this topic, however, this thesis examines several significant knowledge gaps and, in particular, the performance of partially hydrated GCLs when permeated with synthetic municipal solid waste leachate. In addition, the components affecting moisture retention or loss of GCLs when subjected to simulated thermal cycles are examined for GCLs that were deconstructed by cutting of the needle-punched fibres to allow each component to be studied in isolation. The findings from column hydration tests performed on GCLs placed on a silty sand subgrade of varying density, compaction conditioning time, and grain size distribution due to silt aggregation are reported. For the isothermal tests, the columns were stored at 25±4°C for four weeks. For cyclic tests, the column airspace was subjected to thermal cycles up to 60°C for the fifth week. Results suggest that silt aggregation can inhibit moisture uptake if not sufficiently broken down by compaction and a GCL that appeared to retain moisture on a typical subgrade lost moisture when placed on a subgrade characterized by high silt aggregation, low density, and long conditioning times. A GCL with fine-grained bentonite that initially lost moisture under thermal cycles also began to retain moisture when the bentonite was ground to a powdered form. The swell index and cation exchange capacity of the bentonite and the hydraulic conductivity of various GCLs (in terms of bentonite granularity, geotextile type, and construction) were obtained after prehydration at 5% and 16% subgrade water contents. The lowest hydraulic conductivity was for a thermally treated GCL with powdered bentonite (4.5 x 10-11 to 8.4 x 10-12 m/s) and the highest was for a non-thermally treated GCL with coarse-grained bentonite (6.0 x 10-08 to 5.1 x 10-10 m/s). The significance of subgrade water content to GCL performance was mostly due to cation exchange and varied with the type of GCL and permeant.