Factors affecting GCL hydration under isothermal conditions

The hydration of different GCLs from the pore water of the underlying foundation soil is investigated for isothermal conditions at room temperature. Results are reported for three different reinforced (needle punched) GCL products. Both a silty sand (SM) and sand (SP) foundation soil are examined. GCL hydration is shown to be highly dependant on the initial moisture content of the foundation soil. GCLs on a foundation soil with a moisture content close to field capacity hydrated to a moisture content essentially the same as if immersed in water while those on soil at an initial moisture content close to residual only hydrated to a gravimetric moisture content of 30-35%. The method of GCL manufacture is shown to have an effect on the rate of hydration and the final moisture content. The presence or absence of a small (2 kPa) seating pressure is shown to affect the rate of hydration but not the final moisture content. The GCL hydration did not change significantly irrespective of whether a nonwoven cover or woven carrier GCL rested on the foundation soil

Physical modelling of nonwoven/nonwoven GCL shrinkage under simulated field conditions

A physical model was employed to evaluate the potential shrinkage of a needle-punched nonwoven/nonwoven geosynthetic clay liner (GCL) subjected to simulated daily heating/cooling cycles (23°C for 16h; 60°C for 8h; 23°C for 16h etc. at the top) modelling solar exposure conditions of an overlying black geomembrane and simulated field moisture conditions where the GCL must rehydrate with water from a silty sand foundation soil with an initial moisture content of 16-17% in a closed system. It is shown that when the GCL was allowed to hydrate to 110% gravimetric moisture content before being subjected to daily heating/cooling cycles there is a relatively rapid increase in maximum shrinkage strain to 1.7% (corresponding to shrinkages of ~75mm for a 4.42m wide roll) over the first 5 cycles and 3.8% (~170mm) over the first 14 cycles. The shrinkage rate then decreases giving a maximum shrinkage strain of 4.5% (~200mm) after 50 heating/cooling cycles. Experiments are conducted for the GCL with different initial gravimetric moisture contents and it is shown that the initial moisture content at the time the heating/cooling cycle commenced has a substantial effect on shrinkage. Consistent daily cycles of the magnitude examined are shown to suppress GCL shrinkage, but a cool period during which the GCL can hydrate from the subsoil, followed by significant daily thermal cycles, increases the shrinkage. The implications with respect to field shrinkage are discussed

GCL hydration under simulated daily thermal cycles

The hydration of three different geosynthetic clay liners (GCLs) subjected to daily thermal cycles was examined for a range of subsoil conditions. It was shown that daily thermal cycles can significantly decrease the equilibrium gravimetric moisture content of the GCL to as low as 15% of that under isothermal conditions in the worst case. For silty sand (SM) foundation soil with an initial gravimetric moisture content of 16%, the type of GCL had a significant effect on the daily variation in moisture content which ranged between 13% for one type of GCL and only 2% for another. The effect of these daily variations in moisture content on susceptibility to shrinkage is discussed. The initial moisture content and associated matric suction of the foundation soil was shown to have the dominant effect on GCL hydration. For GCLs over silty sand with initial moisture contents, wfdn, of 5, 10 and 16% and initial suction levels greater than their air entry value, the daily thermal cycles controlled GCL hydration at the end of the thermal cycle to moisture contents of between 14 and 30% and the GCL equilibrium moisture content was relatively insensitive to the initial foundation moisture content over this range. However when the foundation moisture content increased to wfdn = 21% (just below field capacity and the saturated moisture content) the GCL moisture contents increased to 113 to 127% (depending on GCL). Results are also reported for a GCL on poorly graded sand (SP) at 10% initial moisture content and the effect of the grading curve (and the related water retention curve) is discussed. The results of this study highlight the potential complexity of interpreting shrinkage of GCLs at the same site let alone at different sites