Clay membrane testing using a flexible-wall cell under closed-system boundary conditions

Membrane behavior represents the ability of porous media to restrict the migration of solutes, leading to the existence of chemico-osmosis, or the flow of liquid in response to a chemical concentration gradient. Membrane behavior is an important consideration with respect to clay soils with small pores and interactive electric diffuse double layers associated with individual particles, such as bentonite. The results of recent studies indicate the existence of membrane behavior in bentonite-based hydraulic barriers used in waste containment applications. Thus, measurement of the existence and magnitude of membrane behavior in such clay soils is becoming increasingly important. In this regard, the development and evaluation of a flexible-wall cell used to measure the membrane behavior of clay soils under closed-system boundary conditions is described. The advantages of a flexible-wall cell include complete control over the state of stress existing within the test specimen and the ability to back-pressure saturate and consolidate the specimen prior to membrane testing. Use of the developed flexible-wall cell is illustrated via tests conducted to measure the membrane behavior of a geosynthetic clay liner (GCL), a manufactured hydraulic barrier containing sodium bentonite. The results indicate that the membrane efficiencies of duplicate specimens of the GCL measured using the flexible-wall cell are both reproducible and similar to, but somewhat lower than, those previously reported for the same GCL using a rigid-wall cell under the same closed-system boundary conditions.