In situ Performance of a Clay-Based Barrier

Bentonite, a processed clay product composed primarily of the clay mineral montmorillonite, has been used or specified for use as a barrier material in a variety of waste containment facilities. When compacted and applied in an earthen waste containment barrier the surface-active clay mineral montmorillonite can impart the desirable properties of low hydraulic conductivity, low contaminant diffusivity and chemical retardation. Moreover, depending on the rigidity of confinement, the dry density of bentonite within the barrier and pore fluid/contaminant chemistry, bentonite barriers may develop up to tens of MPa of swelling pressure against rigid confinement. Bentonite/soil mixtures with bentonite content ranging from 100% bentonite, in a highly compacted form, to 5% bentonite, by dry mass proportion, have been specified for use as barrier materials. The ‘soil’ component may be a specifically designed sand-aggregate. The design of the particular ben-tonite/soil mixture will depend to a large degree on the physical and geologic setting of the proposed barrier, the nature of the wastes coming into contact with the barrier material and barrier construction procedures. For example, barriers for underground waste containment may use 100% highly compacted bentonite in situations in which the bentonite will be rigidly confined by the surrounding rock and rigid restraining structures such as concrete plugs and bulkheads.