Influences of degree of saturation and stress cycle on gas permeability of unsaturated compacted Gaomiaozi bentonite

Abstract

Compacted bentonite is usually used as the backfill material in underground storage of radioactive waste due to its swelling properties and low permeability. To understand the effects of the degree of saturation and stress cycle on the barrier properties of compacted bentonite, gas permeability tests were performed on highly compacted bentonite with different dry densities, water contents and stress cycles. The testing results show that the degree of saturation and stress cycle both have significant influences on the gas permeability of unsaturated compacted bentonite samples. As the saturation or confining stress increases, gas permeability of the samples is decreased by no more than 3 orders of magnitude. Moreover, under the condition of the same dry density, the greater the degree of saturation, the faster the decline of gas permeability of samples during loading; under the condition of the same degree of saturation, it shows that larger initial dry density can cause gentler decrease of the gas permeability of samples. A slight recovery in gas permeability was observed during unloading process of confining stress, and the decline of gas permeability caused by increasing confinements is mainly irreversible. Different from saturated/nearly-saturated samples, gas migration in the low/medially saturated compacted bentonite sample is considered to be dominated by single-phase advection in the connected pores of the unsaturated zone. The present work implicates that the effect of the degree of saturation and the stress path on the gas permeability of the backfill material should be taken into account during the construction period and early stage of operation of the radioactive waste.