Technical Report NTB 85-36

Highly compacted bentonite is known to let gas through at a very low rate but earlier investigations have indicated the possible existence of a critical gas pressure at which the gas forms highly conductive passages through the clay. The present study was primarily aimed at testing the hypothesis of the critical pressure concept by measuring the gas flow as a function of the applied, successively increased gas pressure, and by trying to identify microstructural evidence of how gas percolates clay.

Eight tests were run with MX-80 bentonite saturated with strongly brackish Nagra water, the density of the water saturated bentonite ranging from 1.7 to somewhat more than 2.1 t/m³. In all the tests it was observed that gas break-through, manifested by a largely increased conductivity, took place at a gas pressure of the same order of magnitude as the swelling pressure.

One of the samples was analysed with respect to the microstructure and it revealed the existence of discrete gas-filled voids, some of which released gas when the trimmed clay specimen was placed into the embedding substance used for the microstructural analysis. This confirms that gas percolates through a number of narrow passages that are formed when the gas pressure is sufficiently high. In soft and medium-dense bentonite the "Capillary retention" is probably the major resistance to gas propagation, while in dense bentonite the penetrating gas has to make its way by displacing clay aggregates. In the latter case the critical gas pressure is therefore logically very high and closes to the swelling pressure.