Gas flow in compacted clays
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Date
1998-09-01T00:00:00Z
Authors
Halayko, Krista Sophia Gelmich
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Abstract
Compacted clays and clay-sand mixtures are being used increasingly to isolate wastes from the biosphere. Clay buffers are commonly used in landfills and are proposed for use in the Canadian nuclear fuel waste disposal concept. This study will investigate the effect of high gas pressures on gas migration through compacted clay materials. Illite, illite/sand and bentonite specimens, 50mm in diameter and 24mm thick, were tested in a gas breakthrough apparatus with a capacity of 10 MPa. The inlet pressure was either increased in steps or was held constant. Gas breakthrough was said to occur when a response was noted at the outlet side of the specimen. In the forty-two illite/sand specimens tested with effective clay dry densities, $\rho\sb{\rm c}$, ranging from 1.30-2.10 Mg/m$\sp3$, gas breakthrough was found to be between 0.2 and 6.4 MPa. For most of the fifty-six increasing pressure tests on bentonite ($\rho\sb{\rm c}$ = 0.6-1.2 Mg/m$\sp3$) however, the upper capacity of the test equipment was reached before gas breakthrough was observed. The increased resistance to gas breakthrough is because of an increased proportion of bound water in bentonite which blocks all but the largest continuous pores. Below a certain degree of saturation, no resistance to gas flow was observed in either clay due to the fact that continuous gas pathways existed. This threshold was $\approx$85% for illite and $\approx$93% for bentonite. Capillary and advection theories were compared with the test data. The advection theories were found to provide a more realistic representation of the real system and an estimate of gas breakthrough pressure within an order of magnitude for increasing pressure tests.