Swelling characteristics of some British mudrocks

Abstract

One-dimensional swelling tests were conducted on seven undisturbed and remoulded heavily-overconsolidated clays and indurated mudstones. The samples cover a wide range of properties in terms of geological age, mineralogy, plasticity and diagenetic bonds. It is found that swelling proceeds faster than is predicted by Terzaghi's theory of swelling at low overconsolidation ratios (OCR), but it is slower at high OCR values, probably due to the prominence of secondary swelling. The results show that swelling is of a progressive nature, regardless of plasticity and diagenetic bonds. The coefficient of swelling decreases and the ratio of secondary to primary swelling increases with increasing OCR. At high OCR's, the former drops by more than an order of magnitude below the coefficient of consolidation; the latter can be as high as 0.6 within a log-cycle of time. Progressive softening and failure phenomena in mudrocks are considered to be due to the combined effects of progressive swelling and breakdown of diagenetic bonds. Swelling pressure is found to be indicative of in situ stress conditions. The average swelling index generally increases with pre-consolidation pressure in remoulded samples. Three types of diagenetic bonds are postulated: carbonate cements, viscous-adsorbed-water-type, and mechanical adhesion. Bonds tend to decrease the swelling index and so increase the coefficient of swelling. The fabric of the samples is far different from that assumed in the double-layer theory, which can qualitatively predict the physico-chemical forces involved in swelling. Dead volume, in which the double-layer force cannot operate, is shown to be large. The results of tests using n-decane as pore fluid to suppress osmotic swelling indicates that mechanical swelling is more than 50 to 60 per cent of total swelling in the samples. Both mechanical and total swelling indices are linearly correlated with liquid limit among remoulded samples, except for the Fuller's Earth clay which contains at least 65 per cent smectite (expandable clay).