This paper presents suction and strength characteristics for a dense, compacted, unsaturated sand–bentonite mixture under a variety of preparation and stressing conditions. Suctions were determined experimentally using thermocouple psychrometers and the filter paper method. They are shown to be related to water contents, saturation, dry densities and osmotic agents. The influence of the initial suctions on strength was evaluated using quick undrained triaxial compression tests, here called ‘constant-mass’ tests. Examination of the suction response to applied external stresses was carried out using stress-controlled triaxial tests along selected stress paths. These tests measured suctions in tests where the mean stress p and deviator stress q were changed systematically to give a series of constant values of Δq/Δp. The results showed that suction decreased as mean stress loading increased. Suction changes appear to be produced only by the mean stress component of the stress tensor and not by the shear stress component. In this compacted sand–bentonite material, for the pressure range at which the tests were performed (p′≤3 MPa), changes in suction produced by changes in mean stress are largely reversible.

Laboratory testing has investigated how gases can break through compacted specimens of illite, bentonite, and sand-illite or sand-bentonite mixtures. Specimens were formed with a wide range of initial clay densities, water contents and degrees of saturation. Tests were done using two different test procedures. In one, equal increments of gas pressure were applied at constant time intervals until break-through was observed. In the second, the pressure was held constant, and the time required for break-through recorded. Results show that the pressure at break-through increases with clay density and decreases with degree of saturation. When the degree of saturation is below about 85% in illite and clay-illite, and 93% in bentonite and sand-bentonite, there is only a small resistance to gas migration. Above these degrees of saturation, break-through pressures rise sharply. In an approach that differs from some others that have been reported, it is postulated that gas migration is only possible when its pressure is higher than a Gas Entry Value (GEV) that is related to capillarity effects in the largest pores of the material. Thereafter, the rate of advance of the gas-water interface depends on advective flow, that is, on the pressure (hydraulic) gradient across the specimen. Analysis shows that times to break-through should decrease inversely with pressure increase and this was observed in the experiments. Tests were also done on specimens made with non-polar paraffin instead of water. This inhibited the development of bound water in diffuse double layers (DDLs) and led to break-through at much lower pressures. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract Oil and gas wells can develop gas leaks along the casing years after production has ceased and the well has been plugged and abandoned (P&A). Explanatory mechanisms include channelling, poor cake removal, shrinkage, and high cement permeability. The reason is probably cement shrinkage that leads to circumferential fractures that are propagated upward by the slow accumulation of gas under pressure behind the casing. Assuming this hypothesis is robust, it must lead to better practice and better cement formulations

'Throughout our inquiry we were continually told at each place which we visited that it was the most favourably situated for fishing, and that the adjacent waters were teeming with an inexhaustible supply of fish. But, with rare exceptions, we found few west coast boats at sea, and at few places could we obtain evidence of any systematic fishing by west coast boats.' This was the observation of the members of the Walpole Commission as they moved through the north-west Highlands in 1890. They were asked to decide whether there were sufficient stocks of fish in nearby seas to allow increased employment in fishing and, if so, what measures should be taken. Like many before them, they found a cruel irony about the place of fishing in Highland life on the one hand, evident wealth in the sea and a people in desperate need of any possible source of income, and on the other, a tiny and partly wasted effort of fishing. Why did the people of the Highlands, often in deepest need, so persistently neglect the great resource on their doorstep when strangers from the east coast of Scotland, from England and from continental Europe found good takings in the west coast seas. The readiest explanation to occur to both Lowland Scots and to the administrators of Victor­ ian England was a temperamental incapacity of Highlanders to make full and efficient use of available resources, whether of sea or of land. But other more subtle and more sympathetic reasons could be found the remoteness of the area from the main markets, the nature of the land system which kept the people trapped in a fatal mixture of occu­ pations, lack of capital to allow even the beginnings of improvement, a tyrannical social system which removed from the ordinary man both the incentive and the means to improve, the lack of a merchant class, the hidden traps in some of the evidently easy forms of fishing in the treacherous conditions of the area. These explanations had been advanced many times in the nineteenth century but perhaps it was in the quartercentury following the Walpole Commission, a time of much change for the crofterfisherman, of opportunity and partial response, that the student can best probe the true