Impact of pore-pressure cycling on bentonite in constant volume experiments

Document available in extended abstract form only. The SKB safety case for a KBS-3 repository highlights the potential importance of future successive glaciation events on repository functions. One particular uncertainty is the likely affect of elevated pore-water pressures on barrier safety functions. Over the repository lifetime such changes in pore-water pressure are likely to be cyclic in nature, as successive glacial episodes lead to loading and unloading of the engineered barrier. For a clay-water system with the pore-water in thermodynamic equilibrium with an external reservoir of water at pressure, pw, the total stress acting on the surrounding vessel can be expressed as: (1) σ = Π + αpw where Π is the swelling pressure and α is a proportionality constant. We present results from a series of laboratory experiments designed to investigate this relationship, in the context of glacial loading. Blocks of pre-compacted Mx80 bentonite were manufactured by Clay Technology AB (Lund, Sweden), by rapidly compacting bentonite granules in a mould under a one dimensionally applied stress (Johannesson et al., 1995). The blocks were then sub-sampled and cylindrical specimens prepared for testing (120 mm in length and 60 mm in diameter). The experiments were conducted using a specially designed constant volume cell, which allows the evolution of the total stresses acting on the surrounding vessel to be monitored during clay swelling (at three radial and two axial locations). A high precision syringe pump was used to maintain a constant applied pore pressure within the bentonite, while the rate of hydraulic inflow, and consequent stress development, were monitored to determine the point at which hydraulic equilibrium was reached. During the tests each sample was subjected to an incremental series of constant pore-pressure steps, with all samples experiencing at least one loading and unloading cycle. The resulting average total stress data yield alpha values in the range of 0.8-1.0. For all tests, the swelling pressure was clearly observed to decrease with increasing pore-water pressure. However, the sensitivity of Π to changes in the applied pw was also shown to decrease asymptotically as pw is increased. In addition, our results indicate that pore-pressure cycling may lead to a persistent elevation in the average swelling pressure of the clay, despite a consequent reduction in the applied pore-water pressure. The impact of this behaviour on factors such as the movement of gas and the mechanical properties of the engineered barrier may need to be considered when assessing repository performance during interglacial and glacial events. However, such hysteric behaviour is not always observed and the cause of this remains unclear. Further testing will help to elucidate the nature of the relationship between the pore-water pressure, swelling pressure and the total stress in such systems