Abstract The Large Scale Gas Injection Test (Lasgit) is a field-scale experiment designed to study the impact of gas build-up and subsequent migration through an engineered barrier system (EBS). Lasgit has a substantial experimental dataset containing in excess of 26 million datum points. The dataset is anticipated to contain a wealth of information, ranging from long-term trends and system behaviours to small-scale or ‘second-order’ features. In order to interrogate the Lasgit dataset, a bespoke computational toolkit, designed to expose and quantify difficult to observe phenomena in large, non-uniform datasets, has been developed and applied. Presented results focus on the investigation and interpretation of second-order events occurring in close proximity (temporally and spatially) to a known macro-scale gas flow event that occurred during the second gas injection test. The similarity of the investigated event to dilatant flow observed in laboratory experiments is noted, as is the evidence for localized flow pathways in the bentonite EBS. The sensitivity of the toolkit’s ability to highlight second-order events is also evaluated.

Abstract The Swedish concept for geological disposal of radioactive waste involves the use of bentonite as part of an engineered barrier system. A primary function of the bentonite is its ability to swell when hydrated by its surroundings. One particular uncertainty is the impact on this function, resulting from deviations in pore-water pressure, p w , from expected in situ hydrostatic conditions. We present results from a series of laboratory experiments designed to investigate the form of the relationship between swelling pressure and p w , for compacted Mx80 bentonite, from low to elevated applied water pressure conditions. The experiments were conducted using constant volume cells, designed to allow the total stresses acting on the surrounding vessel to be monitored (at five locations) during clay swelling. The results demonstrate that swelling pressure reduces non-linearly with increasing p w , becoming less sensitive to changes at elevated pressures. After cyclic loading a marked hysteresis was also observed, with swelling pressure remaining elevated after a subsequent reduction in applied water pressure. Such behaviour may impact the mechanical and transport properties of the bentonite and its resulting performance. However, such hysteric behaviour was not always observed. Further testing is required to better understand the causes of this phenomenon and the controls on such behaviour.

Abstract The Large Scale Gas Injection Test (Lasgit) is a field-scale experiment designed to study the impact of gas buildup and subsequent migration through an engineered barrier system. Lasgit has a substantial experimental dataset containing in excess of 21 million datum points. The dataset is anticipated to contain a wealth of information, ranging from long-term trends and system behaviours to small-scale or ‘second-order’ features. In order to interrogate the Lasgit dataset, a bespoke computational toolkit, designed to expose difficult to observe phenomena, has been developed and applied to the dataset. The preliminary application of the toolkit, presented here, has resulted in a large number of phenomena being indicated/quantified, including highlighting of second-order events (small gas flows, perturbations in stress/pore-water sensors, etc.) and quantification of temperature record frequency content. Localized system behaviour has been shown to occur along with systematic aberrant behaviours that remain unexplained.