Use of numerical simulations to assess hydraulic and mechanical measurements of bentonite–sand buffer in an in-floor borehole

Abstract

As part of the process of developing concepts for isolation of used nuclear fuel in a deep geological environment, a large in-ground experiment, the Isothermal Test (ITT), was conducted at Atomic Energy of Canada Limited's Underground Research Laboratory. This installation was designed and constructed to examine how a large mass of dense unsaturated bentonite–sand buffer would take on water from the surrounding rock. The ITT was installed in a 1.24-m-diameter by 5-m-deep emplacement borehole drilled in the granite 240 m below the ground surface. The lowermost 2 m of this borehole was filled with an in situ compacted bentonite–sand buffer material. This buffer material was capped by a 1.25-m-thick plug of high-performance concrete and the installation was left undisturbed for 6.5 years. During this time, the pore pressures, total pressures, water uptake and temperature in the buffer and the surrounding rock were continuously monitored. Numerical simulations were conducted using CODE_BRIGHT to model the evolution of the pore pressure in the buffer and the surrounding rock as well as mechanical response of the buffer. The simulated water seepage from the rock to the emplacement borehole was compared with measured results. The simulated pore water pressure in the rock, the pore water suction in the buffer and the earth pressure in the buffer were compared with data measured during the test and inferred from end-of-test water saturation measurements. Graphs of the simulated displacements in the buffer are presented.