In situ behaviour of a stiff layered clay subject to thermal loading: observations and interpretation

Abstract

The paper presents an interpretation of an in situ heating test carried out on Opalinus clay in the Mont Terri underground laboratory. Opalinus clay is a stiff, strongly bedded, Mesozoic clay of marine origin. When subjected to thermal loading, saturated stiff clays exhibit a strong pore pressure response that significantly affects the hydraulic and mechanical behaviour of the material. The observations gathered in the in situ test have provided an opportunity to examine the integrated thermo-hydromechanical (THM) response of this sedimentary clay. Coupled THM numerical analyses have been carried out to provide a structured framework for interpretation, and to enhance understanding of THM clay behaviour. Numerical analyses have been based on a coupled theoretical formulation that incorporates a constitutive law especially developed for this type of material. The law includes degradation of bonding by damage. By performing three-dimensional computations, it has been possible to incorporate anisotropy of material parameters and of in situ stresses. The 3D simulation has proved able to furnish a satisfactory representation of the development of the in situ test and of the main observed patterns of behaviour. A sensitivity analysis has also been carried out to examine the potential effect of various key or uncertain parameters. The critical examination of test observations and the results of the numerical analyses have allowed the classification, by differing degrees of significance, of the various coupled phenomena present in the problem.

Modelling a heater experiment for radioactive waste disposal

“…The thermally induced pore pressure results from a full coupling of THM processes. In particular, the permeability of the rock and its stress-strain behaviour (as reflected in the value of the Young's modulus if elasticity is assumed) significantly influence the absolute value and the dissipation rate of the pore pressure (Gens et al, 2007;Selvadurai and Nguyen, 1997). The model underestimated the maximum pore pressure by 30%.…”

International Collaboration Activities in Different Geologic Disposal Environments

“…The HE-D experiment was conducted at the Mont Terri URL between March 2004 and June 2005 by heating of Opalinus Clay from two heaters placed in a horizontal borehole (Wileveau, 2005;Gens et al, 2007). About 30 temperature sensors, 10 water pressure sensors, and 3 extensometers were placed around this heating borehole (Figure 6-1).…”

Coupled Thermo-Hydro-Mechanical Behavior of Natural and Engineered Clay Barriers

“…The tightness, low thermal conductivity, and deformability (compressibility), associated with clay-rock generally results in strong THM couplings, including poroelastic responses and the so-called thermal pressurization, especially when pores are water saturated (Gens et al, 2007;Rutqvist et al, 2014a, Yu et al, 2014. Thermal pressurization occurs when the pore-water is heated in the porous clay in which the solid phase has a much lower thermal expansion coefficient than water.…”

“…In the study presented here, the geomechanical behavior of the bentonite was modeled with the BBM, using properties corresponding to the FEBEX Bent, whereas clay host rock properties were derived from the Opalinus clay at Mont Terri (Gens et al, 2007). The simulations were conducted in a 2D plane strain model as shown in Figure 9.12.…”