Long term loading of the gallery lining in a clay host rock

Document available in extended abstract form only. Repository construction and operation in a clay host rock - in particular in poorly indurated clays like the Boom Clay - require a structural support of the host rock excavations for several reasons. Apart from the operational safety, a well-designed structural support also minimizes the perturbation (or excavation damaged/disturbed zone) of the host formation. Furthermore, it is essential if reversibility would be a requirement in the phased development of a repository. The design of such a support depends, amongst others, on the stress state around the lining. In this paper we present the results that have been obtained through monitoring since the extension of the HADES underground research facility in 2002. The design of this extension has been based on a wedge-block lining with pre-cast concrete segments. To assess the stresses inside the lining segments, several segments were instrumented with embedded vibrating wire strain gauges during casting. The first phase of the extension was the construction of the Connecting Gallery in 2002. Strains have been monitored since then and allow assessing the stresses inside the lining, as well as deriving the stress state of the host formation around it. Based on this monitoring set-up, a similar approach was performed for the construction of the PRACLAY Gallery in 2007 where in addition to the strains, the contact pressure between the lining and host formation and the load between the segments are being monitored directly. The measurement set-ups of both the Connecting Gallery and the PRACLAY Gallery show a similar stress state, thereby increasing our confidence in the interpretation of the measurements. The strains measured in the Connecting Gallery show that the wedge-block lining presents an almost instantaneous strain increase followed by a gradual increase with time as shown in Figure 1, which represents the evolution of the measured strains with time at the intrados of the lining for different rings of the Connecting Gallery. From the strain evolution, it is clear that the response of the lining to the load exerted by the host rock is not isotropic, but rather exhibits a sort of symmetric response with respect to the vertical. A higher increase of strains is observed for the segments located on the sides while it is limited for the segments at the top and at the bottom of the ring. In order to interpret the experimental results, a 2D plane strain modelling with the non-linear finite element code LAGAMINE is proposed. The modelling simulates the excavation followed by a stabilisation period corresponding to the time between the end of the construction and the present time. To reproduce the pressure exerted by the clay host rock on the lining, contact elements are used. The clay host rock is modelled with an elasto-plastic constitutive law, in our case a Drucker-Prager criterion. The lining is modelled with different constitutive laws with increasing degrees of complexity of the model used for this material. A linear elastic law and a time-dependent elastic law, to take the creep behaviour of the concrete into account, are used. The use of an elastic law is very simple and allows a first look at the mechanisms acting in the concrete during the 10 years of operation. But this kind of law presents some limitations, for example, the Young modulus is not ageing, while it is observed experimentally that this parameter decreases with time. With the anisotropic response of the liner in mind, a next (and logical) step in this modelling is to consider the anisotropy of the Boom Clay itself. As a main outcome of this modelling, the order of magnitude of the stresses in the lining will allow us to confirm (or not) the design of the Connecting Gallery. Ten years after the construction of the Connecting Gallery, all strain gauges are still functional and continue to monitor the lining strains. They show an interesting potential for long-term monitoring of underground structures in a real repository. (authors)