Rock displacements during the buffer mass test at Stripa

As a part of the Buffer Mass Test, rock mechanics measurements were performed with the aim to assess the interaction between the highly compacted bentonite buffer material and the surrounding rock. The measurements included long term monitoring of vertical rock displacements in four holes drilled adjacent and parallel to heater hole No. 5. Displacements were expected to occur as a result of the thermal load applied to the rock, and of loadings exerted by the confined swelling of highly compacted bentonite. The ISETH Sliding Micrometer, a portable precision borehole extensometer, was used for the measurements. This manually operated instrument provides accurate determinations of the distance between measuring marks, preinstalled at 1 m intervals along the borehole. By repeatedly applying this technique throughout the operation time of the Buffer Mass Test, the time dependent distribution of displacement along continuous vertical profiles, extending from the tunnel floor and down to levels below the zone influenced by the test, was recorded. The results show a teporally and spatially uniform thermal expansion behaviour of the rock. The maximum vertical strain, occurring at the heater level, is about 50x10-6 and the resulting heave of the tunnel floor is of the order of 0.16mm. There is no indication of discontinuous rock deformation. Excluding the uppermost 1 m near the tunnel floor, it appears that the swelling pressure developing in the highly compacted bentonite and reaching a maximum value of about 5 MPa, did not cause any measurable vertical displacements. (author)