Lateral Decompression Behaviors of a Hard Claystone in Excavation-Damaged Zone of Galleries

We propose a new type of mechanical tests, named the lateral decompression tests, to evaluate the mechanical properties of the surrounding rocks in the shifts based on elastic stress solutions of circular openings. We carried out lateral decompression test with a developed autonomous device to evaluate the mechanical properties of the Callovo-Oxfordian (COx) claystone, the hosted rock and geological barriers of the repositories for radioactive waste disposal in France. In the tests, the samples were firstly subjected to a hydrostatic stress that corresponds to the representative in situ mean stress (p = 12 MPa). Then, the samples were loaded to a possible failure at the given constant mean stress (∆p = 0). The results show that the COx claystone failed in the lateral decompression tests at the in situ mean stress of 12 MPa. A shear band controls the failure pattern in lateral decompression tests, which is similar to that in conventional triaxial tests. However, the failure prior to rupture is more brittle than the failure in conventional triaxial tests. The failure of the COx claystone under the tested mean stress suggests that the excavated galleries need external supports to keep its stability after tunneling in the COx formation.