Suction effects in deep Boom Clay block samples

Various laboratory investigations have been carried out on Boom Clay specimens that were trimmed from blocks extracted during excavation of the Underground Research Laboratory in the Boom Clay at a depth of 223 m. Various questions arose concerning the quality of the specimens and the best procedure to adopt prior to running triaxial tests. Special concern related to the depth at which the blocks were extracted, to the corresponding stress release, and to the resulting suction. The swelling observed in Boom Clay specimens when releasing suction by putting them in contact with saturated porous stones in the triaxial cell also caused some concern because of possible alteration of the natural initial microstructure, with consequences for the mechanical properties of the clay, and more particularly the overconsolidation ratio. In this paper suction effects in deep Boom Clay block samples are investigated through characterisation of the water retention and swelling properties of the clay. The relationship between suction and stress changes during loading, and unloading sequences are also examined by running oedometer tests with suction measurements. Finally, the effect of suction release under an isotropic stress close to the estimated suction is investigated.