  Extensive investigations have been and are being carried out on a stiff clay
from an underground research laboratory located at Mol (Belgium) called Boom
clay, in the context of research into deep nuclear waste disposal. Suction
effects in deep Boom clay block samples were investigated through the
characterisation of the water retention and of the swelling properties of the
clay. The data obtained allowed an estimation of the sample initial suction
that was reasonably compatible with the in-situ state of stress at a depth of
223 m. The relationship between suction and stress changes during loading and
unloading sequences were also examined by running oedometer tests with suction
measurements. A rather wide range of the ratio s/sigma 'v (being s the suction
and sigma 'v the effective vertical stress) was obtained (0.61 - 1), different
from that proposed by Bishop et al; (1974). Finally, the effect of suction
release under an isotropic stress close to the estimated sample suction was
investigated. A slight swelling (1.7%) was observed and further compression
provided a satisfactory value of the overconsolidation ratio confirming the
suggestions of taking some precautions before putting a swelling sample in
contact with water as suggested by Graham et al. (1987). The various
experimental data gathered in this study finally evidenced a relatively good
state of conservation of the block sample used.
