This paper presents an experimental investigation on the hydromechanical behaviour of a partially saturated soft rock (a porous chalk), the Chateau-Landon chalk. Such conditions correspond to those of a chalk mine known as Royer located in the North-centre of France in Paris basin, subjected to shrinking, drying and potential flooding cycles. To study this rock, we apply a method usually used in soils whose hydromechanical behaviour is strongly modified by changes in suction, according to the degree of water saturation. Different degrees of saturation are imposed by controlled relative humidity conditions with continuous measurement of physical parameters. Hydrostatic compression and conventional triaxial compression tests are performed under drained conditions for saturation degrees up to 100% under low confining pressures. The obtained results have allowed to show fundamental aspects of the chalk behaviour. Correlations between water saturation degree, confining pressure and the mechanical behaviour of the chalk are discussed.