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In the present work, an attempt has been achieved to describe the effects o f ageing and of water on the hydro-mechanical behaviour of chalks from underground quarries of northern Parisian basin. Physical and mechanical properties were obtained as a function of distances from the pillar face (horizontal core drillings, from the wall to the centre) and relative humidity (Hr) of air. It has been stated that physical and mechanical properties change with depth in the pillar: the density and compressive strength of chalk tend to decrease from the pillar face (at least 1 m from the wall) to the centre, contrary to the classical observations on the ageing of geomaterials. Uniaxial and triaxial compression tests under controlled relative humidity put into light the sensitivity to water (and then relative humidity in the quarry) on the mechanical strength and deformability of chalks.