The effect of suction on the hydro-mechanical behaviour of chalk rocks

Abstract

The aim of this paper is to study the effect of suction on the hydro-mechanical behaviour of a partially saturated soft rock (a porous chalk) under low confining stress. Such conditions correspond to those of the vertical chalk cliffs of the English Channel, which are subjected to spatial and temporal variations of the water table under climatic variations. We apply to this material a method usually used in soils, whose hydro-mechanical behaviour is strongly modified by changes in matric suction (or capillary suction), according to the degree of water saturation. Under controlled matric suction conditions, wet chalk undergoes volume changes due to pore infilling and drainage. Relationships between matric suction and the mechanical behaviour of the chalk are analysed on the basis of drying–wetting cycles, variations of maximum strength, secant modulus during uniaxial unconfined compression and confined triaxial tests. Mechanical and drying–wetting tests were carried out on dry, saturated and moist natural chalk samples. Drying–wetting cycles result in two types of behaviours according to the range of imposed matric suction, with irreversible paths in the [ s, S r] plane for high gravimetric water contents (i.e. matric suction values between 0.1 and 6 MPa) and reversible paths for low water contents (i.e. high values of matric suction, larger than 6 MPa). For chalk samples initially at their natural water content, the behaviour is intermediate between those of the dry and saturated chalk samples. Comparison of the behaviour of the two chalk formations highlights differences on wetting, which could be linked to the different pore size distributions from one chalk formation to the other. The results of uniaxial and triaxial compressive strength tests at various water contents show elastic–brittle behaviour. Brittle failure occurs at very low water contents. The secant modulus reaches 600 MPa for dry chalk samples and decreases to 250 MPa for quasi-saturated samples. Concerning triaxial tests, the envelopes of maximum strength (failure criteria) show a noticeable shift, with an apparent cohesion varying by a factor of 2 to 3 from dry to saturated chalk. The effect of matric suction is interpreted using an effective stress approach, based on a simple model for granular soils, by considering the expression of the intergranular forces between two particles in an idealised medium at the microscopic scale. It makes it possible to describe the behaviour of chalk in relation to the variations of suction. The capillary effect is well marked at very low values of water saturation, whereas it disappears at high water content values.