Experiments on sandstone and limestone rock samples are described in which acoustic compressional wave speeds (Vp) have been measured during triaxial compressive testing. Due to pre-existing cracks and pores hydrostatic compression of rocks is well known to cause an increase in Vp. Additional uniaxial compression in triaxial tests on a dry sandstone causes a further increase in Vp until new, dilatant crack growth is initiated (marked by substantial acoustic emission - AE), when Vp reaches a peak value. Further dilatant cracking as the differential uniaxial compression is still further increased causes a reduction in Vp, which, at lower confining pressures, continues at a steady rate until fracture occurs, when Vp reaches a steady value. At higher confining pressures, when cataclastic flow occurs, dilatant cracking continues to large strains and is accompanied by a continuing reduction in Vp. In a dry fine-grained limestone the behaviour is the same at temperatures below 200°C, but at higher temperatures there is no evidence of dilatancy as plastic flow becomes dominant. In water-saturated sandstone tested under undrained conditions pore-fluid flows associated with dilatancy cause clear changes in the stress-strain and Vp behaviour.
It has long been known that hydrostatic compression of rocks to pressures of a few hundred MPa causes an initial substantial increase in acoustic wave speed as the rocks compact due to the closure or elimination of crack porosity (Birch 1960, Anderson & Liebermann 1968, Christensen 1974, Kern 1978). This has been investigated theoretically by O'Connell & Budiansky (1974) and by Hudson (1981). Dilatancy in rocks under triaxial loading has been widely recognized since the early work of Brace and his colleagues (e.g. Brace et al 1966), and there have been several investigations of the effect of such dilatancy on acoustic wave speed (Nur & Simmons 1969, Gupta 1973, Ramana et al 1974, Rummel 1974, Lockner et al 1977, Soga et al 1978, Rummel & Frohn 1982, Masuda et al 1987). These studies have shown that the value of Vp in the direction of the maximum compressive stress (σ1) increases at first as σ1 increases, but peaks when σ1 reaches about 50 per cent of the failure stress, and then decreases markedly as failure approaches. These changes also depend on the confining pressure. The change of acoustic wave speed under triaxial loading is paralleled by fluid permeability changes (Mordecai & Morris 1971, Zoback & Byerlee 1975), and both are consistent with compaction and dilatancy effects caused by the closure of microcracks approximately normal to σ1 and the opening of new cracks approximately parallel to σ1. Clearly these wave speed and permeability changes are tensor effects related to the applied stress tensor and to the tensor representing any preexisting crack distribution. Therefore, in principle triaxial stress can cause the development of physical property anisotropy in the case of those properties which are affected by the presence of cracks (i.e. elastic and transport properties). Such load-induced elastic anisotropy has been investigated theoretically by Horii & Nernat-Nasser (1983).
