An energy approach to sleeve fracture stress measurements

Sleeve fracture is an attempt to determine the in situ stresses in rock using a pressuremeter (or borehole dilatometer). The method simulates hydraulic fracture in that a section of a vertical borehole is pressurized to the instant of tensile failure in the rock. Sleeve fracture is unlike hydraulic fracture in the sense that once a fracture is created, the membrane of the pressuremeter prevents the pressurizing fluid from penetrating the fracture. This has certain implications for mine safety. Stephansson (1983) believed that the pressure at which the fracture reopened would correspond to the minor principal horizontal stress. If this were true, the initial fracture pressure, the reopening pressure, and the tensile strength of the rock would provide sufficient information to calculate the two horizontal principal stresses in the rock, at the test location.; This thesis shows that the reopening pressure cannot be used to determine the in situ stresses in rock. A new theory of sleeve fracture is proposed in which the minor principal stress is related to the work done in propagating the fracture created in the first pressure cycle. Theoretical elasticity and elastic fracture mechanics provide a relationship between stress intensity factor and crack length. The release of elastic strain energy due to fracture propagation is, in turn, related to the minor principal stress. The energy released during sleeve fracture is measured by calculating the change in area under the pressure volume curve between successive pressurization cycles.; In this thesis, a boundary element computer code was developed to illustrate that the energy approach was applicable to an elastic material. Results from experiments on plaster and limestone blocks suggest that fracture mechanics can indeed be used to estimate the fracture lengths during sleeve fracture. However, the quantity of energy released during the experiments was much larger than the amount predicted using the new (fracture energy) theory. It is speculated that the energy in excess of the theoretical quantity is due to the development of an annulus of microfractures in the region near the pressurized borehole.