Use of viscoelastic analysis to measure stress in a rock mass by the method of relief of a core with a central borehole

Abstract

1. The constructed plasticity curves BCD and EFG (see Fig. 1) enable one to assess the type of stress-strain state of a core as a result of its relief, on the basis of the instrumentally measured deformation parameters (−u) and a. Thus the presence of plastic deformations of inverse sign leads to residual tensile stresses σϕ0 > 0, σz0 > 0, Fig. 3); depending on the strength characteristics of the rock, this may entail cracking of the core in a radial direction or scabbing in planes perpendicular to the core axis [8]. 2. After the type of state of the core has been established, formulas (3.7), (3.16), and (3.20) enable one to determine the load q and stress σz (along the borehole axis) in the rock mass before relief. If the loads q and σz are known, these formulas may also be used to determine the elastic constants of the rock mass. 3. The appearance of regions of platic deformations of inverse sign in the core leads to considerable errors in the determination of the load q from formulas (3.7) of elasticity theory. This is readily seen by the example of the determination of stresses in a coal mass with the following characteristics:E=2·104 kg/cm2, v=0.3, 2Te=150 kg/cm2, β=0.1 when R0=2,5 cm and R1=15 cm. Let the change in the diameter of a measuring borehole Δd be 600 μ and leta=0. Compare (−u)=300 μ anda=0 with the plasticity curve EFG and make certain that plastic deformation of inverse sign occurred during relief. According to (3.16), q=112 kg/cm2, whereas by (3.7), q=89 kg/cm2. 4. Incorporation of the parametera (deformations along the borehole axis) in the calculation scheme enables one to obtain information on the direction of the measuring borehole. Thus, if boreholes are driven in different directions, one of the principal directions of the state of stress in the rock mass must correspond to the maximum value of the parametera.