Optimization of the Design and Use of a Triaxial Strain Cell for Stress Determination

A mathematical model is given for stress determination in inhomogeneous rock with special reference to a triaxial strain cell. The practical effects of variations in the form and the positioning of the straingage rosettes is considered. Four-element rosettes are shown to provide an estimate of the basic measurement variance that, although sensitive to small scale inhomogeneities, is insensitive to anisotropy of the rock. The application of this estimate to an analysis of variance of the data is outlined. A cell incorporating three four-element strain-gage rosettes spaced circumferentially in the borehole at 120-deg spacing is shown to be highly efficient in providing estimates of the stress components with relatively low variances. In addition, the covariances are very low, and this lessens the complexity of expressing and understanding the complete error pattern.