Analysis of vibrating-wire stress gage in soft rock

An analysis for the in situ elastic response of a vibrating-wire stress gage in soft rock (E<105 Mpa) has been accomplished with the use of the finite-element method and approximate analytical procedures. The vibrating-wire stress gage is not a true stressmeter in that it is somewhat dependent on rock-modulus changes. Conventional procedures for calibration include the assumption that this dependency is linear with respect to rock modulus. In this analysis, a nonlinear relationship is demonstrated and an approximate mathematical expression is derived for it. Good agreement is obtained between theoretical calibration factors and test results on laboratory-scale samples with the stressmeter.