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The NX-borehole jack is a widely used tool for estimating the deformability of rock masses in-situ. It applies unidirectional pressure to the walls of a rock borehole by means of two steel platens. In theory, the analysis of test data is straightforward: the modulus estimate requires a record of applied hydraulic pressure versus borehole diameter change, and a knowledge of the rock's Poisson's ratio. In practice, the above procedure, using the original theoretical formula, frequently has resulted in computing a material modulus that was demonstrably too low.
Today, it is finally understood how one or more of the following factors can provide a low modulus estimate: (1) the longitudinal outward bending of the jack plates which gives artificially high readings of the displacement gages and (2) the mismatch between borehole and jack platen radii, which would negate the full platen/rock contact assumed in the original analysis, and (3) tensile cracking of the intact rock or tensile opening of natural joints around the borehole under the jack.
The detailed discussion of the theoretical developments is contained in a separate paper, which should be a complement to this draft. This document outlines a suggested procedure for testing with the NX-jack, and analyzing the test results in a medium which is assumed to be isotropic, linear, clastic, and homogeneous.
Head, Geotechnical Group, and Manager of Unconventional Gas Program, Lawrence Livermore National Laboratory, Livermore, CA
Stock #: GTJ10503J