ABSTRACT:
Plate-bearing tests and borehole-jack tests are examined by means of finite element analysis to determine critical parameters affecting the test results. Two-dimensional and 3 dimensional continuous and jointed rock programs are first applied to the plate test. It is shown how the ratio of plate to rock modulus, the rock anisotropy and the plate geometry, influence the deformations. Correction factors are given corresponding to typical values of some of these parameters. Rock breakage and jointing are not considered here. A complete analysis of their influence is beyond the scope of this paper. Then, the NX-borehole-jack is modeled in 3-dimensions. It is shown for the first time how the measured deformations vary with the ratio of stiffnesses of the jack plate and of the rock. The effect of tensile rock cracking is again shown to be non-negligible. A non-dimensional set of results is given which enables one to use the jack with confidence, in rocks of various stiffnesses.
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Current techniques to measure the deformability of rocks in place are the plate bearing test (references 1,3,6,16,22), the borehole jack test (7,10,11,12,15), the flat jack test (1), and the dilatometer test (7,18,21). The last two involve the interaction of a thin and flexible membrane of steel or rubber, with the rock; it does not influence the analysis, if one assumes pressure boundary loading of the rock. The first two are the most critical because a thick steel plate is interposed between the pressuring agent and the rock: they are the object of this study plate tests can be very large in size, with pressures of several hundred tons applied on areas of several square feet. Borehole jack tests are more modest in size and hence can be repeated more readily, at different locations and at different orientations. The two kinds of tests should be considered as complementary. Although the principles involved are straightforward, it will be shown that a simpleminded application of plate bearing or jack formulas may give results which differ from reality by several hundred percent.
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Two and three dimensional analyses were made (figures 1 & 2) using programs JTRK2D and JTRK3D (13,19). The 2-D geometry was an axisymmetric configuration. The 3-D geometry consisted of a square plate, or footing. Considering the very large number of cases which could be obtained from combinations of various parameters, a choice had to be made of which ones were potentially critical and which of those could be included in the limited scope of this paper. It was decided that the following would be reported: - the effect of o Es/Er (= Esteel/Erock)
- the effect of y: 2 plate thicknesses were used: 1 and 2 inches; plate diameter was 8 inches. A circular pyramid shape plate was also analyzed.
- the effect of y: the rock was assumed to be either isotropic, or transversely isotropic. In the latter case, the ratio En/Et was varied from 1/3 to 3, where En and Et were the moduli as shown in figure 3. The angle of anisotropy ¿ was also varied.
