The corejacking test is a field test designed to measure the response of salt to known boundary conditions. A 1.0-m-diameter salt core is externally pressurized using curved flatjacks placed in the annulus surrounding the core. Time-dependent deformation of the salt is measured by monitoring the diametrical closure of an inner borehole. A finite element elastic analysis of the jack loading system has been performed. The purpose of this analysis was to determine if the original assumption of axisymmetric loading' is correct. Six loading cases were investigated, each representing either hypothetical or previously encountered jack configurations.
The corejacking test has been developed to provide field data on the time-dependent response of rock salt when subjected to load and elevated temperature. The primary purpose of the test is to provide data that can be used for verification of the numerical modeling techniques used to predict the behavior of salt under various conditions. Therefore, the test was designed to be axisymmetric so that it could be easily modeled. Small salt specimens have been extensively tested under controlled conditions in laboratory settings. Field tests have also been performed in salt to measure creep behavior, but have often necessitated broad assumptions with regard to boundary conditions. The boundary conditions of the corejacking test can be determined with only minor assumptions if axisymmetric loading is achieved. This paper is dedicated to the examination of the assumption of axisymmetric loading.
The corejacking test consists of a large hollow cylinder of rock salt to which constant external pressure and temperature are applied (Van Sambeek, 1981; Stickney, 1981). Response of the salt to this thermal and mechanical loading is monitored by measuring the closure of the inside of the cylinder as a function of time. The cylinder has an outside diameter of 1 m, an inside diameter of 0.2 m, and an overall length of 1 m. The hollow cylinder is formed by drilling into the mine floor with a large diamond coring bit. The drilling results in a 1-m core in the mine floor encircled by an annulus 30-mm wide. A 0.2-m diameter borehole is then concentrically drilled in the 1-m core, also using a diamond coring bit. Curved flatjacks, descriptively termed corejacks, are placed in the annulus surrounding the salt core and grouted in place. Constant external load is applied to the core by hydraulic pressurization of the jacks. A plan view of the corejack placement scheme is shown in Figure 2. The tests have been performed at both ambient and elevated temperatures. Closure of the inner (0.2 m) borehole is measured for the test duration using a three-point inside micrometer. The borehole diameter changes are measured at nominal elevations of 0.25 and 0.5 meters below the top of the core. At both elevations three fixed measurement pads are anchored to the borehole wall at 120º azimuthal separation. The tool is self-centering in the borehole and reads the gross borehole diameter directly with a resolution of about 0.005 mm.