INTRODUCTION
The Waste Isolation Pilot Plant (WIPP) is being developed in southeastern New Mexico by the United States Department of Energy as a research and development (R&D) facility to demonstrate the safe disposal in salt of radioactive wastes resulting from defense activities. As part of the WIPP R&D program, a series of in situ tests will be performed to determine the behavior of drifts and storage rooms in the creeping salt medium (Matalucci, et al, 1982). Data obtained in these tests will be used to evaluate and improve numerical models used to compute the structural response of these drifts and rooms. Stress has been proposed as one of the parameters to be measured in the tests, and borehole inclusion stressmeters have been included in the instrumentation package. Gages of this type provide an indirect measurement of stress change in that they actually monitor borehole displacements which are then related to changes in the in situ stress away from the borehole. The relationship between stress and displacement is highly dependent upon the constitutive behavior of the rock in which the gage is emplaced. This relationship is well understood for rocks with linear stress-strain behavior (Fossum, et al, 1976), but, for creeping rocks like salt, the relationship has yet to be determined. Thus, inclusion gage response in salt must be investigated fully before stress measurements from the WIPP in situ tests can be used for comparisons with computed stresses. The calculations presented in this paper were performed as part of an investigation of inclusion stressmeter performance in salt. They focus on the strain gaged stressmeter (SGS) (Cook and Ames, 1979) and on a hypothetical solid cylindrical inclusion with variable stiffness Issues addressed include the effects of creep and the effects of gage stiffness on stressmeter output for various loading conditions. The computational procedures used to calculate gage response are described first and are followed by a presentation of the computed responses of the inclusions to several loading conditions.
COMPUTATIONAL PROCEDURE
A brief description of the SGS shown in Figure 1 is needed as background to the computational procedures used here. This gage is an adaptation of the vibrating wire stressmeter (Hawkes and Bailey, 1973) with especially large platens for emplacement in salt and a columnar strain gaged plug instead of a vibrating wire as the transducer. The gage is designed to be most sensitive to loads applied parallel to the columnar plug. When the SGS is placed in a borehole, load is transferred from the borehole to the platens and subsequently to the columnar plug. Deformation of the plug is monitored by a strain gage whose output is related to the far field stress state around the borehole by means of a laboratory calibration. Calibration (Cook and Ames, 1979) essentially consists of placing the SGS in a borehole drilled into a solid block of salt and applying uniaxial loads in the direction of the columnar plug. During calibration the load is changed rapidly so the effects of creep can be ignored, and a relation between the applied uniaxial load and strain gage output is obtained.
