A deep hole in situ stress measurement device has been designed, built and laboratory tested. The device is capable of measuring 9 components of strain in the side walls of 6-7 inch diameter boreholes in rock. It is designed to be used in boreholes up to 2500 meters deep at temperatures up to 100°C and fluid pressures up to 250 bars. The instrument will be lowered into and retrieved from the borehole on a 7 conductor armored cable and is securely locked into position by double acting hydraulic jacks above and below the measurement units. Hydraulic power to run the device comes from an intensifier, the low pressure end of which is driven by the ambient fluid pressure in the borehole. Electric power comes from a self-contained battery pack. The entire apparatus is constructed as a waterproof bomb so that the electric motors and the data transmission package are operating in a dry environment. The strain gage signals will be amplified and fed up a cable to the surface and recorded on standard equipment. A standard borehole surveying instrument will be fixed to the stress measurement device to indicate the orientation of the strain gages relative to geographical coordinates. A major feature of this system is that no drilling rig or derrick is required over the hole, only a wire line truck carrying the cable reel, power supplies, and read out instrumentation. We will not have to try to transfer signals up or down a rotating drill string.
The measurement principle used is the trepanning of three three-element strain gage rosettes. The strain gage elements are friction bonded to the side walls of the borehole.
The principle of strain relief stress measurements by drilling a small hole in the center of a strain gage rosette, or trepanning, is well established and has been widely used in experimental stress analysis. A previous study has experimentally determined the stress concentration factors for interpreting results of this type of test at the flattened end of a borehole. The same sort of experiments were done as a part of this project for tests performed in the walls of a borehole.
Data analysis proceeds from a solution for the strains in the side walls of a cylindrical hole in an infinite solid subjected to a general polyaxial stress field. The experimentally determined factors are applied to this solution to account for the size and position of the strain gages and trepanning hole and the efficiency of the friction bonded strain gages.
Resolution of the system ultimately depends upon the quality of the data transmission system. This is currently under development but hopefully resolution will be approximately 3 bars in a rock with an elastic modulus of 3 x 105 bars.
We have also investigated residual stresses in rocks and are attempting to determine their influence on the response of the deep hole device.
The purpose of this investigation was to develop and build an instrument to accurately measure primary rock stresses in the side walls of long (up to 2500 meters) 6 1/2 inch diameter drill holes.