Preceding papers have quoted data from absolute stress determinations with a borehole deformation gage utilized in stress relief techniques. This paper will describe the development of a biaxial borehole deformation gage, field techniques, procedures used for data reduction, and some typical results. Stress relief techniques are predicted on the premise that the stressed mass is prefectly elastic, homogeneous and isotropic. Rock, especially sediments such as those at White Pine, vary considerably from ideal conditions but given reliable strain measurements, valid approximate analysis of absolute stress values is possible. The decision to develop a biaxial instrument was made because it was felt that it is more economical and reliable to measure borehole deformation in three directions in a single over-coring operation.
Essential requirements for a satisfactory borehole deformation gage are, to measure deformation accurately, to be stable electronically for the period of time necessary for overcoring, to have sufficient range to measure all normal changes in diameter of boreholes, to be easily installed and to be of a rugged design to withstand the forces involved in core fracture during overcoring. The following design specifications were selected:
1. The probe should fit in an AX borehole - 1.937 inches nominal diameter.
2. The range of measurement should be at least 02 inch.
3. It should be possible to increase the range of measurement to 15 inches by manual adjustment.
4. Accuracy of measurements should be 20 micro-inches.
5. Contact force of measurement probes should be approximately 5 lbs.
Small linear variable differential transducers were selected as the sensing elements for the borehole deformation gage. They have proven their dependability under adverse conditions in other branches of industry. The transducers selected have a linear response range of .020 inch and are capable of measuring accurately deformation of 20 micro-inches. The transducers are approximately 1-1/2 inches long and have a diameter of 3/8 inch so that when equipped with necessary adapters .the unit fits across an AX diamond drill hole. They operate satisfactorily off battery power and have the added advantage of being a stock item, so that when damaged replacements are easily obtained. Early in the work it was evident that the transducers would not provide adequate contact pressure with the sides of the borehole to prevent movement of the instrument during overcoring. For this reason a rubber packer ring is utilized to provide anchorage. Setting tools currently used are designed for setting the instrument up to a 10 ft. depth. The probe unit which was used in early development work is shown diagramatically in Figure 1. This unit was a radius-measuring gage and this feature alone caused it to be unsatisfactory. Any movement of the probe body resulted in erroneous deformation measurements. Knowledge obtained from this unit led to the development of the design now being used. The major improvement is the change to direct diametrical measurement to eliminate the effect of probe movements.
