A stress calculation method was proposed for the new stress measurement technique using the borehole slotter device. The 2D borehole slotter is an instrument which is used in HQ boreholes (96–104 mm diameter), and allows calculation of 2D stress tensor in a plane normal to the borehole axis. This instrument enables calculation of in-situ stress tensor using three boreholes without the need for mounting strain gauges on the borehole wall. The borehole slotter technique requires cutting of half moon shaped slots at different orientations parallel to the test hole axis using a small diamond impregnated blade and monitoring of the strains which are relieved normal to the slot direction. The aim of this paper is to evaluate the effects of key test parameters on the slotter test results. A precise 3D numerical model of typical slotter test condition was constructed using the FLAC3D code. The effects of variations in rock mass deformation modulus on the strain/stress relaxation, and thus borehole slotter test results, were investigated numerically. The validity of numerical findings were discussed and compared against practical observations. Accordingly, the borehole slotter tests conducted at Bakhtiari dam site-Iran was simulated numerically. A very good agreement was achieved between numerical and field test results. The obtained numerical results provided valuable insights in selecting the governing in-situ stress condition from the recorded field data.
Borehole slotting is a relatively new stress measuring technique which is designed for quick operation in boreholes and for high density measurements at reasonable cost. This method was developed by Helmut F. Bock Research as a substitute for other means of insitu stress measurements, which are considered to be expensive and unreliable. The aim of developing this method was to avoid the high cost associated with other in-situ measurement methods and also to increase the accuracy of field measurements (Bock, 1993).
Since the slotter technique has proven reliable in the field and has had certain commercial successes, attention is paid to improve the accuracy of the calculated stresses by considering complexities such as rock mass behavior and heterogeneities at the measurement point. The main goal of this paper is to demonstrate the mechanisms involved the in-situ stress measurement using the slotter technique. The process of in-situ stress measurements conducted at Bakhtiyari dam-Iran was successfully modeled demonstrating the validity of numerical approach in simulating the process.
Borehole slotter is based on the principle of local stress release. In this method by means of a pneumatically driven diamond saw slots of up to 25 mm deep are cut into the wall of a borehole of about 96–101 mm diameter (INTERFELS Corporation. 1993).
(Figure in full paper)
Before, during, and after slotting, the circumferential strain is measured at the borehole wall in the vicinity of slot and the tangential strain is measured by a specially developed recoverable strain sensor (Fig. 1) (Foruria, 1987; Bock, 1984). The strain gauge is mounted at 15 degrees from the slot location according to the INTERFELS test procedure recommendation.