Strain measurements with the borehole slotter

ABSTRACT In the past, the doorstopper method has been used successfully for determining in situ tectonic stresses. Here an example is given about the application of the borehole slotter in the domain of tectonic stress determinations, which is an innovative strain relief technique without overcoring. Data are presented that test the reliability of the strain measurements by the borehole slotter under well‐known stress conditions, that indicate the reproducibility of borehole slotter strain data compared with those from doorstopper measurements, and that demonstrate the utility of the borehole slotter even for the determination of weak tectonic strains. In northern Switzerland the borehole slotter was used in a horizontal borehole under lithostatic loading conditions only, in a rock slope area with very heterogeneous stress conditions, and in a vertical borehole where tectonic stress should be dominant. The results show that the borehole slotter is a reliable technique, which allows precise determination of in situ strains. In addition, borehole slotter measurements are less time‐consuming and therefore less costly than most other stress‐measuring techniques—an important aspect in a future of reduced research budgets. Summary The borehole slotter is a strain relief technique, using a recoverable strain sensor that need not be glued to the rock surface and need not be overcored. It is quicker to use than most other stress or strain measuring techniques, especially in comparison to standard overcoring techniques (Fig. 14). The borehole slotter supplied convincing results with respect to principal stress orientations, in good agreement with: (i) principal stress orientations related to lithostatic stress conditions (Dittingen 2 site), (ii) palaeostress orientations during the main stage of Jura folding only 13 to 7 Ma (Laufen 1 site), (iii) the results of neighboring door‐stopper measurements (Dittingen 1 site). The borehole slotter tests do not supply stress magnitudes directly. Thus, the elastic properties of the rock mass have to be determined independently. Stress ratios from borehole dotter tests in Dittingen 2 are realistic. In conclusion, the borehole slotter is a fully developed probe to measure in situ strains in boreholes. It allows the reliable determination of a large number of data within a short time even in weakly stressed rocks. This makes the borehole slotter a powerful tool for the determination of contemporary regional tectonic stress fields.