First experiences with the C.S.I.R. triaxial strain cell for stress determinations

Abstract

The C.S.I.R. triaxial strain cell allows determination of the three-dimensional stress field in one drill hole. This instrument was tested in the laboratory and in the field to develop testing techniques and to discover areas of possible pitfalls. Compared with the biaxial of stress determination the triaxial method is more efficient and theoretically more desirable to use, however it requires better ground conditions. The waterproofing of electrical contacts and the cleanliness required for strain gauge work underground is an area which requires experience. Some field tests indicate a low stress level parallel to the drill hole which was initially attributed to the particular drill-hole geometry used during the tests. A finite element analysis showed however that for an average rock type the stress component parallel to the drill hole at the point of measurement was reduced by only 3 per cent. During the three field tests, all the overcored cylinders cracked in the vicinity of the strain cell. The existence of a crack across the drill hole prior to overcoring offers an explanation for the low level of measured stress parallel to the drill hole. To check how well the instrument measures strains in three dimensions, a column of aluminium blocks 36 in. high was loaded in uniaxial compression. The strains measured with the triaxial strain cell indicated good agreement (within 5 per cent) between applied and calculated stresses. To facilitate potential use of the equipment and analysis of the data a computer program calculating the tensor of best fit and the error of the tensor components was written and is given in the Appendix.