New stress relief concept for in-situ stress measurements in rock and its implementation in two recoverable stressmeters : Bock, H; Foruria, V; Lequerica, R Proc 4th Australia-New Zealand Conference on Geomechanics, Perth, Western Australia, 14–18 May 1984V2, P498–508. Publ Barton: Inst of Engineers, 1984

Abstract

At James Cook University, research has been carried out for some time now on new in-situ stress measuring techniques. Ideally, these new techniques should overcome some of the major obstacles of the presently established overcoring and hydraulic fracturing techniques. Three essentials have been identified which must be met by any new stressmeter: 1) reuseability; 2) use of standard boreholes, without relying on overcoring, and 3) allowance for a large number of measurements over relatively short distances, even in jointed rock masses. The stress relief concept adopted is to produce local relief by creating a radial crack or slot in the borehole wall according to the principle of St Venant, practically full stress relief will occur in close proximity to such crack or slot. Initially, the aim was to produce a radial crack by applying very high directional loading within a borehole (borehole jack fracturing). Very severe technical and theoretical problems, however, were encountered with this approach. As a consequence, an alternative stress relieving technique was developed. Instead of fracturing boreholes with high-pressure hydraulic cylinders, the technique of slotting borehole walls with a diamond saw was developed. The new technique proved to be very successful in producing local stress relief at the desired points of the borehole wall. Details of the two prototype stressmeters (borehole jack and borehole slotter), based on the principle of local stress relief, are presented. Their performance in both laboratory and field tests is delineated. It is concluded that, while the borehole jack is only of limited value for in-situ stress measurements in rocks, the borehole slotter has a great potential for carrying out quick and cheap 2-d tests. It is also useful for stress logging of boreholes. (Author/TRRL)