ABSTRACT: For any subsurface construction, the stress scenario, determined by local topographic and geological features, needs to be understood and determined by a suitable stress measuring programme. Along with other factors, the stresses are likely to have significant bearings on behaviour of underground excavations. Although several methods have been suggested by the International Society for Rock Mechanics (ISRM) for measuring stresses in rock, every method is based on certain assumptions and with certain limitations. The findings from stress measurements in various underground excavations in the Deccan Traps formation in western Maharashtra by the flat jack method are presented and compared with the published data from wordwide stress measurements. The relationship for the variation of in situ vertical stresses with depth has been found to be in agreement with the published data, whereas variation of in-situ stress ratio with depth shows a considerable amount of scatter, as expected at shallow depths.
Introduction The rock mass at depth is subjected to stresses resulting from the weight of the overburden and locked-in stresses of tectonic origin. When an opening is created in the ground, the stresses in the vicinity of the new opening are re-distributed. The stresses induced around the opening are a function of the in situ stress field and the shape of the opening. There are several civil engineering situations where knowledge about the state of stress can be integrated into engineering design, viz: when choosing the orientation for a cavern, it is best to align the long dimension parallel to the greatest principal stress, and, in cases where initial stresses are very high, the shape of the opening needs to be designed to minimise stress concentrations. In addition, there may be places where in situ stresses are greater than 25% of the unconfined compressive strength of rock. In such situations, excavation activities can trigger rock failure, varying from spalling or slabbing in the rock surface, to severe rock bursts in which explosive failure of significant volumes of rock can occur. Thus, for any type of shallow or deep underground excavation, knowledge of the in situ stress state is essential for planning the excavation layout, shape, the sequence of excavation and the design of an adequate support system. The purpose of this paper is mainly to explain in detail both induced and in situ stress measurement by the flat jack method, and to describe trends in the results obtained from seven tunnels / underground openings in the Deccan Traps rock formation (comprised mainly of basalt and breccia rock varieties) in Western Maharashtra.
Stress Measuring Methods Of the several methods devised for measuring in situ stresses, three methods namely over coring, hydraulic fracturing and the flat jack method have been widely used (ISRM, 1987). All these methods are based on the assumption that the rock mass is linearly elastic, homogeneous and isotropic in the zone influencing the measurement. The USBM type bore hole deformation gauge and the CSIRO type cell are over coring methods where in situ stresses are measured based on the stress relief around the bore hole. The relief of the external forces by overcoring causes changes in 1)borehole diameter for the USBM deformation gauge and 2)strain on the borehole wall for the CSIRO type cell. If the elastic properties of the rock mass are known, the changes in borehole diameter or strain can be converted to in situ stress in rock. For both the methods, since measurements must be made near a free surface likely to be subjected to the effects of local topography,rock anisotropy and natural fracturing, strain relief is determined over very small areas. Moreover, if the strata are jointed and the core breaks during overcoring, the methods may not be successful. Further, since the elastic modulus of the rock mass is much lower than the material modulus, the use of the material modulus for calculation can introduce some error. The hydraulic fracturing method is a suitable technique for measuring stresses at considerable depths. This method involves applying fluid pressures to a test section in a bore hole isolated by packers. Here the fluid pressures required to generate, propagate, sustain and reopen fractures in rock at the test horizon are measured. By relating these pressures to the existing stresses in the plane perpendicular to the bore hole, and tensile strength of the rock, the two orthogonal stresses in the plane are evaluated. In this test the drill hole direction is assumed to be a principal stress direction which may not always hold good. Moreover, the vertical stress has to be taken to be equal to the overburden stress which may be an over simplification, particularly for measurement of stresses at shallow depths.
A simpler technique which can enable a number of measurements to be made easily and economically and is comparatively less time consuming, is the flat jack method, which is suitable for even weak rocks.