The absolute stress at any point in a rock mass or in any solid body for that matter, may be represented by six components of stress, each independent of the other. These consist of three normal stresses and three tangential (or shear) stresses acting on three mutually orthogonal planes oriented relative to a known system of axes. The determination of stress therefore, involves the determination of six unknowns and this requires six measurements. Since stress as such, cannot be "measured" directly, it is usually deduced from measurements of strain although other means are possible and have been used. If strain is measured, six measurements will have to be made such that, in three orthogonal planes, two measurements are made in each plane. This could be achieved, for instance, by measuring the strain in each of six boreholes almost intersecting at the point at which the stress is required. It could no doubt also be achieved in three boreholes in each of which two strains in two directions are measured. It is, in fact, possible that it could be achieved in a single borehole because the end, sidewalls and "roof" and "floor" provide surfaces for six strain measurements in three orthogonal planes. No practical strain measuring instrument has yet been devised to accomplish this nor has much thought been given to the interpretation of the strain readings so obtained. This, nevertheless, is an approach to which, it is believed, further attention should be paid. Most borehole rock stress measuring instruments devised up to the present time, employ only one borehole and simplifying assumptions have to be made to interpret the results. Usually, it is assumed that the major principal stress which is the compressive stress resulting from the weight of superincumbent rock acts in a vertical direction. The direction of one of these is also usually assumed, as, for example, to be parallel to the face, and the borehole is hence drilled perpendicular to this direction. With most present-day rock stress measuring devices, strains are measured in the borehole in a plane normal to the axis of the borehole - the vertical major principal stress and the minor principal stress acting in a plane normal to the axis of the horizontal borehole. No attempt is usually made to assess the magnitude of the principal stress acting alone the axis of the borehole. In this paper a trepanning rock stress measuring technique is described in which the assumption referred to above is made, namely, that the direction of the borehole coincides with one of the minor principal stresses. Strain measurements, however, are made on the flat end of the borehole which is therefore the principal plane of the other minor principal stress and of the major principal stress but it is not necessarily assumed that the major principal is vertical.
The trepanning stress-relieving technique developed by the author, is in principle, similar to that used by Mohr 1), Olsen 2) and Slobodov 3).
