A wide variety of borehole gages have been developed for measuring rock stress. They all operate on the principle that stress changes around a borehole result in deformation that can be measured by the gage and interpreted in terms of the stress change either by calculation or calibration. In general terms, gages that comply with the borehole deformations are referred to as "deformation meters" and those that restrain the deformations by reason of their rigidity are referred to as "stress meters." The main advantages claimed for stress meters are: their "stress" sensitivity is much less dependent on the rock modulus than that of deformation meters, and they are clamped firmly in the borehole.
The use of each of the borehole gages developed for stress determinations in rock in the main has been confined to the laboratory responsible for its development. To be generally accepted a gage must meet the following specifications:
1) Gages to be left in place to measure stress changes must be relatively cheap, and if possible recoverable, so that large numbers can be used to obtain an overall picture of the changing conditions. This, of course, is not necessarily the case for recoverable gages designed for absolute rock stress measurements where usually only one or two gages are involved.
2) For use in the adverse environmental conditions encountered underground, it is essential that gages be robust, stable, and unaffected by high humidity, corrosive water, or dust.
3) Highly qualified technicians are rarely employed specifically for rock mechanics studies in mines or on civil engineering sites and therefore gages designed for measuring stress changes must be easy to set and simple to read. Again this does not necessarily apply for absolute rock stress measurements, which can be made only by highly skilled operators.
4) The sensitivity of tile gage must be commensurate with the use to which tile results will be put. There is little , advantage in designing a highly precise and sensitive gage at the expense of simplicity or cheapness if its readings are to be interpreted only in general terms.
The photoelastic properties of glass have made possible the development of a wide range of gages 1 which in many respects meet the foregoing specifications. This chapter describes a photoelastic unidirectional meter with a diametrically loaded glass cylinder as the transducer element which has been developed primarily to measure stress changes. It will be referred to as the PI T stress meter to distinguish it from the biaxial hollow cylinder photoelastic stress meter which has been described in detail elsewhere. 2 In modified form the PU stress meter has been successfully used to measure absolute rock stress3. Details of the latest developments of the method are given in this chapter.
The PU stress meter consists essentially of four components( Fig. la): a thin stainless steel tube A, a stainless steel reversible wedge mechanism B, a glass cylinder C, and a circularly polarizing light source D. Fig. lb shows the assembled meter and setting tool.
