The initial prototype development of a new, high-technology, re-usable and relatively low cost instrument is described. The instrument uses an optical technique to measure borehole deformation, and microprocessor control to transmit the data in digital form via an infrared signal to the operator.
On décrit le développement du prototype d''un nouvel appareil à technologie avancée, réutilisable et relativement peu cher. L''appareil utilise une technique optique pour mesurer les variations de diamètre de trous de sonde, et se sert de contrôle par microprocesseur pour communiquer à l''opérateur les données en forme numérique au moyen d''un signal infra-rouge.
Die erste Entwicklung fortgeschrittener Technologie arbeitenden, kostengünstigen Gerätes wird beschrieben. Verfahren, um Verformungen des Bohrloches zu Mikroprozessoren um die Daten in digitaler Maschinenführer weiterzuleiten.
The principle of borehole deformation measurement as a rock stress measurement technique is well known. The best documented of the devices using this method is the U.S. Bureau of Mines borehole deformation gauge (BDG), described originally by Merrill (1967). If at least three diametral length changes due to stress relief by overcoring are measured in a plane normal to the borehole axis, the principal strains (and hence stresses) may be calculated for that plane. For determination of the complete stress tensor, at least three non-parallel overcored installations are required in isotropic rock; more, if the rock is anisotropic. Present methods, including the BDG, measure changes in hole diameter by means of the deflection of strain gauged cantilevers pressing rigid probes against the wall of the hole. They present no resistance to borehole deformation, and hence the inference of planar stress is relatively straightforward (using the formulae contained in Obert and Duvall, 1967, pp 413–417, for instance). The cantilever/strain gauge response to diametral displacement must be calibrated before use. This system has a resolution of measurement, after calibration, of -6 about ±1 × 10−6 m, equivalent to ±26 microstrain in a 38 mm diameter (EX) hole. This is ''relatively insensitive when compared with electrical resistance strain gauges which nominally have a resoluton of ±1 to 2 microstrain under favourable conditions (e.g. no creep or amplifier drift). Such a system, like most instrumentation, has both advantages and disadvantages when it comes to practical stress measurement in the field. The advantages are: the instrument is re-usable, and is capable of multiple measurements in a single installation hole without great difficulty; it is stable compared to epoxy devices, because it is more or less unaffected by temperature changes in the rock or the drilling water. Problems mainly arise from the obvious practical and economical constraints caused by the need to overcore three holes to obtain a single three-dimensional stress value. Other disadvantages arise from the need to calibrate the device before and after installation and, in common with all strain gauge techniques, the data is obtained in the form of a very small analogue signal which must be transmitted by cable and is thus subject to electrical interference in a typical mining or construction environment. In practice, this has mainly nuisance value, however, and can be overcome in most instances by the use of shielded cable, short lengths, careful choice of measuring site and so on. (The very need for an electrical cable, until now regarded as essential, causes practical problems at every stage of handling, installation and overcoring. It virtually prevents instruments like the BDG from being used as stress monitoring devices). The natural and obvious tendency has been to develop instruments which measure the complete strain tensor during a single overcoring operation to reduce both the time and cost factors associated with stress measurement. Most of these employ strain gauge arrays in some form, either cemented directly to the wall of the borehole (Leeman/CSIR triaxial cell) or else in a hollow inclusion or solid inclusion configuration. These are commercially available for the most part and are so well known that it is not felt necessary to describe them further here. It is interesting to note the results of comparative overcoring field tests in closely jointed rock (Gregory et al., 1983). The BDG was found to give the best performance, along with the CSIR doorstopper device, on the basis of consistency of results. Also, due to its simpler setting-up and installation procedures, the BDG technique enables a greater number of measurements to be carried out in a given time period, providing a larger sample of stress data and hence better confidence in the results than the other techniques tested. The BDG was also considerably more stable than the others, being relatively unaffected by temperature-induced creep. (The other instruments were: the CSIRO hollow inclusion device; the Swedish LuH triaxial cell (similar in concept to the Leeman cell); and the less well-known solid photoelastic cell developed at the University of California at Berkeley).
