1 INTRODUCTION
Fracture and crack surveying in boreholes, one of the classical tasks in rock engineering, is enjoying renewed interest in recent years. This interest stems from new types of rock engineering endeavours, such as isolating nuclear and toxic waste, enhancing oil and gas recoveries and tapping geothermal energy resources. Most of these projects require new standards in the evaluation of the hydraulic and mechanical properties of rock and its state of fracturing and cracking.
A second reason for the renewed interest is that fractures and cracks can impair some common borehole tests. In fact, all those tests which are evaluated by employing the theory of linear elasticity have to be scrutinized. Examples are pressuremeter and jacking tests, seismic logging and stress measurements by means of overcoring, hydraulic fracturing and borehole breakout analyses. Fracture logging of boreholes and/or drill cores is common practice to substantiate the test results.
This paper illustrates an enhanced method of crack detection in borehole walls. It reports on microcracks which escaped detection when employing conventional techniques. The key in identifying the microcracks was the conjunctive use of two innovative borehole instruments:- the "borehole slotter" for stress measurements and a new modified borehole jack for deformability measurements in rock.
2 SETTING
2.1 Test Site
The tests, referred to in this paper, were carried out in standard HQ-Boreholes of 96 mm diameter in the rock immediately downstream of the Burdekin Falls dam, Queensland/Australia. Andesitic lithic tuff of undifferentiated Devonian-Carboniferous volcanics crops out across most of the 600 m wide river bed, typically as a gently undulating surface with large areas of stepped, flat-topped slabs 0.5 to 3 m thick (Fig. 1). The slabs are bounded by two sets of subvertical joints and are separated from adjoining slabs and the underlying rock by subhorizontal sheet joints. All rock exposed in the river bed is either fresh or fresh with limonite-stained joints (Armstrong, 1987). The intact rock is extremely strong. On average, the UCS is 260 MPa and E 80 GPa.
Figure 1. River bed rock downstream of the Burdekin Falls dam. (available in full paper)
2.2 "Borehole Slotter"
The borehole slotting stress measuring technique was developed in recent years at James Cook University (Bock and Foruria, 1984; Bock, 1986; Foruria, 1987). It is a completely self-contained testing technique which allows numerous 2-D stress measurements to be carried out in boreholes without the necessity for overcoring.
The borehole slotting stress measuring technique is based on the principle of local stress relief. A half-moon shaped radial slot is cut into the borehole wall by means of a small diamond-impregnated saw (Fig. 2). The saw is pneumatically driven and is part of the borehole instrument ("borehole slotter"). Before, during and after slotting, tangential strain is measured at the borehole wall in the immediate vicinity (within a 15 degree arc) of the slot where practically full stress relief occurs. The tangential strain is measured by an innovative recoverable sensor of high sensitivity (Azzam and Bock 1987a). The sensor and its application device are part of the "borehole slotter".
