Field and Laboratory Tests in Rock Mechanics by L. G. Alexander
In the test, the measurement made is of the extension of 8-ft. drill holes (Fig. 12). In each alternate hole, bolts adapted for extension measurements (Sec. 9) are tightened and grouted. Between these are installed free bolts, anchored but untightened, for drill-hole extension measurements on an unrestrained rock surface (Sec. 9).
The rock stress measurement was done at twelve sites; four sites for each of three directions being measured. One of the objectives of these tests was to measure any difference in rock stress caused by close blasting, as any changes could be of importance in the interpretation of stresses measured in the surface of rock. For this purpose six of the twelve sites were placed close to the tunnel heading (Fig. 12).
Fig. l2. —Location (schematic) of Rock Bolts (8 ft.) and Stress Sites around Tooma-Tumut Tunnel (14 ft.).
The drill-hole extensions measured are shown on the schematic drawing (Fig. 12). The results show that 4 ft. from the heading the displacements were so small that the rock must be considered undisturbed.
At 1 ft. from the heading the displacements were large, and some readings were lost by overbreak. Under all the grouted bolts, the rock was found to have been fissured, and the tension increased in the bolts. The significant reading from the free bolts is that at the crown of the tunnel (0.015 in.) where the rock remained unfissured and sound.
Comparison with neighbouring bolts indicates that this may he taken as the elastic rock expansion, whilst results on the other sites are greater by varying amounts of fissuring.
Results for rock stress were:
v = 600 p.s.i., t = 730 p.s.i., l = 1,370 p.s.i.
(tunnel overburden 750 ft.).
The formula given above is used to compute the extension of the 8-ft. drill holes, and the values for v and t and the observed extension inserted. The elastic modulus obtained for the sound rock at the crown is 1.5 × 106 p.s.i. (small corrections for longitudinal tunnel stress and the test condition that the full theoretical extension is not measured, would both give a lower result for E). That no rock loosening occurred is judged both from the superficial soundness, and from the significantly low changes (+7%) of stress observed in test sites 1 and 2 only 2 and 4 ft. from the heading. Such small changes made it unlikely that deep shattering occurred.
The value obtained for E is low compared with that from the stress tests (7.5 × 106). In this test, however, the number of observations of drill hole extensions from sound rock areas is of course inadequate. In further tests, the grouted rock bolts only need be used, as they provide the simplest method of measuring the extensions of the drill holes.
