Field and Laboratory Tests in Rock Mechanics by L. G. Alexander
9.—Measurement of Rock Deflections
Rock Deflections using Untightened Rock Bolts:
In the early stage of excavation of Tumut 2 Power Station machine hall, it was desired to obtain measurements of inward deflection at the roof abutments as a result of excavation below abutment level. As survey methods were not suitab1e at this stage, the method used was to measure the extension of long holes (approx. 56 ft.) driven horizontally out from the abutments. Hollow bolts, made from four 14-ft. bolts, were anchored at the far end of these holes by a grout pocket (grout fed through the hollow bolt).
Fig. 13.—Rock Deflection Measurement Installation
Movements of the rock surface (Fig. 13) relative to the free end of the bolt were observed with a dial gauge. The deflections obtained (Fig. 14) are of the surface relative to a point 56 ft. back in the rock. The relation of this to the total surface movement may be investigated on the same models as used for photoelastic tests. From theoretical considerations for a circular tunnel, the extension of the bolt-hole is expected to be approx. 2/3 of the total surface deflection.
Fig. 14.—Rock Deflection—Abutments of Tumut 2 Power Station.
The use of shorter untightened bolts for rock deflection measurements is not recommended. Tests with 8-ft. bolt length in the Tooma-Tumut tunnel showed that anchor slip can occur (see Fig. 12, in which one bolt gave a large negative result) after blasting close to an ungrouted bolt.
Rock Deflections using Grouted Hollow Rock Bolts:
Hollow rock bolts may be used for bolt tension measurements. In the hole is inserted a 3/16-in reference. rod, welded to the bolt at the anchor end, and with a stainless steel tip at the free end (approx. 1 in. from the end of the bolt). The end of the bolt has a stainless steel bush for insertion of a dial gauge (Fig. 15).
