Rock bolts have been used widely for reinforcing relaxed zones around tunnels or caverns, and proved to be very effective. many researchers have carried out experimental or theoretical studies on rock bolting (Yamamoto 1978, Bjurstrom 1974 and others), and its mechanism of reinforcing has been becoming disclosed gradually.

This paper describes some results obtained by model experiments concerning rock bolting in jointy rocks. In the experiments, special attention was paid to bonded or unbonded rock bolting, as well as the amount of prestress, setting angle and kinds of materials, etc.

Outline of experiments

A test piece simulating a rock joint consist of a pair of concrete blocks BLI and BL2 (Fig. 1). The size of a block is 7.5 cm thick and a contact plane AB (7.5 cm × 7.5 cm) of the two blocks is considered to be a rock joint in experiments. Each block has a bolt-hole of 2 mm (tight hole) or 40 mm (loose hole) in diameter. Rock bolts are made of piano wire, mild steel or brass, and the diameter of them is 2 mm. In case of a tight hole, there exists no allowance between a bolt-hole and a rock bolt, the condition of which would be a similar one of a bonded type bolt. In the other case of a loose hole, an emphasized condition of an unbonded type bolt would be produced.

The procedure of an experiment is as follows. After giving prestress po to a rock bolt by a screw jack, a load is applied by a universal testing machine, which causes a sliding load T on a rock joint AB. Increasing the load T gradually, the blocks BLI and BL2 move relatively in opposite direction each other and yield sliding displacement δ. The tensile force P of a rock bolt becomes larger and larger due to a sliding displacement and finally a break off of a rock bolt occurs. Forces T and P are measured with load cells and δ with a differential transducer. Recording of these values are made by a 2 pen-type X-Y plotter.

Cases of experiments

A friction resistance of a rock joint AB was measured first, and the value of 39 degrees of friction angle ф was obtained. The uniaxial compressive strength of blocks was 544 kgf/cm2. ExperLmer.r.swere carried out for 6 cases of setting angle θ (see Fig. 1) of a rock bolt (0, 5, 10, 15, 20 and 30 degrees), and for 2 cases of prestress Po of a rock bolt (40 and 60 % of a tensile strength of a rock bolt material). Three kinds of materials of a rock bolt were used, i.e., piano wire, mild steel and brass, and the tensile strengths Pt of them were 589 kgf, 243 kgf and 165 kgf respectively. A diameter of a bolt-hole was 2 mm or 40 mm as mentioned before.

(Figure in full paper)

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