The main objective of this experiment was to fasten the fragile and precariously hanging cliff rocks to the firmer substrata with steel rock bolts. To ensure the safety of the cultural relics in and outside the caves, as well as of personnel, successful drilling, grouting, and bolt anchoring methods were essential. First, a series of holes, 12-60 mm in diameter, was drilled to a depth of 3-15 m using an electric rock drill with a guiding track. Techniques and equipment were also developed for bolting after grouting.
Two kinds of rock bolting tests were performed: (1) tensile strength of steel rods bolted and grouted into the cliff face, and (2) shear strength of the conglomerate with concrete grouting and steel rock bolts.
Sixty-six types of rods were tested in situ for their tensile strength. Commercial cement mortar and no. 16 manganese spiral steel, 16-32 mm in diameter, were found to provide a good anchoring effect. This steel anchor has a maximum tensile yield strength when the bolting length is twenty-five times the diameter of the rod. After unfavorable factors were taken into account, such as the property of the rock and the quality of grout filling, the bolting length was set at thirty to fifty times the rod diameter.
To obtain test results that approximated actual conditions, bolted and unbolted conglomerate blocks were comparison tested in situ (Fig. 3), with the bolted samples anchored directly to the cliff face with horizontal and inclined rods. A hydraulic jack was used to shear the blocks. Results were as follows:
First, both horizontal and inclined rods produced a remarkable improvement in the crack-resistance and ultimate-rupture loads of the conglomerate (Table 1). Blocks reinforced with two steel rods of 20 mm diameter showed average increases of 36% in crack-resistance load and 100% in ultimate-rupture load.
A second important effect of the steel-rod reinforcement was improvement in the sudden shear rupture characteristics of the conglomerate. Unbolted blocks ruptured under a 10 t load, whereas the bolted blocks only fractured. These tests indicated that bolted blocks can sustain 21-55% more load than unbolted blocks.
Bolting also increased the maximum shear displacement of the conglomerate. Tests found the shear displacement of the bolted blocks to be five to nine times that of the unbolted conglomerate before rupture.
In summary, conglomerate reinforced with steel rock bolts in combination with cement grouting showed higher tensile and shear strengths than unconsolidated conglomerate.
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