UNSTABLE ROCK AND ITS TREATMENT SNOWY MOUNTAINS SCHEME
Use of rock bolts in patterns
Single bolts or small groups of bolts can be made to hold up isolated blocks of rock by pinning them back to more stable rock. It is necessary to calculate the potential rock load, and the bolts necessary to hold it.
However a more significant application of rock bolting is the systematic use of rock bolts in patterns as rock reinforcement. With a suitable bolt spacing together with a suitable ratio of bolt length to bolt spacing the influence of adjacent bolts can be made to interact to create a continuous rock structure of considerable strength out of a loose mass of broken rock or detached joint blocks. This has been demonstated by experiments on models made of rock particles and by photo–elastic experiments (Figs. 7 and 8).
Fig. 6.—Groutable rock bolts with cone-and-shell anchor.
The more important general guides which have been developed for the installation of bolts in patterns in jointed rocks are the following.
1. The ratio of bolt length to bolt spacing should not be less than about 2. This is to ensure an adequate degree of interaction between the bolts in a pattern. The bulbs of pressure beneath the plates interact, with the formation of a zone of uniform compression with a thickness equal to about one third of the length of the bolts.
2. The length of the bolts should be at least three times the width of the joint blocks. This is to ensure that bolts are anchored to joint blocks embedded sufficiently deeply in the rock mass. The outer layer of joint blocks is loose and possibly close to fallout. Its loose appearance is usually the reason why support is considered to be necessary. The second layer of joint blocks may also be considerably loosened.
3. The compressive force applied to the rock mass by the tension in the bolts should be sufficient to hold the loose layers of joint blocks firmly in contact. For example, consider a pattern of rock bolts 8 ft long spaced at 4 ft centres, installed with a tensile force of 15,500 lb in each bolt (Fig. 9). This produces an average compression in the zone of uniform compression, in the direction of the bolts, of about 6-7 lb./sq.in. This pressure would be sufficient to hold together, in contact, a layer some 5-8 ft thick of loose rock or completely detached joint blocks in the roof of an excavation. It is to be noted that since the tensile force in each bolt is constant (15,500 lb in this case) irrespective of bolt length, the average compressive force applied to the rock mass decreases rapidly with bolt spacing. For example bolts at 5 ft spacing would exert a compression of 4.3 lb. sq./in. equivalent to the weight of 3-7 ft of detached rock. Therefore if the bolt spacing is made too wide even if the length is increased appropriately, the pressure exerted on the rock becomes too small to be useful.
