For example, stresses as much as 5500 psi were measured at one site in the power station. lf the joints are rough or irregular, tightly closed, and cemented, the surrounding layer of jointed rock may be strong enough to resist movement along its joints, The sites for flat-jack tests, selected by inspection and sounding, were usually in rocks of this type.

More commonly, however, the jointed rock around the excavation is in varying degrees of looseness. Apparently the rock mass fails by sliding along joints in toward the excavation, accompanied by slight rotation of blocks, by crushing of irregularities along joint planes, and by shearing or crushing of parts of interlocked blocks. These slight movements permit the stresses in the outer layer of rock to become wholly or partly relieved, and the high stress is transferred to the layers behind. These in turn may partly fail, but to a lesser extent. The highly stressed zone thus migrates back from the surface of the excavation until eventually the mass becomes strong enough to carry the load. This is brought about by a decrease in the difference in the minor and major principal stresses and also probably by the accompanying increase in the strength of the rock under triaxial conditions.

In this manner, a “decompressed” zone may develop around an excavation. Analytical studies and experimental investigation of such zones have been described by J. Talobre (1957, p. 231).

ROCK-BOLT INVESTIGATIONS

Because of the character of the jointing of the rock mass, and from experience with preliminary excavations, it was evident that parts of the openings would require support during construction

The two forms of supports considered were conventional steel supports and rock bolts.

The manner in which bolts function in comparatively soft rocks, particularly those encountered in coal mining, has been the subject of many investigations, but relatively little published information was available concerning rock bolts in hard jointed rocks such as granite and gneiss. The Snowy Mountains Authority has therefore undertaken a program of investigations to study how rock bolts function in hard, jointed rock, and to find techniques for their installation. Results of these studies have been described by T. A. Lang(1957).

Action of Rock Bolts

In jointed rock, individual rock bolts should be installed so the bolt makes an angle with the normal to the joint that is less than the angle of friction; this increases the friction on the joint by increasing the normal reaction across the joint. But, more important, by holding the blocks together and preventing opening and rotation, they maintain the interlock of rough surfaces, and by this means bring into play the strength inherent in the interlocked rock. This effect can completely overshadow the friction effects,

It is well known in mining experience that there is a time factor involved in the failure of jointed rock - either complete failure leading to collapse of rock into the openings, or partial failure resulting in in loosening of the surrounding rock, in the interval between the time of the blast that forms the opening and the time when the rock mass either collapses or reaches stability, it is assumed that there are progressively increasing or decreasing amounts of movement along joints. For bolts to be most effective, it is desirable that they be installed as soon as possible in order to control initial movements.