UNSTABLE ROCK AND ITS TREATMENT SNOWY MOUNTAINS SCHEME
D. G. Moye
1. By the use of one or more tapered washers between the plate and the nut; or more recently, by the use of a hemispherical washer in combination with a plate having a matching hemispherical depression centred on the hole in the plate.
2. The tension in all bolts is checked indirectly with a torque wrench immediately after installation. All bolts are rechecked with a torque wrench before grouting. Some check tests have been made of the tension in sample bolts by pulling on the end of the bolt with a hydraulic jack.
D. R. Stephenson (Broken Hill South Lid., Broken Hill, Australia):
1. How are the zones of “uniform compression” shown in Fig. 9 modified by the natural stresses existing in the rock prior to installing the rock bolts?
2. Is it possible for this zone of uniform compression to be changed to tension (in the tangential direction) for example over a more or less circular opening mainly loaded in a vertical direction?
D. G. Moye: The rock surrounding an underground opening before bolting is in some state of stress, depending on many factors, including the primary stress existing in the rock mass before the opening was made, the stress concentrations produced by the opening, and the ratio of rock strength to rock stresses; if less than 1 the rock fails and the stresses are redistributed.
In strong rock under deep cover the intensity of the rock stresses in the area of the zone of uniform compression would commonly be greatly in excess of the few pounds per square inch produced by the installation of bolts, and the bolts would only have a minor effect on the stresses in this particular area.
In the particular case mentioned, of the circular tunnel, bolting probably could not prevent the development of tension in the tangential direction in the crown. However, if the rock failed in tension, bolts could still be very useful in maintaining the interlock between rough fractured surfaces.
Rock bolts are often used where the rock around an opening has already partially failed along natural joint planes or artificial fractures and is more or less detached from the influence of the stress field of the surrounding rock mass; in the extreme case it is about to fall and is mainly influenced by its own weight. In these circumstances the superimposed compression due to the rock bolts may be of the same order of magnitude and capable of acting as an important stabilizing factor.
I. G. Tulloch (Hydro-Electric Commission of Tasmania, Hobart, Australia): Referring to a comment by the author on the history of rock bolting and its early use in the St.\ Joseph Lead Mines, Missouri, it is of interest to record that rock bolting of railway cuttings was done by the Emu Bay Railway Co., Tasmania, about 1900 and the work is still in good condition.
