ABSTRACT
A three dimensional bolting system has been developed to make most of the properties of the rock material usable and to reduce considerably the number of bolts, yet retain the necessary factor of safety.
Just as a dome or a hyperbolic paraboloid roof is much more economic in material than a flat roof, so it is possible to design a three dimensional form of rock bolting with similar properties. The number of bolts is reduced to the same ratio as the number of reinforced bars can be reduced in a domed roof relative to a flat one. Furthermore, the saving in th tension elements in a three dimensional structure is only one part of the economic advantages. Another is the better use of the compressive material, making the best of it. Thus, the thickness of the concrete in the a.m. dome, or in fact any other three dimensional structure is very much less than in an ordinary two dimensional one. The analog in a cavity in rock is not direct but the corresponding compression of the rock due to the prestressing of the bolts makes it possible to support large spans, otherwise not economic, if at all possible, particularly in soft rock. The three dimensional geometry suitable for the cavity is very restricted in the choise of shapes compared to that used in structures. Out of the very few possible, a special shape has been chosen, one which has optimal advantages, namely the envelope of “one sheet hyperboloid”. The particular shape has many important properties, one of which is the equal distribution of stress, another is the possibility of forward inclination of the bolts which permits the pre-strenghtening of the rock in advance of the face, etc.
The method thus ensures that the best advantage of the compression strength of the rock is obtained, acting as a reinforced concrete or prestressed concrete, with the difference that there is no need for the concrete itself to be introduced. The existing rock does just that. The bolts, all inclined at a constant angle are the substitute to the reinforcement, taking care of tension in bending and shear.
The method has been patented and can be used in large tunnels, caverns, underground chambers and is particular suitable for soft or medium hard rock.
