Rock tunnelling machines have the ability to achieve record breaking advance rates in all kinds of competent rock however their performance in weak and fractured rock depends upon their specific design. A number of case histories of machines operating in zones of adverse ground are presented to define both the engineering properties of rock affecting machine Performance and the nature of the tunnelling problems encountered. This provides basic data for designing and selecting machines for specific projects; planning Site investigations; using geological data to its maximum advantage; and gives an insight into ways in which high progress can be maintained once machines are installed. Two main types are discussed namely, the roadheader, or track mounted boom-ripping machine which employs pick cutting tools, and the hard rock tunnel boring machine(TBM)employing disc Cutters. Both types are designed to excavate rock as their primary function with the installation of support as an integral but secondary operation. The machines are however capable of excavation in more variable ground conditions as discussed in the paper.
The selection of a rock tunnelling machine for a project depends upon a number of actors including the availability of suitable plant, the length of the longest continuous drive in rock and the contractors familiarity with advanced methods of excavation. A period of 9–12 months is normally required for the manufacture of Purpose-built machines however a greater number of ready-made machines are becoming available on the international market, and expertise can often be supplied with these.
A number of cost benefits are directly obtained from such an investment in terms of advance rates of 2–4 times those typically achieved by drill and blast excavation, lower support costs and minimal ground disturbance. The latter has particular advantages for urban and water tunnels as blast vibrations and friction head loss problems are reduced. Excavation by machine gives minimal overbreak, thus for tunnels where lining is required a large direct cost saving can be made. Each tunnel is however unique and must be judged on its own merits. Geological conditions will largely dictate the feasibility of installing a machine and the success of a particular design.
For each of the case histories discussed the geology of the tunnel is shown on a wall section and the corresponding delays occurred at the same locations are shown on the delay chart directly below. This chart gives the time in shift working hours required to excavate unit length of tunnel in 5 or 10m sections. Tunnelling rates (in hours/metre)in adjacent competent rock have been projected across the main delay zone, and the shaded area above this therefore provides a measure of delays attributable to both the geological feature and the type of machine employed.
Figure 1 shows the geological section of a 4m diameter tunnel excavated by a medium-heavy weight roadheader.
