Tunnelling - an extract from “Basic Environmental and Engineering Geology” by F.G.Bell

Accordingly, tunnelling can be an uncertain and sometimes hazardous undertaking because information on ground conditions along the alignment is never complete, no matter how good the site investigation. Estimating the cost of tunnel construction, particularly in areas of geological complexity, is uncertain.
A pilot tunnel is probably the best method of exploring tunnel location and should be used if a major tunnel is to be constructed in ground that is known to have critical geological conditions. It also drains the rock ahead of the main excavation. If the inflow of water is excessive, the rock can be grouted from the pilot tunnel before the main excavation reaches the water-bearing zone.
Reliable information relating to the ground conditions ahead of the advancing face obviously is desirable during tunnel construction. This can be achieved with a varying degree of success by drilling long horizontal holes between shafts, or by direct drilling from the tunnel face at regular intervals. In extremely poor ground conditions tunnelling progresses behind an array of probe holes that fan outwards some 10 to 30 metres ahead of the tunnel face. Although this slows progress it ensures completion. Holes drilled upwards from the crown of the tunnel and forwards from the side walls help locate any abnormal features such as faults, buried channels, weak seams or solution cavities. Drilling equipment for drilling in a forward direction can be incorporated into a tunnelling machine.
Machine tunnelling in rock uses either a roadheader machine that moves on a tracked base and has a cutting head, or a tunnel boring machine (TBM). Excavation with a TBM is accomplished by a cutter head, equipped with an array of suitable cutters, which usually is rotated at a constant speed and thrust into the tunnel face by a hydraulic pushing system. The stresses imposed on the surrounding rock by a TBM are much less than those produced during blasting and therefore damage to the perimeter is minimised and a sensibly smooth face usually is achieved. What is more, overbreak normally is less during TBM excavation than during drilling and blasting, on average 5 per cent as compared with up to 25 per cent for conventional methods. This means that less support is required.
The rate of tunnel drivage is an important economic factor in tunnelling, especially in hard rock. Tunnel boring machines provide increased rates of advance and so shorten the time taken to complete tunnelling projects. Indeed, they have achieved faster rates of drivage than conventional tunnelling methods in rocks with unconfined compressive strengths of up to 150 MPa. Consequently, tunnels now are excavated much more frequently by TBMs than by conventional drill and blast methods. The unconfined compressive strength commonly is one of the most important properties determining the rate of penetration of a TBM. The rate of penetration in low strength rocks is affected by problems of roof support and instability, as well as gripper problems. Problems associated with rocks of high strength are the increased cutter wear and larger thrust, and hence cost, required to induce rock fracture. The rate of penetration also is influenced by the necessity to replace cutters on the head of a TBM, which involves downtime. Cutter wear depends, in part, on the abrasive properties of the rock mass being bored. Whether a rock mass is massive, jointed, fractured, water bearing, weathered or folded, also affect cutter life. For instance, in hard blocky ground some cutters are broken by the tremendous impact loads generated during boring. Moreover, the performance of TBMs is more sensitive to changes in rock properties than conventional drilling and blasting methods, consequently their use in rock masses that have not been thoroughly investigated involves a high risk.

Extract from Basic Environmental and Engineering Geology by F.G.Bell
Whittles Publishing 2007 (RRP$150.00)
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