Read the text and define recent trends in rock tunnelling

Transport tunnels are horizontal underground passageways, which come to the ground surface at both sides. It is essential that any tunnel project start with a comprehensive investigation of ground conditions. The results of the investigation allow for the proper choice of machinery and methods for excavation and ground support. As tunnels are dug in various types of underground conditions, from soft clay to hard rock, it is necessary to reduce the risk of encountering unforeseen ground and water conditions in the early stages and make the best use of them.

Tunnels are typically divided into two categories – rock tunnels and soft ground tunnels, the latter covering all non-rock ground conditions. Modern standard rock classification systems categorise anticipated rock conditions in term of hardness, extent of fracturing and type of required support. For very large openings and particularly for non-circular shapes, the New Austrian Tunnelling Method has become the method of choice for rock tunnels. In softer rocks, excavation usually progresses using a roadheader, a machine with a rotating cutting head mounted on the end of an excavator arm.

For longer, circular tunnels, the most common excavation method is the use of a tunnel-boring machine equipped with a full face rotating cutting head, which cuts a circular cross section in rock. TBM's tend to be expensive and require considerable set-up time, but they can provide very efficient tunneling excavation. For example, on the Channel Tunnel, progress rates of 300 m/week were achieved. Progress rates of this order will never be attainable with the blast method, even with modern computer-controlled drilling jumbos and improved explosives. Most soft ground tunnels require an initial temporary lining system followed by a final permanent lining. This method is used in soft ground but it requires very close control during construction. In many instances, particularly when tunneling below the water table, the excavation face would not be stable if not supported. These are conditions where either compressed air or modern tunnel boring machines must be used.

In some cases, conventional desk and site studies will not produce sufficient information to assess the blocky nature of rocks, the exact location of fault zones or softer ground. This is a particular concern in large diameter tunnels. Virtually all public transport tunnels are in this category. To overcome these problems, a pilot tunnel, or drift, may be driven ahead of the main drive. Alternatively, horizontal boreholes may be used ahead of the advancing tunnel face.

Tunnel excavating can start with a vertical opening, which is usually called a shaft. A complex of connecting tunnels and shafts is often associated with underground chambers. True tunnels and chambers are excavated from the inside and then lined as necessary to support the adjacent ground. Tunnels may also be started from the bottom of a vertical shaft or from the end of a horizontal tunnel driven principally for construction access and called an adit. The sequence of excavations in the cross section area through hard rock is shown in fig. 17.1.

Tunnelling starts from the deep adit (1) with temporary supporting (2). Then vertical and inclined footways (3) from the deep adit are dug towards the top adit (4). The next step is to open the calotte, i.e. driving the top tunnel area (5). If the ground dome tends to be fractured, it requires supporting immediately after excavation (6). Then the middle bench or stros (7) is driven and finally the side benches or strosses. (8) Soil is transported out and the whole tunnel is given a final lining.

Rock tunneling is the most ancient excavation method and the following distinctive features determine it:

1. The tunnel cross section area is divided into portions which are driven in turns, i.e. the individual excavation phases for heading, bench and invert are used.

2. In the driven areas a watertight lining is made of bolted segments with sealing gaskets to prevent water entering while the face drivage may be lengthened.

3. Temporary timber lining supports the loose rock.

So-called cut-and-cover tunnels (more correctly called conduits) are built by excavating from the surface, constructing the structure, and then covering with backfill. Cut-and-cover is a simple method of construction for shallow lying tunnels where a trench is excavated and roofed over. A strong overhead support system is required to carry the load of the covering material. Two basic forms of cut-and-cover tunneling are available: bottom-up method and top-down method.

Bottom-up method: A trench is excavated, with ground support as necessary, and the tunnel is constructed within. The tunnel may be of in situ concrete, precast concrete, precast arches, corrugated steel arches and such, with brickwork used in the past. The trench is then backfilled, with precautions regarding balancing compaction of the backfill material, and the surface is reinstated.

Top-down method: In this method, side support walls and capping beams are constructed from ground level, using slurry walling, contiguous bored piles, or some other method. A shallow excavation is then made to allow the tunnel roof to be constructed using precast beams or in situ concrete. The surface is then reinstated except for access openings. This allows early reinforcement of roadways, services and other surface features. Excavation machinery is then lowered into the access openings, and the main excavation is carried out under a permanent tunnel roof, followed by constructing the base slab.

Shallow tunnels are often of the cut-and-cover type (if under water, of the immersed-tube type), while deep tunnels are excavated, often using a tunneling shield. For intermediate levels, both methods are possible.

Hard-rock tunneling often uses the traditional “drill and blast” method which requires the following stages: blast holes drilling; explosive charge tamping into blast holes; explosive charge igniting; removing toxic blast fumes from the face by ventilation; inspecting the blast area looking for unexploded charges in the tunnel face. Safety is always the first consideration, and the blasting crew keeps an eye on the ventilation requirements. Force fans, which clear the blasting fumes, ventilate the face and then the spoil is loaded into standard tippers for transportation to the entrance where it is reloaded into trucks for disposal.

Soft rock tunneling needs busters and drill pluggers, blast-hole drills and loading machines, which are most efficient for: sand, sandy loam and other types of loose ground. Broken rock is loaded into skips driven by electric locomotives, which run along narrow-gauge tracks. The gauge in tunnels ranges from 600 to 750 or 900 mm. Sometimes the broken rock is moved away by a conveyer belt.

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