Read the text to have an idea of state-of-the-art TBM’s.

Almost each tunnel route involves short stretches of running sand or water bearing sands, swelling clay, permafrost or other sorts of bad and hazardous ground conditions which has a great influence on the choice of special driving methods. The areas of high water inflow or stretches of high-pressure water may also be encountered during the drive. Besides, the ground can be riddled with pockets of methane gas. Slow progress through poor geological formations called for new ideas and modern technology, which had to satisfy the requirements of many different ground conditions and minimizes all risks.

The problem of tunneling under a river had defied the engineering imagination for centuries because of the difficulty of preventing water from seeping in and collapsing the tunnel heading. In 1818, M. Brunel built a giant iron casing, or shield, that could be pushed forward through soft ground by means of screw jacks, while miners dug through shutter openings in the face. Brunnel’s shield, rectangular in plan, was successfully employed in driving the world’s first underwater tunnel (Wapping-Rotherhithe Tunnel) under the Thames. The tunnel originally measured 366 m in length, with the cross section measuring 7 by 11 m. It was opened to traffic in 1843. It was used only for pedestrian traffic until the 1860s, when it was converted to railway use. Brunnel was knighted for his engineering feat. The tunnel has been in use, as part of the London Underground (the Tube), since 1913. The old tunnel underwent refurbishment in the late 1990’s.

Modern tunnel shields are powerful sealed steel cylinders pushed forward by hydraulic jacks. They are designed for driving tunnels in soft ground, especially under rivers or in water-bearing strata. The machines are easily operated to ensure a fast and safe breakthrough in the most difficult ground conditions. TBM’s are used to escape the risk of toppling and to reduce great labour efforts required for temporary supporting and excavation of separate areas. Their diameter ranges from 1 to 10 m and depends on the tunnel’s function. The total shield length reaches 30 or even 50 m (fig. 18.1).

The essential parts of the shield are the skin, the cutting edge, the pockets, the bulkheads and the tail. The skin usually consists of several curved plates. Two or three ring girders stiffen the skin plate against distortion. There are large holes or jack pots in the rear ring, through which the pushing jacks extend. Horizontal and vertical frames or diaphragms divide the front part of a shield into pockets. The diaphragms stiffen the shield structure against distortion from eccentric loads on the cutting edge. The cutting edge is made of segments, bolted to the front end of the shield. The individual segments may be removed and replaced if damaged by collision with boulders or other obstructions. When the tunneling is through hard rock the cutting wheel is equipped with milling cutters. Milling cutters are the rotating disks of a rock-cutting tool made of a hard alloy. When the jacks are pressed to the rock with great force they split it under the rotating cutting wheel and push the shield forward. The tail is the part of the shield extending to the rear, within which the primary lining is erected.

Shield tunneling allows full-face driving without dividing the heading into separate areas, and the advance rate may be 15-30 m per day. One of the basic shield tunneling problems is the ground frontal pressure and friction between the ground and the cylinder skin when the shields advance. Hydraulic jacks, braced against the end of the completed lining are mounted along the shield perimeter therefore reducing motion friction (fig. 18.1). Proper operation of the shield requires great skill and experience because various forces can deflect it. The system of driving must be flexible enough to meet all anticipated ground conditions. It is very difficult to drive a smooth curve with a shield. The primary lining on curves is made up of tapered rings. Sometimes every ring will be tapered, but it is a more common practice to install tapers in every second of the fourth ring.

Shield tunneling has many advantages for excavation. However, it has some disadvantages as well. First, there are non-mechanical shields and the miners have to use hand tools for ground excavation, especially when loading the muck and for the lining. Second, only small shields can be assembled in the shop and shipped to the job site in one piece. However, most shields must be assembled in situ, at the tunnel portal or at the bottom of the shaft because of their size. Assembling the shield in situ requires a special mounting platform, which serves as the launching trench for the TBM. These operations are very expensive and labour consuming. Besides, shield starting and dismounting is rather costly.

Tunnel boring machines (TBMs) can be used in a variety of conditions, from hard rock to soft water-bearing ground. They prove to be effective at sections with severe tectonic pressures and where the rock behaves more like loose material. Some types of TBMs have pressurized compartments at the front end, allowing them to be used below the water table. This pressurizes the ground ahead of the TBM cutter head to balance the water pressure. The operators work in normal air pressure behind the pressurized compartment. The Herrenknecht Mixshield meets these requirements, and is equipped with a peripheral cutting wheel drive, leaving the centre of the machine free so that excavated material can be removed either by conveyor belt, screw conveyor or through pipes. Until recently, a TBM with a diameter of 24.87 m was used to bore the Green Heart Tunnel in the Netherlands. Nowadays, even larger machines exist. The trend in tunnel boring machine design is towards automation. A robotic tunnel lining erector has been used in Japan. Development continues in non-intrusive investigation techniques such as geophysical investigation.

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