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Read the following text and fill in the gaps with the words from the boxes below.

Tunnel boring machines (TBMs) and associated back-up systems can be used to automate the tunneling (1) __________ . A tunnel boring machine (TBM) also known as a "mole", is a machine used to (2) __________ tunnels with a circular cross section through a variety of soil and rock strata. They can bore through hard rock, sand, and almost anything in between. Tunnel (3) __________ can range from a metre (done with micro-TBMs) to almost 16 metres to date.

Tunnel boring (4) __________ are used as an alternative to drilling and blasting (D&B) methods in rock and conventional 'hand mining' in soil. A TBM has the (5) __________ of limiting the disturbance to the surrounding ground and producing a smooth tunnel wall. This significantly (6) __________ the cost of lining the tunnel, and makes them suitable to use in heavily urbanized areas. The major (7) __________ is the high cost. TBMs are (8) __________ to construct, and can be difficult to (9)__________. However, as modern tunnels become (10) __________ , the cost of tunnel boring machines versus D&B-method is actually less—this is because tunnelling with TBMs is much more efficient and results in a (11) __________ project.

There are various TBMs that can (12) __________ in a variety of conditions, from hard rock to soft water-bearing ground. Some (13) __________ of TBMs have pressurized compartments at the front end, allowing them to be used in (14) __________ conditions below the water table. This pressurizes the ground ahead of the TBM cutter head to balance the water pressure. The (15) __________ work in normal air pressure behind the pressurized compartment, but may occasionally have to enter that compartment to renew or repair the cutters. This requires special precautions, such as local ground treatment or halting the TBM at a position free from water. Despite these difficulties, TBMs are now (16) __________ more than the older method of tunneling in compressed air, with an air lock/decompression chamber some way back from the TBM, which required operators to work in high (17) __________ and go through decompression procedures at the end of their shifts, much like divers.

Until recently the largest TBM built was used to bore the Green Heart Tunnel (Dutch: Tunnel Groene Hart) as part of the HSL-Zuid in the Netherlands. It had a (18) __________ of 14.87 metres (48.8 ft). Nowadays even larger machines exist. All of these machines were built at least partly by Herrenknecht. Nowadays, the largest diameter of TBM is 15.43m. This TBM (19) __________ built by Herrenknecht for a recent project in Shanghai, China. The machine was built to bore through soft ground including sand and clay. The largest diameter of TBM constructed for hard rock excavation is 14.4m. This TBM was (20) __________ by The Robbins Company for Canada's Niagara Tunnel (21) __________ . The machine is currently boring a hydroelectric tunnel beneath Niagara Falls.

(22) __________ TBMs typically consist of the rotating cutting wheel, called a cutter head, followed by a main bearing, a thrust system and trailing support mechanisms. The type of machine used depends on the particular geology of the project, the amount of ground water present and other (23) __________ .

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