C+S March 2020 Vol. 6 Issue 3 (web)

ter, and a cutting wheel that used for the excavation of ground. This machine is frequently used in ground that consists of gravel and soil, but it has a limited use in clayey ground mass 5 . It provides support to the face of tunnel in front of the machine by using the pressurized fluid, applied on the basis of surrounding ground permeability 6 . Cut-and-Cover The cut-and-cover technique, in which a trench is excavated (cut) at a shallow depth and then backfilled (covered), is often used for the con- struction of sub-surface, shallow tunnels. At a depth of 18 meters and more, the cut-and-cover method is commonly used for the construction of rapid transit tunnels. At a depth of 10 to 14 meters, this method can be more practical and cheaper than underground tunneling 7 . However, this method has the significant disadvantages of longer construction duration, construction easement requirement, and high surface dis- tortion 8 . It is also limited in its route, as it cannot pass under surface structures and buildings, and can only be used in locations where there is clear space above, such as roads and greenfield sites.

Photo: Railsystem.net

Selection of Tunnelling Methods In selecting a method of tunneling, various factors need to be taken into account 11 . The following chart details the relative advantages and disadvantages of the various tunneling methods.

Photo: Railsystem.net

Drill and Blast The drill and blast method dates back to the early 1600s and is suit- able for both weak-strength rocks (e.g. chalk, clay, and marl) as well as high-strength rocks (e.g. quartz, basalt, gneiss, and granite). It is suitable for non-circular cross sections and tunnels of comparatively shorter length, where a TBM is not considered suitable for use. The drill and blast method consists of several steps such as drilling blastholes, charging boreholes, tamping, blasting, fumes extraction by ventilation, mucking, and support installation 9 . The main drawbacks of the drill and blast method are the vibrations and shockwaves from the blasting process. These make it an unpopular choice in heavily populated urban locations. The drill and blast tech- nique has the added disadvantages of intense noise, gases, dust, and flying debris. As a result, both workers and machines must be evacu- ated from the working area 10 .

Moving Material The movement of materials frequently has a significant impact on the people who live in and travel through the affected project areas. Dis- posal of excavated material in urban locations is problematic. Delivery of construction materials creates traffic congestion during construc- tion, no matter how well this is managed. Delays make people late for


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