TUNNEL EXCAVATION METHODS

Posted on Feb 13, 2010 in Construction Technology & Machines

Drill and blast tunnelling

Most of Gautrain’s rail track tunnels are excavated by means of drilling and blasting.

Several holes are drilled into the rock. These are then filled with explosives. The explosives are detonated. The explosion causes the rock to collapse and this lengthens the distance of the tunnel. The circular shape of the tunnel is achieved by carefully calculating the position and the depth of each hole to be drilled. The shape of the tunnel is also affected by the timing of each explosion. The drill and blast process is repeated until the desired length of the tunnel is reached.

Before the invention of Tunnel Boring Machines, every tunnel in the world had to be excavated by drilling and blasting. Today, both methods can be used simultaneously.

Automated tunnelling technology

A Tunnel Boring Machine (TBM) can be described as a high-tech moving factory. A computerised cabin automatically controls and drives this massive machine underground.

Designed in Germany to specifically cope with complex underground conditions south of Rosebank along Oxford Road, this moving factory will started boring a tunnel towards Park Station in January 2008. Geology in this area is affected by a high water table as well as different degrees of hard rock, sand and soft, water clogged soil. German engineers custom built a mixed face Earth Pressure Balance Shield TBM for Gautrain.

Urban tunnelling requires that the ground surface is left undisturbed, especially since tunnelling has to be done under deep infrastructure foundations. Another advantage of TBM technology is its ability to produce a smooth tunnel wall that is cost-effective and time efficient.

Imbokodo, with her 145m long back up system comprising 13 trailers, will bore a three kilometre long tunnel with an excavated diameter of 6.8m and a minimum ground cover of 15m at some places. She weighs 885 tons. The remainder of the 15km tunnel between Johannesburg Park Station and Marlboro Portal will be excavated using conventional drilling and blasting.

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