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Tunnel excavation using Tunnelling Boring Machines (TBM) has grown up a lot in the last decade.
One of the most serious challenges when using TBM, is the diaphragm wall (or launching camber) which is built at the beginning of the tunnel to allow the TBM to be lowered inside it to start boring and at the end of
the tunnel to recover the tunnelling machine.
Very often launching chambers are built to a great depth (20m to 40m below the ground) and have big diameters, moreover they have to stand strong ground and water pressure; for this reason the walls are built with a
consistent thickness (1 to 2 m) and are reinforced with enormous steel rebars.
Breaking of this wall by the TBM is a risky operation which can seriously damage its cutting tools.
So in most of the cases, before the TBM start to boring the tunnel, breaking of the wall is done manually as well as the cutting of the reinforcing steel rebars.
This operation requires a lot of time and can be dangerous, overall in case of presence of water into the ground. The more the workers break the wall reducing its thickness, the more the water counter pressure can
suddenly break into the launching chamber, causing flows of water inside it.
This is the reason why nowadays a "smart solution" offered by Fibre Reinforced Polymer rods (in general glass fibre) is always more adopted.
The technique consists of substituting the internal steel reinforcement bars of the wall with composite materials bars having a high tensile strength but low shear strength, which allow the TBM to bore through the
wall section easily and without running any risk for the cutting tools.
Commonly, a section (cage) of the reinforced wall having the diameter of the TBM is reinforced using FRP materials (see figure 1), while the upper section of the cage and the lower section of the cage is built with
steel rebars and connected to the FRP cage by overlapping steel and glass fibre bars.
Building and lifting of the cage become a fast operation thanks to the light weight of FRP material, which is assembled on a steel frame until placed in position by mean of cranes. Before lowering the FRP cage into
the ground, the steel frame is removed and the whole structure is lowered.
The same reinforcing technique can be used for the walls of the tunnel stations which will be built and bored by the TMB along the tunnel route, without loosing anytime to break them manually.
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