1.2m thick diaphragm wall gives way to Metro tunnel

Adnan Al Hammadi, Director of Construction, said, “This is a significant achievement. It’s one of the most critical stages during construction of the tunnel. Breaking through a reinforced concrete wall of a thickness of 1.2 metres is a complex task. This procedure has to be done slowly to make sure no damage is done to the TBM and surrounding walls.”

A TBM consists of a shield (a large metal cylinder) and trailing support mechanisms. A rotating cutting wheel is located at the front end of the shield. There is a chamber behind the cutting wheel where, depending on the type of the TBM, the excavated soil is either mixed with slurry (slurry TBM) or left as it is (earth pressure balance or EPB shield).

The choice for the type of TBM depends on the soil conditions. Systems for removal of the soil (or the soil mixed with slurry) are also present. Behind the chamber there is a set of hydraulic jacks supported by the finished part of the tunnel which are used to push the TBM forward. Once a certain distance has been excavated (roughly 1.5-2 metres), a new tunnel ring is built using the erector.

The erector is a rotating system which picks up pre-cast concrete segments and places them in the desired position. Behind the shield, inside the finished part of the tunnel, several support mechanisms, which are part of the TBM, can be found: soil removal, slurry pipelines if applicable, control rooms, and rails for transport of the precast segments etc.

TBMs are used as an alternative to drilling and blasting methods. It has the advantages of minimum disturbance to the surrounding soil and produces a smooth tunnel wall. This significantly reduces the cost of lining the tunnel, and makes them suitable for built-up areas.

Adnan added: “The TBM is designed to first excavate the soil, support the ground and build rings. The external diameter of the TBM is 9.5 metres and the total tunnelling distance to be covered is 1,500 metres, up to BurJuman.

The second shaft for Al Wugeisha 2 (TBM 2) at Union Square is complete and the second TBM is being assembled. Al Wugeisha 2 will travel from Union Square to Al Rigga, and from there to City Centre. At BurJuman, the shaft for receiving the first TBM has already been completed.