Al Wugeisha, the first TBM successfully breaks through the diaphragm wall at Union Square

This journey will take six months.

Adnan Al Hammadi, Director of Construction said, "The successful TBM wall break through is a significant achievement. It is one of the most critical stages during construction of the tunnel. Al Wugeisha had to break through a reinforced concrete wall of a thickness of 1.2 metres. This procedure has to be done slowly to make sure no damage is done to the TBM and surrounding walls."

A tunnel boring machine (TBM) consists of a shield (a large metal cylinder) and trailing support mechanisms. At the front end of the shield a rotating cutting wheel is located. Behind the cutting wheel there is a chamber where, depending on the type of the TBM, the excavated soil is either mixed with slurry (so-called slurry TBM) or left as-is (earth pressure balance or EPB shield). The choice for a certain 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 meters), a new tunnel ring is built using the erector. The erector is a rotating system which picks up precast 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, rails for transport of the precast segments, etc.
Tunnel Boring Machines are used as an alternative to drilling and blasting methods and has the advantages of minimum disturbance to the surrounding soil and producing 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 diametre of the TBM is 9.5 metres and the total tunneling distance to be covered is 1500 metres to Burjuman. The break through the wall is done using part of the TBM as its total length is 80 metres while the dimension of the shaft from where the machine is launched is 25x 25 metres. Once the whole machine is complete, the tunnel operation will be faster."

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. While at Burjuman the shaft for receiving the first TBM has been completed.

Other progress at Union Square includes the completion of soil excavation and the construction of the roof and concourse slab.