Al Wugeisha3- the third Tunnel Boring Machine arrives today

The TBM will run from Al Ras to Burjuman for the Green Line which is the second phase of the Metro project.

A TBM is used to excavate tunnels in a circular cross section through a variety of geologies. It is manufactured to bore through hard rocks or sand layers and almost anything in between.

Mr. Engineer Adnan Al Hammadi, Director of Construction Department - Rail Agency, RTA said, "The Earth Pressure Balance (EPB) type TBM designed and assembled for Dubai Metro has a diametre of 9.56 metres, is 82 metres long and weighs 1,000T. The Cutter head will have a revolution speed of 0.1-1.82 rev/min.

"The boring for the project is schedule to commence in December, 2007 at Al Ras towards Burjuman for the Green Line. The tunneling will pass under the creek up to a length of 2.5 km and at a maximum depth of 30 metres," he added.

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.