Adits connecting annex shafts and a bored tunnel are constructed for the Dubai Metro Red Line for the purpose of ventilation and passenger escape. A critical part of the design is to mitigate the risk of face instability due to the mining of adits to connect the bored tunnel and annex shafts, which is carried out in calcareous sandstone, whose strength and stiffness are equivalent to very denselycompacted soil or extremely to very weak rock material. Overall instability due to global shear failure of the adit faces is classified as acceptably low in risk, but local piping failure due to the possible existence of weak layers of cohesionless materials could lead to overall stability problems. To limit water ingress and piping risks, dewatering via deep well pumping and horizontal drains has been proposed. Pumping trials have been carried out, with back analyses performed to create calibrated seepage models.
The Road & Transport Authority (RTA) is constructing the Dubai Metro Red Line, first railway line in the United Arab Emirates. The Red Line, together with the Green Line, civil works are being built by JTMJV (Japanese Turkish Metro Joint Venture). Atkins is the Contractor's Designer responsible for all tunnel design including temporary and permanent works.
The line, which is 52.5km in length, has 26 stations, two depots, and three annex shafts. Two earth pressure balance tunnel boring machines (EPBMs) were used to construct a 5.3km-long underground section.
The Dubai Metro project includes three annex shafts, each of which has two adits connected to the bored tunnel to provide ventilation (ventilation adits) and an emergency escape from the bored tunnel into the shaft (escapeway adit). The ventilation adits have a span of 8.2m and are 7.5m in height. The escapeway adit is smaller, with a span of 3.7m and a height of 5m.
The size and shape of the adits were optimized to minimize the bending moment generated on the bored tunnel lining and to encourage the arching effect of the ground, especially for the ventilation adits, the height of which is almost the same as that of the bored tunnel. A section of the adits is shown in Figure 1, viewed from within the tunnel.
(Figure in full paper)
The adits for annexe shafts 1 and 2 were constructed underneath existing roads, whereas the adits for annex shaft 3 were constructed underneath an existing six- storey building. It was important therefore to prevent excessive ground settlement due to adit excavation, especially for annex 3. The layout of annex 3 is shown in Figure 2. The annex shafts were constructed using permanent diaphragm walls with the top-down excavation method. The bored tunnel was constructed by the EPBM, which passed beside the annex shafts. The adits were constructed by the mechanical mining method with lattice girders and shotcrete for temporary support. Permanent support was provided by cast in-situ reinforced concrete.
(Figure in full paper)
A temporary ring beam bracing system was installed within the affected section of the bored tunnel.