Large sections of the Doha Metro will be underground: 11.3 km for the Red Line North, 12.05 km for the Red Line South, 16.6 km for the Green Line, and 13.3 km for the Gold Line. In addition, tunneling is required for the Lusail Tram. Tunneling works will be accomplished through the 'cut and cover' methodology for the LRT and by the use of Tunnel Boring Machines (TBMs) for the Doha Metro.
Cut and Cover Technique for the LRT
Cut and cover tunnelling is a method used to construct tunnels primarily in locations where the tunnel profile is shallow; therefore, excavation may be done from the surface. Two types of 'cut and cover' methods will be used:
1. Bottom-Up involves the installation of walls down to the bedrock; whilst the walls are in place, soil between the walls is removed and the tunnel floor is poured along with the tunnel sidewalls from the bottom-up. A roof is then constructed after the completion of the tunnel.
2. Top-Down first involves trenching the tunnel out before the final installation of the walls and floor. A
slurry wall provides the necessary support as the tunnel is excavated, albeit temporarily, until the final structures are installed.
Tunnel Boring Machines for the Doha Metro
TBMs are a high-tech mechanised alternative to conventional methods of design-and-build mining, drilling and blasting tunnels through everything from soft ground to hard rocks. They form circular tunnels through the rock, and can install concrete linings along the drilled sections to reinforce the bored tunnel and stabilise the ground.
Although relatively slow moving (about 13 meters per day), TBMs are ideal for Doha; they interfere very little with our vibrant and always active city life. Needing only one point of entry, they can drill right below the city streets, practically unknown to the crowds above. They are also ideal for long lengths of uninterrupted tunnelling, as required by the Doha Metro project.
TBMs also have a small environmental footprint, and will have very little impact on the infamously high water levels that lie beneath our capital. Dust pollution associated with construction sites will be virtually non-existent as all work will take place approximately 20 meters below the surface.
Prior to tunnelling, 'Pre-Condition Surveys' are carried out on buildings along the route. Key Monitoring Points will be installed and actions taken to ensure that all works are carried out under the safest conditions. As with all tunnelling works, safety is paramount.
Due to the country's geological makeup, Qatar Rail has chosen the 'Earth Pressure Balance' (EPB) type of TBMs. These are ideal for use in cohesive soft ground and rock conditions. Their name is derived from the fact that the pressure exerted by the machine itself is in equilibrium to the weight of the earth above it. While the machine drills through the ground, grout is injected straight into the ground to fill the overcutting gaps and stabilizing the ground.
By the end of the first quarter of 2015, 25 TBMs have been commissioned and tunnelling work has commenced across all lines: four TBMs are dedicated so far for the Red Line North Underground project, five for Red Line South, ten for the Green Line, and six for the Gold Line. By tradition, TBMs are each given a ‘name’; in the case, Qatar Rail has elected to name each boring machine after geographical districts within the country (i.e Lebretha, Al Gharrafa, Al Rayyan, and so forth).