Both bores of the longest tunnel in the United Arab Emirates (UAE) are now complete.
The 1.3km long, twin-bore tunnel is the most challenging element of an 11km arterial road linking the settlements of Diftah and Shis in the Hajar mountains.
Contractor General Mechanical Company (GMC) initially cut a 34m2 pilot tunnel at both eastern portals and, as an additional precaution, shored the roof over the first 6m with 12 support frames, infilled with a 500mm thick layer of sprayed concrete, and rock bolted at 1.5m spacings.
The cutting was then enlarged with the top heading excavation measuring 13m by 6.5m for a length of 400m, and GMC opted to drill and blast the 45m2 lower bench. A Sandvik DT820 jumbo drill rig drilled the holes for the blasting and rock bolting.
The road, which cuts through slopes, under mountains and across dry riverbeds, forms part of a strategic plan to connect the container port of Khor Fakkan on the UAE’s east coast with Sharjah city on the west coast.
Consultant Halcrow is working for the Government of Sharjah to design and supervise construction of the road, which involves building 9km of dual, two-lane carriageway through the mountains, the 1.3km long twin bore tunnel and a 1km long link road.
The tunnel bores were blasted through the rock using explosives, and stabilised with rock bolts and spray concrete, before being lined with a waterproof membrane and permanent concrete inner shell.
Rock throughout the mountain range is predominately serpentised peridotite, a type of gabbro rock that is particularly hard and abrasive. However, conditions varied considerably, with some sections being very fragmented.
At each face GMC drilled up to 108 holes with a variety of spacings and diameters, with depths varying from 1.5m to 3m.