Waiting for go on Diftah-Shis drives

Preliminary work has started on a 1.3km twin bore two-lane tunnel for the Diftah-Shis highway in Sharjah, United Arab Emirates, but the scheme has been held up over delayed approval for onsite storage of explosives.

The cost of tunnelling work on the road project has already jumped from about US$19M to a forecast US$27M, following the addition of a second bore plus cross passages and increases in the cost of construction materials.

Iranian main contractor General Mechanic confirmed that the first blast, which was due to take place on 5 May, has been delayed awaiting resolution of the explosives storage problem.

Phil Cooper, project manager with scheme designer and construction supervisor Halcrow, said an onsite store for explosives was needed as the tunnel site was remote. However, despite the remoteness, the store would need a permanent guard supported by round-the-clock armed police presence.

General Mechanic had already shotcreted and installed rock bolts above the two portals on the western side of the tunnel when T&TI was on site. A Tamrock jumbo drilling rig and two smaller Atlas Copco drilling machines are working on site. Two more jumbos, one Tamrock and one Atlas Copco machine, are also due on the project.

The contractor’s technical manager, Yaghoub Alipour, said two jumbos would drive the tubes while a third was to be kept in reserve.

Alipour plans to start driving with one blast per day on the project and expects to increase the frequency to two or three blasts daily, at least one in each bore.

He forecasts an advance rate of 2.5m every 24 hours on the 13m wide bores. But extensive overbreak is anticipated given the poor ground conditions - the gabbro is heavily fractured with RQD of 45%-50%.

‘For now, the excavation line is 1m inside the form line,’ Alipour said on site last month. He added that this plan might be adjusted depending on experience with the excavation, once started. The tunnel will have temporary support of sprayed concrete lining with rock bolts and lattice girders. The permanent lining will be cast insitu mass concrete.

Cross passages will be built every 158m, using breakers and possibly drill and blast, especially as the 7.5m high twin bores diverge at the western end until they are spaced 36m between edges. No difficulties with groundwater are anticipated as any water encountered would quickly drain out of the inclined drives.

General Mechanic hopes to negotiate a further contract to extend the highway from Shis to Khor Fakkan, which would include a similar sized tunnel.

The contract has been put out to tender and several contractors have prequalified, but there still might be the possibility of direct negotiation, noted Cooper.

The highway between Diftah and the port of Khor Fakkan will become a major arterial route allowing faster transit of cargo between Khor Fakkan and Sharjah city, and bypassing the narrow Straits of Hormuz.

Site preparations are underway for a twin bore road tunnel on the Diftah-Shis road, UAE Diftah-Shis 1 Drill and blast for the bores await explosives storage permits Diftah-Shis 2