Boring the Diftah-Shis Tunnel through Hard Rock Mountains in UAEAlthough just 11.5 km long, the proposed Diftah-Shis road and tunnel project, partially located in both Ras Al Khaimah (RAK) and Sharjah in the United Arab Emirates, has already been acknowledged as one of the most difficult construction projects in the Middle East. Passing through a virtually inaccessible mountain range featuring one of the region's hardest rock and steeply-sided V-shaped valleys often less than 1 m wide at the base, the project has already experienced 4 m deep flash floods.The new 29 km-long route will, on completion, provide a faster route for goods and materials being transported from the busy port facility of Khorfakkan to Dubai and Sharjah. The existing road network from the port through Fujairah is very busy and slow moving. It will also improve transportation for villagers of Shis and surrounding remote villages passing through a mountain range which, until now, had proved to be virtually impenetrable.Rock throughout the range is predominantly serpentised peridotite, part of the gabbro family of rocks and is particularly hard and abrasive.Road works started on Phase 1 in December 2004 in a 42-month contract. Starting at Dafta, near Masafi some 29 km northwest of Fujairah, at approximately 370 m above sea level, the new mountainous two-lane dual carriageway will rise to 600 m above the sea level at the west portal over 6.15 km to the twin 1.27 km-long tunnels and onto the village of Shis, totalling 11.5 km. The government of Sharjah awarded the project contract to Iranian contractor General Mechanic Co. (GMC) under supervision of Halcrow International. Visit www.generalmechanic.com and www.halcrow.comSince starting earthmoving operations, the contractor has received four design variations changing road alignments and more than doubling the earthworks with an increase of 1.2 million cubic metres.General Mechanic has also been awarded a new 3.5 km extension from the exit of the tunnel to the Shis interchange requiring more than 1.3 million cu m cuttings of which approximately 60% will be used for infill. Phase 2 is currently under design and will continue from Shis to Khorfakkan covering a further 19 km and feature 5 km-long twin tunnels.Access roadsThe mountain formation features numerous high peaks in close proximity dropping into steep but often shallow gorges unlike a conventional mountain range with peaks spaced widely apart. The constant undulations on Phase 1 has caused major problems. Even the use of GPS techniques was unable to provide the exact desired location. Often it was necessary to prepare a 500 m-long access route to reach a peak above it and begin excavating down to the desired point.The access roads are prepared by a Komatsu dozer and ripper where space permits. Often it is necessary to widen the path with a Hitachi 35 t excavator fitted with one of two purpose-ordered Sandvik G88 breakers. Once the pass is sufficiently wide, the dozer is re-engaged to clear the access. The breakers continue to provide essential service breaking through rock outcrops to permit access for the dozer on steep gradients often at 45 degrees and more. Excavations are now underway to gain access to the east portal from Shis. This is proving just as difficult. Visit www.komatsu.com and www.hitachi-c-m.comTo ensure the slope stability once the access roads and the route of the highway have been prepared, GMC are preparing a series of benches, using Rammer breakers to remove overhangs and ensure a smooth surface finish. Visit www.miningandconstruction.sandvik.comInitially the dozer continues to remove any loose material. Three Sandvik hydraulic surface drills, including the latest CHA560 plus a second and a CHA560 operated by subcontractor Gulf Rock Engineering Co. are then used to drill on the 1:1.5 slope. The CHA560 is hydraulic surface crawler drill designed for drilling 51-89 mm-diameter holes up to 25 m in depth, using R32, T38 and T45 extension drill steels. It is fitted with a HL510 hydraulic rock drill, 5.5 cu m/min air compressor and powered by Caterpillar 3056E diesel engine. The top bench is generally in two steps to provide a height of between 12 m-30 m with the rigs drilling at an inverse 76º angle. Thereafter two four 12 m-high benches are prepared. Blasting has been taking place on a weekly basis with a minimum of 150 holes drilled. The Sandvik hydraulic drills feature Sandvik T38 extension drilling tools including 64-76 mm diameter button bits. Visit www.cat.comTwin tunnelsTunnelling has recently got underway with GMC relying on a Sandvik twin boom drill jumbo. A pilot tunnel covering 34 sq m was initially cut at both eastern portals, as a precaution for the uncertainty of the rock condition at the open face. At both ends of the tunnel, Class 6 type rock had been predicted which was generally very fragmented. Visit www.sandvik.comCarried out over the first six metres and as an additional precaution the roof was shored with 12 support frames and infilled with shotcrete to a depth of approximately 50 cm. Additionally the roof was also rock bolted using jumbo's cradle 1.5 spacing.The cutting was then enlarged with the top heading excavation measuring 13 m x 6.5 m which will continue for the first 400 metres. GMC then plan to begin drill and blast on the 45 sq m lower bench, with a reduced floor width of 12.6 m.At the eastern portals, both tunnels are 12 m apart and by the western portal, the distance will have increased to 42 m.A 2.5 m/day advance is planned for the top bench using a single daily blast depending on the rock conditions. Six different categories of rock condition predicted but generally, Class 3 will be experienced ensuring the 2.5 m advance.Face drillingTwo Sandvik jumbos and one Atlas Copco are working on site. Two will drive the tunnels while the third will be kept as back-up. It is planned to start driving with one blast per day and is expected to increase the frequency to two or three blasts daily, at least one in each bore. Visit www.atlascopco.comGenerally, GMC are drilling 70-80 holes at the face with a variety of spacings and diameters. For example, 30-40 cm along the contour line using 45 mm diameter drill bits and 57 mm-diameter bits in the face holes. Holes are drilled to a depth of 3 m in Class 3 rock reducing to, for example, 1.5 m in Class 5. It is taking approximately 5 minutes to drill each 3 m hole in the hard grade, Class 3 rock. It is necessary to use different type carbide grades depending on the class of rock condition being experienced. Crews are achieving three face drillings with each Sandvik drill bit and 1,500 drill metres with the 4 m Sandvik drifter rods.Throughout drilling, the rock conditions also determine the tunnel excavation radius. For example, in Class 2, 7.045 m is specified. For Class 3 it is 7.07 m increasing to 7.295 m for Class 5/6, determining the thickness of shotcrete stabilization.Despite the hardness of the rock, it is expected to be fragmented and rock bolting with types 1, 2 and 3 support is specified along the full length of both tunnels.Initially GMC are concentrating on tunnelling from the west portal but as accessibility is improved from Shis, the contractor is intending the open the east portals and tunnel for the final 300 metres, with breakthrough planned for late 2008.WadisThe new highway in Phase 1 is predominantly routed through wadis (dry river beds) and this too is not without its problems. The contractor is on constant vigil throughout the rainy season even when there is no sign of rain. A 4 m-deep torrent of water from rainfall a couple of valleys away destroyed a number of prepared access roads, even though the sun was shining above the site. The constant threat of flooding therefore means that the contractor must park all equipment at higher levels overnight and when not in operation.The contract specifies that all wadis must be retained where possible which means placing culverts up to 140 m long whenever it is necessary to reroute. It is also sometimes necessary to reroute the wadi with a permanent diversion. 50-51/07.