|
MEEF |
|
|
|
|
|
There
will be a constant threat of flooding Covering just 10 km, the
proposed Dafta-Shis road and tunnel project, partially located in
both Ras Al Khaimah and Sharjah, in the United Arab Emirates, is
arguably 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 a 4 m deep ‘flash flood’.
The Government of Sharjah awarded the project contract to Iranian contractor General Mechanic Co (GMC) under supervision of Halcrow. It involves the construction of a 10 km mountainous dual carriageway including 1.2 km twin tunnels joining Dafta to the Shis village of Khorfakhan in the Emirate of Sharjah.
Road work started on phase one in December 2005, in a 42-month contract. Starting at Dafta, near Masafi some 29 km northwest of Fujeirah, at approximately 370 m above sea level, the new dual lane road will rise to 600 m above the mean sea level over 6.15 km to a 1.27 km long tunnel and onto the village of Shis, totalling 11.15 km.
Phase two, currently under design, will continue from Shis to Khorfakhan covering a further distance of 19 km. It will also include either a single 3.9 km long twin-tunnel or three shorter tunnels.
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 Khorfakhan to Dubai and Sharjah. The existing road network from the port through Fujeirah 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 predominately serpentised peridotite, part of the gabbro family of rocks and is particularly hard and abrasive.
The
mountain formation features numerous high peaks in close proximity
dropping into steep but often shallow gorges. The constant
undulations cause major problems, as even the use of global
positioning system techniques cannot provide exact desired
locations. Often it may be necessary to prepare a 500 m long access
route to reach a peak above it and begin excavating down to the
desired point.To ensure the slope stability once the access roads and the route of the highway had been prepared, GMC is preparing a series of benches, using Rammer hammers from Sandvik to remove overhangs and ensure a smooth surface finish.
Initially the dozer continues to remove any loose material. Three Tamrock hydraulic crawler drills, including a CHA560 and CHA550 belonging to subcontractor Gulf Rock Engineering C are then used to drill on the 1:1.5 slope.
The top bench is generally drilled in two steps to provide a height of between 12 m – 30 m with the rigs drilling at an inverse 76º angle. Thereafter two to four 12 m-high benches are prepared.
Blasting has been taking place on a weekly basis with a minimum of 150 holes drilled.
Both Tamrock hydraulic drills feature Sandvik T38 extension drill steels including 64-76 mm diameter button bits.
The new highway in phase one is predominantly routed through wadis (rivers) and this too is not without its problems. The project recently experienced a 4 m deep torrent of water from rainfall a couple of valleys away destroying a number of the prepared access roads, even though the sun was shining above its own 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.
A recent example includes 18,000 m3 being excavated to prepare a permanent 160 m long diversion alongside the in-filled highway above the old wadi.
On the first 1 km from Dafta, the sub-grades have been laid to allow access to the contractor’s site offices and to the foothills.
By completion of the phase one highway it is estimated that more than 1.5 million m3 of cut will have been carried out and more than 1 million m3 of in-fill laid.
“We are building a road but this isn’t just a road project, we are virtually building a dam … but a dam would be easier,” says a GMC spokesman. Backfilling between 29-35 m will not be uncommon along the approximate 10 km of highway.
The twin 1.27 km long tunnels will feature an approximate 100 m2 cross-section to ensure a traffic envelope of 9.3 x 5.5 m. Drill and blast techniques will be used with 3-boom Tamrock hydraulic drill jumbos being shipped from Iran. All three will use Sandvik rock tools.
GMC’s plan is to start road construction from the west and east sides simultaneously working the tunnel from both ends. However, as the access to the Shis side (east) is quite difficult, the contractor has no option but to open a pilot tunnel from the west allowing it to transfer the equipment and manpower to the other side to begin tunnel excavations from both ends. For the construction of the tunnel, it has been decided to split the cross-face into three sections.
The top half, divided into two sections each measuring 40 m2 will be drilled to a height of 6 m to provide a total 13.5 m wide bench. The remaining lower 4 m high bench will then be drilled, blasted and excavated. (December 12)