Breakthrough schedules set for longest tunnels in UAE

The project requires cutting about 2,870,000m³ of material and filling with about 3,175,000m³ of cut material and imported fill. Much of that is used backfilling to depths of between 29 and 35m along the route of the new highway So the project has been likened to virtually building a dam instead of a road.
The new drill rig has allowed Iranian contractor GMC to increase tunneling duties and advance rates on the twin Difah – Shis tunnels in the UAE. Breakthrough on the 1.2 km eastbound tunnel had been set for last month and in the westbound tunnel a December breakthrough was anticipated.
Road and tunnel project
Although just 11.5km long, the proposed Diftah-Shis road and tunnel project, partially located in the Emirates of both Ras Al Khaimah (RAK) and Sharjah (Khorfakkan side), has 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 rocks and steep-sided V-shaped valleys often less than 1 m wide at the base, the project has experienced a series of 4m deep flash floods as well.
The project contract was awarded to GMC under supervision of Halcrow International (HIP). In addition to building the 11.5km mountainous dual carriageway, it also includes two 1.2km tunnels linking Diflah to Shis.
Starting at Diflah, near Masafi, some 29km northwest of Fujairah, at about 370m above sea level, the new dual lane road will rise to 600m at the west portal more than 6.15km towards Shis. More twin tunnels to the village of Shis, take the total to 11.5 km.
Since starting earthmoving operations, the contractor has received a series of design variations changing road alignments and more than doubling the earthworks with an increase of over 1.2 million m³.
Design variations
Since the start of road works in phase 1 in December 2004, GMC has also been awarded a new 3.5km extension from the exit of the tunnel to the Shis interchange, requiring more than 1.3m m³ of cuttings of which approximately 60% will be used for infill.
Under phase 2 of the project, currently under design, the highway will continue from Shis to Khorfakkan covering a further 19km and feature 5km long twin tunnels.
The new 29km 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.
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.
Twin tunnels
At the eastern portals both tunnels are 12m apart and at the western portals the distance will be 42m.
Rock throughout the mountain range is predominately Serpentised Peridotite, part of the gabbro family of rocks, and is particularly hard and abrasive. However throughout the length of both tunnels, the contractor is experiencing a wide variety of rock conditions varying from class 2 to class 6.
As a result GMC initially cut a 34m² pilot tunnel at both eastern portals according to the project technical manager, as a precaution against the uncertainty of the rock conditions at the open face, where they initially encountered very fragmented class 6 rock.
Over the first 6 m, and as an additional precaution, the roof was shored with 12 support frames and infilled with shotcrete to a depth of about 50 cm. The roof was also rock bolted at 1.5m spacings.
The cutting was enlarged with the top heading excavation measuring 13 x 6.5m and continuing for the first 400m. GMC then opted to drill and blast on the 45m² lower bench, with a reduced floor width of 12.6m and providing a ramp up to the top head – using two older twin jumbo rigs including a Sandvik Axera.
New jumbo
With delivery of the new Sandvik jumbo, GMC increased its excavation rates using the DT820 twin boom rig, for up to 80% of all drilling and rock bolting duties.
A two boom electro-hydraulic jumbo rig for fast and accurate drilling in tunneling and cavern excavations of 12 – 110 m² cross sections, it is fitted with TB 90 universal booms, to ensure optimally shaped coverage and full automatic parallelism. The booms can be used for cross cutting and bolt hole drilling too.
Face drilling
Generally GMC is drilling up to 108 holes at the face with a variety of spacings and diameters. For example, 30-40 cm along the contour line, using 45mm diameter drill bits and 57mm dia bits in the face holes. Holes are drilled to a depth of 2 to 3 m in class 3 rock, reducing to, for example, 1.5m in class 5.
According to project manager Arash Foroozan Yazdani, GMC had hoped for an increase in class 2 and 3 rock. “Unfortunately we seem to have been hitting mostly class 4, meaning advance rates of less than 2 m, resulting in an average of 55m a month. It is taking about 5 minutes to drill each 3m hole in the hard class 3 rock,” he said.
He continued, “It is necessary to use different type carbide grades depending on the class of rock. Generally we are achieving 1100 drill metres with each drill bit, which in these rock conditions, we feel is excellent,” he added.
Throughout drilling, the rock conditions also determine the tunnel excavation radius. For example in class 2, 7.045m is specified, class 3 is 7.07m and class 5/6 is 7.295m for.
Despite the hardness of the rock, it is proving to be fragmented and rock bolting with types 1, 2 and 3 support is specified along the full length of both 1.27m tunnels, utilising a resin instead of grouting.
Initially GMC concentrated on tunneling from the west portal but as accessibility is improved from Shis, the contractor has opened the east portals and tunnel. By July 2009 GMC had excavated 1005m from the western portal and, using the older Sandvik rig, excavated some 10 m from the eastern portal continuing for up to 100 m in readiness for the breakthrough.
On the second tunnel 950m has been excavated for the top heading.
As work continues on the bottom heading GMC is using a Sandvik CHA660 top hammer rig and employing vertical drilling techniques with Sandvik 64mm drill bits to drill 2.5m deep holes at 1.8m spacings.
Wadis
The new highway in phase 1 is predominantly routed through wadis (dry river beds) and this too is not without its problems.
According to Arash Foroozan Yazdani, the contractor is on constant vigil throughout the rainy season even when there is no sign of rain. A 4m deep torrent 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.
The contract specifies, that all wadis must be retained where possible, which means placing culverts whenever it is necessary to reroute. It is also sometimes necessary to reroute the wadi with a permanent diversion.
Close to the westbound tunnel, culvert 6 is the project’s longest at 170m long. Constructed in-situ in 10m lengths x 7m high segments, it will take 20m of infill to reach the new highway level. In total there are six culverts and an underpass plus a further eight at Shish.
Slope dressing
To ensure slope stability for the access roads and the route of the highway, a series of benches are being prepared by sub contractor, Gulf Rock Engineering Co, utilising a fleet of three Sandvik CHA550 hydraulic surface crawler drill rigs.
Designed for drilling 51 to 89mm diameter holes up to 25m deep, the CHA550s are using Sandvik R32, T38 and T45 extension drill steels, including 64-76mm dia button bits to drill a minimum of 150 holes per weekly blast.
The top bench is generally carried out in two steps to provide a height of between 12 and 30 m with the rigs drilling at an inverse 76° angle. Thereafter two, three or four benches are prepared.
Gulf Rock Engineering
Gulf Rock Engineering is responsible for all surface drilling and blasting for the project’s roads. The company has also provided the blast engineering service for all underground work to GMC since its first blast in 2007, charging all the holes.
Currently they have two crews working two faces a day. Gulf Rock has also been responsible for most of the drilling for geotechnical works outside the tunnel portals.