The tunnel ... being built using the NATM.
Tunnel blasts way to UAE record
General Mechanical Company has created the UAE’s longest tunnel through a virtually inaccessible mountain range between the emirates of Ras Al Khaimah and Sharjah using the New Austrian Tunnelling Method.
01 October 2009
THE UAE’s Diftah-Shis Highway – a route that will link the port of Khor Fakkan on the east coast of Sharjah emirate to Diftah in the central region – blasted its way into the local record books at the end of last month as the project team achieved their first of two tunnel breakthroughs – and in turn created the UAE’s longest rock tunnel.
Passing through some of the highest and steepest ranges of the Hajar Mountains, the project forms a strategic road link – enabling goods shipped to the port of Khor Fakkan to be distributed throughout the UAE.
Halcrow, as consultant to Sharjah’s Directorate of Public Works, is responsible for the design and supervision of the road tunnel – involving the construction of approximately 9 km of dual two-lane carriageway mountain road, a 1.3-km twin-bore tunnel and a 1-km link road.
The tunnel has been blasted through the rock using explosives and the New Austrian Tunnelling Method (NATM), which involves stabilisation of the tunnel with rock bolts and spray concrete (shotcrete), before being lined with a waterproof membrane and permanent concrete inner shell. A second tunnel breakthrough is scheduled for early November.
The contractor, General Mechanical Company (GMC), has been counting on its giant drill rig to achieve the breakthrough schedules for the tunnel construction. The purpose-ordered Sandvik DT820 two-boom jumbo drill rig, delivered by local distributor Dynatrade, has enabled the Iranian contractor to increase tunnelling duties and advance rates on the tunnels by a month.
The project passes through a virtually impenetrable mountain range, posing a challenge that project manager Arash Foroozan Yazdani sums up as “like building a dam…only harder”.
The proposed Diftah-Shis road tunnel project is partly located in the emirate of Ras Al Khaimah and partly in Sharjah (the Khor Fakkan side). The project extends 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 and hence has experienced a series of 4-m-deep flash floods.
Starting at Diftah near Masafi (29 km northwest of Fujairah) at approximately 370 m above sea level, the new dual carriageway will rise to 600 m above sea level at the west portal over 6.15 km to twin 1.27-km-long tunnels and onto the village of Shis.
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 more than 1.2 million cu m. Since the start of road works in Phase One in December 2004, GMC 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 per cent will be used for infill.
Smooth traffic
Under Phase Two of the project, currently under design, the highway will continue from Shis to Khor Fakkan, covering a further 19 km and featuring a 5-km-long twin tunnel. The new route will facilitate faster transportation of goods from the busy port facility of Khor Fakkan to Dubai and Sharjah. The existing road network from the port through Fujairah is very busy and slow moving. It will also open up a new route through the mountain range for villagers of Shis and surrounding areas that until now had proved to be virtually impenetrable.
The twin tunnels
At the eastern portals, both tunnels are 12 m apart and at the western portals the distance will be 42 m. Rock throughout the mountain range is predominantly 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 says it initially cut a 34-sq-m pilot tunnel at both eastern portals as a precaution against the uncertainty of the rock conditions at the open face where it 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 approximately 50 cm. The roof was also rock bolted at 1.5 m spacing.
The cutting was enlarged with the top heading excavation measuring 13 m by 6.5 m and continuing for the first 400 m. GMC then opted to drill and blast on the 45 sq m lower bench, with a reduced floor width of 12.6 m and providing a ramp up to the top head – using two older twin jumbo rigs including a Sandvik Axera.
With the delivery of the new Sandvik jumbo, GMC has increased its production rates, using the DT820 twin-boom rig for up to 80 per cent of all drilling and rock bolting duties.
The Sandvik DT820 is a two-boom electro-hydraulic jumbo rig for fast and accurate drilling in tunnelling and cavern excavations of 12 to 110 sq m cross-sections. Large optimum-shaped coverage and full automatic parallelism is ensured as it is fitted with TB 90 universal booms, according to a spokesman for Sandvik Mining and Construction. The booms can be used for cross cutting and bolt hole drilling. The unit’s basket boom is described as being ideal for bolting and utility works.
Face drilling
Generally, GMC is drilling up to 108 holes at the face with a variety of spacings and diameters – for example, 30 to 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 2 to 3 m in Class 3 rock reducing to, for example, 1.5 m in Class 5.
According to Yazdani, GMC had hoped for an increase in output 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 55 m per month,” he says.
“It is taking approximately five minutes to drill each 3 m hole in the hard grade Class 3 rock,” confirms a drilling operator. “It is necessary to use different types of carbide grades depending on the class of rock condition being dealt with. Generally, we are achieving 1,100 m with each Sandvik drill bit which, in these rock conditions, we feel is excellent,” he says.
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 and 6.
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 1.27-m tunnels utilising a resin instead of grouting.
Initially, GMC concentrated on tunnelling from the west portal but as accessibility is improved from Shis, the contractor has opened the east portals and tunnel.
As work continues on the bottom heading, GMC is using a Sandvik CHA660 top hammer rig and employing vertical drilling techniques with Sandvik 64 mm drill bits to drill 2.5-m-deep holes at 1.8 m spacing.
Wadis
The new highway in Phase One is predominantly routed through wadis (dry river beds) and this too is not without its problems.
The contractor was on constant vigil throughout the rainy season even when there was no sign of rain for fear of flash floods. For instance, 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 whenever it is necessary to reroute. It is also sometimes necessary to reroute the wadi with a permanent diversion. Culvert 6, located close to the westbound tunnel, is the project’s longest at 170 m long. Constructed in-situ in 10-m lengths and 7-m height segments, it will take 20 m infill to reach the new highway level.
In total, there are six culverts and an underpass along the route in addition to a further eight at Shis.
On completion of the original Phase One highway, it is estimated that more than 3,175,000 cu mm of infill will have been laid, with 2,870,000 cu mm of cuttings – the balance being imported to the site. Backfilling between 29 m and 35 m is not uncommon along the route of the new highway.
Slope dressing
To ensure slope stability of the access roads and along the route of the highway, a series of benches are being prepared by subcontractor Gulf Rock Engineering Company (GREC), utilising a fleet of three Sandvik CHA550 hydraulic surface crawler drill rigs.
Designed for drilling 51 to 89 mm diameter holes of up to 25 m deep, the CHA550s are using Sandvik R32, T38 and T45 extension drill steels including 64 to 76 mm diameter 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 m and 30 m with the rigs drilling at an inverse 76-degree angle. Thereafter two, three or four benches are prepared.
GREC is responsible for all surface drilling and blasting for the project’s roads leading from Diftah to the portal and from the Shis portal to the village of Shis. 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, has two crews on site working two faces a day.
The company has also been responsible for most of the drilling for geotechnical works outside the tunnel portals.
