The Shis Tunnel links the towns of Diftah and Shis and is part of a larger plan to link the eastern container port of Khor Fakkan to Sharjah City on the west coast of Sharjah, Unitd Arab Emiartes (UAE). It is the longest tunnel in the UAE, and also the highest at 575m above sea level. The construction is being overseen by Halcrow Middle East for the Government of Sharjah Directorate of Public Works. It is an international effort, with the design undertaken by Halcrow’s specialist hard-rock team based in Melbourne and additional geotechnical input came from Halcrow’s Dubai office.
The tunnel is set in a hostile landscape. Not only does it cut through a mountain with a peak reaching almost 900m above sea level, it also traverses wadis – dry riverbeds typical of the area, that are prone to flash flooding during the rainy season.
The twin bore tunnel has a straight horizontal alignment and will hold two 3.65m-wide traffic lanes, 1m-wide hard shoulders and 1m-wide footways upon completion. The tunnel is horseshoe shaped with an arch inner radius of 6.575m. Its vertical alignment is generally 2.0 per cent falling west to east, and a crest curve near the west portal means that there is a High Point inside the tunnel. There is a vertical gradient limit to mitigate heavy trucks’ exhaust pollution and prevent overheating. It has a traffic headroom envelope of 5.5m, with an allowance for overlaying the pavement.
“The tunnel bores diverge to the east in order to avoid a wadi at the east portal,” says Paul Cooper, project manager. “The portal positions are dictated by topography. We followed existing wadis up as high as possible and established portals where cuttings ceased to be economic and where there were suitable faces.”
Gaining access
Halcrow initially wanted to start works in January 2004 but political difficulties prevented the firm getting on site until December that year. “Although the whole road and tunnel is being paid for by the Sharjah Government, the western half of it is in the emirate of Ras al-Khaimah. This meant that as well as our normal permissions from the Government of Sharjah, we had to go to the Ras al- Khaimah land division,” explains Cooper. The firm also needed to be granted permission to establish a compound andthat involved dealing with the local wali – a mayor of a small town or village. “There was a lot of toing and froing and liaison,” he adds. “We also had to deal with the services authority to make sure there were no objections from the services in order to get permission to construct.”
However, these issues did not result in a delay as the contract was not signed with an official start date until the start of works in December 2004. The next challenge Halcrow faced was reaching the site. It took nine months to reach the tunnel portal, which is only 5km from the site office at the western end. “The first couple of kilometers are wide wadi beds which are reasonably horizontal. They are even driveable with a four wheel drive,” says Cooper. “From there it goes up in to a deep, deeply incised valley which has about 45 degree natural rock side slopes going down in to a point, with zero width at the bottom. In order to fit the road up the wadi, putting to one side the change in alignment, we had to fill the wadi and cut the sides away to create a platform wide enough for a dual carriageway.” It took several months to create a highway that was wide enough and at a reasonable gradient for delivering machinery to the portal.
Conditions on site were tough. “There’s not a breath of wind up there in the summer. After about April until October, it’s a very, very unpleasant place to be outside for any period of time. The rocks will get too hot to touch during the day,” says Cooper.
Tough ground
The geology of the area is as exciting as the challenges faced by Halcrow during the bore. Andrew Crowe, one of Halcrow’s engineering geologist says: “The twintunnels traverse igneous, ultra-basic peridotites derived from the Upper Mantle which form one of the major geological units of the ‘Hajar’ or ‘hard rock’ mountain range. This north-south trending range flanks the east coast of the Emirates and was over-thrust in an east to west direction against the Arabian Shield during the late Cretaceous (120Ma) forming the Oman- UAE Ophiolite Sequence.”
Both the olivine-rich peridotite derivatives of ‘Harzburgite’ and ‘Dunite’ form the typically dark greenish-grey rock found within the tunnels. The intact rock is predominantly strong to very strong, fresh to slightly weathered and fine to medium grained.
Discontinuities in filled with hydro magnesite, serpentine, calcite and clay minerals delineate the characteristically blocky to very blocky rock mass. Single and multiple occurrences of weakness zones of up to 300mm thick were encountered during excavation of the twin alignments. The maximum overburden thickness above the tunnels at the axis of the steep-sided ridge system through which they burrow is 270m.
Excavation
Works on the tunnels were carried out 24 hours a day in shifts. In order to create the 110m2 horseshoe-section tunnels, drill and blast excavation using the NATM was coordinated into two stages via a top heading and bench sequence. A 7m high arched opening formed the top heading excavation, which was driven a minimum distance of 150m ahead of the bench. Electronic blasting of the tunnel faces was carried using a classical wedge-cut drill pattern. Emulsion-based Powerdyne cartridge explosives were used in the central and easer sections of the face and 40g PTN-based detonating cord used on the profile holes.
Following each blast cycle, fragmented rock from the front of top heading was removed using a 5t loader and three to four. 20t Volvo dump trucks. This typically took between three and four hours. To date, approximately 250,000m3 has been excavated. The spoil goes in to general rock fill for the rest of the project. Project manager Cooper explains: “It’s good competent rock and the only fill we have for the earthworks on this project is rock.”
Geological mapping and rock mass classification using NGI ‘Qsystem’ was carried out on a round by round basis at the tunnel face after each blasting/mucking cycle. Qsystem also formed the basis for rock support design for each advance of the tunnels using pre-determined classes IVI for the systematic support of the tunnels. Advance lengths of the top heading face ranged from 1.25-3.5m, again being dictated by rock support class.
Spraying of a primary shotcrete layer provided immediate support of the exposed rock of the arched profile for each incremental advance of the top heading. A remotely operated Meyco Potenza shotcreting unit, capable of spraying 12m3/hour was used to apply the fibrere in forced shotcrete, with accelerator additives applied at the nozzle to ensure early in situ strength of the mix at the tunnel face.
Finally, rock bolting using 4.0m-long full column resin-anchored rock bolts formed the next stage of installation for the integrated temporary rock support lining. Drilling and spinning installation of the bolt units was carried out by a 2009 model Sandvik DT820 twin-boomed tunnel jumbo, as were the drilling of the blasting holes in the tunnel face.
There is daylight through both tunnels after breakthrough on the eastern side in late 2009. At the west portal looking east, the left tunnel is down to formation level, has had its temporary rock support shotcrete and rock dowels, and is about to be waterproofed and lined. The eastbound tunnel, the tunnel on the right, is 60-70 per cent down to formation and the remaining is still just on the top heading.
Handling explosives
Cooper says drill and blast methods were chosen as hard rock tunnelling with a TBM is extremely specialised and suitable machines are scarce. Furthermore, drill and blast is a technique understood by both the client, design team, and the main contractor, General Mechanics, which was responsible for all the tunnel and road works. The only aspects of the project subcontracted to external agents were the reinforced concrete and anything explosive related.
The necessary explosives posed a major issue to the project. “The authorities are very touchy about explosives,” says Cooper. “Initially we were having to import them from the Ministry of the Interior in Ras al-Khaimah. It was necessary to plan a blast, order the explosives, transport them to the site under armed guard in a convoy, use them and then burn off anything left. It was forbidden to take any explosives back to the compound.”
Later in the project the teams were granted permission to build a compound onsite, which streamlined the blasting process and speeded progress. Encased by a large fence and under 24-hour guard from Ras al-Khaimah police, with a network of closed circuit TV and floodlights, the compound allows General Mechanics to get explosives as and when required, and removes the previous need to place orders days in advance.
Weather or not
When working in the UAE, searing heat is to be expected. However, it was not the only adverse weather condition that Halcrow faced during the project. Seasonal rains proved damaging to progress, as aggressive storms caused havoc on site. “When it rains it really, really throws down. That rain then comes and washes away anything not moved to higher ground,” says Cooper.
“The contractor lost compressors and concrete mixers. One memorable time, they had set up reinforcement to do a head wall at the end of one of the culverts. The rain came and swept away and rendered useless about 35t of reinforcement. It just picked it up and threw it down the wadi – it’s not just water, it’s got rocks in it.
“Rain comes in November to December, followed by another bout around February.
What caught the contractor out one year was that we had a summer storm. You can tell when it’s going to rain as the clouds bubble up over the mountains, but the ferocity with which the storm dropped and the amount of water that came down the wadi caught everyone unawares.”
Cooper has one recommendation for safety on site during such deluges: “Stay out of the water. It’s fast flowing and can be a metre or two metres deep. And it’s got rocks and trees and dead sheep and all sorts in it! You really don’t want to be in the water when you get a flash flood.”
The size of a wadi can vary from a small v-shaped trickle at the top of a mountain to hundreds of metres wide. The Difta portal section is near a wadi that is almost 200m wide.
Despite the threat of flash flooding, and with the exception of relatively minor water inflows (<0.5l/min) occurring after periods of infrequent rainfall, the tunnels have remained dry throughout the excavation period. “We encountered no water at all during the construction period,” says Cooper. “We knew the water table tends to be very low, because down at Difta where there is some irrigation, the contractor had to replace some of the wells that were buried by the road embankments. They had to sink wells hundreds of feet into the bottom of the wadi. We were expecting to find perched water tables in the mountains, but in the end we encountered none at all. That was a bit of a surprise.”
The project is an exciting one, with many surprises and twists and turns. However, for Cooper, the most dramatic part is the construction itself. He says: “The great drama is having drawn it and looking at the drawings and seeing the size of the earthworks. You go ‘wow, that’s big earthworks’, then you go there for a site visit, and you see the colossal scale of the road cuttings, and you look up and see 60 or 70m above road level, there is a tiny, tiny little yellow excavator, pecking away at the rockface. That’s something I find astounding, that the contractor managed to get someone up there with a machine. I find the ability of people with machines astonishing.”
At present there is no electricity supply at the tunnel portal other than what the contractor is generating, but the electricity authority wishes to lay a cable providing 11kv up to the portal. This will provide the power for both the tunnel and the necessary street lighting through a system of transformers and feeder pillars. Cooper adds, “The Electricity Authority has jumped at the idea of a tunnel through the mountain. It means that they can put high voltage through to Shis village.”
Shis village will benefit greatly from the new tunnels. At present it is geographically cut off from the rest of Sharjah and can only be accessed by driving through Oman. Not only is the tunnel crucial to the region as Sharjah is the only emirate having access to both the Arabian/Persian Gulf and Gulf of Oman coasts, but upon its completion, scheduled for late 2011 to early 2012, the Diftah-Shis tunnel will reduce the current journey distance between the two locations by approximately 25km.
“The tunnel will mean that Shis can be accessed without leaving the UAE. Occasionally we get locals wanting to use the tunnel as a short cut, so it’s a constant battle with the man on the gate who has to explain to them, ‘we’re blasting, there’s rockfall – it’s dangerous and you’re not authorised on this site’,” says Cooper. But do they listen? He laughs: “Quite a few have made it through! They just make a run for it.”
Escavation within the NATM was co-ordinated in two stages Halcrow spent nine months reaching the tunnel portal The twin tunnels are the highest in the UAE at 575m above sea level The travelling formwork for the cast insitu tunnel lining
