For a small country the Middle Eastern emirate of Qatar has huge ambition. It is currently preparing to embark upon the world’s biggest tunnelling project as it seeks to deliver 59km of twin bored tunnel for a new city wide metro project. At peak this will see 26 TBMs simultaneously boring through conditions which are expected to include dolomitic limestone, chalky limestone and mudstone, to deliver four new lines of light rail.

The metro project is a key element of the Qatar Rail Development Programme which also includes a 400km heavy rail link that will connect the country with the wider GCC railway, and a 38km light rail network around Lusail City to the north of Doha. When complete in 2026 the metro will consist of 236km of line and 96 stations but to achieve this it must successfully delivering the first and most challenging phase of works – the underground lines.

Ticking clock

To make things even more challenging Doha is in a race against the clock. In 2022 the country will be the first ever Middle Eastern state to host the FIFA World Cup. Newly constructed football stadia will be connected with the first phase of the new public transport system ,which will consist of a red line running from the airport in the south through to Lusail to the north of Doha, the Green Line that runs west out to Education City, and the Gold Line that runs south west out to Al Rayyan.

This means that the project must be completed by 2019 to ensure that it is fully operational by 2022 and so far the scheme is progressing well. After a huge international procurement program that saw Qatar Railways present its scheme all over the world, the first major design and build tunnelling contracts were awarded in late May 2013. Qatari companies are included in all of the consortia, which was a requirement from early bidding stages.

The first contract is the northern section of the red line which involves 13km of 6.17m i.d. twin bore tunnels. It was awarded to a consortium led by Italy’s Impregilo, with Korea’s S.K. Engineering & Construction Co, and Qatar’s Galfar Al Misnad Engineering & Contracting. The scope of work includes design and construction of seven underground stations between the proposed Msheireb Underground Station and Doha Golf Course via Doha West Bay. For this, four TBMs are proposed to bore the required rail tunnels. The tunnels will be built at an average depth of 20m below ground.

The second contract is for the southern section of the red line running from Doha south to the Sheikh Hamad International Airport. It is slightly longer that the northern section with 13.8km of twin bore, and also includes a 1.2km single operational link. This was awarded to a consortium led by Qatari Diar Vinci Construction (QVDC) and includes Korea’s GS Engineering and Construction Corp, and Qatar’s AIDarwish Engineering. The package includes construction of six underground stations and is expected to use five TBMs boring at up to 50m below ground level.

At 16.6km the longest twin bore to be awarded so far is the green line which was won by Austria’s Porr in joint venture with Saudi Bin Ladin Group (SBG) of Saudi Arabia and Qatar’s Hamad Bin Khalid Contracting Company. The package also includes eight underground stations and will run at approximately 20m below ground.

A final tunnelling contract for the 15.2km Gold Line is still to be awarded. Meanwhile Korea’s Samsung C&T with Brazil’s Obrascon Huarte Lain and Qatar Buildings Company will build two major stations at Mshreib and Education City.

Given the size of the scheme it is not surprising that 20 consultants are engaged in various roles on the project including US firm Jacobs as project manager for the Red Line, US consultant Hill International as project manager for the Green Line, and a joint venture of US firm Louis Berger with France’s Egis Rail is project manager for the Gold Line.

As is usual for major tunnelling projects the underground works have been tendered as design and build contracts leaving the ground condition risk with the contractors. This is in direct contrast to neighbouring Abu Dhabi where the Abu Dhabi Sewerage Services Company chose to retain this risk on its 40km deep sewer tunnel to make the project more attractive to international firms, and reduce bid costs.

Qatar Railways Company (Qatar Rail) told Tunnels: "Successful contractors are required to undertake further investigations on award of contract to verify the information and assumptions prior to design and tunnelling arrangements finalisation. Tenderers have been positively encouraged to identify risk and price accordingly.

All ground risks therefore ultimately sit with the contractor consistent with the ethos of a lump-sum, fixed-price design and build contract intended to provide QRC with certainty from the outset," it said in a statement, however the organisation says that contractors who can outperform the contract will reap the benefits. "Conversely, and consistent with the certainty requirement, QRC will not seek to limit benefit derived from better than anticipated conditions."

New ground

Qatar does not have a history of underground tunnelling of this scale which makes the ground investigation all important for contractors. "Tunnelling in Doha is going to be difficult, there is no doubt about it," says a senior tunnelling consultant close to the scheme. "The ground is not well known and some of it is karstic limestone, which has voids in it as a result of water passing through and wearing it away." This phenomenon can create large voids in the ground and can also lead to what the consultant describes as "strange steep rock cliffs below ground". The city also has a high water table.

"There are numerous design and construction challenges for the Doha Metro, related to the expected ground and groundwater conditions," it says. "These are the spatial variability of key geotechnical properties, such as the degree of weathering and fracturisation of the encountered geomaterials; the possible existence of medium karstic features, such as karstic depressions and in some cases voids; and the presence of numerous perched aquifers, which during excavation of the cut and cover pits will call for detailed dewatering."

Qatar Rail says that all of these factors will be further investigated by the contractors in order for them to better understand the geo-mechanical behaviour of the encountered ‘ground’ types; evaluate the expected tunnelling conditions through the project network; and apply the most appropriate techniques to ensure the highest safety and quality standards.

Although the tunnelling contracts have only just been awarded, enabling work had already begun onsite. The Porr/ SBG/HBK group has been undertaking the enabling works contract which was awarded in mid 2012 involving the utility diversions and site hoarding to prepare packets of land allocated to the project. From a tunnelling perspective, there are currently 2 micro-tunnelling machines working at the Msheireb Station location and tunnelling will continue until the end of August 2013. Qatar Rail says that the most difficult challenge for the contractor on this section so far has been to excavate in a very congested area with numerous existing underground utilities, including pressurised potable water pipes and live electrical and communication cables.

One of the tougher challenges is to stabilise the side walls of the excavated shaft having to support unstable soil layers. Beyond the challenges of ground conditions and logistics a more immediate requirement is for the contractors to source the multitude of tunnel boring machines. With average order times being around 12 months and projects being undertaken simultaneously the industry could be in for a TBM boom. Experts expect to see slurry or EPBM machines used in Qatar. Despite the challenges however the prospects for Qatar are good.

Thanks to recent mechanised tunnelling in neighbouring Abu Dhabi and Dubai and Saudi Arabia the region has begun to build up its expertise in underground construction, ably assisted by the world’s leading firms.

However for Doha the clock is ticking and the race to complete the world’s largest simultaneous bore is now on