Abu Dhabi is building a deep-level sewage interceptor tunnel, following the model used in Singapore. It is the first time the tunnelling method has been used in Abu Dhabi.

Typically situated 20-100 metres below ground, these large-diameter pipes use gravity to transport wastewater away from busy cities and into treatment facilities in quieter areas. The pipes are deep enough to bypass other underground utility services, but need huge pumping stations to lift the flow back to the level of the treatment works.

Large-diameter pipes use gravity to transport water away from busy cities to treatment facilities in quieter areas

The principle of wastewater transfer is nothing new, but utility tunnels are typically less than 3 metres in diameter. The new breed of interceptor tunnel can be double this size, giving a huge amount of water storage capacity. This is useful for cities such as Jeddah where flash flooding is an issue, and at the same time it provides a transfer route for sewage.

Reducing water surcharges

Taking wastewater away at such a deep level and huge capacity offers many advantages including a reduction in odour and a lower chance that the system will surcharge and flood the streets. These considerations were instrumental in Abu Dhabi’s decision to implement its $1.6bn Strategic Tunnel Enhancement Programme (Step).

Progress on all three tunnel boring machines employed on the sewer project has been excellent

Bill Brundan, CH2M Hill

Starting on Abu Dhabi Island, a 41-kilometre tunnel will run south to the mainland, descending from 24 metres below ground level to a depth of 80 metres. Together with a new major pumping station, almost 43km of connecting sewers and other improvements, the Step project will upgrade the emirate’s previously strained sewage network.

The project was first introduced to the market back in 2008, when MEED reported that the prequalification process had begun in August. To date, more than 12,000 metres of the new sewer tunnel have been excavated using earth pressure balance (EPB) machines. It is the first time the technology has been used in Abu Dhabi. All six of the scheme’s major contracts have now been awarded. “When the first tender documents went out at the end of 2008, design and build had never been tried by the government,” says Shahzad Orakzai, manager of Abu Dhabi Sewerage Service Company’s (ADSSC) programme management department.

So far the decision has proved a success. “Design and build has worked very well,” says ADSSC managing director Alan Thomson. “It allows freedom for the contractors and consultants to bring their expertise to the table.”

The organisation also decided to adopt the programme management approach used by Singapore for the construction of a $3.65bn, 48km deep-tunnelled sewer system in 2008. US consultant CH2M Hill was appointed to the Abu Dhabi project management role in February 2009.

The six design-and-build contracts were split into two main areas. The first consisted of three tunnelling contracts for the 41km sewer. The other three contracts were for the smaller-diameter connecting sewers and the pumping station, which will send the wastewater to the new Al-Wathba treatment works. The contracts attracted more than 45 expressions of interest.

Good response

“We were very pleased with the response, which was better than we expected in some respects,” says Robert Marshall, Step programme manager at CH2M Hill. “At the time we were doing this, there was a lot of work worldwide, but the size [of the project] was large enough to attract international contractors.”

The team narrowed the firms down to a list of 15. In September 2009, Italy’s Impregilo became the first contract winner, securing the $242m package T02. Involving three major work shafts, three additional access shafts and a total of 15.5km of tunnelling, this section forms the centrepiece of the sewer and is being undertaken in three separate bores using EPB machines supplied by Germany’s Herrenknecht.

To date, the site team has made good progress in excavating through ground consisting of mudstone and gypsum. The contractor describes the conditions as “good ground with no significant geological difficulties”.

By the end of February 2012, Impregilo had completed 11.8km of the tunnel, equivalent to 76 per cent of its total length. By mid-March this was closer to 86 per cent.

“Progress on all three tunnel boring machines (TBMs) has been excellent,” says Bill Brundan, resident engineer for contract T02 at CH2M Hill. “Each machine has been averaging 579.5 metres a month over the last six months, which is as originally planned. Record production rates have been achieved: 46.2 metres over one day, 218.4 metres over a week and 775.6 metres over a calendar month. These are going to be good targets for the other two tunnelling contracts.”

Grouting issues

As is usual in tunnelling projects, the first section of excavation had teething problems. Before the machines could get up to speed, issues with the grouting process, which holds the tunnel ring segments in place before a full ring is completed, led to some movement of the segments and localised cracking.

“This was due to a combination of factors concerning the grouting system,” says Richard Graham, project manager for Impregilo. “It was a case of ensuring the correct procedures were being followed and defining the correct parameters for pressure and volume according to the advance rates we were trying to achieve. We consider it part of the commissioning process; you never start at high speed.”

Impregilo now has two more TBMs at the deepest part of the project. These are part of the $198m T03 contract awarded in November 2010. The first TBM has begun excavation and the second was to begin boring in early April. “It is expected that boring rates similar to the T02 machines will be achieved,” says Graham.

At the northern end of the new tunnel, South Korea’s Samsung C&T has also begun boring the first 5.7km length of the 16.1km section under contract T01, which was awarded in February 2011. Japan’s Kawasaki is supplying the machines and two more EPB machines are set to arrive at the site before the end of April. “The first few hundred metres of the first TBM drive will be most interesting,” says Mike Murphy, resident engineer for T01 at CH2M Hill. “This is where we sort out the machine’s teething problems, while passing underneath Maqta Creek from Abu Dhabi Island to the mainland.”

This section of tunnelling had previously been an area of concern for the team when ground investigations showed a variation in the conditions beyond the mudstone and gypsum that have characterised the rest of the bore. “By the creek we found some very soft clay, which would have had an impact on the tunnel design, so we did some further investigation,” says Lee Pearson, construction manager at Samsung C&T. Fortunately, a survey showed that the clay was just an isolated pocket.

As with the other contracts, Pearson says the firm expects to tunnel in good ground. However, engineers are taking precautions to ensure they are prepared for potential voids. “We have a ground prediction system that looks about 12 metres ahead and will show any anomalies,” he says. “If we come across a big void, we can put probes out and do forward grouting.”

Slower tunnel progress

Although the tunnelling is now well advanced, less progress has been made on the link sewer and the pumping station contracts. Despite seeking bids in 2009, awards were not approved by the Executive Council until December 2011. The contracts were only signed in January.

The global economic crisis was one reason for the delay in making the awards. Abu Dhabi held back as the local effects of the downturn were examined and its $10bn bailout of Dubai was executed. “We were looking to complete the project by the end of 2012,” says Thomson. “Now we hope to do this by the end of 2014, but that may be optimistic and it is being reviewed.”

Abu Dhabi’s sewer construction experience is already changing local perceptions of tunnelling. Previous cut-and-cover techniques were seen as disruptive and many were fearful that the emirate’s underground space was littered with massive voids.

“The decision-makers perceived that there were huge cavities everywhere,” says Orakzai. “When we started discussing a solution to Abu Dhabi’s sewerage needs in 2006, the first reaction of our senior members was negative. They were worried that the roads and houses would collapse and wanted to do something smaller. It was very difficult to cross that barrier.”

Six years on, however, these experiences are being shared across the region. Qatar, for example, is now starting its own tunnel interceptor project, similar to the Step scheme. In Saudi Arabia, the US’ Aecom has been appointed to consider Jeddah’s wastewater requirements. Abu Dhabi itself could see yet more tunnelling as part of its planned metro project.