Dubai Metro recycling tunnelling knowledge

29 April 2012

Dubai Metro offers many lessons to the other cities planning underground urban rail networks, chief among them the value of understanding ground conditions before boring commences

Doha, Kuwait, Abu Dhabi, Baghdad and Riyadh are currently planning metro networks, which will involve tunnelling to bypass and protect buildings and infrastructure. As planners consider how best to tackle contracts, they will keep in mind the lessons learned on the construction of the pioneering Dubai Metro, which completed the last of its six tunnel bores in December 2008.

The 52-kilometre Red line of the metro opened in September 2009, followed by the 22.5km Green line in September 2011. Both lines have underground sections, with 12.6km of tunnelling in total.

Abdulssamie Haimoni, a consultant to the chief executive officer and deputy director in the rail projects construction department at the Dubai Roads & Transport Authority (RTA), oversaw the construction of both lines. Speaking at MEED’s Qatar Tunnelling Conference in Doha in December, he said that tunnelling was a major success for the metro scheme and has more benefits than are immediately apparent.

Tunnelling benefits

“To other clients, I would say do more tunnelling,” he said. “When people evaluate the difference between tunnel and elevated sections, they do it on a cost basis, but if they allow for all the costs of working above ground, they will find that they add up at the end of the day.”

The expense comes from challenges such as service diversions, traffic issues and difficulties in getting work permits, but they are less of an issue beneath the earth. “Tunnelling is all hidden in the ground, it doesn’t affect your city and you are likely to have less programme problems than when you are working above the ground, if you do it right,” said Haimoni.

[When working on the Dubai Metro project], we watched and analysed everything on a daily basis

Abdulssamie Haimoni, RTA

These advantages only come from projects carried out correctly, he added. This means considering all the risks before beginning a tunnel bore. Compared with other construction activities, tunnelling carries unique risks that must be considered at an early stage. “Planning and risk management are the recipes for success,” said Haimoni. “You can’t do proper planning or risk management if you are working with people who haven’t done this type of work before. Bring in experts from the planning stage itself.

“It is too late if people submit tenders using information that is not adequate, use the wrong type of machine for the ground and it all ends up in a big fight. Bring the experts in early because they can tell you [where exactly to tunnel]. That can make a huge difference.”

As a project progresses, this also means making sure risk management exercises are carried out by engineers and tunnellers themselves. “Don’t just do [these exercises] for quality assurance; give them to the tunnelling engineer to manage with the tunnelling contractor,” said Haimoni. “Involve the people on the ground. You can then respond quickly to issues.”

Unforeseen ground conditions are a key risk to any tunnelling project. An unexpected event, such as a major cavity, a rock fall, or simply ground conditions that do not meet the original design criteria, can throw a project severely off its programme. Tunnel boring machines (TBMs) are carefully designed to suit the type of ground they are expected to encounter.

Careful project consideration

Haimoni urged clients to carry out a thorough analysis of the ground before embarking on construction. “Make the unforeseen foreseen,” he said. “Consider everything possible regarding the ground and make sure that you and your machine are ready and prepared. This is the cheapest way to do it. A bit of good thinking and expenditure early on can prevent a lot of hassle later and ensure success.”

The expertise on the Dubai Metro project came from a range of sources, including the RTA itself, lead designer Atkins from the UK, and design and build contractor Dubai Rail Link Consortium, made up of five firms led by Japan’s Mitsubishi Heavy Industries. Fellow Japanese firms Mitsubishi Corporation, Obayashi and Kajima Corporation were part of the team, as was Turkey’s Yap Merkezi. 

Tunnelling statistics
Total bore length (metres)12.6
Machine typeEPB machine
Internal diameter (metres)8.50
External diameter (metres)9.3
TBM diameter (metres)9.56
Cutter head rotation (rpm)0.86-1.82
Average rate8 rings (12m) a day
Weight (tonnes)940
Length (metres)82
EPBM=Earth pressure balance; TBM=Tunnel boring machine; rpm=Rotations a minute. Source: RTA

Design and build is the dominant contract arrangement for tunnelling schemes. The contractor carries most of the project risk, but is also given the freedom to manage the work. Unlike in a traditional turnkey contract, the contractor has control over the detailed design as well as construction.

Haimoni said it is still important for the client to remain involved in key decisions, which on the Dubai Metro required the approval of the client’s engineer and, ultimately, the client itself.

“I was there at every step to make sure critical decisions were made collectively,” he said. “We did not allow the contractor or consultants to take risky decisions on our behalf.”

Despite the close involvement of the client, Haimoni said the metro team never needed to reiterate contract terms to force decisions through. Communication, negotiation and mutual respect prevailed on the project.

In some cases, this meant compromise. The Dubai Metro team did not always agree on the construction methods to be used. In one case, there were differences of opinion over the most appropriate way to handle the TBM’s breakthrough into a station box at the end of its bore. This was one of the most difficult stages of the work and required careful planning to ensure that the sudden breakthrough of the machine was not accompanied by a flood of water or material that could cause a collapse.

The contractor’s preferred method was to carry out grouting ahead of the excavation to make sure there was no flow of water or materials into unprotected areas. This is the standard technique. But the client disagreed.

“I happen to have a lot of experience in grouting,” said Haimoni. “[After studying] the soil characteristics, I knew this method would not work. Testing then proved that the grouting would not be enough to stop the flow of water.”

As a compromise, the client allowed the contractor to use its preferred method of construction, but insisted that an alternative plan be put in place ready to be immediately employed if the grouting proved insufficient.

Changing construction methods half way through an activity can mean months of delay, but this prior preparation meant the Dubai Metro team could move straight into the alternative technique.

The contractors filled the station box with water so that when the TBM broke through the wall there was no pressure difference between the two excavations and therefore no risk of collapse.

“Because the contractor was ready, it could do the task quickly and break through the wall underwater, and that was done successfully without any problems,” said Haimoni. “If we had stuck with the grouting, then, after testing, the contractor would have been re-grouting, which might not have worked. That takes time and cost and there is a risk of collapse, which would have delayed the project as well.”

Another difference of opinion occurred regarding the face pressure at the head of the TBM. In order to accelerate progress in some areas, the contractor felt it was safe to reduce the pressure. But Atkins and the client insisted it be kept at a higher level to control ground movement and protect buildings.

“You can move faster at a lower pressure, but then there is more risk of settlement,” said Haimoni. “We kept a consistent pressure and had virtually no settlement – less than 5 millimetres throughout – which is practically a new record.”

Analysing tunnelling performance

When work began, the settlement was about 20mm. In anticipation of this, the tunnelling was started in a quiet area so the processes could be refined and performance improved. The first section of a tunnel bore presents an opportunity to modify techniques as the team compares performances and ground conditions to what had been predicted. “Don’t have your learning curve under the most important building [on the route],” said Haimoni.

He offered a last word of advice: “The most important lesson is that knowledge gained should be recycled and used to its full extent. The tunnelling work here was relatively easy because everyone recycled information from other tunnelling jobs. Spend money on experienced people who can take decisions and understand the implications.”

Clients should form a close relationship with the project team and bring in key stakeholders early on in the scheme to ensure their support. “We watched and analysed everything on a daily basis,” Haimoni said. “I would recommend that for everyone. Keep watching and stay informed.”

Key tunnelling lessons

  • Invest time, money and effort in planning
  • Carry out extensive ground investigations to enable a robust TBM design
  • Ensure tunnellers and contractors are involved in risk management
  • Use experienced teams
  • Recycle knowledge
  • Engage stakeholders from the start

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