Engineers had to tunnel within millimetres of bridge foundations for a busy interchange extension in Saudi Arabia’s capital - overseen by a design team based in England. Natalie Hardwick reports
In Saudi Arabia’s busy capital city of Riyadh, the King Abdullah Road is an essential transport artery. Increasing demand and the fact that the road is already running at full capacity meant that local transport agency, Al-Riyadh Development Authority (ADA), needed to take action.
So in October 2009 ADA commissioned a £186m expansion of the road.
The work, in two phases, will widen the road to cope with a dramatic increase in traffic from 190,000 vehicles daily to 520,000.
In the first phase this will include new carriageway underneath another major road which crosses at right angles on the King Fahd flyover.
To minimise repeated future disruption the widening also includes installation of an underground box beneath the new road sections for a planned light rail system which will go in later.
But that was easier said than done since the box must be installed exceptionally close to the flyover columns’ pad foundations.
It was also in a restricted space and with low headroom.
But making this much more difficult was the fact it had to be done while the six-lane road above remains live - the King Fahd bypass traffic is too important to be disrupted for possibly many months.
The work was awarded to contractor Saudi Oger, which appointed London-based Buro Happold as designer.
The consultancy firm selected experts from its different strands to create a design solution.
Brian Jackson, a designer in the ground engineering division of Buro Happold, says: “Our department and the bridge engineering department worked together to find a solution to the expansion that fitted well with the constraints of the structure and the ground. What we came up with was quite a radical solution.”
The project leader from Buro’s bridge engineering department was Dr Nasser Khawandi.
“The King Abdullah project is more complex than your average tunnelling job,” he says. “This is where you see structural and ground expertise at Buro Happold come together.”
Initially, a number of workshops were held between client, contractor, designers and developers to ensure that everyone was fully briefed on how Dr Khawandi and his team were to tackle the restraints.
“We developed the solution using a series of meetings so that everyone had a say in the decision making,” says Dr Khawandi. “We also needed to stress the importance of the risks to the clients, and of the mitigation measures we would employ.”
Ordinarily a railway tunnel could be built using an open excavation, approaching the flyover from both sides and joining in the middle.
However, the fusion of the tunnels beneath the King Abdullah caused more complex problems.
“The tunnel here was to be cut very close to the two existing shallow foundation piers of the flyover,” says Dr Khawandi. “We are talking 70mm on one side, and 135mm on the other. It is extremely tight.
“We created diagrams and graphs to illustrate how the excavation would commence in great detail to guide the site workers through each stage”
“Usually there is a traditional solution - that you use a continuous pile wall,” he adds. “But the problem we faced is that you can’t fit a piling rig underneath the flyover that is of the sufficient height and strength to drill through relatively strong rock.”
The limestone rock beneath the road was weathered, but still troublesome to penetrate.
“A small rig wouldn’t have the strength to drill the piles deep enough,” explains Dr Khawandi.
“It was too much of a risk. The contractor said ‘no - if we start drilling and can’t get the piles down to the right length, what will we do then?’”
The solution was to build the tunnel remotely.
Cut and cover tunnel sections were built offsite, comprising two, three or four reinforced concrete portals, in lengths ranging from 180m to 700m.
These were brought to the site, where the ground was to be excavated to a depth of 13m, and slotted in place.
The second part of the solution was to ensure that there was absolutely no disruption to the flyover or its pad foundations during the excavation process.
The King Abdullah flyover section of the road was built in the early 1980s using premade segments of two or three metres that were then assembled on site.
It is post-tensioned, which makes it sensitive to settlement.
As a result, it required careful analysis and close monitoring for even the most superficial movement during tunnelling work.
The structure was assessed using Lusas finite element software tailored for bridge projects, which uses 2D and 3D beam analysis to provide fast linear static analysis.
“We had to assess the stress condition of the bridge,” says Dr Khawandi.
“How much can it tolerate movement? And how can you predict how far it will settle once you begin work tunnelling?
The answer to these questions then drives what you are allowed to do with the excavation.”
Monitoring work began in November 2009.
“We prepared the foundations of the flyover by placing nine monitoring points on each side of the bridge’s piers,” says Dr Khawandi. “The lasers logged any kind of slight movement and fed the results directly back to the Buro office in England using the Lusas program.”
While one side of the tunnel lay 75mm from the flyover foundations, an extra 65mm measured on the other provided the team with the leeway to erect a small section of supportive piling.
“We found a window of opportunity and decided we could erect five piles down one side of the tunnel,” says Dr Khawandi. “Then we could apply post-tensioning here to counter the existing stress. When the piles were in place we could begin to excavate. We had a construction team in Saudi working on site. Brian and I were based in England, so were involved remotely.
“We created diagrams and graphs to illustrate how the excavation would commence in great detail to guide the site workers through each stage, and constantly received the laser results which showed in millimetres any settlement or displacement of the pier.
“Geologically we had layers of gypsum, some of which had dissolved, causing the rocks to collapse and creating a gravel limestone formation that is weak to moderate in strength”
“Onsite, workers were constantly vigilant. After tunnelling one metre, they would stop and monitor the ground movement using Plaxis 3D tunnel software to track any
deformation or instability,” adds Dr Khawandi.
“In the original workshops we had set a limit on the maximum amount of movement allowed in agreement with the ADA and Saudi Oger - a very low figure of 10mm. If the Plaxis reading showed that the movement had exceeded this, mitigation measures would be put in place. If not, the workers would continue to excavate down to another metre, and so on.”
Despite not having any interaction with the bridge, Dr Khawandi and his team were able to take advantage of the existing piers.
“We decided that the original bearings would remain intact, and we would maximise the use of the piers as they are sloped,” he explains.
A piled load transfer structure was introduced which hugged the bridge piers below the bearing shelf level.
“This created a wedge effect,” continues Dr Khawandi. “This means that if there was any settlement during excavation, the existing foundations would be grabbed by the wedge and load would be transferred to any installed piles to prevent any further movement.”
Approaching the project from a geotechnical perspective, Brian Jackson was faced with some separate issues.
“To work out how to negotiate around the pad foundations, it was essential that we identify the ground conditions,” he says.
“Geologically we had layers of gypsum, some of which had dissolved, causing the rocks to collapse and creating a gravel limestone formation that is weak to moderate in strength. We then took this into account during the monitoring process.”
Upon completion of the tunnelling work in February, results showed that recorded movements were well within the limits set during the original workshops.
Phase One of the work on the King Abdullah Road is now complete, and Phase Two is now set to expand a further stretch of the road by building additional bridges.
Successfully supervising the project over 3,000 miles away from Riyadh was one of the main achievements for Dr Khawandi.
“Monitoring the work from England via a live internet feed of data was challenging,” he admits.
“In addition, completing the work in Saudi Arabia itself is quite a hurdle as we’re dealing with a very different working culture to how projects like this work in the UK.
“For instance, the concept of using Construction Design and Management (CDM) regulations is entrenched in England. Wherever there is a risk assessment to be done, they are used naturally.
“But such regulations are practically unheard of in Saudi, so you need to clarify the importance to the contractors of doing this kind of risk assessment and keeping each part involved. The risks were shared so it is very important.
“This was far from a run-of-the-mill job,” concludes Dr Khawandi. “This project needed information being constantly fed back and forth. And most of all it needed the very best quality control possible.”
Facts and Figures
Developer: Al-Riyad Development Authority (ADA)
Contractor: Saudi Oger
Designer: Buro Happold Bridge Engineering Group
Value: SR698m (approximately £186m)
Total length: 5.2km
Start date: October 2009
Tunnel completion: February 2011