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Probing into the heart of London

Network Rail has ambitious plans to transform London Bridge station into a state of the art interchange but before work could get underway, a major ground investigation was called for. Claire Symes reports.

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Construction of the glass edifice that is the Shard at London Bridge station means that many commuters’ eyes have been drawn skyward for the last year or so. But even if they had kept their eyes on the ground, they might have missed signs that a £4.7M ground investigation was underway.

Over the past two years, Soil Engineering has been carrying out a phased ground and structural investigation out of public view and during night time possessions so station operation was not disrupted.

Nonetheless, the travelling public will soon start to see the results of the investigation as work to transform London Bridge begins. When the work is complete in 2018, the station will be capable of handling 90M passengers a year - 35M more than its current capacity.

According to Network Rail, the plans are part of the second stage of the Thameslink programme to unlock capacity on London’s rail network, and allow more services to travel to and through the capital.

The aim is that by changing the way the station works, a critical railway bottleneck will be removed while creating better pedestrian connectivity between Tooley Street and St Thomas Street.

London Bridge Station was built in 17 phases between the 19th and 20th century around a central viaduct spine, which was gradually widened through the addition of other arches as the station grew.

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Each phase of development resulted in modifications to the brick arch viaducts which carry the rail lines above street level, so an in-depth ground investigation was essential in order to be sure the plans would work. Nonetheless, Soil Engineering’s involvement in the scheme started off as a much smaller role.

“The work on the project started with a £500,000 contract to carry out structural and geotechnical investigations at London Bridge Station in June 2010,” says Soil Engineering site agent Andy Melling. “The aim was to provide information for the upgrade of the station, which will result in reconfiguration of the through platforms to increase the number to nine, in addition to the six terminus platforms.

“The work focused on establishing the ground conditions, presence of groundwater and the strength and type of the existing foundations.”

This first stage of the work primarily used trial pitting techniques to expose the footings and look at arch connections, as well as carry out investigations at track level. This phase was completed in March 2011 but Network Rail added to the scope of the project, which has resulted in a total of five phases of work being carried out by Soil Engineering.

The second phase expanded the work carried out in the initial phase. The third phase added investigation of five bridge structures. Phase four saw archaeological investigations added to Soil Engineering’s brief and the final fifth phase covered installation and testing of geothermal technology.

The total package of work ended up being £4.7M and took Soil Engineering into structural and archaeological work.

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While the main challenge was to keep the station operational throughout the work, the site team also faced a number of logistical issues. Locating utilities was a major undertaking and the team found a number of unrecorded, disused sewers during the work.

In addition, the Jubilee Line runs directly below and follows the alignment of the station’s main viaduct. The tunnels are located 25m below ground and with some of the boreholes in phase one and two going to 45m depth, careful location was vital.

Part way through phase one the focus changed and led to Soil Engineering working in a live rail environment. The company drew on expertise from its parent company Vinci to assist with designing the temporary works called for by this part of the work.

“Network Rail wanted information about the platform and trackbed construction, which called for trial pitting to be carried out at track level to a depth of 4m,” says Melling. “This would have involved a long track possession and would have been challenging, so we suggested building a platform over the track that could be accessed from the platform from which we could operate a Terrier window sampling rig.”

The result was a platform built from polystyrene that could quickly and easily be lifted into position by hand. The temporary platform allowed the rig to be driven directly from the station platform onto the temporary structure.

“Working on the scheme has really sharpened our knowledge about how to get the best out of trial pits and directional drilling”

According to Melling, this was an important element to the solution as the suspended platform was not considered strong enough to support a crane capable of lifting the rig down to track level.

Soil Engineering hopes to put the platform design to use on other site investigation contracts but the company also believes that there is a potential market for it to be used to extend platforms.

Some of the trial pits excavated by Soil Engineering have been up to 6.65m deep but in some areas the depth of the pits was reduced by use of other techniques.

“The original plan was for all pits to expose the underside of the foundations but instead we proposed an alternative of digging to the top of the footing and then installing tubes and reinstating the pit,” says Melling. “The tubes were then used to drill concrete cores to determine the foundation thickness at a later date.

“This approach meant that we were able to minimise disruption to station operation and was also quicker and safer for the site team. Excavation of the trial pits could have taken up to 10 days and another 10 days to carry out the probing work, whereas the angled coring could be carried out in one shift.”

Soil Engineering only had a three to four hour slot each night to excavate and reinstate pits in the public areas of the station so, even with an alternative approach agreed, a temporary reinstatement method was needed.

The solution was to construct a frame with a manhole cover that could be put in place each morning to allow passengers to travel round the station unimpeded but which could be removed the following night to enable work to restart quickly.

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“Working on the scheme has really sharpened our knowledge about how to get the best out of trial pits and directional drilling,” says Melling.

In addition to adding structural, archaeological and rail environment working to Soil Engineering’s skill set, the project has also taken the company into the field of demolition.

“We have had to clear some areas under the arches to enable archaeological investigation to be carried out in areas where new piles will need to be installed to support new viaducts,” says Melling.

Before the station was built, the site was covered by housing dating back to the Medieval period. But in earlier times, the site was closer to the banks of the Thames and archaeologists believe the area to have been formed by a number of islands and marshes.

There was also the possibility that a wharf built in the area during the Roman occupation of Britain could be uncovered.

Work on the geothermal testing called for use of a low headroom rig to drill to 150m in order to test the concept. This testing presented considerable challenges in the form of providing sufficient ventilation for the work in addition to the handling issues associated with drilling to 150m using 150mm diameter, 1.5m long drill strings.

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The bore had to be fully cased after flushing fluid disappeared while the rig drilled through the fractured chalk. With the test bore completed, the geothermal testing was carried out over 48 hours and proved that the concept was feasible.

But to provide the heating and cooling needed by the redeveloped station, around 100 boreholes would be needed.

According to Melling, the way the work has been delivered at London Bridge is very dynamic, with Soil Engineering reacting to Network Rail’s changing requirements and building on the knowledge gained in earlier phases of the work.

At peak there were 50 people from Soil Engineering on the project and the boring carried out with a mix of day and night working, as well as with track possessions.

With information from the ground investigation secured, work is now underway to use this for design before work starts on site.

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