Going to ground
July 2012
Balfour Beatty Ground Engineering has been working on the foundations of the extended Blackfriars station with up to 300,000 passengers passing through the work site each day
Passengers taking Thameslink trains through central London are already benefiting from the rebuild of Blackfriars station. The simultaneous construction of two new entrances, four platforms on top of a Victorian rail bridge, and a new Underground station make Blackfriars one of Network Rail’s most challenging projects to date.
Blackfriars is being rebuilt as part of the Thameslink programme, which will deliver longer trains, more seats and more frequent services between Bedford and Brighton through Central London. Once the construction is complete, Blackfriars will be the first ever station to span the River Thames. Two new entrances have been built, north and south of the river, platforms have been rebuilt on top of a Victorian rail bridge, and a replacement London Underground station has been constructed. All of this work has been completed, with some 300,000 people moving through the worksite every day, while underground and overground trains continued to run through Blackfriars and traffic used the river and roads below.
While the glimmering solar panel roof and platform views of the River Thames may steal the limelight above the ground, the complicated geotechnical works hidden beneath the ground deserve an equal dose of praise. Balfour Beatty Ground Engineering (BBGE), the specialist geotechnical division of Balfour Beatty, was appointed to construct the foundations for this prestigious project in central London.
Following demolition of existing buildings, a number of challenges had to be overcome, including working adjacent to a live railway line above the live London Underground tracks, and in small constricted spaces. The multiple railway lines located on the northern end of Blackfriars Bridge meant that access was very difficult, during this period a series of works was in place to maintain passenger flows and minimise disruption to the public.
Completed in two phases (northern and southern), the project comprised more than 700 large diameter secant wall piles and bearing piles ranging from 600mm to 1000mm in diameter and up to 39m deep. A number of these piles were installed in less than six metres of headroom. In addition to this, self drilled micro piles of 187mm diameter were installed to a depth of 18 metres, some within only three metres of headroom.
At the northern end of the site a track protection structure (TPS) and a temporary King Post Wall was installed by BBGE and placed above the London Underground tracks to ensure there was no disruption to existing services and to create a safe working environment. The installation of the King Post Wall allowed the piling platform to be raised to a single level, thereby providing the required safe working space for the large piling rigs without any undue load being exerted on the TPS. Upon completion of these structures, the main piling works commenced from several restricted areas working around the TPS. The south station was built in the space originally occupied by three brick viaducts that supported the railway lines going onto the bridge. The viaducts ultimately had to be demolished, as they could not accommodate the additional forces generated by the longer, heavier trains braking as they approached the station. Reinforced concrete boxes were constructed to support the railway lines and to house the new station.
Urban developments
By its very nature, any development in central London will provide construction teams with a range of challenges. The repeated urban development over several centuries has left a legacy of different foundation solutions buried beneath the ground. The proximity to the river Thames provided further complications, with the river bank having been developed many times throughout the course of history.
During the contract, a number of different obstructions were encountered, these are classed as ‘something that materially affects the progress of pile boring operations that could not have reasonably been foreseen at the tender stage or from the contract documentation’. Obstructions at the Blackfriars site consisted of old brickwork, reinforced concrete, existing concrete piles and existing timber piles, the latter most likely forming part of the original riverside.
Although BBGE retains an extensive range of specialist piling equipment for coring, this particular project required bespoke solutions and to mitigate any potential construction delays, BBGE relocated an equipment technician to be permanently based at Blackfriars. Operations manager Jon Chevin explains: ‘When delivering projects of this nature, meeting key programme dates is essential. We were aware from previous experience that unforeseen obstructions can lead to significant delays, so by having our own equipment technician on site we were able to engineer bespoke tools to overcome obstructions, as and when we required them.’
While this approach ensured that the majority of obstructions were successfully overcome, there were several obstructions that simply could not be cored through. In one case, the team discovered foundations from an earlier railway. Short lengths of railway line had been used as reinforcement in the foundations and despite the best efforts of the engineers it could not be cored through. The solution was to sink shafts to allow miners access to remove the previous foundations and allowing the piling to continue.
Due to these unforeseen obstructions, BBGE took further action in order to mitigate delays to the overall programme. A pattern of double shift working was introduced and carried out in accordance with Network Rail regulations and with approval from the relevant authorities. This was achieved through careful selection of equipment and techniques while maintaining strict control on the environmental impact. These measures brought about the additional benefit of unrestricted engineering hours for the installation of the piles.
Collaborative partnerships
Close working relationships were always going to be integral to the success of this project and none more so during the early development stage.
Dave Mellors, piling director at BBGE, says: ‘We spent approximately 12 months developing the solution to ensure every potential option was reviewed. When working on a project of this nature, it’s very costly if you get it wrong and, therefore, it was important that we developed the scheme correctly. Working in partnership with Network Rail and Balfour Beatty allowed all parties to input their concerns, ideas and proposed solutions, leading to an overall solution which met the required criteria.’
This collaborative approach continued throughout the project with weekly planning and daily logistics meetings and regular project reviews occurring to ensure any issues were rectified before they impacted on the programme. The close working relationship between BBGE, the integrated Balfour Beatty team, Network Rail and the surrounding stakeholders enabled this project to be delivered successfully leaving a lasting legacy for the people of London.
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