When working on a construction project of any size, where it is immediately adjacent to Network Rail (NR) or London Underground Ltd. (LUL) property, it is important to consider the requirement for detailed discussion with Network Rail's Asset Protection and Optimisation (ASPRO) team, Tube Lines or Transport for London.
Our chosen case study for today is the Camden Lock Village project, which on first glance may appear to be relatively straight forward, but as with many projects, this was far from the case.
The Camden site in London, was split into four distinctive areas, by two 150 year-old Network Rail viaducts. These were supported on shallow foundations and crossed an old redundant river bed. The site had a fragile wall on the Regent's Canal to the South, which exerted ground pressure in the opposite direction of the viaduct. In addition, LUL's Northern Line underground tunnels run under Kentish Town Road to the East and Chalk Farm Road to the West. To make the task a little more complex still, the site contained National Grid high voltage equipment and the plan was to excavate a 16 metre deep basement between the two viaducts.
These works required a plethora of detailed meetings and submissions to Network Rail and other stake holders which were critical to the continued progression of the project. Should Network Rail have been unhappy, at any point they had the power to suspend all works on the project.
The images below show the process of forming this cavernous basement, which extended 16 metres deep below the railway lines, whilst also concurrently excavating basements on the other side of the viaduct, but ensuring no disruption to the busy passenger and freight trains which needed to run above unhindered.
As the viaducts had to be limited to a near zero margin of movement and settlement, a vast array of monitoring equipment was placed to monitor the Network Rail assets in real time. This included 6 total stations, 42 inclinometers, 20 wireless tiltmeters, 14 vibration monitoring units and a team of geotechnical experts, engineers and technicians. In addition, due to the potential for surcharge affecting the underground tunnels, a series of tunnel deformation equipment was installed with the approval of London Underground. The data was constantly monitored to track every vibration or movement of the structures. Utilising live CCTV, movements made by passing trains were identified and recorded, which always ensured our construction tolerances were impacting the viaducts less than the train movements.
The agreed sequence for Building C was to install 292 no. 750mm diameter contiguous piles to form the perimeter of the basement box. Internally there were 52 plunge columns encased in a 1,800mm diameter foundation plus a further 34 bearing piles each of 750mm diameter. The piles extended up to 36 metres into the ground.
Prior to basement excavation, the ground floor slab was installed first to provide the lateral support for the viaducts. The 263 metre long capping beam was so deep in some sections that it required sheet piles to support the first phase of the capping beam excavation. The capping beam was formed to incorporate 133 tons of reinforcement, prior to 600 cubic metres of concrete being poured. The plunge columns were interconnected with 376 tons of steel stanchions, prior to decking a 3,400 square metre ground floor slab with a further 450 tons of reinforcement and 1,800 cubic metres of concrete.
Once cured the required top down excavation of 58,250 cubic metres of hard London clay, through mole holes formed in the ground floor slab took a number of months due to the constrained logistical access points.
Simultaneously whilst the deep excavation of Building C progressed, two further large open excavations were completed to Buildings A1 and D/E, between the Southern Viaduct and the Regent's Canal. With 190 linear metres of sheet piling and 180 bearing piles Building A1 had a large span that required the excavation of 11,500 cubic metres of London clay. The phase 1 propping was later replaced with raking propping and a second layer of props in the deeper locations to support the Network Rail assets. Building D/E also progressed with a further 8,250 cubic metres of clay being excavated whilst the basement was also propped.
As the superstructure progressed, the basements were excavated, cast and lined with waterproofing, which then lead to the next tricky task of running services to the buildings, under each viaduct from the Energy Centre in the middle of the development. The services included, foul drainage, surface water drainage, high voltage electricity serving the sub-stations, low voltage power, gas, lighting, district heating, district cooling, sprinkler pipework, water, data cabling for security and communications and telephone.
Due to the constrained routes, the co-ordination and routing of services was critical. However, due to the depth of the installation of many services, special measures such as sheet piling, trench boxes and limited excavation lengths were required. The trickiest element of the installation was where services crossed from one building to another beneath the viaduct arches. In these instances restricted headroom piling rigs installed mini piles to form capping beams through the arches. Once the piles and capping beams had reached strength they were propped and trenches excavated for the installation of services, thus negating the need for directional drilling, but still maintaining integrity for the 150 year old viaducts.
To complete the scheme, 30 viaduct arches were waterproofed, lined and received mezzanine floors and glazing to provide retail spaces.
The final fit out of the shopping centre is currently completing and once the development opens to the public, the vast majority of the projected 80,000 visitors each week will be blissfully unaware of the engineering effort required to construct the scheme.
The team from Chelsea Construction Consultancy provided Quantity Surveying and Project Management throughout the five year project duration. If you require any Pre-Construction, Estimating, Development Management, Commercial, Rail or other assistance, please don't hesitate to contact us.
Whether you need our assistance or that of the team of in excess of 100 contractors that worked on the project, we're happy to help.