HS2 has seen another major milestone as the final 292 concrete piles have been installed this week, to form the foundations for the UK’s longest railway bridge at the site of the Colne Valley Viaduct, with the success acting as the proverbial cherry on the first phase of the record-breaking project’s cake.
The construction process for these foundations has run concurrently with separate teams working from north to south, with the deck assembly beginning last year, seeing more than 500 metres of the viaduct structure successfully completed.
Before these works could begin, an extensive programme of test piling was completed with engineers sinking 12 piles at two locations with geological and structural data from these tests fed back into the design of the viaduct. This testing enabled a 10-15% reduction in the depth of the piles, accompanying time and cost savings.
Upon delivery of the of the overall project, the 2.1-mile bridge will be entrusted to carry the new rail line safely across a series of lakes and waterways near Hillingdon on the north-west outskirts of London.
To accommodate the delivery of the 66 piles in the lakebed, the engineering team had to first construct a kilometer of temporary jetties, with cofferdams to hold back the water around each set of foundations. The jetties are also available for use to transport materials and equipment during the construction of the piers and viaduct deck, thus decarbonising the process through the removal of unnecessary vehicular road usage.
HS2 Ltd.’s Project Client, David Emms said:
“Once complete, HS2 will transform journeys across the UK, help to boost the economy and support the UK’s transition to net zero.
“The completion of the piling for the Colne Valley Viaduct is a major achievement for our whole team and marks the end of almost two years of hard work. With pier construction and deck assembly also well underway, the viaduct is fast becoming one of the most impressive and recognisable parts of the project.”
Atop of each pile group, a concrete pile cap will be used to support the pier, which will in turn support the sizeable weight of the mammoth bridge structure above. The process of pile installation was adjusted from the usual process of hammering them into the ground, instead the holes were bored and backfilled to create the piles.
The viaducts main deck is currently being built via an innovative modular design process, seeing 1,000 separate pieces assembled at a factory nearby to the construction site. The assembly process sees a bridge-building machine lift each piece into the position, before shifting itself forward to the next pier.
The design has been inspired by the flight of a stone skipping across the water, with the infrastructure being set low into the landscape, the widest spans reserved for where the viaduct crosses the lakes, accompanied by narrower spans for the approaches. The elegant design was chosen as it will enable views across the landscape, whilst minimising the viaduct’s footprint on the lakes and help complement the natural surroundings.
To ensure the working methods are conducted in a sustainable way that protects the natural ecosystems collated within the watery depths, HS2 worked closely with Affinity Water and the Environment Agency to monitor water quality and agree working methods. These were subsequently monitored by a team of specialist engineers to ensure minimal disruption to the natural environment.
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