A contiguous sectional flight auger pile wall has replaced a failing sheet pile wall to secure the future of a mainline railway embankment on the east coast of Yorkshire.
The historic Hessle Foreshore, situated in front of the Humber Bridge Country Park on the River Humber in east Yorkshire, has long been used for a variety of leisure activities. It is also a popular destination for walkers, marking the beginning of the Yorkshire Wolds Way National Trail, a long distance walk from Hessle to Filey.
The foreshore forms a natural walkway, separating the Humber Estuary from the main Hull to Leeds railway line, which runs on embankment on this stretch. Unfortunately, tides and poor ground conditions had caused a sheet pile retaining wall supporting the embankment to start to fail. A new means of support was needed to prevent movement of the embankment, which may have led to the closure of the railway.
The solution was a 100m long contiguous pile wall, comprising 143, 600mm diameter sectional flight auger piles to a depth of 15m. This was installed by foundation contractor Roger Bullivant, working for main contractor Construction Marine. The piles were designed by Construction Marine and specified a full depth cage of nine H40 bars in the bottom 10m of the pile and nine H20 bars in the top 5m.
“Contiguous piled walls offer a technically efficient and cost-effective temporary or permanent means of retaining earth structures even in water bearing strata,” explains Roger Bullivant director John Patch.
However, there were some initial problems with the piling, caused by water pressure and voids continuing to develop behind the sheet pile wall. “The wall continued to fail towards the estuary, compromising the consistency of the materials along the path of the proposed pile line,” Patch says.
“Because of these issues, the piling platform was tending to collapse into the open pile bores, which in turn made the platform very unstable. To overcome this, 1m lengths of 660mm diameter temporary steel casing were used to ensure a safe method of work and also guarantee the integrity of the piles.”
After consulting with Construction Marine, it was decided to excavate deeper in some areas and backfill with compacted clay, allowing Roger Bullivant to complete the project without the need for temporary casing, which sped up work.
But further problems arose during cage installation, Patch adds: “The cages were installed in three separate sections and spliced together as the cage was installed. The original plan was to use a 360° excavator to help push the cages to depth.” However, two issues quickly arose.
“First, sand in the bottom 6m of the pile bore meant the freshly placed concrete in this zone was ‘flash setting’ [going off too quickly] and therefore prohibiting the cage installation.
“Second, due to the cage being less robust at the top than at the pile toe, even a small amount of pushing by the excavator bucket was causing the top section of the cage to buckle. Once this happened, we couldn’t push the cage any further,” Patch says.
As a result, several different concrete mixes were trialled on site before a suitable mix was found that would allow successful cage installation to the design depth. But in some areas of the site the cage still required additional force from pushing with the excavator bucket.
“We also trialled full length H40 cages, which solved this problem,” Patch says. “However, as this change had huge financial implications for Construction Marine it was decided to revert back to the original detail and strengthen by adding two additional H40 bars, which worked.”
After overcoming these challenges, work was successfully completed, without any disruption to rail services, Patch says.