The recently completed infilling of Liverpool’s Bramley-Moore Dock marks a significant milestone in enabling works for Everton Football Club’s new stadium.
Liverpool’s Bramley-Moore Dock on the River Mersey opened in 1848 and once served the largest steamships of the period. It is Grade II listed and the largest of five in the Stanley Dock complex. Its brick hydraulic accumulator tower, which provided power to the dock gates and lifting equipment, still stands, retaining the dock’s connection to Liverpool’s industrial past.
Today, Everton Football Club, along with its main contractor Laing O’Rourke, is giving the disused historic site a new lease of life by building a 52,888 capacity stadium on top of the old dock. The stadium is the largest single-site private sector development under construction in the UK.
Since Liverpool City Council gave planning permission for the stadium in late February 2021, the club has taken control of the site and the stadium’s foundations have begun to take shape.
Ground was broken on 10 August, with enabling works beginning soon after. In October, Laing O’Rourke started the process of infilling the vast dock to provide a solid base for the stadium and began installing the first of 2,500 concrete piles across the site.
In late December – more than a week ahead of schedule – the infilling of the 10m deep, 325m long and 125m wide Bramley-Moore Dock with more than 450,000m3 of fluidised sand was completed.
This involved pumping sand dredged from Liverpool Bay and the Irish Sea into the dock via a pipeline connected to a dredging vessel called the Shoalway, moored in the Mersey.
Laing O’Rourke principal engineer Andy Baynton says having the sand in a fluid state so it could be pumped through the pipeline was crucial to the process. But the sand extracted from the dredge zone quickly lost water on its journey to the dock. As a result Laing O’Rourke “extracted water from the Mersey under an Environmental Agency licence and fluidised the sand within the hopper of the dredging vessel via a high-pressure water jet”, says Baynton.
For the first part of the filling process the pipeline was connected to a spreader pontoon with a dissipation bar which could be lowered to the bottom of the dock, where it dispersed the sand/water mixture. Specialist dredging and offshore contractor Boskalis Westminster was employed to deliver the pumping and infilling works.
Laing O’Rourke civil and geotechnical engineering lead Keith Miller says that using a spreader pontoon ensured a very even sand coverage across the full area of the dock. “By using this method of spreading you avoid creating any mud waves and causing instability of the dock silt,” he adds. This is because “the sand fill was being placed in thin layers regularly across the dock base.”
The sand pumped in from the dredger was built up until it broke through the surface of the dock water – this happened first at the western end of the dock in mid-November. By December this sand front had progressed to the east side of the dock and finally displaced all the water.
Now, the sand is being topped up with the compaction process expected to be finished by the end of January.
The ongoing rapid dynamic compaction process is being carried out using a 94t excavator, says Baynton. It has a specially designed 16t vertical hammer attachment with a 2m diameter foot at the base of the hammer.
The excavator’s hammer compacts each point at a rate of approximately 40 blows per minute to create 2m diameter craters 3m apart. The compaction rig follows a predetermined square grid that is uploaded to its onboard computer.
Having hammered a grid of craters, Laing O’Rourke then tests the sand with cone penetration testing (CPT) to determine whether a second pass is needed to achieve the required densities. Baynton says this will likely be the case for areas adjacent to the dock walls “because of the careful way the infilling has been carried out adjacent to these listed assets”.
The spreader pontoon manoeuvring about the dock during sand discharge
Piling work began in October 2021 and is expected to continue until summer 2022. This has so far only taken place along the north and south wharves, while piling for the infilled dock and foundation works to the western stepped terrace leading down to the River Mersey has yet to begin.
Of a total of 2,500 piles that will be drilled across the site, 700 individual 14m to 16m deep supporting piles have so far been installed in the northern and southern wharves. Each pile is 600mm in diameter and uses between 4m3 and 6m3 of concrete.
The piling process involves drilling down to around 16m with a hollow drill. Concrete is then pumped to the bottom through the drill’s stem to replace the bore with a column of concrete. A reinforcement cage is then placed inside the bore.
The piling layout has been designed to avoid the dock walls and prevent damage to the listed assets. The idea is that if Everton FC decides to move stadium one day, the site could be reverse engineered back into a dock.
Heritage assets such as cobbles, capstans, mooring posts and railway lines have also been removed from the north and south wharves and where possible are being preserved – these will eventually return to site as part of Everton FC’s landscaping and public realm works.
Once the compaction process is complete and testing has been carried out, piling in the infilled dock area will start with Laing O’Rourke concentrating on the west end first. Each of these piles will eventually be capped to form the substructure for the stadium.
Across the site Laing O’Rourke has chosen to use continuous flight auger (CFA) piles over rotary piles. As Miller explains, the latter would involve having to case all the way down to the sandstone and add “considerable costs and an awful lot of additional time to the programme”.
However, he says the big issue with CFA piling in the infilled basin – where ground conditions are characterised by soft clays with a hard bearing stratum below – is “over-flighting”, where the auger is rotated too quickly, and more soil is extracted than concrete pumped in to replace it.
In this situation, explains Miller, “you’re not achieving sufficient penetration into the sandstone below the soft clays in relation to the rate of auger rotation. This results in entraining the soft clays and then the sand above it into the pile bore when you’re extracting or installing the auger, leading to ground instability.”
“That’s a matter which needs careful consideration and will require very great expertise from our piling operators when they come to install the piles through the sandfill and the dock silt into the sandstone below,” he adds.
Drone shot showing the scale of the compaction process
The greatest challenge geotechnically, says Miller, was created by the ground conditions themselves – specifically the dock silt beneath the sand infill.
“You’ve got about 9m of sand infill that we’ve placed,” he explains, “and then beneath you’ve got about 1m or so of dock silt – a highly compressible material – and then you’ve got sandstone bedrock.”
While the sand is a uniform material, the dock silt “is just there – you can’t do anything with it other than wait for it to settle itself”, he adds.
Excavating the silt, which Miller says is “contaminated as a consequence of the industrial usage of the do ck over its operational life”, would have been costly and time consuming.
So the challenge for Laing O’Rourke’s team has been working out the infilling programme to fit in with the consolidation of the dock silt. Determining the shear strength of the dock silt is crucial to this and will be key to determining when piling operations in the infilled dock can be carried out, explains Miller.
Expanded Geotechnical, which has carried out similar work in infilled docks, also provided advice to the project team on the minimum shear strength the dock silts need to achieve before they can safely carry out their piling operations.
The team was able to mitigate significant risks associated with the dock’s ground conditions by carrying out two years of extensive planning under a preconstruction services agreement before the start of the enabling works. This included extensive surveying work and time spent considering the ground conditions “in great detail”.
Image looking north west showing progression of the sand fill from west to east while piling and substructure works continue on the north and south wharves
Sand infill and dock silt monitoring will continue right through the project. The contractor has already carried out some piezocone penetration (CPTu) tests through the fill and the deposits to determine if “the correct relative densities have been achieved and the shear strength of the deposit”, says Miller.
As well as measuring the strength of the dock silt, Miller says Laing O’Rourke is “also doing porewater pressure dissipation tests with the CPTu, the purpose of which is to give an indication of how much primary consolidation has occurred with the dock silt”.
“That aspect will have major ramifications on, for instance, deciding what drainage detail we’re going to install.”
Dock silt monitoring and testing then has to be tied into surface settlement monitoring, 90% of which “is going to come from the dock silt – the sand fill itself isn’t going to settle much in the future”, says Miller.
After tests have been done to confirm the completion of the sand compacting process, the site will be reprofiled to make it ready for a working platform – a new 450mm to 600mm thick temporary roadway, made up of reclaimed materials, laid on the solid sand base. This will allow heavy machinery to begin the piling process for the stadium’s east and west elevations.
Following the reprofiling, a series of settlement monitoring points will be implemented across the dock fill that will determine when the rate of settlement is sufficiently low for the contractor to continue with follow on activities, such as drainage.
“We can cope with a working platform which is settling. We can simply top it up to the right level.
“But once you start installing assets within the ground which are going to settle with the ground such as the drainage then we can’t do anything about that.
“So, we’ve developed a suite of technical solutions to ensure that we get this bit right.”
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