Ground Engineering Associate and Job Leader Jonathan Rowe explains how Bramley-Moore Dock will be infilled in preparation for the construction of our new stadium.
‘Nil satis nisi optimum’ - ‘Nothing but the best is good enough’ - is perfectly reflected in the engineering solutions that have overcome the challenges to infill Bramley-Moore Dock.
The Grade II-listed dock is approximately 10m deep and filled with 8m of water. At the bottom of the dock basin there is up to 3m of soft, blancmange-like silt that has built up as a result of it being an operational dock.
To dredge or not to dredge?
Typically, before infilling within a marine environment, the dock would be dredged in order to remove the soft silt. However, our team asked the question, ‘Would it be possible to keep the soft silt in place and infill on top?’. Not only would this reduce the overall construction programme, but there is a huge sustainability benefit, something the Club had stressed was an important factor. By not having to dispose of the silt, we subsequently reduced the total import volume by 75,000m3 – enough to fill 30 Olympic-sized swimming pools.
The next step was to select and test the infill material. In this instance, it was sand dredged from Liverpool Bay and the Irish Sea, about 25 miles north of Bramley-Moore Dock. Samples were prepared in a laboratory and this allowed us to make predictions about how the sand would perform at different depths.
But what next?
Calculations were complete, and the analysis had been undertaken - surely the sand could now get pumped in? Not quite. A carefully planned sequence of works was needed and this was successfully done through close collaboration between the Club, the divers from a specialist marine contractor, Laing O’Rourke and Buro Happold.
1.0 Dock Raking
To minimise disruption to the piled foundations, any metallic objects and obstructions within the soft silt had to be identified and recovered. Once we knew what was down there, a giant rake dragged from the back of a purpose-built boat moved objects towards the east wharf. The recovered objects were then lifted ashore, documented, and appropriately disposed of.
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WATCH: BRAMLEY-MOORE DOCK INFILL BEGINS
2.0 Risk Mitigation Survey
Next, a remote operated vehicle was deployed to the bottom of the dock. This was part of a Risk Mitigation Survey to identify any remaining metallic objects that could cause harm, such as the Second World War anti-aircraft shells that were found, expertly discharged and disposed of. Finding such devices is common in docklands developments across western Europe and by dealing with these devices early in the process it means the project risk is reduced from a health and safety and programme perspective.
3.0 Marine Life Protection
To protect marine life before the infilling, fish were removed and new habitats and rafts for sea birds were created. To ensure the fish did not re-enter Bramley-Moore Dock, a silt curtain was constructed at the entrance to Sandon Half-Tide Dock to the north.
4.0 Dock Wall Protection
Remedial works to the Grade II listed dock walls (above and below the dock water level) were undertaken to preserve their condition. This was an imperative step for the proposed development as it protected the dock walls for future generations.
5.0 Temporary Dock Isolation
A temporary dock isolation structure was required in the north of Bramley-Moore Dock to retain the dock infill and to ensure the sand remained within it. The design went through many iterations and the final result was a sustainable underwater prism constructed from quarry obtained stone. This solution had the benefit that the temporary isolation structure could be modified to the required angle and size of the permanent structure, with additional considerations added for scour and wave protection.
6.0 Dock Infill
Measuring approximately 325m long and 125m wide, Bramley-Moore Dock requires more than 450,000m3 of sand to infill it, taking into consideration ongoing settlement and bulking factors.
To do this, sand dredged from Liverpool Bay and the Irish Sea is transferred via a trailer dredger. The dredger then gets moored approximately 300m away from Bramley-Moore Dock in the River Mersey with sufficient water depth allowing for the operation of discharging the sand to not be affected by the tide.
Once moored, the trailer dredger is connected to a discharge pipeline network that is floating on the surface of the shore. The dredged material is fluidised in a hopper, and then hydraulically pumped via the pipeline over the River Mersey wall to a spreader pontoon in Bramley-Moore Dock.
Finally, the sand is hydraulically placed into the dock via the spreader pontoon and the water is displaced into a neighbouring dock. The process continues until the reclamation of land is above the dock water level.
7.0 Compaction, monitoring and evaluation
To ensure that the settlement of the dock infill is controlled and measurable, rapid dynamic compaction will compact the upper six metres of the infilled sand by dropping a 16 tonne weight at a frequency of 60 times per minute.
A land roller compactor completes this process.
We then validate that the right densities have been achieved with penetration tests from the very top of the newly compacted sand to the bottom of the basin.
As a result, the detailed construction and operation programme will be reviewed to establish how, when and what influence the infilling has had on the proposed construction process. It also determines the return periods of relaying the pavements, the hard standing and the pitch surface, all of which are ground bearing.
So, what have we learnt?
It is clear that without a detailed understanding of the site, obtained by reviewing historical information, interpreting survey results, and undertaking detailed design, we could not lead on the innovative engineering solutions. This has very much been a team effort by everyone involved.
I have thoroughly enjoyed being part of the Everton new stadium team on behalf of Buro Happold. We have worked with some incredibly talented people and led the infill design from concept through to construction. I am excited to see the project progress through all the different phases. Once completed, the Buro Happold ground engineering team will hand over the dock to our expert structural engineers, which will include the next exciting stage of seeing the stadium come out of the ground.
I hope you have enjoyed reading my update and you have a bit more knowledge on how Bramley-Moore Dock is being infilled in preparation for Everton’s stunning new home.