Innovative secant piling has cut time and cost during construction of a deep pump station shaft at a north London sewage treatment works. But it wasn’t without risk. David Hayward reports.
A decision to install a massive secant piled retaining wall to form the perimeter of a sewage treatment works’ inlet pumping station shaft has cut temporary works costs by 20%.
It has also slashed a valuable five weeks off the construction programme at the north London project.
What’s more, the challenging shaft design, resulting from the use of 29m long piles, has triggered further innovative construction solutions, saving yet more time and money.
“Our latest target was to capitalise on the 10-day Christmas shutdown through early completion of a crucial concrete ring beam support needed in the shaft,” explains Gavin Symonds, contracts manager for contractor J Murphy & Sons.
“By allowing the concrete to fully strengthen while we enjoyed the break, shaft excavation could restart on day one of our return to site.”
The construction of the new pump station and inlet works at Deephams Sewage Treatment Works in Edmonton will make the nearby River Lee cleaner and healthier for aquatic life while improving the riverside environment.
The location and design of the £35M-plus inlet works has been restricted by the location and route of the 20m deep sewer that it must connect up to.
“Installing such a deep secant-piled shaft wall is pushing the boundaries of what is acceptable in the UK.”
The obvious construction choice for the 18.5m diameter shaft, founded 24m deep in water-bearing Thanet Sands, was to use diaphragm walling and Murphy tendered on that basis.
Once on site, the team looked again at the constricted location and reconsidered the construction method.
The only other option - secant piling - would require 29m long male piles driven through adjoining females either side and into the variable strength clays and gravels, to found in the cohesionless sands.
Assuming the female piles were formed of standard 20N/mm2 concrete, boring the males to such considerable depth - and to the required 1 in 200 verticality tolerance - was initially considered tough going and risky.
“Installing such a deep secant-piled shaft wall is pushing the boundaries of what is acceptable in the UK,” says David Beadman, director for Murphy’s geotechnical consultant, Byrne Looby Partners. “Around 19m depth would be considered the absolute limit.”
But also on board was the contractor’s own piling division which brought experience of working in Ireland where use of lower strength concrete for female secant piles is often successfully adopted.
Armed with this confidence, the team opted to trailblaze a UK first and halve the concrete strength of the 24m long female piles to just 10N/mm2, making it considerably easier to drill the longer males.
The relevant British Standard, BS8110, does allow such lower strengths for unreinforced concrete - as the female piles are - though, as Beadman points out: “UK practice tends to adopt a typical 20N strength for such piles.”
As a bonus, Murphy hired a brand new £1.9M Bauer BG40 piling rig.
With its impressive 40tm torque, use of the rig at Edmonton was correctly predicted to considerably ease the drilling operation.
It has proved a worthwhile addition, with the original 17-week piling programme cut by five weeks.
To help ensure such long piles would stay within the required verticality, a hefty concrete guide wall was laid to 5mm accuracy and the 76-pile shaft perimeter set out on two radii.
As the 1.2m diameter males are drilled through adjacent females, creating a total 320mm overlap, the slightly larger concrete mass of the female being cut into on the inside of the pile circle creates the possibility of pushing the descending male out of alignment.
This could result, as Beadman puts it “with the pile popping out of the circle so destroying the shaft’s required waterproofing”.
To remove this risk, engineers adopted the novel approach of setting out the radius of each male pile exactly 65mm inside that of the females.
This places each male pile on a straight line between adjacent females.
Another innovation is the C8/10 concrete mix design, which demands a balancing act between conflicting needs.
The shaft’s secant pile perimeter is technically all temporary works, as a structural 600mm thick internal concrete lining will later take over ring compression forces.
But the piles’ deadweight is still needed to act as flotation resistance in the wet lower ground strata, and the females’ C8/10 concrete is a real minimum and maximum strength.
“Not only has piling proved 20% cheaper, it has also saved us an estimated six weeks on the diaphragm walling tender programme.”
Additionally, the piles have to resist the corrosive effects of the surrounding ground’s high sulphate content - a demand that pointed to extensive use of sulphate-resisting cement.
The mix adopted blends these two requirements.
Sulphate resisting cement is not used, but more than 90% of the conventional cement is replaced by a secret mix of granulated blast furnace slag and admixtures.
With all the piling due to be complete in late November, Murphy’s Gavin Symonds reflects on the success of the contractor choosing secant piling.
“Not only has piling, at around £1.22M, proved 20% cheaper, but it has also saved us an estimated six weeks on the diaphragm walling tender programme,” he says. “Plus, the piling itself has been completed five weeks faster than planned.”
Contributing to this claim was the decision to bring in a second set of pile casings to maximise rig use.
While one pile was being concreted, the BG40 was starting to drill a second bore.
This increased pile rates from four and a half to eight a week, and saved almost three weeks.
Symonds calculates the basic financial gains of such a move: “The £9,000 hire cost of the casings, set against taking the rig off hire three weeks early - saving us £85,000 - illustrates the obvious win-win solution,” he says.
Similar innovation resulted when solving the conflict of a 4.2t pile rebar cage, 29m long, having to remain stable in its founding medium of waterlogged sands.
It could not be allowed to move while either being surrounded by pile concrete or during follow-on casing removal.
The simple solution was to weld a flat steel plate across the bottom of each reinforcement cage to act as a steady base.
With piling complete, the upper 11m of material was dug out.
At this level the secant walls needed internal support.
But, instead of installing conventional temporary steel walling supports, Murphy is opted to cast a narrow 2m wide section of permanent concrete lining around the shaft to act as a ring beam.
Concern over achieving, at this excavation stage, the required concrete finish on the lining, plus the need to easily incorporate the beam into the main lining as it is later cast upwards from the shaft bottom, meant the beam was formed only half the lining’s full 600mm thickness.
Continual innovation such as this has ensured the project has continued in the new year with no setbacks.
Use of secant piling to form the 24m deep shaft calls for complex structural design.
A row of horizontal pipes, penetrating the piles just 6m down the shaft, means that upper sections of the wall act not in ring compression but as a cantilever with forces transferred down below the pipes.
At 11m deep, there is a need for a compression ring and it is here that the first section of half thickness permanent concrete were cast to double as a temporary works beam.
Two further similar beams are needed at 16m and 20m depths.
Aecom is Murphy’s permanent works designer.