Temporary works specialist Mabey Hire has successfully completed one of its largest ever excavation support schemes as part of a major energy from waste project in Derby.
Interserve Construction is currently building an energy from waste facility for Derby City and Derbyshire County Councils. The new £145M plant, due to open in April 2017, is expected to divert up to 98% of residents’ residual waste, and generate enough electricity to power 14,000 homes.
When the facility is up and running, waste delivered to the site will be deposited in the reception pit where it is held before being sorted for processing within the plant. Construction of this pit began last summer, with temporary support provided by Mabey Hire’s proprietary waling beam and propping systems.
concrete pump night shot
It is one of the largest excavation support projects ever undertaken by Mabey Hire, according to regional sales director Pat Flannery. “Our major projects division undertakes a lot of basement propping in the centre of cities, like London and Birmingham,” he says.
“But in terms of the physical size and depth – and the tonnage of equipment – this is one of biggest we’ve done. With five levels of support, you can appreciate there was a significant volume of equipment within the excavation.”
The pit was excavated within a 39m long, 19.2m wide and 12.3m deep steel sheet piled cofferdam, with subcontractor Ivor King Piling installing the 600mm wide sheet piles to a depth of 19m. To get the piles through competent mudstone, Ivor King pre-augured to a depth of up to 18m using a twin rotary cased auger. The piles were then installed to full depth using a Bauer RTG 21T universal piling rig.
derby waste sheet piling
With the sheet piled wall complete, civils subcontractor Clumber Construction began excavating the material and installing the five levels of frame required to support the excavation as it progressed to formation. The top frame was constructed using Mabey Hire’s Super Shaftbrace waling beam, propped with its Super Bracing Strut 400; the remaining four frames consisted of Supershaft Plus waling beam propped with Super Bracing Strut 600.
Mabey Hire’s Super Shaftbrace waling system was developed typically for supporting large sheet piled excavations, while the Supershaft Plus was designed specifically for high load capacity and even larger excavations. Supershaft Plus can resist bending moments of up to 2,300kN/m, allowing the beam to cater for much greater waling loads while maintaining substantial prop centres, which provides greater working room within the excavation.
Both waling beam systems were designed to work with Mabey Hire’s Super Bracing Strut, which can provide unsupported spans from 2m to 31m. The Super Bracing Strut was primarily designed for providing intermediate propping within an excavation, although this has now expanded to other applications such as the propping of capping beams and other reinforced concrete structures.
supports for sheet piling
The Super Bracing Struts have square cross-sections. At Derby, the 400mm square strut was used for the top frame and the 600mm section for the lower frames. They were installed as straight horizontal props, but can also be used in raking applications. The cofferdam design was developed over a number of months, with Mabey Hire’s engineering team working in collaboration with Interserve’s temporary works designers.
Interserve site temporary works and constructability manager Rhys Jones explains why the steel sheet piled solution was chosen: “Due to an Environment Agency requirement for a membrane to surround the pit, contiguous/secant piling was ruled out, as there would have to be a lining wall built in front of the piles. The lining would then have required a concrete facing applied to the inside of the pit while maintaining the lining integrity. Essentially the secant walling would have only been a temporary works option incorporated into the permanent works.”
Sheet piled solution
Having ruled this out, the best solution was to build the concrete lined pit within a sheet piled cofferdam solution, supported by as few frames as possible. Jones says the advantage of a proprietary system over a fabricated steel frame included immediate availability and – in line with Interserve’s commitment to reducing its carbon footprint – the equipment would be removed and reused elsewhere when the job was finished.
Interserve provided Mabey Hire with information from two borehole locations to aid the design of the temporary support. The boreholes helped to identify the expected groundwater level, as well as the prevailing geotechnical conditions.
There was some variability in the ground conditions across the excavation, as it is partly located in a backfilled tannery settlement pond. This clearly showed up as variability between the boreholes and the differences in strata levels.
Discussions continued into spring 2015, with the design being refined as the construction programme was firmed up. “There was a lot of collaboration between our engineering department and Interserve’s temporary works team,” says Flannery.
Constraints on the support system included the fact that it had to work around the permanent pile design, which put space constraints on constructing the permanent structure inside the excavation.
props suppot sheet piles
“Through early involvement with the reinforced concrete contractor Bell Formwork Services, as well as some minor design tweaks from the permanent works designer GHD Livigunn, this was overcome,” says Jones.
But the biggest challenge in the design and installation of the excavation was groundwater. “Prior to commencing the works, various ground investigation elements were undertaken, in addition to the various historical site investigation reports for the site,” says Jones.
New boreholes were undertaken, trial holes dug, and perforated steel tubes were sunk into the area to be excavated in an attempt to verify the location of groundwater.
concrete pour night
“Unfortunately, none of the data could conclude the exact water profile for the excavation, and thus the design had to take a conservative approach until such time as we had undertaken partial excavation. Buildability was another challenge, as the sheet piles had to be designed as light as possible to economise on the cost, but also so they could be pre-driven to the required depth.”
All of these issues had an impact on the temporary support design, but the early discussions enabled them to be overcome within the required timescale.
“By engaging with Mabey Hire at an early stage, they were able to understand the project requirements in full,” says Jones. “There was a collaborative approach from the onset between their engineers and our technical services department. It allowed Mabey to fully understand the project requirements, which led to a more economic outcome during the detailed design phase.”
Once Mabey Hire got the go-ahead to provide the support system, a lot of work went in to make it happen, says Flannery.
props free up workspace
“We had a logistical challenge of preparing the equipment in such a manner that each frame was clearly identifiable for both our internal operations team and, more importantly, the site team.
“To do that we needed to pull together lots of different departments within Mabey Hire: this meant a collaboration between our engineering, operations, transport and fabricating teams.”
In all, Mabey Hire supplied around 450t of equipment to support the excavation.
Every frame was colour coded, and every component was numbered, “so the guy on the ground who was installing the beams knew exactly where on site it needed to go”, says Flannery. Deliveries were phased to fit in with crane availability, and the equipment was single handled straight into the excavation.
three levels of support
“We worked really hard as a team to ensure that, when the equipment arrived on site, everything went together as per our discussions with the site team,” he adds. “Speed of installation was a critical component.”
Mabey Hire supplied an on site demonstration team to support Clumber during the installation. “It was very much a joint approach between us and Interserve,” says Flannery. “We designed and delivered it together. And because there was such early collaboration between all parties, the design was worked out at an early stage, which resulted in no last minute surprises.”
Mabey Hire also created a complete building information modelling (BIM) model of the excavation support system, using its in house Revit library of components.
“The reason behind this was to provide a clear and visual representation of what the cofferdam would look like from both an external and internal viewpoint” explains Flannery.
The whole Derby and Derbyshire Waste Treatment Centre development has no requirements for works to be BIM compliant, but Interserve chose to incorporate the reception pit temporary works into the project’s BIM strategy. The model included the various phases of the project, allowing the site teams to visualise the complex installation and removal sequence of the support frames around the excavation, concrete construction and backfill operations.
“We utilised the BIM design in our Q&A procedures in terms of complex design,” says Interserve senior temporary works design engineer Shaun May. “It is not currently a mandatory requirement for temporary works, but the industry and Mabey Hire’s procedures are heading that way – so this was the first step.
“There were huge benefits in the visualisation from the subcontractor in getting a feel for how the excavation would look and, in particular, the sequencing of the scheme in line with our hold points,” he adds. “Mabey Hire’s sequencing drawing was printed out large, laminated, and various copies cable tied to the edge of the excavation. This, in turn, aided the health and safety of the scheme as the site operatives had prior knowledge and understanding of the purpose of the scheme, allowing them to plan accordingly.
“It also highlighted possible construction constraints we could encounter on site through the process. It brought up questions from all levels of the site team that ordinarily may not have been raised.”
The base of the reception pit is 1,700mm thick reinforced concrete, poured continuously over more than 24 hours. Mabey Hire’s design allowed the three lower frames to be removed after the base had been cast, as this then acted as a prop against the sheet piles.
Two concrete lift pours
The reinforced concrete walls of the reception pit were designed to be poured in two lifts, each 6m high. The walls in the lower half are between 1,200mm and 1,400mm thick, and the upper half between 800mm and 1m. With the lower three levels of frames removed, the first wall lift could be poured without any frames obstructing the shutters.
Once the first lift was finished, the excavation was backfilled to this level, allowing the upper two levels of frames to be removed and the remaining sheets to stand in cantilever while the top 6m high wall section was cast.
As a result, the bottom frame was in place for a total of around five weeks and the upper levels progressively longer, with the top frame in place for around four months.
The wall sections are now complete, and the sheet piles are being extracted using an ABI TM16/20 leader rig. The entire project is due to be completed in March 2017.
The £145M Derby and Derbyshire Waste Treatment Centre in Sinfin, south Derby, is a mechanical biological treatment facility and on-site gasification plant which will use household waste that would otherwise go to landfill to generate electricity.
Derby waste treatment scheme
The electricity – enough to power around 14,000 homes – will be supplied to the national grid.
The project is being delivered by Resource Recovery Solutions (Derbyshire) (RRS), a special purpose vehicle that has a 27-year, £950M Public Private Partnership (PPP) contract with the two local authorities. It worked with the councils to secure planning permission for the new facility. RRS has raised non-recourse debt funding for the PPP through the UK Green Investment Bank and two international banks, BayernLB and Sumitomo Mitsui Banking Corporation.
RRS appointed Interserve to construct the plant, which is due to be completed by April 2017, at which point Shanks will take over as operator.
“This facility will help both councils significantly increase their recycling rates, reduce the amount of waste being sent to landfill sites and boost Derbyshire’s local economy,” says Interserve chief executive Adrian Ringrose.
Councillor Dean Collins, Derbyshire County Council’s cabinet member for highways, transport and infrastructure, adds: “The county council is facing massive budget cuts and our current landfill bill is one we cannot afford in the future. We do not know how much landfill will cost in coming years, but it certainly won’t cost less than it does now.
“This waste treatment facility will give us certainty about the cost of managing waste in the future, help us to manage our budgets and protect us from future rises in the cost of landfill – including Landfill Tax, which is currently set at £82.60 a tonne.”
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