Aarsleff lays foundations for sludge treatment
11:26 06 Oct 2009
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Sludge from South West Wales is being turned into electricity to power the region. But before the process can begin, specilaist piling contractor Aarsleff must develop foundations to support the vast digester tanks. Contract Journal reports.
FactFile
- Project: Afan advanced sludge treatment centre
- Value: £27m
- Main contractor: Morgan Est
- Piling contractor: Aarsleff
- Piling contract value: £600,000
- Consulting engineer: Arup
- Sludge treatment technology provider: Cambi
As part of a review of its energy strategy, Dŵr Cymru Welsh Water is building two technologically advanced sewage sludge treatment centres. They will use an advanced anaerobic digestion process to maximise bio-gas production and generate 'green' electricity, significantly reducing Welsh Water's carbon footprint. One of these plants is being built at the existing Afan Waste Water Treatment Works alongside the Corus steelworks at Port Talbot, South Wales.
In addition to the sludge already generated at the treatment plant, dewatered sludge from across South West Wales will be transported to the new sludge treatment centre. It will then undergo thermal hydrolysis treatment followed by anaerobic digestion.
The final product will be a high-quality sludge cake, free of pathogens and ideal for application to agricultural land. The digestion process will produce large quantities of biogas, principally methane, which will be burnt in Combined Heat and Power engines.
The new £27M plant will also generate up to 3.1MW of electricity, with any excess to the works' requirements being diverted into the national grid. The heat recovered from the CHP engines will be re-used in the thermal hydrolysis stage.
The new Afan plant is being built within the existing site boundary and requires a very compact design, which includes two side-by-side 18m diameter reinforced concrete sewage digester tanks and an adjacent sludge cake storage building.
The site is underlain by a thick sequence of wind blown sand and compressible alluvium, with weathered coal measures at a depth of about 30m. While raft foundations were considered suitable for many of the proposed structures, piled foundations were necessary for the heavily loaded digester tanks and cake storage building and these were all supported on tubular steel piles made up from drill casing recycled from the oil industry.
Piling specification
The main civil works consulting engineer Arup produced the piling specification and identified settlement criteria for the structures and a regime of dynamic and static load testing of the piles. Specialist piling contractor Aarsleff won the piling works in open tender. The company, working for main civil works contractor Morgan Est, was responsible for the design of the piles and opted to use second-hand oil well drilling casing, and provided materials test certification for every 30t of tubes to satisfy material quality requirements.
Preliminary probing and pile testing was carried out by Aarsleff, to establish the likely driving resistance and pile performance prior to the main piling works. The testing highlighted an unexpected complication with the performance of the piles.
"There was some surprise with the performance of the preliminary test piles being much better than anyone anticipated," says Aarsleff piling manager Philip Chippindale. "We looked at the settlement at the pile toe and elastic compression of the pile with load applied and expected the pile to move about 30mm, but it only moved 10mm."
Aarsleff found that a combination of circumstances enabled this enhanced performance to be achieved, including greater skin friction due to material plugging inside the pile early in the drive and causing the surrounding material to be displaced and densified, internal skin friction, and an enhanced section modulus of the pile due to plugging, which reduced the elastic compression of the pile.
Chippindale adds: "However, densifying the soft to loose material at shallow depths would theoretically increase the value of negative skin friction and offset the improved performance in areas subject to surcharge loads.
"As piles had been tested to two and a half times the safe working load, and the settlement characteristics were linear, Arup and Aarsleff were able to take a global engineering view, which took into consideration the conflicting theories and test results."
"The test piles performed better than anticipated, due to enhanced skin friction," adds Arup project engineer Terry Gordon. "The test data was used to re-assess the impact of negative skin friction at 24 piles which were considered to be within the zone of influence of the adjacent raft foundations.
"To allow for the estimated negative skin friction, the working load of these piles was reduced from 700kN to 375kN, requiring installation of an additional 24 piles. We also increased the allowable total settlement of the affected parts of the structures from 20mm to 30mm and this was accommodated within the structural design."
Aarsleff, using its largest rig, the Junttan PM26 LC and fitted with the five tonne version of the Junttan HHK 9A accelerated hammer, is installing over 460 of the threaded and collared steel tubes for the two digester tanks and sludge cake storage building.
Morgan Est and the main mechanical and electrical contractor Imtech Process are scheduled to complete the Afan advanced sludge treatment centre by March 2011.
Anaerobic digestion and power generation
After reviewing its energy policy, Dŵr Cymru Welsh Water decided to reduce its reliance on energy intensive thermal drying of sludge at three plants in South Wales in favour of anaerobic digestion (AD) and power generation. The aim is to be able to generate five mega watts of green power by March 2011.
After start-up, the plants will play an important role in reducing Dwr Cymru Welsh Water's carbon footprint by 35,000 tonnes of CO2 per year, the equivalent to 15% of the total carbon footprint of the organisation.
The thermal hydrolysis pre-treatment involves the sludge being heated to 170°C, sterilized and then anaerobically digested at digester loading rates that are two to three times that of conventional. An advantage of this treatment process is that a high volume of biogas is yielded (mainly methane).
The treated sludge will be dewatered and processed into a high quality sludge cake that is free of pathogens and will be used across South Wales as an agricultural fertiliser.
The thermal hydrolysis pre-treatment technology provided by Norwegian biomass specialist Cambi is designed to encourage a well dewatered digested product in excess of 30% dry solids that is non-smelly and easy to store and use.
Piling on the pounds
Centrum Pile has undertaken a third expansion of its production facility in Newark, making what it claims will be the UK's largest manufacturer of continuously reinforced precast concrete piles.
The new factory, costing £6.7m to the Aarsleff subsidiary, will provide Centrum with an 80% increase in production capacity to a maximum of 30,000 linear metres per week of continuously reinforced concrete piles.
Terry Bolsher, Centrum Pile managing director, says the investment is designed to ready the company for a return to house building. "This is a major and significant investment for us, especially at a time when the construction industry, and the piling and foundations sector in particular, is going through such a deep recession," he says. "However, this considerable expansion is for the long term and I am confident that we will be able to take advantage of the upturn when it comes, just as we did in the last recession.
"Since moving into the Newark premises in the early 1990s, and coincidentally during the last recession, over £11M has been invested in the acquisition of land and capital equipment. Centrum Pile was subsequently formed in 2000, as a subsidiary to its specialist driven piling contracting parent Aarsleff Piling, and now approximately 45% of Centrum's production is for customers other than Aarsleff."
The piles produced all come with a CE mark of conformity, a boast that Centrum believes it is the only manufacturer in Europe able to make.