A secant piled shaft design eliminated temporary works from a stormwater storage project at Bridgwater in Somerset
Construction of one of the UK’s deepest unbraced secant piled wall shafts helped speed up excavation of a new underground stormwater storage tank through challenging ground in Bridgwater Bay, Somerset.
Wessex Water’s Burnham-on-Sea Bathing Water Improvement scheme is a £39M project to upgrade sewerage infrastructure, with the aim of improving water quality for the seaside town of Burnham-on-Sea on the Severn Estuary.
The project will reduce the number of combined sewer overflows discharging wastewater into water courses during storms, by increasing the capacity of the sewerage network to cope with extra flow and to store water. Sewage treatment will also be improved across the network.
Work is being carried out at a dozen sewage pumping stations and treatment works along the mouth of the River Parrett. In the town of Bridgwater, underground storage tanks are being built along with new pipelines to pass more water to treatment. Ultraviolet equipment is also being installed to improve the quality of water being discharged.
In Bridgwater, the Bristol Road sewage pumping station has been replaced with a new pump station containing an integrated 3,000m3 of stormwater storage, reducing the amount of water flowing into the River Parrett during times of intense rainfall.
Wessex Water appointed south Wales-based Lewis Civil Engineering as principal contractor to build the pump station and stormwater overflow tank, under a £6.5M design and build contract.
The pumping station is tightly squeezed between the A38 Bristol Road and the River Parrett, so Lewis Civil Engineering’s consultant Alan Auld Group decided that the safest and most practical design for the tank was as a 26m deep, 15.5m diameter circular secant piled shaft covering a sixth of the site.
Cementation Skanska was chosen to carry out the detailed design and construction of the shaft, under a £1.4M contract.
“The original shaft design included substantial temporary works,” explains Cementation Skanska southern projects director Graeme Mitchell. “Instead, we proposed an alternative design using hoop stress to provide temporary support. This eliminated the need for internal propping, which meant excavation would be faster and construction significantly cheaper.”
Cementation Skanska had previously used the approach to build a 4,500m3 storage tank on the Salford 172 Project in Manchester for United Utilities.
“Our experience at Salford not only meant we were sure a hoop stress design could be adapted to the ground conditions at Bristol Road, but also gave Wessex Water and Lewis Civil Engineering the confidence it would work,” Mitchell says.
Ground conditions were a challenge, he adds. “Beneath a thin layer of made ground we had to pile through about 26m of estuarine alluvium, principally water-bearing sands, soft clays, with a band of coarse gravel at the base, and into the underlying Mercia Mudstone, which was weathered at the top.”
The River Parrett drains the southern part of the Somerset Levels into the Severn Estuary. As a result, the groundwater level was high and linked with the tidal river, so it was vital to minimise water ingress to the shaft during construction.
The shaft was constructed from 66 interlocking 1.18m diameter hard/hard rotary bored secant piles installed to a depth of 30m and founded in competent Mercia Mudstone. Piles were installed with temporary casing, in an effort to exceed the target design tolerance of 1:200 verticality and 25mm in plan.
“We needed the piles to generate sufficient hoop stress to resist external soil and water loads, which meant ensuring they remained interlocked over 29.5m of their total 30m length.” Mitchell explains.
“Even at the design tolerance, theoretical interlock at the pile toe could be as little as 45mm, so we wanted to achieve better verticality. This also meant we could minimise the amount of water flowing into the shaft during excavation.”
Concrete mix trials were also carried out with a local supplier, to ensure concrete strength gain was fully understood before work began.
After a guide wall was cast, piling was carried out by the firm’s largest rig, a Bauer BG42. Using this machine, along with all of its support equipment, presented logistical challenges for such a small site.
“Getting the rig on site was the first step, as we could not cause any hold-ups on the A38,” Mitchell says. “We then worked with Lewis Civil Engineering to coordinate plant movements meticulously, minimising disruption to local residents and businesses.
Pile installation, including casing placement, was continually monitored using conventional surveying methods as well as self-levelling lasers.
“We developed a bespoke magnetic mounting system that allowed us to fit these to the digging tools, Mitchell explains. “This allowed us to undertake as-built surveys of the pile positions prior to concreting, giving everyone confidence in the integrity of the shaft prior to, and during, excavation.”
Pile integrity and interlock were also checked as excavation progressed. These checks showed that, on average, Cementation achieved better than 1:300 verticality, well in excess of the project’s target tolerances.
Three-dimensional laser scanning was carried out to provide an accurate, as-built, picture of the shaft once the final depth had been reached and a concrete blinding layer cast in the base, 26m down. Lewis Civil Engineering then installed the shaft’s permanent precast concrete lining.
Construction finished this spring and the shaft came on line in May. The Burnham-on-Sea Bathing Water Improvement scheme is due to be finished in 2018.