Groundforce Shorco solution helps keep treatment works running during major upgrade
Mott MacDonald Bentley (MMB) has the £56m contract to modernise the Longbridge Water Treatment Works in Warwick, almost doubling treatment capacity from 734 litres/sec to 1,261 litres/sec. The scheme involves construction of four 32m-diameter final settlement tanks and one new 27m-diameter primary settlement tank.
The project also involves the diversion of all untreated wastewater flowing into the works from the existing 1,800mm-diameter pipeline into a new tunnelled pipeline feeding the new treatment infrastructure.
To do this, MMB must install a new concrete structure around a length of the existing pipeline within a deep excavation, whilst keeping the pipeline live. The new pipeline enters the excavation perpendicular to the existing one. Once the new structure is completed, the design-and-build contractor will cut the existing concrete pipe using specialist wire-sawing equipment, allowing the sewage water to flow down the new 50m tunnelled pipeline.
The excavation itself measures 10.8m long by 10.4m wide and is 8.48m deep. Poor ground conditions and a high water-table necessitate a robust support system.
MMB brought in Groundforce Shorco at an early stage of the design development to ensure the optimum temporary works solution.
To line the excavation, 12m-long L605 steel sheet piles were driven into the ground with approximately 3m of toe-in beyond formation level to ensure maximum stability.
The depth of the excavation, combined with heavy lateral loads from the weak surrounding soil, required four hydraulic frames, comprised of Groundforce Shorco’s Super Mega Brace beams, to be installed at different levels. The existing pipeline passes across the middle of this excavation, below the third frame and above the bottom frame.
With the pipeline exposed and the ground excavated from beneath it, some support was necessary to counteract the self-weight of the concrete pipe and the sewage water that constantly flows through it.
“Pipe support was necessary to stop the pipe from bending and cracking when it no longer had any ground below it to support it,”
confirms Groundforce Shorco design engineer Sam Leggott.
The solution was to suspend eight heavy-duty lifting slings from one of Groundforce Shorco’s Mega Brace beams. The beam was positioned directly above the pipeline’s axis, allowing the slings to pass underneath the pipeline and tensioning them to take the weight.
“The weight that the straps had to support was 49.5kN/m,”
says Mr Leggott.
“In the layout that we suggested, the maximum load a single strap would hold was around 77.5kN.”
Maintaining the integrity of the suspended pipeline was essential to the success of this operation, explains MMB site agent Burhan Adi.
“All of the wastewater from Warwick flows through that 1,800mm-diameter pipeline into the treatment works. This concrete pipeline is estimated to be 80 years old, and has to continue flowing throughout the whole construction process. It cannot be interrupted. Installing these sheet piles and heavy duty frames around such a pipeline comes with challenges that require strict control measures. Extra precautions have been implemented during these works, such as surveying the existing pipeline daily and monitoring for any movements, even up to 1mm.
“Credit to Groundforce Shorco who worked with us throughout to develop a solution that was buildable, and when we came across challenges were ready to problem solve, review designs and visit site to offer technical support.
“When all the new infrastructure is completed, we will cut through the existing pipeline using specialist wire-sawing equipment and the wastewater will then be diverted through the new pipeline and into the new treatment process.”