- Project Upgrading Bicester Sewer Treatment Works as part of AMP6
- Principal Contractor Lewis Civil Engineering Ltd
- Project Lead Ray Hind, Technical Sales
- Key Benefit MGF provides a cost-effective solution through close working relationships with the contractor.
Introduction
This particular part of the AMP6 infrastructure is for a new final settlement tank (FST), which measures 25m in diameter with varying depths of up to 7m. Site works commenced in October 2020 on behalf of client MWH and contractor Lewis Civil Engineering Ltd who carried out the project.
This was an important tender to secure for MGF as it was a new site in the AMP6 framework that was based in the South East. Our partnership approach we take with our customers and emphasis on early involvement was a key differentiator in securing this project.
The Solution
The project required a design that would facilitate dewatering methods to mitigate the risk of adverse weather conditions affecting the ground. To expedite the works and provide a central dewatering point, the central pump and connecting pipework was undertaken first within a smaller 5.7m(l) x 5.7m(w) x 7m(d) multi-braced fully enclosed cofferdam. The deeper incoming and outgoing pipework was also constructed at this stage within mechanical box supported excavations through the use of our Trench Box systems.
During early optioneering stages a multi-framed fully open span octagonal cofferdam was proposed for the FST construction, one of the largest MGF have sanctioned. This provided the customer with the opportunity to reduce the bulk excavation footprint and structural backfill costs.
Following the award of the contract, additional targeted site investigation was specified and undertaken resulting in further value engineering to support the project requirements.
Based upon improved confidence in the overall ground model a revised solution was adopted in the form of a single propped driven cantilever solution. At the final stages, following consultation with the site team and with an agreed resequencing of the initial structural backfill stage a low propped cantilever solution (LPC) was designed and implemented, to enable early off hire of the entire internal bracing and provided significant cost efficiency to the customer. Changing of the FST base detail, from a mass fill base to a piled solution, required additional checks and liaison with an external third-party checker to re-affirm the low prop cantilever suitability.
As one of MGF’s largest to date, the resulting octagonal chamber measured at 25.2m sheet to sheet diameter, formed by 7m GU13N sheet piles and internal bracing comprised of 8no x 10.1m legs of 406UC Brace, to negate the need for any internal cross propping.
The Verdict
MGF were able to successfully provide the customer with a sustainable solution delivered through the combination of a resilient design approach, adapting to progressive changes made to the works as well as working alongside the customer throughout the entire process. These elements resulted in the reduction of the overall bulk excavation, dewatering and structural backfill requirements.
Further, the LPC approach enabled a cost-effective ‘hire’ solution, requiring only a single top frame for a much shorter duration than planned at the estimation stage. This saved the contractor time and allowed them to move on the next stage of the project earlier than previously intended. In suitable ground the LPC solution can provide a very resilient, efficient and sustainable design solution.
