UU gets inventive in tight spot
An unusual site and a tight time frame could have spelt trouble on a complicated project. But close partnership working and creative thought ensured successful and timely delivery, as Atkins' senior project team leader, Martin Meadows, explains
The Wigan Inlet Works scheme is a maintenance project to replacing failing assets, safeguard plant performance and ensure discharge is in line with current Environment Agency standards. In September 2005, UU awarded solutions identification and development phase design to GCA.
GCA's target was to review the initial solution, develop and agree cheaper and safer alternatives, and produce design information suitable to agree a cost, all by the end of December 2005. To achieve construction spend by the end of the financial year, design commenced in parallel with pricing, ensuring an integrated, teamwork approach to the delivery of this challenging project.
Additional problems occurred when the proposed site became unavailable. This left GCA with a challenge, as the only available land was a small triangular plot adjacent to the existing operational inlet works.
Inlet works solution
Numerous options were considered. The team finally developed an option that both worked and fitted within the tight constraints of the unusually shaped site. The plan was to move away from the traditional in-series detritor layout and incorporate an unconventional clustered grit detritor arrangement.
The main elements were:
- Main sewer intercept chamber and connecting 2.4m diameter sewer
- A series of coarse bar screens upstream of 9.0m deep wet/dry well-pumping station
- Elevated fine screens structure with interconnecting channels to the clustered grit detritor arrangement
- Process buildings incorporating main MCC
- Washpactors and collection skips/conveyors
- The process areas are odour controlled
The 3D fly-through model also proved useful with United Utilities operators and the contractor. It was used to gain a full appreciation of the project solution and its complexities from a construction and health and safety point of view. It increased operator confidence by demonstrating access and operability issues. The model was also linked to GCA's construction programme, demonstrating to UU that the works could be constructed safely within the tight confines of the site and to the company's programme dates.
Hydraulic design was also key to the success of this project. Atkins' hydraulic specialists developed a range of hydraulic profiles for various flow scenarios and operational maintenance conditions. The flow split to the clustered detritor arrangement proved challenging, as an equal flow split between the detritor inlets and outlets was required to ensure optimal plant operation.
In parallel with the hydraulic design, civil and mechanical general arrangements were being prepared. GCA decided, due to significant impact on design, to have a scaled hydraulic model prepared for the full inlet works. This would be from interceptor chamber through wet-well to fine screens channels, through the detritor arrangement.
GCA commissioned the BHR Group in Cranfield to prepare the models. Various flow scenarios and operational conditions were simulated together with the complex flow split arrangement to the detritors. The results demonstrated that the hydraulic design for the plant was correct. It verified the flow split to the detritors was within 5% of the calculated values, and that the inlet pump sump arrangement was satisfactory.
Construction of the deep inlet pumping station in the tight confines of the triangular site posed another risk to the design and construction team. There was limited space to install a traditional cofferdam between the tree line and the operational inlet works. This was recognised early in the design process and piling subcontractors were brought on board early to discuss the use of secant bored pile walling as both temporary and permanent works.
Piling subcontractor Bachy enabled the design of the pumping station to progress at speed - the 33m long x 20m wide, split-level inlet arrangement (9m and 4m deep respectively) required the installation of 110 piles of 900mm diameter and 95 of 600mm diameter piles. Once installed, the internal face of the secant bored pile wall was lined to form the permanent works.
GCA design team worked closely with Bachy and the site construction team to
optimise the pumping station layout, which in turn maximised construction savings. Structural design took into account the preferred method of construction with respect to installation of capping beam, temporary propping and excavation maximising work areas.
The design of the high-level inlet screens/detritor structure was simplified to aid construction. Flat soffits were adopted throughout, which aided detailing and fixing of reinforcement. And the various changes in level were formed internally with benching. The large volume of concrete needed was reduced by using polystyrene void formers, while implementation of full-height formwork was maximised by the use of pull-out bars and wall couplings where applicable.
The Galliford-Costain Atkins joint venture is co-located within the Integrated Alliance Southern Area Framework offices in Warrington. The close proximity and good working relationships have allowed this project to resolve technical and operational issues swiftly.
In addition, the GCA team produced technical drawings and specifications, obtained prices and agreed a solution, all within a six-month period. Inventive design was encouraged, as demonstrated by the clustered detritor arrangement. This was one of the main reasons the project progressed so rapidly, while saving UU more than £10M on the original AMP3 solution.
Construction is now almost complete apart from the final tie-in, demolition, roads and finishes. Mechanical and electrical installation works are advanced. Commissioning and dry testing began in mid-November as planned, and the site is on target to meet its February 2008 opening date.
To date 200,000 man-hours have been worked on this project without a lost time accident. This is testament to the delivery team's dedication and commitment to maintaining standards of health and safety.