Almost within cheering distance of the hallowed grounds of the Twickenham rugby football stadium in west London lies one of the capital's largest sewage treatment works - Mogden STW. And there is much to shout about there, regardless of the fortunes of English rugby, since the current civils package for the upgrading works remain on programme for a successful completion in January 1999.
Mogden sewage treatment works is being upgraded and extended by Thames Water Utilities to meet new regulations concerning effluent discharge levels into the nearby River Thames. The whole scheme is due for overall completion in December next year. The site serves a population of 1.7 million people in west London, and has a total throughput of 540M litres/day.
At the heart of the works are the construction of a sizeable and complex new inlet structure, a series of primary settlement tanks, aeration lanes and six final settlement tanks, together with associated pipework and mechanical and electrical installation.
Thames Water's project team carried out the initial process design and appointed Black & Veatch (UK) as main contractor in 1996. The two companies are working under a partnering agreement, with design works being undertaken jointly by Black & Veatch and Thames Water Technologies, Engineering. In June 1997 an enabling works contract worth pounds 800,000 was awarded by Black & Veatch to Edmund Nuttall, to be followed by the pounds 7.1M main civils subcontract. This involves construction of the primary and final settlement tanks and aeration lanes, without interruption to existing operations of the plant.
First area for Nuttall to tackle involved construction of three flowsplitting chambers. Next came two 125m by 45m by 5m aeration lanes, six 21m internal diameter final settlement tanks and a final effluent discharge outfall culvert. Lastly, over 2km of pipes are being placed.
A teamwork arrangement was agreed for the contract involving the Black & Veatch design team and Nuttall working together on improving buildability and value engineering.
Construction of the rectangular primary settlement tanks and the aeration lanes was carried out using specially fabricated steel shutters. The repetitive nature of the construction lent itself to using steel formwork, as major time and cost savings could be achieved in eliminating the re-facing required for timber formwork.
Tower cranes were the most cost effective and practical solution for transporting formwork, reinforcement and other materials and for placing the concrete in the walls. The bases of the tanks were constructed using mobile pumps.
The primary settlement tanks are each 50m long, by 30m wide, and 2.75m deep. Two static tower cranes, with a capacity of 6.35t at a radius of 40m, were erected on the bases of two of the tanks. The 126m long, 46m wide aeration lanes were configured to allow a tower crane with a capacity of 4.4t at 50m radius to travel the length of the tank. Programming constraints limited the available length of crane rail to 70m and the wall construction sequence had to be carefully planned to ensure that no wall would end up outside the reach of the crane.
To improve production, Nuttall checked and determined that the loading capacity of the aeration lane base was adequate to withstand the loading imposed by a 40t Andes crawler crane. This meant reinforcement and formwork could be fixed and erected while the tower crane was occupied with placing concrete. The successful co-ordination of the two cranes resulted in a turnaround of two 15m long, 4.8m high walls per day.
One of the most challenging aspects of the job has been construction of the final settlement tanks, designed with a 30degrees slope and an internal diameter of 21m. Because the tanks had such a tight internal diameter and were very close together, a 30t, 360degrees excavator with a long reach arm was used.
Following excavation of the general profile, the sump was excavated and the 500mm, 90degrees duckfoot bend inlet installed and concreted in place. Correct locating of the duckfoot bend was critical, as it became the base point for the setting out of the remainder of the tank.
Following installation of the bend and construction of the 1.8m diameter sump, the sloping base of the tank was constructed. Initial concern over slumping of the concrete during its compaction was overcome by close liaison with the ready mix concrete supplier to ensure tight control of the concrete.
The sloping base was constructed in three lifts.
The perimeter walls were constructed using a proprietary steel formwork system that was sufficient to cast a quarter of the tank circumference in one pour. Initial problems of achieving the radius were overcome by introducing additional props and turnaround of the formwork increased from one pour every four days to a pour per day.