Preston Waste Transfer
Overview
The contract involved the design and construction of a new Waste Transfer Station (WTS) due west of Preston at Wallend Road. To protect the WTS from a 1 in 200 year flood from the adjacent River Ribble, the level of the site had to be on a raised earthworks plateau approximately 2.7m higher than the existing ground level formed by the importation and deposition of granular material.
The Works
The scope of the work comprised:
- Construction of concrete hard standing.
- Highway access works
- Installation of services
- Fabrication and erection of a steel framed building.
- Construction of an administration office
- Provision of weighbridge and office
- Jet wash facilities
- Associated surface and foul water drainage.
Various contractor design elements were also required under the contract, along with substantial value engineering in relation to the piled foundations, the reinforced earth platform and the structural steelwork, which were all subject to contractor proposals to the betterment of the project.
Key Challenges
The increased loading from the 2.73m high platform was predicted to result in unacceptably large post construction settlements that would result in a serviceability failure.
In order to force the settlement to take place within a short time period pre-fabricated geosynthetic vertical drains (PVDs) up to 10m deep were installed on a 1.2m triangular grid and the site surcharged by 1m.
Traditional one dimensional consolidation tests were augmented by Cone Penetrometer Tests with pore pressure measurement (CPTU) and dissipation testing to derive the vertical and horizontal coefficients of consolidation. Differential settlement was controlled by the use of 500mm high 22,000m² Geocell Mattress foundation using PP biaxial geogrids and HDPE uniaxial geogrids. Continuous monitoring using 17 settlement plates across the site was implemented to manage the consolidation process with all settlement being built out during the construction phase. The delivered solution was a finalist in Ground Engineering Awards.
The site investigation identified tidal flat deposits up to 9m deep to be present across the site comprising loose to medium dense silty sand, with layers of soft alluvial clay underlain by a layer of dense sand & gravel.
Added Value
EWCE were able to shorten the construction programme by installing the building piles and pile caps whist importing the fill materials to the site whilst erecting the structural steel within pre-constructed ‘manholes’ at the pile cap locations following backfill operations in the locality of the structure. This saved a significant amount of time on the programme and effectively secured the contract programme.
Through working closely with the supply chain, EWCE were able to offer changes to the proposed steelwork arrangements, which both saved in the cost of the structural frame and reduced the number of foundations and piles required to support it.
The use of recycled aggregate resulted in a significant cost saving to the client and reduced the amount of raw materials used with a significant reduction in the carbon footprint (2,930 tonne of CO²) and use of recycled aggregate (all with Environment Agency consent).