The Lancashire Waste Network comprises the treatment of household waste from the 15 local authorities at 4 Central Waste Treatment Facilities supported by 11 Waste Transfer Stations. The Preston Waste Transfer Station (WTS) will process 60,000 tonnes per annum of mixed waste with future expansion to include a 26,000 tonnes pa composting plant.

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.

In order to protect the WTS from a 1 in 200 year flood from the adjacent River Ribble the level of the site had to be raised by up to 3m. The increased loading from the 3m high platform was predicted to result in unacceptably large post construction settlements which 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 a 500mm high 22,000m2 Geocell Mattress foundation using PP biaxial geogrids and HDPE uniaxial geogrids.

Continuous monitoring using 17No settlement plates across the site was implemented to manage the consolidation process with all settlement being built out during the construction phase.

The PaSCoE delivered solution was a finalist in the 2010 Ground Engineering Awards.