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Bushey Water Treatment Works
The Challenge:
The former Veolia Water Central Ltd offices and water treatment works at Three Valleys Way was to be redeveloped for housing. The site covers an area of approximately 4.2 hectares and required excavation, remediation and soil improvement in places down to the 7m depth of fill. The work included the breaking out of former water treatment works infrastructure, including nine underground reinforced concrete softening tanks and associated pipe work. The tanks had previously been backfilled with made ground that was to be removed and processed for re-use as engineered fill. The project involved very deep excavation of soil, backfill, encapsulation and stabilisation.
The Approach
Cognition was contracted by Persimmon Homes to develop the backfill design and testing and then carry out the earthworks. In accordance with the design. The objective was to process and re-use existing fill to save on disposal and import of materials and produce a geotechnical suitable platform for piling and construction. The key elements of the works included;
- Excavation of former water treatment plant to expose concrete treatment tanks;
- Carry out environmental monitoring, including vibration, to demonstrate minimal impact on local community and neighbouring buildings;
- Removal of Japanese knotweed infested soil, encapsulate and bury deep beneath site in accordance with EA guidance;
- Stockpiling and processing of all arisings to remove waste items and produce geotechnically uniform material for backfill;
- Stabilisation and backfill laboratory design;
- Breakout of all obstructions and crushing of concrete for re-use;
- Backfill in accordance with design specification;
- Geotechnically test all layers including testing at regular intervals using plate bearing and nuclear density gauge tests;
- Produce compliance report;
- Stabilise where required, using combination of binders;
- Perform longer term skip load tests to ensure no settlement.
The Outcome
Cognition's strategy enabled 60,000m3 of soil to be processed for re-use, delivering considerable savings on disposal and import and minimising traffic and disruption in the local area. The heavily reinforced concrete water tanks were broken out and the concrete crushed and re-used. The stabilisation design mix was developed to optimise backfill and compaction. The geotechnical testing proved the backfill would not be subject to long-term settlement and the site was able to be re-used for a large residential housing scheme.
