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Windmill Shopping Centre, Widnes...

It was proposed to construct a new shopping park at Widnes, as part of the construction Terrain Geotechnical Consultants Ltd (TGCL) were appointed to design an embedded retaining wall. It was proposed to construct a secant bored pile wall using 500mm diameter continuous flight auger (CFA) bored piles at 700mm centres, retaining a height in excess of 5m.

The sequence of works outlined by the main contractor indicated that the wall would be in cantilever for an extended period of 13 months before being propped by the floor slabs. The retained cohesive material would potentially tend towards the drained condition over this period and would normally require temporary propping. Due to the length of time and the large scale of the project temporary propping was prohibitively expensive.  Longer cantilever piles could have been installed designed for the drained condition but again at considerable additional expense.  As a way forward we suggested the option of using the observational method as outlined in CIRIA R185.

Monitoring was carried out at regular intervals using accurate survey methods, the wall displacements observed from the monitoring were compared with displacements predicted using PLAXIS finite element analysis. Following CIRIA R185 a traffic light system of trigger levels was derived, each level passed resulting in an action, using this method it was possible to monitor the displacements and only prop the wall if an appropriate trigger level was breached. The wall behaved as predicted by PLAXIS and by using trigger levels it was possible to reduce the frequency of monitoring with the option to increase monitoring should a trigger level be breached.

The use of the observational method at this site resulted in a successful bid for our client with a considerable saving for the ultimate client and elimination of disruption to the building programme.

Terrain Geotechnical Consultants Limited
Telephone: +44 (0) 1422 842 682 | Fax: +44 (0) 1422 847107 | Email: info@terraingeotech.com
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