Creative earthworks, including phased surcharging, is addressing geotechnical and geo-environmental issues in the redevelopment of Barking Reach for housing.
Barking Reach, on the lower reaches of the Thames in Essex, is typical of a riverside industrial site. But the London Borough of Barking & Dagenham, together with Bellway Homes, has a vision for this derelict wasteland.
Working as the Barking Reach Company, they are of developing 200ha into a 6000-home riverside village - with a mix of affordable 'key worker and market housing', schools, shops and transport connections.
Hyder Consulting is working on the strategic engineering aspects of this phased 12 year development and is providing the conceptual model for foundations, drainage, roads and other services. The plan is to release the remediated site to developers in tranches, to work up the proposals in detail.
Geologically the site is typical, with 5m to 7m of alluvium including up to 2m of peat, overlying 2m to 3m of river terrace gravels. The underlying solid geology is the familiar Tertiary sequence of London Clay, Woolwich and Reading Beds and Thanet Beds. The chalk aquifer underlies the site at depth, but is not a major issue.
The project, says Hyder's project director Dr James Apted, is dominated by the site's past uses - particularly the Barking Reach power station, which left a huge volume of pulverised fuel ash over most of the site. 'Although not severely contaminated, ' he says, 'you have to be on the look out for localised nasties.'
Redevelopment has to address two key problems - compressibility of the soft alluvium, particularly the peat layers, and the contamination issue.
Hyder's general approach is to level and then isolate by capping the existing on-site materials. It will then build up the site level, by up to 4m, to create a new development platform.
By incorporating phased surcharging into the earthworks, Hyder has come up with an integrated way of dealing with both settlement and contamination.
Surcharging the ground with temporary stockpiles of fill means that by the time the developers move in, most of the site settlement will have taken place.
This will allow the option of raft foundations for the two storey houses, while residual differential settlements will be within the design tolerances of the roads and services.
Hyder is controlling the phased surcharging and typically expects to leave mounds in place for eight months, although it is currently experimenting with higher surcharge loads to see to what extent this will accelerate settlement rates.
'Essentially it is about observation and control to make sure the settlement measures are effective, ' says Hyder's resident engineer James Long.
Over the course of the development some 2M. m 3of fill will be imported to the site. Conveniently nearly a quarter of this will come from the Channel Tunnel Rail Link contract 250. CTRL goes into tunnel adjacent to the site and all the spoil from Barking to Stratford section will be delivered to Barking Reach directly by conveyor.
Apted is very enthusiastic about this arrangement: 'It's putting into use material that would otherwise need to be removed - now that is sustainable.'