Contaminated ground beneath the surface of two new housing developments has resulted in an unusual foundation technique being adopted. GE reports.
An innovative technique that uses vibro-stone columns (VSCs) with concrete plugs has brought significant savings for clients at two residential developments in south-east London.
In Orpington, Kier Partnership Homes is constructing three, three-storey residential blocks, while in Lewisham, Higgins Construction is building 10 houses and a three-storey apartment block for affordable housing organisation Circle Anglia.
In both schemes the presence of underlying aquifers would normally rule out the use of stone columns, which could potentially create preferential pathways for contaminants to leach from fill material into underlying groundwater.
However, by constructing a concrete plug at the base of each stone column, contaminant pathways are restricted.
Ground improvement specialist Pennine, which is carrying out the work, claims this technique will save its clients in excess of £50,000, and lower the carbon footprint by using less concrete than originally expected.
The concrete plug prevents the migration of contaminants through it, and thus blocks the pathway.
Certain contaminants may dictate a specific concrete mix, and for very aggressive contaminants this system may not be an option.
Pennine south east regional manager Adrian Mercer explains: “At Orpington the client and design team were originally planning to pile this site, but our early involvement enabled us to offer a design and build ground improvement solution using 290 VSCs up to 6m deep.
“We placed and compacted a plug of concrete at the base of each column and then constructed the stone column on top to give a safe ground bearing capacity of 150kN/m².
“The site is a former tile works, and the ground comprises 6m of backfilled clay pits over a thin band of river alluvium. Underlying this are river terrace gravels and upper chalk, which is classified as a major aquifer by the Environment Agency.”
At Lewisham the firm installed 500 VSCs up to 5m deep on top of concrete plugs to give a safe bearing capacity of 125kN/m².
Here the site is next to the River Ravensbourne, so careful assessment of the potential impact of any works on the surrounding ecology was paramount.
The site is overlain by up to 3.5m of contaminated fill with high levels of lead, arsenic and zinc.
Beneath the fill are natural gravels, and beneath them a chalk aquifer for public drinking water, designated a source protection zone (SPZ) by the Environment Agency.
Vibro-concrete plug technology was developed in the UK during the 1990s in response to concerns raised by the Environment Agency about situations where conventional vibro-stone column technology was being considered for brownfield sites with underlying sensitive groundwaters or aquifers.
The concern was due to the potential for stone columns to provide pathways for contaminant migration from the made ground into the underlying strata.
The technique is now recognised by the Environment Agency, and is mentioned in Environment Agency Report NC/99/73 (2001) - Piling and penetrative ground improvement methods on land affected by contamination: Guidance on pollution prevention.
Although the report details other foundation solutions that can be used in such ground conditions, most of them have associated risks in some form or another.
The advantage of the stone column with concrete plugs solution is that it significantly reduces costs and programme.
Furthermore, once the ground has been treated it is then possible to adopt lightly reinforced, ground-bearing foundations, far cheaper and quicker compared to reinforced concrete pilecaps, ground beams and suspended slabs.
Vibro-stone columns are a displacement method and therefore there is no spoil to dispose of, which can become an important factor when dealing with contaminated ground.