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A series of SIs revealed vibro stone columns and grouting would keep a new housing development above ground.

Site workers are using compaction grouting and vibro stone columns to strengthen ground for a 152ha housing estate on a chalk hill in Berkshire, south east England.

Consultant PL Ross Associates opted for a combination of compaction grouting to treat the solution features and vibro stone columns to improve disturbed soil, after talking to SI consultant Southern Testing and contractor Keller Ground Engineering.

Dissolution features or swallow holes develop over considerable time, usually as a result of acidic ground water perched above the clay that percolates its way down to underlying chalk. Over time the voids become loosely infilled with granular or cohesive overburden material. The strata comprises a thin mantle of clay over up to 8m of sands and gravel over chalk.

The demolition of the derelict Canadian Red Cross hospital compounded the swallow hole problem.

Removal of its old foundations disturbed near surface soils, meaning the proposed development would require deep trench fill or piled foundations. The dissolution features also required foundations to span over any affected area or some form of ground treatment to be carried out.

Initial SI and a geophysics survey, carried out by Southern Testing and consultant STATS respectively, indicated chalk solution features at some locations on site. Although the survey pointed to likely locations of swallow holes, developer Countryside Homes elected to use Southern Testing to carry out further SI involving an intensive dynamic probing exercise to map out the extent of the problem.

'The benefi of having such detailed soil information was that it allowed us to focus on those areas actually requiring treatment, ' says Keller geotechnical engineer, Dominic Gibbon.

Probing confi rmed that badly affected areas would require ground treatment involving compaction grouting of the chalk features down to depths indicated by dynamic probes, which extended up to 3m below surface level.

Compaction grouting introduces a very stiff grout into the ground in a series of bulbs that displace and compact the loose soil. Fluid grouts are not used to treat chalk dissolution features as they would run the risk of re-activating unstable ground. Grouting was done up to 2.5m outside the building footprint.

Site workers improved the bearing pressure of soil above the grouting zone by installing vibro stone columns.

'Using compaction grouting meant we were able to treat the source of any future settlement, ' says Gibbon.

'Vibro provides uniform bearing pressure in disturbed near surface soils.' The design team specified a uniform bearing pressure beneath the plots of 125kN/m 2. Although both treatment processes have similar objectives, vibro induces high energy compaction whereas compaction grouting uses high pressure to displace soil. The latter is benefi cial when work is done close to existing structures.

The £250,000 contract also required Keller to perform load tests, measure grout takes and monitor ground heave by a system of laser targeting. Keller has installed the 859 stone columns and was off site at the beginning of this year.

Cliveden Village will comprise 135 two storey homes and apartments and is being jointly developed to form a sustainable new village community by the National Trust and Countryside Properties.

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