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Case study: Case Study: Ground Improvement, Eco AD Plant, Chapel Lane, West Parley

CLIENT: Mildren Construction

CLIENT:

Mildren Construction

Scope of works:

As part of a new energy from waste facility, our client was contracted to form foundation bases for several bulk storage structures, including a series of 29.0m diameter tanks applying a peak bearing pressure of 130kPa to the supporting soils.

A maximum permissible total slab settlement of 35mm was specified, alongside a maximum specified angular distortion criterion.

The shallow soils comprised up to 5.0m of Firm to Stiff Clay Fill, over Medium Dense to Dense Sand of the Branksome Sand formation.

Adopting a high area replacement ratio for vibro replacement stone columns installed full depth through the Clay Fill, our estimated tank settlement under an applied bearing pressure of 130kPa was 46.6mm.

Options to limit tank settlements to within tolerable limits included reverting to a traditional piled approach, the use of rigid inclusion ground improvement, or vibro stone columns with pre-consolidation using static surcharge.

A sufficiently large stock of aggregates was available on site to facilitate static surcharging, and this was selected as the most cost-effective method for achieving a satisfactory design solution.

Using GeMech’s in-house ground improvement design process, a settlement of 12.7mm was estimated under an applied pre-consolidation surcharge. We estimated that this consolidation settlement would occur within a week of placement, owing to the beneficial effects of the closely spaced high void ratio stone columns acting as radial drains. An induced settlement of 12.7mm from static surcharging, was sufficient to reduce estimated post construction slab settlement to within the specified limits.

Settlement monitoring conducted during the static surcharging period recorded an average induced settlement of 11.0mm occurring within 5 days of the commencement of surcharge placement – in reasonably close agreement with our design estimates. The recorded average settlement remained stable for a 3-day period prior to removal of surcharge.

The marginally smaller magnitude of induced settlement recorded, compared to our design estimates, is likely to result from conservatism in our assessment of operational stiffness in the Made Ground under the applied stresses and associated strains, indicating that our initial total settlement estimate of 46.6mm was marginally conservative.

In addition to providing the aggregate required for static surcharging, the Client was able to provide a suitable quantity of graded aggregate, from the material available on site, for construction of the stone columns. This avoided any requirement for importing in the order of 2,700t of material.

A successful project, completed on time and within budget, with a comparatively low embodied carbon and lower total cost in the ground significantly lower than a traditional piled solution.