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Columns tackle lagoon loads

Use of reinforced soil to construct two aeration lagoons at a Yorkshire sewage works accommodated ground movements and provided a more cost effective solution.

Construction of two aeration lagoons using reinforced soil was a significant factor in the award of the contract to Taylor Woodrow Construction (Northern). The contractor won the design and construct project to redevelop Yorkshire Water's sewage treatment plant at Worsborough, South Yorkshire because its reinforced soil proposal was more suited to the ground conditions and was cheaper than a conventional reinforced concrete structure.

The site lies on the flood plain of the River Dove, between the river and a disused railway line. Ground conditions varied across the site, with near-surface soils consisting of very soft alluvium (typical undrained shear strength 10kPa to 15kPa) on the flood plain and competent glacial clay on the flank of the valley in the northern part of the site.

Available space was very restricted and it was necessary to locate the lagoons partially on the flood plain and partially over the valley slopes.

The aeration process precluded construction of the inside faces of the lagoons in earth embankments at conventional slopes, but the process requirements were satisfied by the construction of the lagoon sides in reinforced soil at slopes of around 1:2 (horizontal: vertical).

The considerable amount of ash and clinker fill on site was used in the reinforced soil, which reduced the load increase on the alluvium.

Nevertheless, the alluvium was far too weak to support the shear stresses imposed by the reinforced soil structure. This difficulty was solved by constructing stone columns below the embankments where they were over alluvium.

Conceptual design was undertaken by Neil Smith, director of Applied Geotechnical Engineering and consultant to Taylor Woodrow Construction. This included additional ground investigation to determine the extent and properties of the alluvium and the ash fill.

This established the properties of the ash to be particularly suited to the intended application, having a high angle of friction (4051degrees) and a very low unit weight (max dry density 1.12Mg/m3-1.25Mg/m3).

Trial loading was carried out on a 2.4m high embankment formed of ash to conventional (27degrees) side slopes in an area treated by stone columns - detailed design having been carried out by the specialist contractor Bauer.

Average bulk density of the trial fill was 1.51Mg/m 3.The performance of the embankment was satisfactory and showed that the primary consolidation settlement of the alluvium would be completed within about 10 days of completion of loading.

Predicted creep settlement between fitting out of the lagoons and the end of the 50 year life of the facility was 25mm, resulting in overall settlements which were well within the tolerance determined for the facility.

Each lagoon was 21.5m by 40m in plan with a common central dividing wall and 4.6m high embankment. Stone columns of nominal diameter 0.6m to 0.7m were installed at 1.4m to 1.5m centres below the southern part of the lagoons.Penetration refusal was encountered when the vibrating poker reached the glacial soils.

Detailed design of the reinforced soil was carried out by geosynthetic manufacturer and supplier Tensar.The slope was reinforced with 14 layers of geogrid 55RE SS20 and 40RE.

A design criterion for the ground treatment was that post-construction settlement of the embankment crest should be less than 70mm.

No differential settlement was observed at the boundary of the treated ground and the monitored performance has proved satisfactory.

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