Aeration Lagoon, Worsborough

 

Client & Main Contractor: Taylor Woodrow Construction Ltd

Stone columns support reinforced earth banks for WwTW lagoons

Aerial view of Worsborough lagoon showing works in progress

A scheme to accommodate two large aeration lagoons on a small site with poor ground conditions was devised by AGE for a design and construct project to redevelop Yorkshire Water's sewage treatment plant at Worsborough, South Yorkshire. The use of reinforced soil to construct the lagoon sides and stone columns to carry the load provided significant savings on the tender design of a conventional concrete structure.

Cross section diagram showing plans for Worsborough lagoon works

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 ˜ 1:2 (H:V).

Cross section diagram showing plans for Worsborough lagoon works

Ash and clinker fill available on site was used in the reinforced soil, thereby reducing 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. AGE also designed the 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 (40°-51°) 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 (27°) 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/m3.

The performance of the trial 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 required tolerance. Each lagoon was 21.5m by 40m in plan with a common central dividing wall. The lagoon sides were 4.6m high. Stone columns of nominal diameter 0.6m were installed at approx l.4m 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 8820 and 40RE. A 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.