August – October 2018


As part of a larger scheme, Wessex Water required the installation of two large diameter storage tanks in Wimborne. The two tanks were to be installated in different sites, at different times, due to construction constraints around the local school. The first shaft installed was a 9m diameter tank that was 9m deep through stiff clay. The second shaft was 7.5m in diameter, installed to a depth of 8m through similar ground conditions.


While excavating to form the collar to the first shaft, MDA encountered very poor ground that was not identified within the ground investigation. The stiff clay was 2m below ground level with made ground and the ground water level being above this clay layer. This combination meant that the made ground was very wet and needed to be excavated within a cofferdam.

MDA installed an octagonal cofferdam system down to the clay layer which enabled us to safely construct the concrete collar and the first two rings of the shaft. With the concrete collar installed and cured, we were able to backfill around the edge of the collar and remove the temporary works while still maintaining access to be able to excavate the shaft and construct the rings using the caisson method.

A 3t excavator was used inside the shaft to trim the edges and to feed the material to the larger excavator on the surface, however once the 22t excavator reached a depth of 5m, MDA reverted to the use of our 360 degree swivel grab to remove the bulk of the material.

With the shaft excavated to the required depth, the concrete plug and base were installed to form a water tight structure, MDA set up temporary scaffold platform in order to core the inlet and outlet orifices for the pipework connections to the remaining network within the sewage treatment works. The final part of the works was to caulk all the joints and bolt holes before lifting on the segmental PCC cover slab to complete the works.

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