June – October 2017


With increasing demands on the Sewage Treatment Works (STW) in Calne, Wessex Water (WW) decided to upgrade the network and employ MDA to install two new storage tank. The first was a 5.5m ID shaft, 9m deep and the second shaft a 3.66m ID shaft, 5m deep. Both of these shafts were sunk using the caisson method.


As MDA are a strategic delivery partner for Wessex Water, we understood the importance of keeping the existing STW fully operational while our works were being undertaken. In order to do this, we had to plan all deliveries, craneage, concrete pumping and network shutdowns in collaboration with WW.

The 5.5m ID shaft was excavated so the first ring and concrete jacking collar could be installed. Once the jacking frames were set into the pour, the C32/40 concrete was delivered by local suppliers and pumped into the collar and left to gain strength.

While this shaft was curing, the smaller shaft could also be excavated with the same steps being followed to ensure the jacking collar could be sufficiently cured before placing any load onto it.

The 5.5m ID shaft was then constructed using back bolted segments to build the ring on top of the existing ring which provided edge protection and ensured that we were not working over an open excavation. This method was used until the base of the shaft was reached at 9m where the concrete base plug and benching for formed ready to receive the associated pumping system and pipework.

The 3.66m ID shaft was also constructed using the caisson method but with one-piece rings so the construction was much more efficient. Again, the concrete base plug was installed and benching to suit the STW’s requirements.

All works were carried out without accident or injury ahead of schedule and ensuring that there was no time lost for either party whilst the works were ongoing.

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