Holdfast Training Services Trial Blog

My site is increasing the capacity of a Wastewater Treatment Works (WwTW) near Rochdale to cope with the growing population and to prevent occurrences of discharging untreated waste into the local Brook, (surprisingly common with 126 recorded spills from this site alone since 2020). The site consists of multiple new reinforced concrete (RC) structures that have required numerous cofferdams in order to install pipework and build the foundations for the various treatment tanks.

The ground investigation (GI) was sufficient enough to understand the basic risks in the ground. Firstly, the site has a risk of contamination given its use, the borehole log records a summer ground water table located approximately 2m down and the lithology is principally soft sand, overlaying very soft sand. This indicates that with the excavations varying in depth between 3-6m the risk of water ingress must be addressed.

As with many projects, the principal driver on the site is cost. Pressures on management across all parties is evident. They need to keep costs as low as possible as the project is significantly in the red (the principle contractor is circa £5 million for this site and around £20 million across the £150 million works package with estimates of the figure growing to £40 million). However, this pressure is resulting in repeating mistakes, losing time and incurring further costs.

It seems at times the management are in a fantasy world like Alice in Wonderland where they are requesting and accepting Temporary Works Designs (TWD) without careful evaluation, in the hope that somehow the conditions have significantly improved. The designs address the requirement for dewatering, stating it is the clients responsibility to reduce the ground water level below formation level prior to starting to excavate. Additionally, it identifies ‘fine ingress’, saying the client is to ensure suitable control measures are in place to reduce and to monitor existing structures due to loss of fines given the granular nature of the soil.

As the purpose of the cofferdam is to have people working inside it, it must principally be safe. But secondly, the cofferdam needs to remain dry enough to enable the works to be completed. Knowing the the ground has high permeability the decision to install ‘open’ cofferdams that are not sealed the whole way round, and in the most recent case with sheets without clutches is questionable. Dewatering systems were installed to draw down the water table, but seems to be designed in isolation, with little link between that and the cofferdam design.

As a result of the open coffer design in coarse grain soil with high permeability within the ground water table, each cofferdam has flooded and sink holes have formed due to fine migration between the sheets. Understandably, the cost of a ‘closed’ cofferdam is greater, however, the pressures to reduce losses and minimise costs are overshadowing the benefits of saving time and cost by preventing flooding and the risk to personnel of sink holes injuring them or destabilising other nearby structures.

Has anyone else found the commercial pressures are further increasing problems?