Concrete slabs supporting both a 68-tonne reservoir of liquid oxygen (O2) and the 4 sludge aeration unit evaporators subsided into the water treatment plant of a paper mill. In noting the strong inclination of one of the evaporators, the subcontractor assigned to monitor the specific installation decided to neutralise the equipment and temporarily supply oxygen to the plant by lorry. During the week, the O2 supply was consumed in order to drain the reservoir. The operator set up a safety perimeter and notified the mill’s personnel of the intervention schedule while distributing a specific set of guidelines in order to avoid any violent shock to the ground (loader bucket, etc.).

The subcontractor procured the live rolling stock for the mill’s backup supply and evaporator de-icing, then the O2 supply of the treatment plant was halted on 16th February in order to install the tapping necessary to connect with the lorry; the safety perimeter was extended during the corresponding intervention. At the same time, the effluent flow rate to be processed was restricted.

The installation was de-iced in order to release the slab, which had been built in 2 stages: a first part in 1993 during implementation of an anaerobic effluent treatment process, and the second part in 2001. According to the operator, water infiltration was the cause of this incident, as the frost and ice formed subsequently caused the lifting of both slabs. As regards the site location choice, the operator indicated never having encountered this type of problem since the initial construction more than 10 years prior.

In coordination with the subcontractor assigned to install the liquid oxygen, a specialised firm conducted a design engineering study as a preliminary step to building a new supporting slab.

During a site visit a few days after the incident, the classified facilities inspectorate remarked to the site operator a lack of protection on the mobile O2 cistern that had been temporarily set up onsite.