Stewarton WWTW Final Settlement Tanks

CHALLENGE
WGM Engineering received a phone call early February 2010 from the Scottish Water WWT Team Leader responsible for Stewarton WWTW. The extreme temperatures and heavy frost during the winter had caused rapid deterioration of the concrete walls of the final settlement tanks. This resulted in the complete failure of one bridge mechanisms, with the other in imminent danger of failing. We were asked to provide proposals for a permanent solution to restore the failed tank back to operational status as quickly as possible and then carry out similar repair work to the second tank.

SOLUTION
Various options were discussed with the client and WGM proposed a bespoke solution of, casting a new concrete wall on the outer perimeter of the existing tank and extending the motor carriage to run on the new wall head. Our submitted proposal included structural design of the new wall and a combination of Civil, Mechanical and Electrical input which would all be delivered in house. WGM were appointed Principle Contractor and control of the area was transferred for the duration of the project. Work commenced on site immediately and the area was fenced off from the rest of the works. Handrails were removed from the tank and located on top of the existing wall head to provide temporary edge protection for the tank. The bridge was jacked up, electrically isolated and the motor drive carriage removed for modification. Civils works commenced on site with the location of existing services and excavation round the tank. The outer perimeter of the existing wall was scabbled and drilled for tie bars and the steel reinforcement which was partially fabricated off site positioned and tied. The curved shutters were manufactured on site and installed ready for placing the concrete.

The mechanical elements of the motor drive carriage on tank No 1 were found to be badly damaged and required a complete refurbishment. SW were able to supply some spare parts from stock while we sourced the additional parts and machined new shafts for the drive wheels.

Tank 1 was ready to place the concrete after just 2 weeks on site, but unfortunately the extremely low temperatures resulted in a seven day delay. To allow work to continue the decision was taken to progress the civil work on the second tank while it was still operational. This required an amendment to our Safe System of Work, as we were unable to provide temporary edge protection on the top of the existing wall.

The concrete for the first tank was placed when the weather improved and after seven days curing the modified / refur-bished motor carriage was returned to site and installed. Power was restored to the bridge and it was tested and returned to operational status.

The concrete was placed to tank No 2 while it was still operational and only after tank 1 had been proved was the motor carriage removed for modification. The carriage was in better condition than tank 1 and only required new bearings which were fitted during the modification work. Again the motor carriage was returned to site and installed after the 7 day curing period for the concrete.

The handrails were refitted to both tanks and the general area tidied up before removing the security fencing and returning the area back to Scottish Water control.

CLIENT COMMENTSM
Having lost one final tank and with the other in imminent danger of failure we needed a quick fix and contacted a number of company's to provide proposals for a solution. We were extremely impressed with WGM,s option which offered a fast long term solution and demonstrated best value. The fact that WGM are a Scottish Water framework contractor and were able to deliver the complete project in-house alleviated any concerns I had over Project Management and Health & Safety of the works. The area was transferred over to WGM as principle contractor, which they secured and provided comprehensive safe systems of work for all aspects of the project. WGM,s ability to design manage and deliver a multidiscipline solution of this nature was very impressive and it is comforting to know that a company with these capability's is available through Scottish Water Frameworks.
Billy Gracie WWT Team Leader

The Weather conditions were extreme during the work and the WGM site team worked relentlessly to get the tanks back up and running as fast as humanly possible. WGM,s presence on site never impacted on any of the day to day operational activities and I am delighted with the finished Job.
Joe Pettigrew Senior WWT Operator