What we found
A 1974 cast in-situ basin, 340 m², leaking an estimated 14 m³ a day into made ground above a rail tunnel. Original plant: four fixed-speed 15 kW pumps and no filtration worth the name. The city was paying to treat mains water and put it into the subsoil.
The concrete itself was sound. Every leak we traced was at a construction joint or around one of the 32 original nozzle penetrations, where the puddle flanges had corroded through. That is a repairable failure, and we said so before we were asked to price a replacement.
The work
Grit-blast, structural repair at eleven joints, then a reinforced liquid-applied polyurea membrane taken up the walls and dressed into a new stainless coping. Every penetration was cut out and replaced with a welded stainless puddle flange — 32 of them, which was the bulk of the programme. Depth came down from 450 mm to 260 mm, which took 65 m³ out of the volume without changing anything anyone can see from the plaza.
Numbers, before and after
Make-up water went from about 5,100 m³ a year to 620. Pump energy went from 61,000 kWh to 19,400, mostly by replacing four fixed-speed pumps with two variable-speed units sized for the actual duty rather than the 1974 duty. The capital cost was recovered in operating savings in the sixth year. Five years on, the city has asked us to survey two more of theirs.
