NASSCO Report – Manhole Committee: ‘I Have Seen It All.’
In December 2013, I was 17 years into my career as a leak and concrete repair specialist — thought I’d seen it all regarding gushing leaks in manholes. One afternoon in Brentwood, Tennessee, challenged that premise.
Kurt Koehn with CK Masonry asked me to assist with a particular manhole rehab expected to be unique and challenging. Of course, I accepted the invitation with complete confidence and made the four-hour drive from Atlanta. This manhole was positioned on the edge of a well-groomed horse pasture and within a few feet of The Little Harpeth River — a setting you would associate with a picnic, not a manhole with its own built-in geyser!
Since 2007, Drew Muirhead, P.E., with the City of Brentwood, and George Kurz, P.E., had been working to monitor and reduce inflow and infiltration (I/I) in the City’s collection system. Using a network of permanent flow monitors and various rehabilitation projects, the monitors measured significant reductions in I/I until 2012-2013. Additional rehab work did not appear to be as effective reducing I/I in the main trunk system.
A difference of 0.5 MGD was measured by subtraction of upstream meter readings from measurements downstream of the horse farm. City staff and contractors inspected manholes along the trunk line and found multiple leaks. One defect located in the invert of the manhole was a hole larger than a man’s foot. Also noticed, water from the Little Harpeth River was moving toward the manhole. Dye testing confirmed a major flow from the river entering into the collection system.
The trunk line was bypassed and the severity of this geyser was visually realized. Initial attempts to stop the flow with proven chemical grouting techniques were not successful due to the extremely high flow and hydraulic pressure. Much of the day and into night, the scene became more intense. Everyone collectively threw out ideas to solve the dilemma. Muirhead asked if the manhole could be filled with sand to overcome the hydraulic pressure, then inject, stop the leak, and vacuum the sand out after.
This gave me an idea — we don’t need sand, we have water! An inflatable pipe plug was used to plug the downstream outlet while grouting ports were established in the invert and plumbed to the surface where they could be remotely operated. The manhole flooded with water and equalized the hydraulic flow from the large defect in the invert. Sandbags were placed into the hole to prevent grout infiltration. The grouting took place beneath 5 ft of water, filling the cavity beneath the manhole, the defect and surrounding voids. The manhole was drained by deflating the pipe plug and the polyurethane plug held tight. The next day, CK Masonry installed a new invert using high-strength mortar and coated the entire manhole with a protective epoxy.
Flow monitoring after the repair of this manhole confirmed a reduction of 0.75 MGD had been eliminated from the system. The proactive flow monitoring and rehabilitation program by the City, along with diligent inspection practices, proved to be valuable and necessary tools in the maintenance of the collection system. A recent inspection of the manhole is proof positive the repairs are holding perfectly after eight years.
Trenchless techniques not only save money and the environment but can be sustainable and considered permanent. I am very proud to have been a part of this team of professionals. Never again will I step onto a project thinking, “I have seen it all.”
Scott Kelly is with Prime Resins and is a member of the NASSCO Manhole Committee.