Emergency Repair Turns to Long-Term Rehab
June 8, 2012No one hopes for a roadway collapse. Infrastructure failure inevitably throws up a cloud of crisis along with its attendant debris. But when two road cave-ins forced the Town of Chesterton, Ind., to take emergency action, utilities superintendent Rob Lovell discovered that such a cloud truly can have a silver lining.
Long Time Coming
In 2008, two key sanitary collection system manholes were failing so badly, they led to two $500,000 roadway collapses. Lovell and his field maintenance chief Jay Iler began searching in earnest for a permanent crumbling manhole fix. They identified spray-on relining as the best long-term solution to what they believed was a corrosion issue.
In early December 2010, Lovell and Iler met with Barry Hubbard of Conco Spray Solutions of Indianapolis to inspect the manholes at 8th and Porter streets and 15th and Morgan. Inspection sheets were completed with pictures indicating dimensions, incoming pipes and necessary rehabilitation.
By this time, the 8th Street manhole had collapsed on one side, and significant hydrogen sulfide (H2S) damage — noted on both holes — was identified as the culprit. The discovery of similar significant structural compromise prompted the inclusion of two additional nearby manholes — one at the water tower at Porter & Cherryhill streets and a smaller one just west of the 8th and Porter intersection — to the project.
Emergency repairs to the two initial road collapses took time. At the end of 2011, the permanent repairs were scheduled so temporary traffic diversion would coincide with local schools’ spring break in 2012, lessening student bus traffic.
Corrosion vs. Structural Repair
With such rampant H2S damage, it became apparent that more than simple corrosion repair was needed. The water tower manhole, 4 ft across and 14.2 ft deep, had concrete walls that would crumble at a slight touch, and the bench and invert needed replacement. At 8th and Porter, the 6-ft x 25-ft precast manhole had the same bench and invert problems, with inches of its walls already sloughed off. This is a critical lift station hub, with a main line running directly to the treatment plant.
The 4-ft x 16-ft manhole at 15th and Morgan was the oldest of the assets, a brick-built structure from which all mortar had been eaten away. It, too, required a bench and invert rebuild, but its major challenge was working around the many lines that dropped into it. At 8th Street west of the intersection, yet another bench-and-invert rebuild was needed in the 4-ft x 5-ft manhole. This one also evinced an extensive leak at the inlet pipe.
On any buried structure, the presence of such active infiltration changes the job from a simple surface corrosion repair to serious structural rehabilitation. Most transport pipes are located in low areas, close to the groundwater table. Many are immediately adjacent to or beneath active waterways, increasing hydrostatic load.
This is calculated as: 2.31 ft of water = 1 psi of hydrostatic pressure at the bottom of the pipe. Extrapolated over a single square yard, this load increases exponentially to 1,296 lbs — more than half a ton.
So, says design engineer Chip Johnson of Sprayroq Inc. — whose Spraywall product was chosen for the rehabilitation — even if there is no apparent structural issue, once water has violated the wall, it will continue to weaken. A thin coating of hydraulic cement — a corrosion repair — will work for a short period, but eventually, the water pressure will blow out that spot patch. The resulting repair may be massive and require substantial surface disruption.
Monolithic Strength Within Budget
Lovell and Iler had done their homework, scouting new repair products at seminars and trade shows. “You always want to be on the safe side when choosing a solution,” says Lovell. “Sulfuric acid doesn’t affect Spraywall, so it was a safe choice to rebuild structural integrity. Conco was our local provider of Spraywall for structural support, which we needed as opposed to a trenched barrier replacement. We did four manholes in one week with Spraywall, whereas a trenched rehab would have taken at least that per manhole.”
Lovell’s department usually has about $50,000 set aside for rehab projects each year. This one cost $34,000. “We’re trying to budget for so many manholes per year. Especially if they’re not yet severely damaged, we have lots of options for corrosion, wear and structural (repair).”
Sprayroq designs its Spraywall applications for water pressure loads — they can be designed along ASTM 1216-09, Appendix XI guidelines — effectively stopping further structural deterioration. Chesterton’s design assumed partial deterioration, so the application would support the hydrostatic needs of the structure without requiring excessively expensive thickness. As application moves toward the top of the structure and hydrostatic load decreases, thickness is reduced to 250 mils and continued to grade. This ensures a monolithic structural repair that eliminates seams, discouraging re-infiltration.
Preparation and Application
The town closed the intersection for about a week using traffic detours. Lovell was at the site often throughout the day and into the evenings. “We stayed late some nights, but my collections crew rotated in and out,” he recalls. “I was heavily involved in direct supervision and kept informed by phone.”
He was concerned about being able to bypass the key hub manhole at 8th and Porter during the process. About $14,000 of the budget went toward plugging certain lines and dropping pumps into upstream points, as well as running aluminum surface lines and constructing a temporary header system that all three pumps fed into. It discharged into a downstream manhole.
All manholes were first power-washed at 5,000 psi pressure to remove corrosion, crumbled concrete, mortar and loose brick. Lovell detailed a collections crew with a Vactor truck onsite to help Conco clean out demolition debris.
BASF Waterplug hydraulic cement was applied to stop leaks. Strong Seal Mortar was used to smooth around pipe connections and joints, and to rebuild the bench and invert areas. Once these applications cured, the Spraywall application was made to create a “manhole-within-a-manhole” seamless structural repair.
It’s All About Results
Some locations had 24/7 operations, staged so the high pressure crew would clean out a manhole, followed by the prep crew and application crew, then move on to the next manhole. Lovell committed an average of five people to helping the contractor with minor traffic control, temporary lift station shutdowns and cleanouts of the wet wells for transport to a treatment facility.
His biggest challenge was controlling lift station flow, solved by installing diversionary directional flow hose socks in the 15th and Morgan hole. This allowed two critical lift stations to continue to pump.
He is happy with the outcome of the repairs, which turned an emergency situation into an opportunity to create sound, long-term structural integrity. He encourages other municipal managers to keep themselves informed about new products and technology, so they might experience their own silver lining scenario.
“Keep an open eye and an open mind, and attend trade shows and seminars,” he advises. “You may discover a new way to approach a problem you’ve been struggling with or that could come up in your near future.”
Mary Shafer is a technical writer and social media strategist for Creative Raven.