Skyrocketing H2S Levels Prompt Emergency Manhole Repairs

By Jeff Griffin, Senior Editor | May 2014, Vol. 69 No. 5
A fully deteriorated manhole at Moody Gardens, Galveston, TX.

“This was the worst wastewater corrosion attack I have seen in my entire career,” said Dupré. “Inspections of the manholes found that 90 to 98 percent of the concrete was completely deteriorated or had lost all structural integrity, eaten up back to the surrounding dirt with no concrete and no rebar. We could not apply epoxy spray liner because there was no substrate remaining. The only thing preventing them from collapse was the concrete and rebar from the pavement.”

Therefore, seven of the manholes required fiberglass inserts. The eight was rehabilitated with a Manhole Mortar lining system, a Southern Trenchless product with 100 percent epoxy protective coating.

Rehabilitation of the manholes was complicated by the fact that the 18-inch main had to remain in service during manhole rehabilitation. In addition, coastal island ground water tables are from two-to-three feet deep, any new excavation would have incurred major costs to deal with dewatering work areas and bypass pumping.

To keep the 18-inch main in service, Southern Trenchless used proprietary flow control devices to control the flow and work without an active flow inside work spaces.

Workers prepare to place a manhole insert.
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“We had to excavate only the top cone portion of the manhole to install the fiberglass insert which was pre-cut to fit and align incoming pipes,” Dupré explained. “Once in position, we pressure grouted the annular space to compact and stabilize the ground around the manhole insert.”

Inserts were provided by Containment Solutions. HDPE risers, and epoxy coating ring and covers were supplied by Ferguson Waterworks.

Quick turnaround

Rehabilitation of the eight manholes was completed in two weeks. “Moody Gardens took immediate action once the manhole problems were identified as a crisis,” said Dupré. “Swift action by the owner, general contractor and management company avoided immediate failure of the system. The decision to proceed with a trenchless construction methods saved time, money and risk of damaging existing utilities.”

By not bypassing, Dupré said estimating savings totaled approximately $20,000 per week. Bypass pumping would have restricted crossings, streets, walkways and other inconveniences related to a bypass system.

Dupré added that issues with the Moody Garden sewer system being linked to the 18-inch city main remain to be addressed.