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Tackling Manhole Rehab In Laredo
Laredo, TX, founded more than 250 years ago, on what today is on the border of the United States and Mexico, has grown to a bustling city of more than 230,000 people with another 660,000 living just across the Rio Grande River in Laredo’s sister city of Nuevo Laredo, Mexico.
The city’s utility department provides water and wastewater services to more than 46,000 residential and 4,800 business customers. Wastewater is collected by a network of more than 395 miles of pipe and conveyed to five treatment plants with a total capacity of 21 million gallons per day (mgd). Treated discharge goes into the Rio Grande River.
As are many American cities, Laredo city officials are addressing the deteriorating infrastructures of its basic utilities. Two projects completed over the past several months have gotten Laredo off to a good start rehabilitating long-neglected manholes in its sanitary sewer system.
Laredo has never had a maintenance program for manholes, said Oscar Martel, E.I.T., who supervised various phases of the project for the city.
“Existing conditions of the manholes were in poor conditions, and replacing them would be expensive,” he continued. “Rehabilitation of manholes through pressure washing and rehabilitation with a cementitious liner and coating of epoxy was a viable alternative. This method is more convenient and extends the life of the structure without replacing it.” Martel now is with Castle Engineering & Testing LLC, Laredo.
The rehabilitation project involved 150 manholes and included CCTV inspection and cleaning of approximately 10 miles of the city’s main trunk line comprising pipe in diameters of 30, 36 and 42 inches.
The project contractor was Southern Trenchless Solutions of Laredo.
“These manholes were in the worst condition of any we’ve seen anywhere,” said Eric Dupré, Southern Trenchless Solutions project manager. “The hydrogen sulfide (H2S) levels are the highest we’ve seen and the system is closed with no ventilation, basically making it one big H2S pressure cooker requiring ventilation and safety equipment to do the work. The amount of hydro blasting to reach solid substrate was between one and three inches, requiring as much as two hours per manhole of hydro blasting with 3,500 psi equipment.”
Dupré said the rehabilitation process for each manhole included reconstructing the bench and inverts, then applying a shotcrete spray allied with ultra-high strength mortar two to four inches thick, followed by spraying on a 100 percent solids Epoxy protective coating from Raven Lining Systems.