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.

Bad shape
“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.

For the rehabilitation process, Southern Trenchless uses trailer-mounted manhole/mortar spray rigs the company designs and builds. Each contains a 350-gallon water tank, 3,500 psi pressure washer, 6,500-Watt generator, mortar mixer, grout pumper and dual pallet material deck.

A manhole crew is comprised of five workers.

“We start by rebuilding the bench and inverts to reshape them so that they funnel all the flows properly from upstream to downstream without materials getting hung up,” said Dupré. “They are slopped so that solids debris falls back toward the center channel flow.”

Next the surface walls are cleaned with high-pressure water until clean, sound substrate is exposed.

“The manholes in Laredo are so deteriorated,” Dupré continued, “that bench building is more involved than when working on large trunk lines, and extra hydro blasting is required, tripling the amount of time usually required for manholes.

“Before applying cement liners, it is necessary to have a saturated surface-dry surface by pre-wetting so that the dry old substrate does not absorb the needed moisture during the curing process of the cement. Once the cement liner is applied, we steel trowel to a smooth uniform surface, a standard American Concrete Institute (API) process.”

When an epoxy top coating is required, a light brush finish is used to provide better bonding and interfacing of the product.

Dupré said a typical 48-inch manhole can be completely rehabilitated with cement liner from top to bottom in one hour. After the cement cures for 48 hours, application of the epoxy coating takes about 45 minutes.

“The average time to do a Laredo manhole was four hours,” he added.

Big bypass
Two types of bypass systems were implemented to avoid interruption of the sewage flow while rehabilitating manholes.

The longest was a conventional bypass system of 6,500 linear feet of 18-inch diameter fused HDPE pipe that ran along the city’s largest storm channel, Zacate Creek, which feeds into the Rio Grande River. The pipeline was connected to three 12-inch Godwin pumps.

“This bypass carried the main 36 to 42-inch trunk line for all the sewers for the entire city, and responsibility for its operation was entirely on our shoulders,” said Dupré. “This was the most nerve racking part of the entire project, because if the bypass line broke, untreated sewage would flow directly into the Rio Grande River, having state, local, and international repercussions. One month after we completed the project, the Rio Grande and Zacate Creek flooded Had we still been in operation, floodwaters would have taken out the bypass line. We got lucky there!”

Dupré said the second bypass was an internal system of 24-inch diameter pipes of a proprietary design developed by Southern Trenchless that operated at the bottom of manholes, allowing the line to remain in service while manhole repairs were made.

“The internal bypass saved money by eliminating traffic control requirements and lane and driveway closures so work did not impact top-side traffic,” said Dupré. “The city really appreciated that.”

Safety procedures included using a four-way gas monitor to check atmosphere levels for safe entry, Dupré said.

“We used an electric air blower to blow in fresh air supply, he explained. “And we used respirators meeting N95 OSHA standards to the prevent inhalation of solid particles. Each man who enters confined space is secured to a tripod and harness.”

Also part of the work in Laredo was CCTV inspection and cleaning 56,000 linear feet of the city’s main trunk line that started at 24-inches in diameter, grew to 30 inches, then to 36 inches and finally 42 inches.

Dupré said the pipe was at least 60 or 70 years old and had never been cleaned. Most of the cleaning was performed with a Vactor 2100 truck developing 80 gpm of flow. When a wall of super-heavy debris was encountered, a Brenford Sewer Hog was called in. The machine can deliver 350 gpm at 2,000 psi and uses sewer water because a water supply from a fire hydrant cannot keep up with the machine’s demand for water.

Dupré said he, Pedro Santos, and partner Ramon Closner established Southern Trenchless Solutions in January 2010 to serve the needs of Southern Texas which had no company specializing in manhole rehabilitation.

“Before we arrived, rehab crews had to come from Houston, Dallas and other Texas cities,” said Dupré. “It was clear a local rehab specialist was needed.”

Dupré said the company’s mission is broader than providing rehabilitation services, extending to research and developing its own methods, materials and equipment so they can serve the industry nationwide.

“We fabricate our own working rigs,” he said. “We have designed and tested our own materials — Manhole Mortar MSP and Ultra Coat Epoxy — which are listed in the 2011 Trenchless Technology Center Louisiana Tech University directory of Currently Available Products and Techniques for Manhole Rehabilitation.

“Our goal is to be a true turnkey operator offering customers a higher-level warranty under one umbrella. Usually a project has an applicator and cement and epoxy manufacturers which makes warranty service complicated because there are several entities. A one-source warranty would eliminate such issues.”

Dupré and Santos are ACI shotcrete certified nozzlemen with combined industry experience of 18 years. “We are dedicated to providing the most effective and cost-efficient solutions for our clients,” Dupré stressed.

Southern Trenchless Solutions, (956) 763-7898, www.southerntrenchless.com
Raven Lining Systems, (800) 324-2810, www.ravenlining.com
Godwin Pumps, (856) 467-3636, www.godwinpumps.com
Vactor Manufacturing, (815) 672-3171, www.vactor.com