Tackling Manhole Rehab In Laredo

By Jeff Griffin, Senior Editor | March 2011, Vol. 66 No. 3

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.