Complications On The Rio Grande

Storm Sewer Trunk Line Is Successfully Rehabilitated Using Two Trenchless Methods
By Jeff Griffin, Senior Editor | May 2009 Vol. 64 No. 5
The access pit for FRP insertion.

Repairing or replacing old, deteriorating utility infrastructure that has been buried in the ground for years and often directly beneath homes and businesses is a challenge that has become common for the nation's utility providers.

Indeed, site conditions often make it impossible to make excavations to install new pipe, leaving rehabilitation the only construction option. Fortunately over the past decade, technologies have advanced to the point that rehabilitation often is the most cost effective method of construction, regardless of surface conditions.

The population of Laredo has grown steadily over the past decades, more than doubling between 1969 and 1990 and reaching more than 217,000 by 2008. One result of this growth has been a strain on the city's storm sewer system, requiring significant improvements. Commercial and residential developments, and in some cases encroachments to storm water easements, has complicated the problem.

A recent project to rehabilitate approximately 1,100 feet of 120 inch outside diameter corrugated metal pipe (CMP) storm sewer trunk line is an excellent example of how trenchless rehabilitation technologies can be utilized. On this project, there also were potential environmental issues because a portion of the site had once been a landfill. Therefore, it was important to limit excavation during construction, and trenchless methods allowed that to be accomplished.

The rehabilitation process reduced the size of the pipe from 114 inches inside diameter to 78 inches, while maintaining capacity due to improved flow characteristics of the structural liners.

"The CMP storm sewer was constructed more than 30 years ago and serves an extensive drainage system through an older section of the city of Laredo near the Gateway to Americas International Bridge which is one of the city's five international bridges to Mexico," said Temple Williamson, P.E., CFM, project engineer, Lockwood, Andrews & Newnam Inc. (LAN), engineering firm for the job.

"The project," he explained, "addresses many utility and environmental issues among which is crossing a former landfill area where the existing storm sewer has a cover depth of more than 20 feet below the surface along certain sections of the project. The existing storm sewer line also transverses densely compacted residential and commercial areas including the Rio Grande Plaza Hotel, a Laredo landmark, before the line outfalls into the Rio Grande River. Additionally, numerous public facilities would be impacted by any open cut construction."

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