Located on the north bank of the Rio Grande River in South Texas is the largest inland port on the U.S.-Mexican border — the City of Laredo. In the last two decades, Laredo has experienced significant growth in its trade volume. The population of Laredo has also increased considerably, exceeding 233,000, according to a 2007 census estimate. This growth, combined with commercial and residential developments, has increased the burden on the City’s storm sewer infrastructure.
One of the primary storm sewer systems for the City is a 114-in. corrugated metal pipe (CMP) storm sewer just few blocks from the Gateway to the Americas International Bridge. This 114-in. CMP trunk line serves an extensive drainage area of almost 130 acres in one of the older sections of Laredo before out falling into the Rio Grande River. Constructed more than 30 years ago, the storm sewer trunk line had deteriorated considerably and was in poor condition.
In December 2007, the City commissioned Lockwood, Andrews & Newnam Inc. (LAN), a planning, engineering and program management firm headquartered in Houston, to evaluate approximately 1,100 ft of the storm sewer line and provide recommendations to cost-effectively replace or rehabilitate it.
The project presented a number of environmental, utility and structural challenges. The first segment of the line was constructed through a former landfill area. However, no environmental reports remained regarding the landfill. Along certain sections, the storm sewer had a cover depth of more than 20 ft below the surface. The storm sewer line also crossed densely compacted residential and commercial areas including the Rio Grande Plaza Hotel, a Laredo landmark.
After coordinating with the City to find all available record drawings and GIS data, LAN conducted a manned entry of the CMP storm line to assess the existing condition of the pipe.
“The CMP storm line had experienced deflections in a number of locations resulting in a severely egged-shape pipe. In one location, the height of the pipe had been reduced to more than 67 percent of the original diameter,” said LAN project manager Temple Williamson, P.E., CFM. “There were evident signs of buckling of the CMP liner. The majority of the bottom portion of the pipe had deteriorated considerably.”
The team also found that the liners had delaminated and separated from the pipe wall at numerous locations resulting in obstructions to the flow. Significant voids behind the pipe wall were also visible at several locations where the liner had failed. Furthermore, large amounts of debris had settled at the upper section of the line, obstructing view of the pipe invert.
Following the condition assessment, LAN conducted a preliminary analysis. Three alternatives were evaluated:
- Fiberglass Sliplining: Inserting a fiberglass reinforced plastic (FRP) pipe into the existing pipe
- Spiral-Wound Pipe Renewal (SWPR): Inserting PVC strips with rib steel wound in place into the existing pipe
- Open Cut Installation: Removing existing 114-in. CMP and replacing with an 8-in. by 6-in. single box culvert
The team determined that using open-cut construction would create complications, given the depth of the storm sewer trunk line and the number of surrounding utilities. The trenchless methods minimized the impact to the surrounding areas and removed the need for excavation in an area formerly used as land fill. Additionally, the materials used for the two trenchless methods provided improved hydraulic characteristics that allowed the use of a smaller 78-in. diameter pipe than the existing 114-in. CMP, while maintaining the storm sewer’s capacity at the same time.
“We took into account a number of factors, including the construction cost, environmental concerns, impact to surrounding utilities, depth of the storm sewer trunk line and the limited right of way for the upper section of the project,” said Williamson. “It was clear to us that the most cost-effective solution would be to rehabilitate the existing line rather than remove and replace it.”
Consequently, LAN recommended rehabilitating the 114-in. storm sewer line using both FRP and SWPR.
Design and Construction
Following LAN’s recommendations to rehabilitate the old line using trenchless methods, the City selected Huff & Mitchell Inc., Cypress, Texas, as the general contractor, Glen Crawford as the project manager and Boring & Tunneling Co. of America (BorTunCo) as the contractor for sliplining. U.S. Composite Pipe South LLC, were the suppliers for FRP and Sekisui SPR Americas LLC, were the suppliers for SWPR.
To fulfill U.S. Department of Homeland Security (DHS) requirements, the team designed a special outfall at the Rio Grande. Specific provisions were also included in other areas of the project to comply with DHS stipulations, as well as to reduce impact to local residents and businesses.
The lower section of the 114-in. storm sewer, which had only 6 to 8 ft of cover, allowed for the easier insertion of sliplined fiberglass pipe. Approximately 300 ft of fiber-reinforced, thermosetting polymer mortar pipe (FRP) was sliplined in 20-ft joints along the lower section. The entire FRP section was installed from one insertion pit in one week after which grouting began. A special grouting technique that used high-strength cement grout to fill the holes and cracks on each side of the liner while tying the liner to the old pipe completed the operation.
The upper section of the storm sewer, which had more than 20 to 25 ft of cover, was more conducive to the use of spiral-wound PVC pipe. Approximately 800 ft of highly stiff, low-weight plastic liner was spirally wound into the existing pipe using a hydraulically self-running winding machine. Successive PVC strips were interlocked together by the winding machine, forming the structural liner. The annular space between the liner and the existing pipe was then grouted. The result was a strong composite pipe integrated with the existing pipe.
Just prior to the construction phase, the pipe experienced further deflection and loss of further material, causing major collapses in the pavement surface above the pipe. The latest deflections in the pipe eliminated the ability to reshape the pipe and caused LAN to downsize the recommended liner.
However, the improved roughness coefficients of the liners allowed the rehabilitated pipe to maintain the same hydraulic capacity as the original pipe. Regarding the collapse of the pavement, the City of Laredo immediately set up barriers to protect the public until construction could be completed to fill the void. In the case of one of the larger voids, it was used as the location of a manhole and as an access point for sliplining the host pipe.
Since the area for the lower section of the project is in a flood plain of the Rio Grande River, construction was scheduled at the beginning of October at the end of the wet season. The area had been dry preceding the projected starting date of construction. However, heavy rains came about the time work started, delaying the project from the beginning. The problem was compounded by the Mexican government using this time to release water owed to the United States causing the project area to be submerged longer than expected.
“We considered constructing a coffer dam, but we found that would take too much time to get the permit from the International Boundary and Water Commission,” said Williamson. “The lower section of the project was installed once the water level of the Rio Grande receded in one week and then grouting operation began.”
The construction of the upper section of the project offered its own challenges. A large 20-by-20-by-15-ftvoid was discovered above the host pipe and below the intersection of the street above, which could have resulted in a catastrophic collapse of the street. The void, Williamson explained, was caused by erosion from runoff leaking from a 60-in. CMP connected to the host pipe from above by a vertical 90-degree bend. The section below the springline of the 60-in.pipe had completely corroded away, allowing the storm water to erode the bedding around the pipe to create the giant void.
“The City immediately closed the intersection and the pavement was removed to fill the void and replace the lateral based on a design that LAN developed,” said Williamson. “To keep the pipe structurally secure, lining of the host pipe was completed while the void was filled with backfill and the lateral replaced.”
The void was repaired in two weeks and a potential disaster was averted.
The use of trenchless methods helped preserve the service of one of Laredo’s primary storm sewer systems and prevented the catastrophic failure of the 114-in.CMP, while resulting in a timely delivery.
“One of the advantages to allowing the use of two trenchless methods was the increase in the number of qualified bidders, which ensured a competitive bid process for the City,” said Williamson. “The project was the first of its kind for the City and any trenchless contractors who bid the job would be from outside of Laredo. So the City wanted to make certain that there were a number of qualified bidders.”
The City’s initial cost estimates had placed the construction cost of the project at approximately $1.8 million. However, the final construction cost was just more than $1.2 million, resulting in a savings of almost $600,000 to the City. The use of trenchless methods also fast-tracked the construction schedule. The liner for the host pipe including the grouting was installed within two months for the lower section and the entire construction was completed in eight months, including the emergency repair.
Jayendran Srinivasan is a technical writer for Lockwood, Andrews & Newnam Inc., a national engineering firm offering planning, engineering and program management services.