The National Ground Water Assn. (NGWA) announced that ...
Seventy-year-old sewer lines were badly cracked and corroded, and segmented PVC pipe prevented a collapse
After extensively surveying the condition of the East Baton Rouge Parish sewer system, the Department of Public Works for the City of Baton Rouge, Louisiana, concluded that many of the interceptor sewers placed into service over the last seventy years urgently needed repair or replacement.
The use of video inspection and flow monitoring of the reinforced concrete pipe (RCP) and vitrified clay pipe (VCP) in the system revealed that these lines were leaking badly because of cracked and broken joints, severe corrosion and misalignment.
Not only were the lines in danger of collapsing, but also the streets and highways above the lines. Another problem was that many of the leaking sewer lines ran parallel to the Mississippi River and soil infiltration was beginning to weaken the levee system.
Shread-Kuyrkendall & Associates, Inc. of Baton Rouge was retained by the city to evaluate flow studies, review video inspections and prioritize the lines for repair. On the basis of their report, twenty-eight different sewer lines were selected for rehabilitation. Shread-Kuyrkendall determined sliplining and cured-in-place lining could be used to repair the lines. However, Rick Shread, P.E., owner of Shread-Kuyrkendall, noted that sliplining using PVC pipe was the most cost effective method for the project. "Even with the reduction in diameter, PVC has superior friction factors and hydraulics than traditional materials," he said.
Hartec Corporation, a local Baton Rouge contractor, was selected to handle the project. Hartec chose to use Vylon Slipliner segmented PVC pipe. This selection saved the costs of bypass pumping and closure pieces. Although cured-in-place lining was an option, material and installation costs were double that of sliplining.
Hartec called in Line One, Inc. of Lewisville, Louisiana, a sliplining specialist, to assist with the project. Line One performed the pre-inspection, cleaning and sliplining. Hartec constructed the insertion pits, controlled traffic and grouted the annular space between the slipliner and the existing conduit to hold the new pipe in place. Together, the companies sliplined 7,900 linear feet of 24­p54< diameter failing sewer pipe. The repaired sections ranged from 200 to 600 feet and included one section under Interstate 110.
Greg Strudwick, owner of Line One, Inc., said, "It was only a matter of time before all 28 sections of pipe would no longer be able to hold up the ground above them."
Strudwick noted that trunk lines are more vulnerable than other pipelines. "In sewers with diameters larger than 15 inches that have turbulent flow, gases are released promoting acid production at the crown that eats away at concrete. As the concrete is devoured, the pipe's wire reinforcement begins corroding and the structural integrity of the pipe is threatened. Moreover, surface roughness due to pitting slows the rate of flow. When our crews got in there to clean, between the debris and the condition of the walls of the pipe, the line was carrying one-half to two-thirds its capacity."
Although sliplining reduces the inside diameter of the pipe, the reduced inflow and infiltration combined with the smooth interior surface of the slipliner actually increased the level of the flow, according to Strudwick. "When the RCP lines were new, they provided an equivalent Manning's n of 0.013. Prior to rehabilitation, they were probably providing something like n of 0.020," Strudwick said.
According to Chris Hicks, executive vice president of Hartec, in many lines, flow was enhanced 35 to 40 percent after rehabilitation.
Working along the Mississippi River posed some unusual challenges and a tight construction schedule. Excavation is risky near the levee in Baton Rouge when the water table is high. In spring and early summer, the river swells with melted snow from upstream and rules out any underground construction. Even in the fall and winter, digging around the river depends on weather conditions further north. "If there was a flood in Ohio," Strudwick said, "we would have huge amounts of uncontrollable water."
Excavators were used to dig insertion pits and lower pipe sections. Pits were 10­p12 feet wide, 25 feet long and 15­p20 feet deep. Hartec used stacked trench boxes, sliding one-inch thick plates on the open ends and behind the box walls. "With saturated soil, we wanted to make sure it wouldn't flow in under the box walls or around the existing pipe," said Hicks. "It would endanger workers standing in the pit and also eventually migrate into the recently cleaned line."
Since the work took place during Baton Rouge's dry season, no dewatering was necessary. With trench protection in place, workers entered the pit and removed the top of the old pipe using sledgehammers and jackhammers. Cleaning buckets were pulled through the line with a pneumatic winch. After removing the debris from the line, the slipliner was lowered, joints assembled and the pipe pulled into place. The majority of the slipliner was pulled with a winch downstream from an upstream insertion pit. The flow carried some shorter segments of the neutrally buoyant pipe in place downstream.
During insertion, the previously installed joints were left protruding about 2 feet into the pit, and inflatable lifting bags were positioned at the spring line between the host pipe and the slipliner pipe. When inflated, the bags anchored the slipliner train in place during assembly. Workers lubricated the gasket and bell of the slipliner with subaqueous lubricant, lowered and positioned the next section of slipliner and winched the joints together. Closure pieces were constructed in the field when 15 feet or less were required to complete a section.
A lightweight, cellular grout was pumped between the host pipe and the slipliner. Bulkheads were built at the end of each line segment to contain the grout and prevent new sewage from entering the annulus. Three 2-inch tubes were built into each bulkhead. The grout was then fed through the center tube at the 12:00 position. The other two tubes served as vents at the 10:00 and 2:00 positions.
The sliplining project was one of the most extensive projects undertaken by the City of Baton Rouge. The Wastewater Collection Department learned a number of valuable lessons from the experience. Special Projects Engineer Ed Stumpf pointed out that although sliplining is trenchless technology, locations of entry pits have to be coordinated with utilities well in advance of the actual construction.
"They (the utilities) know best what they have down there and how to efficiently deal with it so pits can be constructed on a timely basis," said Stumpf.
Stumpf also recommended educating city officials about trenchless technology. For example, one pit, because of the obtuse angle of the sewer line, extended somewhat into the street, creating a minor disruption to traffic. "That generated some phone calls-but the disruption was nothing compared to what it would have been if some other method of construction would have been used."