The City of Shreveport, Louisiana, is a relatively small community with a population of around 195,000, in 2016.
The City’s sanitary sewer infrastructure includes more than 1,100 miles — 5.8 million ft — of gravity sewer lines and force main and more than 125 sewer lift stations. With years of neglect and reactive, rather than proactive maintenance schedules, much of the City’s sanitary sewer infrastructure is old, undersized and in need of repair and rehabilitation.
Due to sanitary sewer overflows stemming from the aging infrastructure, the City entered into a federally-mandated consent decree with the Environmental Protection Agency and the Department of Justice in 2014. This consent decree requires the City to take remedial measures to eliminate overflows and reduce inflow and infiltration (I/I) in the system.
An Investment for the Future
The consent decree mandates that the City address system deficiencies in five phases, broken out by areas. Over the next 10 years, the City will be investing over $500 million in repairs and rehabilitation, and approximately $250 million in capacity upgrades of their sewer system to comply with the consent decree.
Construction for the first phase is expected to cost more than $180 million, by far outweighing the City’s normal spending on sewer infrastructure. Phase 1 design work started in early 2015, allowing two years to complete design and construction by the end of 2017. The City has made a major investment that will help revitalize the local economy, renew the public infrastructure for the next 100 years and eliminate sewer overflows.
The City established the Clean Water Shreveport program to execute this work. To provide adequate expertise and additional staff in managing the design and construction aspects of the consent decree, the City hired CDM Smith as its program manager. The City also selected four local design firms for Phase 1 engineering: Cothren, Graff, and Smoak Engineering, Balar Associates, Aillet, Fenner, Jolly & McClelland, and Civil Design Group to serve as its design engineers.
The City’s sanitary sewer infrastructure had severe defects; a small rain storm (<1 in.) could easily result in overflows from manholes, into drainage ditches, and backup into homes. While some of this can be attributed to grease build-up in the system or illegal catch basin or drainage ditch connections, the majority of the issues are due to aging infrastructure that has been in place well beyond its recommended lifespan.
The City’s existing sanitary sewer system consists predominantly of vitrified clay concrete and PVC pipe. The vitrified clay pipe has joints every 4 ft, and suffers from crushed pipe, loss of roundness, cracking, broken pipe, sags, light to heavy root intrusion and improper service connections (made as break-ins to the pipe). The concrete pipe has been observed to be severely corroded, with its aggregate exposed. The area is flat with a high groundwater table for a large portion of the pipes and structures, perfect for infiltration into the system.
The City’s practice was to locate the sewers in planter strips, alleyways (some were closed/abandoned), or in easements that run through the backyards of houses. Within these strips and easements are other utilities (power, telephone, cable, storm drainage ditches and pipes) and lots of trees that contribute to root intrusion throughout the sanitary sewer system.
In order to eliminate sanitary sewer overflows, the City’s Phase 1 goal is to reduce infiltration and inflow by 40 percent. The Phase 1 sewer area consists of approximately 1 million lf of pipe, 2,400 manholes and 17,500 laterals. Existing pipe sizes range from 6 to 42 in., but required upsizing in various areas to 8 to 54 in., based on capacity recommendations from a hydraulic model. About 10 percent of lines requiring upsizing are 36 in. or larger.
The initial planning recommendations included:
• 80 percent Open-cut Replacement
• 5 percent Cured-in-place Pipe Liner (CIPPL)
• 15 percent Re-assess Later
The recommendation was to replace 800,000 lf of piping in Phase 1. Local historical bids for open-cut construction in the City have traditionally been low; planning cost averaged $140/LF. On that basis, it was cost-feasible and could be done by small and large contractors.
Why was CIPPL not given serious consideration early on? Because many of the service connections were break-ins or vitrified clay taps at an angle with a smaller than 4-in. pipe shoved inside the tap. The service connections would have to be dug up and spot repaired and the cost for CIPPL would be nearly the same as open-cut. This, in combination with the City’s initial preference to solve almost all issues with its sanitary sewer infrastructure with an open-cut approach resulted in a heavily-weighted recommendation.
However, there were only a few bidders, and the open-cut costs were substantially more, averaging $340/LF. This increased the individual project costs 50 to 60 percent more than the budget. Initial projections pegged the Phase 1 cost at approximately $280 million, a whopping $100 million above the City’s budget for Phase 1 construction. An adjustment was needed rapidly to contain the overall cost.
Change in Approach
Based on previous experience on other programs, the program team was convinced that trenchless rehabilitation was a practical, cost-effective approach that had tremendous success for other municipalities in meeting their consent decree goals. A CIPPL rehabilitation approach could bring down the cost by millions of dollars, with average costs ranging from $80 to $100/LF. The City and the program team moved quickly to reconsider rehabilitation using trenchless construction methods.
The program team brought in engineering experts and rehabilitation vendors (CIPPL, sliplining, coating liners), and added to the team a Shreveport native – retired CIPPL contractor and industry expert Kerry Coker. With more than 30 years in the business, Coker was able to educate the City on the benefits and stability of the CIPPL technology and show a vision of how the system could function for many years in the future. Consequently, the laterals were closely evaluated and many situations that were initially ruled as needing spot repairs were now considered appropriate for lining.
The program team prepared a set of guidance documents to assist the engineers with trenchless design, and conducted workshops with the engineers, City personnel and small, local and minority businesses to encourage participation in CIPPL projects.
CIPPL of mainlines and lateral connections was found to be the most cost-effective means of sewer rehabilitation with major long-term benefits, including some reduction in I/I, elimination of root intrusion and extension of the useful life of the collection system. The City saw the benefit of trenchless construction as a solution – reduced costs, shortened construction schedule, reduced impacts to the community, all while restoring the integrity of the system. The engineers quickly reassessed their projects to use trenchless construction as a cost/schedule mitigation tool.
Some of the benefits observed to date are:
• CIPPL can reduce costs by 50 to 75 percent vs. open-cut
• CIPPL can shorten construction time by 60 percent vs. open-cut
• Pipe bursting in lieu of open-cut whenever feasible greatly reduces the environmental and community impacts.
“The traditional ‘spot repair’ approach that has been used in the past to remove I/I assumes that all defects are visible when conducting CCTV inspections. However, in sewer systems that exhibit large amounts of rainfall-dependent infiltration, a great number of defects may exist which cannot be seen during dry weather CCTV inspections,” said Kunay. “Since not all CCTV inspections can be conducted during wet weather events, a comprehensive rehabilitation approach is the most effective way to ensure that all defects are being addressed. The implementation of this approach is especially important when facing a consent decree which requires a municipality to remove large percentages of I/I by volume in order to come into compliance.”
This comprehensive approach focused on sealing the system as much as possible regardless of pipe condition. Lining of manholes in conjunction with pipe lining was essential to minimize as many potential sources of infiltration as possible, turning the system, in effect, into a jointless conveyance.
CDM Smith has been evaluating and employing numerous technologies throughout the country while tracking their progress. In several national programs that CDM Smith has assisted, I/I reductions as high as 50 to 65 percent in flow and volume have been achieved vs. the national statistic of 10 to 20 percent with CIPPL alone. In these cases, the municipalities have significantly decreased overflows and service backups. This is why a more comprehensive approach was preferred to achieve high levels of I/I reduction by volume.
In a short amount of time, the program and projects were reshaped to result in:
• 25 percent Open-cut Replacement
• 56 percent CIPPL
• 19 percent Re-assess Later
The bar chart below provides a comparison of the initial approach and the trenchless first approach.
Six pump stations will be repaired/replaced, and approximately 500,000 lf of sewer pipe in the first phase will be CIPPL’d, the largest rehabilitation program in the country.
Working with the City on significant outreach to the construction community, Coker was able to stimulate program interest, resulting in competitive bids. “The trenchless industry embraced the City of Shreveport’s efforts and ideas by have six to eight prime bidders on every project. Almost every major CIPP contractor is working on City of Shreveport projects,” Coker said.
Seeing The Benefits Today
Phase 1 construction costs, which were trending upwards of $280 million, have been brought down to approximately $180 million, a reduction of almost $100 million. Costs are expected to decrease further as more trenchless opportunities are found.
This massive effort was only possible with the strong support and leadership of the Mayor and the City’s Department of Engineering and Environmental Services, and Department of Water and Sewerage.
“When comparing the cost of trenchless technology to traditional open-cut, there is a cost savings of roughly $200 per linear foot,” said Mayor Ollie Tyler. “With the amount of work taking place throughout Shreveport to meet the Consent Decree, we’re looking at a savings of at least $100 million utilizing trenchless technology.”
Had it not been for the vision of the mayor and multiple City departments, the program team would not have been able to accomplish this ambitious goal. Thanks to incredible teamwork, the City of Shreveport is leading the way nationally in the use of trenchless technology, creating an infrastructure that will stand the test of time for the 21st century.
Judy Nishimoto and Rajbarath Panneerselvam are project managers, Jonathan Kunay is trenchless technology specialist and Kerry Coker is project/construction manager at CDM Smith. Autumn Permenter is assistant city engineer with the City of Shreveport. Judy Williams is communications specialist at Williams Creative Group.