Sewer Pipe Lining – An Economic Solution for Pipe Rehabilitation By Tawana Albany Nicholas, Virgin Islands Waste Management Authority
Abstract
The United States Virgin Islands Waste Management Authority (VIWMA) oversees the maintenance and operation of the wastewater and solid waste infrastructure in the Territory. St. Croix is the largest of the four (4) islands of the United States Virgin Islands (USVI) with a population of approximately 50,600. The collection system on St. Croix is the largest system in the Territory, and consists of one (1) treatment plant, three (3) pump stations, twelve (12) lift stations, and approximately 102 miles of sewer piping ranging from four (4) inches to thirty (30) inches in diameter. Many of the major interceptors in the system were constructed in the 1970s with a design life of 25 to 30 years, and little maintenance has been performed since.
With scarce funding, VIWMA is constantly looking for cost-effective long term solutions to maintaining the aging wastewater infrastructure and preventing system failures. There are many methods of sewer line rehabilitation available today. The typical ‘dig and replace’ method has been increasingly avoided by many communities due to the serious challenges in excavation and restoration in some localities and the cost effectiveness of this rehabilitation solution. VIWMA has joined this trend and recently worked on its fourth sewer lining project for the island of St. Croix, with other lining projects schedule for next year. This report provides an overview of the solution for the sewer system rehabilitation of the LBJ Force Main, the Williams Delight Interceptor, the Krause Lagoon Interceptor, and the Anguilla Interceptor. The report will discuss the main steps of the sewer lining rehabilitation process. It will focus on the achievements as well as the challenges of each project; and, it will provide a unique perspective on the execution of a sewer lining project in a small community.
St. Croix Collection System Overview
The United States Virgin Islands Waste Management Authority (VIWMA) oversees the maintenance and operation of the wastewater and solid waste infrastructure in the Territory. Overseeing the maintenance repairs, rehabilitation, and replacement of the infrastructure, as well as capital improvements, are part of that responsibility. St. Croix is one of four (4) islands of the United States Virgin Islands (USVI) with a total population 50,600 according to 2010 U.S. Census Bureau data; and its wastewater collection system serves approximately 51.5% of its population. The collection system on St. Croix consist of one (1) treatment plant, three (3) pump stations, twelve (12) lift stations, and approximately 102 miles of sewer piping ranging from four (4) inches to thirty (30) inches in diameter. Many of the major interceptors in the system were constructed in 1970s with a design life of 25 to 30 years; and little maintenance has been performed since. Physical deterioration and system component failures now require rehabilitation or replacement of many of the sewer lines and manholes to help bring the system into conformance with Environmental Protection Agency (EPA) standards.
Trenchless Technology Overview
With scarce funding available, VIWMA is constantly looking for cost-effective long term solutions to manage and maintain the aging wastewater infrastructure to prevent system failures. There are several methods of sewer line rehabilitation available today. The typical ‘dig and replace’ method has been increasingly avoided by many communities “up to 70 percent in the sewer sector and up to 31 percent in the water sector” according to the EPA. The ‘dig and replace’ method involves soil excavation, shoring (when applicable), pipe removal, pipe replacement, soil backfill, and site restoration. Trenchless technologies leave the existing pipe in place and use it as a host for a new pipe or a liner so that many of the steps for ‘dig and replace’ can be eliminated. It is important to note that trenchless technologies are not applicable for all pipe rehabilitation. A pipe that is collapsed or has settlement of grade alignment may need to be dug up and replaced; or abandoned and replaced in a new route. Trenchless technologies are applicable to pipe with cracks, infiltration, joint offsets, and wall deterioration. They are also applicable when a project has limited above and below grade access, has safety concerns for deep excavations, a need to limit the environmental impact, has minimal impact on the public, and has time constraints.
The trenchless technologies available today are Pipe Bursting or In-Line Expansion, Slip Lining, Cured-In-Place-Pipe, and Modified Cross Section Liner.
Pipe Bursting is when new pipe (equal or larger diameter of host pipe) attached to a bursting or expansion head is pulled through the host pipe
Slip Lining is when new pipe of a smaller diameter than the host pipe is pulled through the host pipe and the space in between both pipes are filled with grout. Cured-In-Place is when a liner of the same diameter as the host pipe is pulled or
inverted through the host pipe, inflated, and cured in place.
Modified Cross Section Liner is when a new pipe of the same diameter as the host pipe is deformed, typically in a ‘U’ shape, pulled through the host pipe, and expanded (by heating and pressurization or cool down)
Project Summary
Each pipe rehabilitation method should be chosen based on the pipe and field condition.. VIWMA has not and does not plan to standardize or limit rehabilitations to one type of trenchless technology or limit the material used for each type of trenchless technology. VIWMA’s first trenchless technology pipe rehabilitation project was the Williams Delight Interceptor, consisting of 700ft of 30” diameter pipe. This was a Cured-In-Place-Pipe Liner installation using a felt liner material. The liner was inverted through the sewer line with water and heated with a boiler to cure. The second and third trenchless pipe rehabilitation projects were the Anguilla Interceptor consisting of 2,825ft of 24” and 30” diameter pipe, and the LBJ Force Main consisting of 4,500ft of 24” pressure pipe. This was also a Cured-In-Place-Pipe Liner installation; however the material used was a fibreglass (resin –impregnated fibreglass material). The liner was pulled into the host pipe, inflated against the exiting walls, and cured with Ultra Violet Light.
The Engineering Division of VIWMA recently completed its fourth sewer lining project in St. Croix; the Krause Lagoon Interceptor and Figtree Sewer Rehabilitation, Phase 3 & 4 Project. The project consisted of approximately 5,700ft of 24” diameter sewer lining and rehabilitation of fourteen (14) manholes. The sewer lining material for this project was manufactured by SAERTEX multiCom®. The material was composed of stitch-bonded glass-fibre fabrics and was cured in place with UV light. The sewer line depths below grade ranged from six feet (6ft) deep up to twenty feet (20ft) deep. The average manhole to manhole distance is 400 linear feet; the minimum distance is 155ft and the maximum distance is 600ft.
Inspection of the sewer lines to determine the condition and help set spending priorities is critical. With limited resources, the project scope of work included closed circuit television (CCTV) inspection with recommendation and/or confirmation to proceed with sewer lining. If maintenance funding had been available, cleaning and CCTV work would have been completed prior to completing the request for proposal (RFP) project scope of work. The importance of this became evident when it was discovered that 225ft of sewer line had a larger pipe size and an incorrect slope; that required the ‘dig
and replace’ method for a grade alignment repair. In addition, 700ft of sewer line had excessive debris that required additional work to get cleaned prior to lining.
The main steps of the Krause Lagoon Interceptor and Figtree Sewer Rehabilitation, Phase 3 & 4 Project were:
1. Temporary bypass flow for the sewer line to be repaired. 2. Sewer line clean and inspect
3. Pull liner in Place
4. Secure liner ends to a can. (The cans seal and anchor the ends of the liner.) 5. Inflate and insert ultraviolet (UV) light train. The light train is the curing device 6. Cure (and monitor) the liner
7. Inspect liner
8. Demobilize and remove temporary bypass
The sewer line was cleaned and inspected just a day or two before the liner was installed. A few spot repairs were required; however, the repairs were minor (removal of a section of the pipewall) and completed the same day the liner was installed. The liner arrived on St. Croix approximately eight (8) weeks after an order was placed (three (3) weeks shipping time). The liner was custom manufactured; its thickness is based on the sewer line depth. The contractor was required to verify the manhole to manhole distances during their inspection. Manhole rehabilitation was executed while the liners were being manufactured and shipped to the site. The liners were pulled manually into a manhole and pulled by winch through the sewer line to the manhole on the other end. The cans at each end are installed manually; and care must be taken to prevent the liner from getting over exposed and cured by the sun’s UV light. The light train vehicle came equipped with an air compressor; however, a mobile compressor was required for the long distances to inflate the liner material. With the liner inflated, care was taken to insert the light train to begin the curing process. The curing time is determined by the number of UV light bulbs used and the thickness of the liner. The average cure time was three (3) hours. The liner curing is monitored via video to ensure the liner was completed inflated with no ripples. After the curing is complete and the cans and light train are removed, the liner is cut at the edge or just inside of the manhole wall depending on the existing manhole configuration. A video of the liner post installation is recorded to document the quality of work; and, it documents the new baseline condition of the pipe for comparison to its future pipe inspections.
After the video recording confirmation, the line can be immediately placed in operation. However, depending on the project schedule, the bypass may be kept in service to clean and line an adjacent section of pipe.
Project Lessons Learned
As an Authority for a geologically separated territory, there are additional challenges that may not be present if the project were located in a major city. The challenges for the Krause Lagoon Interceptor and Figtree Sewer Rehabilitation, Phase 3 & 4 Project were related to the isolated location.
To receive economical cost per linear foot pricing, the minimum size for future VIWMA lining projects will be 2000ft. The cost for mobilization and the operation of a bypass line (also known as a pump around) starts to level off with a scope of 2,000ft; therefore the project funding can be optimized when more piping is rehabilitated under the same project.
Picture on Right: Cleaned Pipe Pre-Liner Installation
Picture on Left: Video Monitoring During Liner Curing
The operation and maintenance of a good bypass system is critical. It is important to protect the health and safety of the workers on the project, the public heath, and the environment. Failure of the plug for the bypass system can cause workers’ injuries and serious project delays. The bypass system must be leak free to avoid negatively impacting the public heath and environment. VIWMA best management practice dictates that at least one (1) and sometimes two (2) additional spare bypass pump on site at all times.
Due to the isolated location, another VIWMA best management practice is to keep additional spare parts for equipment on site. The project schedule and efficiency will be negatively impacted if equipment is down for an extended period of time for repairs. If spare equipment parts are not onsite, project workers may become idle because the delays due to the shipment of the required parts. In addition, material may be lost due to the down time; since some materials are time sensitive and the materials’ integrity are vulnerable to prolonged delays.
It is important to also note that after the rehabilitation method is chosen, the VIWMA project specifications require that the contractor is responsible for all labor, materials, and equipment necessary to complete the contract. VIWMA was responsible for the inspection.
Conclusion
Sewer lining and other trenchless technologies continue to advance quickly. Constant research and networking are required to keep up to date in an effort to choose the most cost effective solution for the site specific situation. Despite the anticipated and unforeseen challenges, the Krause Lagoon Interceptor and Figtree Sewer Rehabilitation, Phase 3 & 4 Project proved economically successful. The ‘dig and replace’ not only would have cost more, by more than 20%; but posed more safety concerns with 20ft deep trench excavations, and the project would have taken for time.
Bypass Pump and System Suction
References
1. U.S. Census Bureau, Census 2000 for the U.S. Virgin Islands and the 2010 Census for the U.S. Virgin Islands.
2. www.saetex-multicom.com
3. www.epa.gov
4. Friesenhahn, P.E., Leonard. “The University of Texas Sewr Rehab: Using Trenchless Technologies”. Available from
https://www.utexas.edu/utilities/services/mechanical/APPA-Sewer-Rehab.pdf Accessed 24 June 2015.
5. United States Environmental Protection Agency. Office of Water. Washington, D.C.: Collection Systems O&M Fact Sheet. Trenchless Sewer Rehabilitation. EPA 832-F-99-032. September 1999.
