Rehabbing Double Stack Sewer Service Connections with CIPP

After 45 Years, the cured-in-place pipe (CIPP) process has become the “anchor” of trenchless pipeline rehabilitation. Since the initial installation in 1971, the evolution of materials and processes has brought about new applications for CIPP rehabilitation. It is the continued research and development over the past 45 years that has advanced the capabilities of CIPP, and, today, the resin-saturated liner, inverted in a pipe, is used to make repairs that were yesterday considered impossible.


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All over South Florida, municipalities are investing significant resources in renewing their sewer infrastructure — and for good reason. The groundwater conditions in certain areas of South Florida are atrocious, and the amount of water that can enter a sewer collection system through pipe defects brings a staggering cost to municipalities and their rate payers. Municipalities and their contractors are working diligently to renew and seal their collection system, but not without a unique set of challenges due to the groundwater conditions, infrastructure configuration, and double-stack service connections.

South Florida’s groundwater conditions are mostly running sand/water. When the system was installed, it was advantageous to bring the lateral pipes straight up as to be out of the groundwater table as much as possible. Since hardly anyone has a basement in South Florida, this was easy to do. Normally, you see lateral connections at 3:00 or 9:00, but in this case, the lateral connection is at 12:00 and runs straight up to a bullheaded TEE or double WYE fitting. From there, one pipe runs laterally to the left and another pipe runs laterally to the right. The lateral pipes were then split again by use of a double WYE fitting located near the property line servicing two properties in each side of the street. This unique pipe configuration can be found in municipalities all over Florida. At the time sewer systems were installed, it made good sense, since one lateral connection coming off the main provides service to four properties. This would drastically reduce the number of lateral connections, and the vertical rise to an elevation above the main pipe meant a much shallower lateral trench was needed to install the system.

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Numerous utility agencies have the same challenge, how to repair the notorious “Double Stack” Lateral pipes without the cost and disruption of conventional excavation. LMK Pipe Renewal (a Florida contractor) set out to find a solution to the unique double stacked piping system. The challenge for CIPP would be the ability to measure in detail the length of the riser pipe, as well as determine the orientation of the double WYE or TEE fitting. This information is crucial for manufacturing the custom made liner. Another challenge is presented when selecting the best material for the liner. Any stretch in the material would mean the liner would be too long causing wrinkling. Lastly, end seals by way of fitted CIPP gasket is always considered best practice for a leak-free liner… So, there were plenty of challenges in designing a CIPP solution for the notorious the CIPP Double Stack liners.

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Notorious Double Stack Laterals

Notorious Double Stack Laterals

The Double Stack CIPP Liner

The solution was to use a single piece CIPP that rebuilt the main/lateral connection (including compression gaskets) in accordance with ASTM F2561-16. A main liner tube measuring 20 in. in length was installed in the main pipe and includes a vertical lateral portion (5 to 6 ft long) that forms a T-shape. This T-shaped liner is outfitted with a flange shaped compression gasket that is seated into the lateral connection. The vertical lateral liner portion extends to another upside down TEE (hence the term double stack). From this point, two lateral liner tubes would extend laterally toward properties on opposite sides of the road. Additionally, it is not uncommon for the liner to transition from a 6- to a 4-in. diameter, so the accuracy of these measurements is also crucial. To ensure water tightness, hydrophilic O-rings were attached to the terminating ends of the lateral liner tubes as described in the F2561 industry standard.

The Installation Process

Full-Hoop Lateral Connection Liner outfitted with Compression Gasket

Full-Hoop Lateral Connection Liner outfitted
with Compression Gasket

Pipe Renewal has had numerous contracts where the notorious Double Stack laterals were found, but were historically not included in the CIPP work. The contractor worked closely with LMK Technologies (a supplier of CIPP products) on the equipment/methods used for obtaining the pipe detail and on how that information would be used to manufacture the custom tailored Double Stack liners. The contractor initially performed a pipe survey from the main pipe to the invert of the upper TEE fitting. Additionally, each lateral was surveyed from a clean-out to the center of the vertical riser, or center of the TEE fitting. The necessary information was obtained, a pipe detail was prepared, and the first liner order was placed. The crew then assembled a pipe mockup matching the pipe configuration they surveyed in the ground.

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The testing in the piping mockup was the same as an actual in-ground installation. The liner assembly is saturated with a thermoset resin under a controlled vacuum. The technicians first saturate the two lateral tubes using a calibration roller, then the main tube is saturated. The saturated liner assembly is then loaded into a launching device, towed through the main pipe so the opening in the launcher is aligned with the lateral opening, and for Double Stacks that opening is always at 12:00.

A technician is then outfitted with a lateral sewer camera at each sewer cleanout located at the property line on opposite sides of the street. The image captured by the sewer camera allows the technicians to robotically position the liner and apply air pressure causing the main tube to inflate and the lateral riser tube to invert straight up to the TEE. As the liner inverts, the camera operators pull the lateral camera back, keeping a visual on the inverting liner. As soon as the liner hits the crown of the inverted TEE, the two lateral liners begin to simultaneously invert. The lateral tubes can be 30 ft in length meaning the cleanouts can be 60 ft apart.

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The liners are inverted and cured with steam, which ensures a thorough cure. The liner is then cooled down and the inflatable bladders that press the liner tight against the pipe wall are removed. The result is a single piece homogenous CIPP including compression gaskets that renews the main connection, the riser pipe, and two laterals through one completely trenchless installation. This process was performed time and time again and as one might suspect, there was a lot of fine tuning. Good communication with the liner manufacturer and the ability to work with numerous municipalities in South Florida allowed the contractor the ability to prove a viable solution to the notorious Double Stack laterals.

Since that first installation in 1971, CIPP continues to be the ever changing advancing technology that provides sound engineering solutions for municipalities everywhere. Its versatility and proven longevity make it appropriate for a variety of applications.

Larry Kiest Jr. is CEO and CTO of LMK Technologies.