Challenges in Sealing Large Diameter Lateral Connections

In keeping with the nation’s focus to improve the condition of water and wastewater infrastructure, the St. Louis Metropolitan Sanitary District (MSD) has recently used cured-in-place pipe (CIPP) to renew and seal lateral connections on large diameter trunk sewers.

Sealing lateral connections is a common practice for MSD for reasons well known to municipal agencies and the engineering society. However, when MSD decided to utilize a structural full-hoop main/lateral connection outfitted with a compression gasket for forming an engineered seal, some unique challenges were encountered that will set precedence for many other sewer districts in how to properly renew and seal lateral connections. 

In March 2012, a sewer renewal project that included the renewal of mainline sewers ranging from 8 to 24 in. and the sealing of lateral connections was awarded to Insituform Technologies LLC. The main and lateral pipes were renewed using CIPP, providing MSD and its customers a quality CIPP system that is structural and has a connection gasket seal that is comparable to new pipe. As with most trenchless pipe renovation systems, the advantages include cost-savings, minimal disruption to the public and a green environmentally friendly solution to the typical open-cut replacement methods.

The most important initiative that MSD currently has under way to serve its community is its multi-billion dollar, multi-year Capital Improvement and Replacement Plan (CIRP). This program, the largest and most extensive capital program MSD has ever embarked on,  is meant to expand and rehabilitate MSD’s entire wastewater collection and treatment capabilities, according to data published by the St. Louis MSD regarding its Capital Improvement and Replacement Program.

This project, like most CIPP projects, was just another day in the business of installing a flexible liquid pipe for renewing old decaying leaking pipes. The Insituform crews quickly renewed the main pipes. Insituform had sub-contracted Performance Pipelining Inc. (PPI) is the oldest original licensed installer of the T-Liner lateral renewal system manufactured by LMK Technologies LLC.

The T-Liner process is a cured-in-place pipe that consists of a short (18-in. long) full-hoop mainline liner tube and a lateral tube that is resin saturated and inserted robotically from within the main pipe. The liner assembly is compliant with ASTM F2561-11, which means the interface seal is structural and can be designed by a professional engineer per ASTM F1216, Appendix X1. The liner assembly also included a seamlessly molded neoprene rubber compression gasket that forms a flexible engineered seal, as the gasket expands when exposed to water. Renewing and sealing lateral pipes is a daily routine for the PPI crews. This routine remained much the same on this project until the crew encountered the 21- and 24-in. trunk lines. Up to this point, the project was smooth sailing, but as the pipes increased in size, so did the challenges, which finally led to temporarily shutting the lateral crew down. 

As it goes in the field of trenchless pipe renewal, when it rains, it pours and in this case, as the pipes increased in size, so did a host of issues as the site conditions worsened and the manholes became more difficult to access. Many of the manholes were located in backyards, ravines and thick woods. In one instance, the crew had to create a 75-ft path through heavily wooded terrain just to reach the manhole. However, bad access points are often a challenge for any pipe renewal crew, which is typically overcome by applying proper methods and equipment combined with ample time.

So, even though the manholes were tough to access, it was clearly no reason to halt the lateral work. In addition to tough site conditions, the pipes were also getting deeper and deeper and by the time the lateral crew had made its way into the 24-in. trunk lines, flows were very heavy, which required by pass pumping through backyards, ravines and heavily wooded areas.

While many of the sewer renewal projects on MSD’s CIRP involves the installation of new sewer pipes to accommodate growth, there is also a major focus on using trenchless processes to improve and renew the existing collection system. The rehabilitation focus is primarily on mainlines and lateral connections — two areas that allow excessive amounts of infiltration and inflow (I&I) to enter a collection system.  Some of the many challenges faced during this project were definitely tough terrain and location with proximity to bodies of water.

However, the greatest challenge that led to temporarily halting the lateral sealing works was the difficulties encountered during the robotic positioning and inversion of the lateral liner assembly in the large diameter mainlines.  Much to the surprise of the lateral crew, pipe inspection and measurements identified that the 6-in. lateral connection fittings comprised a 30-degree WYE fitting with a teardrop opening ranging from 10 to 15 in. in length. The opening was so large that the normal 18-in. long-full hoop interface seal was actually gathering during inversion and drawn up into the lateral opening.

Since the lateral liner assembly is robotically positioned using a Schwalm robotic cutter, the crew was readily equipped to grind and remove a few failed liners out of the lateral openings. While the crew was cleaning and preparing the pipe, the LMK engineering department was working around the clock to make modifications to the installation equipment and designing custom-tailored Main/Lateral CIP-liner assemblies to fit the unique lateral pipe connections.

“We had to increase the length of the launching device and the overall length of the full-hoop interface seal while using laser technology to measure and model the large teardrop aperture required in the interface seal,” said Sahar Hasan, lead design engineer at LMK Technologies.

As many entrepreneurs before our time, this team was outfitted with the persistence necessary to overcome the challenges at hand that eventually matured with success. The lateral pipes on this project were robotically cleaned, inspected and renewed 3 ft up the lateral from the main pipe with no cleanout access. In this case, size did matter and as other sanitary districts set their goals on sealing large diameter lateral connections, let the challenges and successes described in this case study be an opportunity for others to succeed. As each of us strive for success, and mature by hitting the mark, the result is success for our communities and the trenchless industry in general.

Kristina Kiest is director of marketing at LMK Technologies LLC.
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