February 2007, Vol. 19, No.2

Problem Solvers

Antiquated Sewers Get Modern Makeover

Problem: Widespread pipeline damage beneath bustling downtown district. Solution: Cured-in-place pipe technology.

 

Until recently, an antiquated system of brick sewer lines in need of rehabilitation lay beneath the city of Des Moines, Iowa. Most of the original combined storm and sanitary sewer lines were estimated to be 80 to 100 years old, and several sections under the downtown district most likely were installed in the late 1800s. More than 3660 m (12,000 ft) of brick sewer lines, ranging in diameter from 460 to 1520 mm (18 to 60 in.), were in need of rehabilitation.

But to repair such an extensive system required forethought, so as not to disturb the daily activities of the nearly 200,000 residents dwelling in this capital city. To ensure minimal disruption, the city chose the cured-in-place pipe (CIPP) technology of sewer rehabilitator contractor Visu-Sewer Clean & Seal Inc. (Pewaukee, Wis.).

Another issue was maintaining the longevity of the system.

“The brick itself has performed well,” said City of Des Moines Project Engineer Jeff Hansen, “but the mortar is failing the test of time.”

To ensure long-term durability, project managers selected liners made with Vipel® L704-FAH resin, a high-molecular-weight, isophthalic polyester engineered by AOC (Collierville, Tenn.) for CIPP use.

“To repair the sewers, we chose CIPP over ‘sliplining’ a thermoplastic liner inside the old pipe,” Hansen said. “Compared to CIPP, sliplining would have left us with less flow capacity and would have caused significantly more surface disruption.” He explained that using CIPP, the city was able to install most of the new liners through manholes that already existed — minimizing the number of additional holes to be dug to accommodate the curves throughout the system.

Resin-Saturated Felt
For this project, the city chose a nonwoven polyester felt liner with a corrosion-resistant thermoset resin by National Liner®. Visu-Sewer uses National Liner CIPP technology licensed from National EnviroTech Group (Houston). The felt liner was saturated, then inserted into the pipes in need of repair. Under pressure, the felt liner expands against the interior of the host pipe. When heated, the liquid resin cures into a solid that encapsulates the felt to produce a new smooth, continuous liner inside the old pipe.

Engineers encountered challenges throughout the Des Moines project. One issue was negotiating the resin-saturated liner through sweeping bends in sections of 900-, 1050- and 1200-mm-diameter (36-, 42- and 48-in.-diameter) sewers. In some areas, they had to install sections of liner where the grade changed as much as 4.6 m (15 ft).

The CIPP process required work crews to monitor the head pressure of the water that was “pushing” the inverting liner. The objective was to keep the pressure high enough to ensure complete and even application against the interior of the host pipe, and low enough to prevent the liner from bursting at its seam. Maintaining a pressure balance was easier when the route was straight or relatively level but called for incremental adjustments during sharp turns and steep grade changes.

Additionally, project staff had to keep the resin from curing prematurely when temperatures in Des Moines approached 38°C (100°F). Heat can cause the “crosslinking” chemistry of the resin to prematurely “kick in.” To help the resin-saturated liners keep their cool until inversion was complete, staff installed air-conditioned tents over the inversion sites. In some instances, they applied tarp-covered ice to cool the sites further.

Engineers then made adjustments when what were supposed to be 900-mm (36-in.) round cross-sections often ended up being 840-mm-wide by 1020-mm-tall (33-in-wide by 40-in.-tall) ovals. To address these situations, staff provided a drawing for each insertion to the liner supplier so that the liner geometry would change at the appropriate locations.

Project engineer Alex Rossebo said that the processing characteristics of Vipel L704-FAH resin had many benefits.

“Wet-out with the Vipel resin was never an issue, regardless of location” Rossebo said. “Wet-out of liners up to 48 in. [1220 mm] was performed at our facility in Pewaukee, Wis. For the 60-in. [1520-mm] liners, three over-the-hole wet-outs were performed onsite next to the Des Moines River. The longest over-the-hole insertion was approximately 1000 ft [305 m].”

Rossebo explained that the resin retains a high modulus of elasticity over time while providing excellent resistance to the corrosive environment of municipal wastewater — meeting the city’s requirements for long-term durability.

Emilio Oramas, AOC business manager for Vipel corrosion-resistant resins, pointed out that “AOC helped pioneer the development of isophthalic polyester resin technology that sets the standard for most CIPP rehabilitation projects around the world. We’re pleased that this technology could give the people of Des Moines ... a major infrastructure upgrade with little disruption to their daily lives.”

Contact Visu-Sewer at (800) 876-8478, visu-info@visu-sewer.com , or www.visu-sewer.com . For more information on National Liner, contact Ray Pavlic of National EnviroTech Group at (800) 547-1235 or info@nationalliner.com , or see www.nationalliner.com . For more information on AOC materials, contact Ben Bogner at bbogner@aoc-resins.com .