In late 2011, three consecutive days of heavy rain overloaded the combined storm/sanitary sewer system in Grimes, Iowa, a city of about 8,000 residents just west of Des Moines. This produced the usual objectionable result: gray water backing up into residential basements.
The city’s municipal water department asked Fox Engineering of Ames, Iowa, to investigate. Founded in 1993, Fox Engineering is an environmental firm that’s served as the city’s wastewater collection and treatment consultant since 1998.
“We looked at the sanitary sewer that runs through a development corridor and found that a 24-inch outfall line was near capacity and subject to added inflow and infiltration,” says John Gade, P.E., a Fox Engineering employee who serves as the city engineer. “We recommended a new line be installed.”
The existing line was made of clay pipe and had been in the ground for nearly 40 years. Fox Engineering recommended installing a new line parallel to the old one. Although the old line would be plugged, it could be temporarily unplugged to accommodate future maintenance on the new line.
Unfortunately, it was winter.
“The actual work started in January and was completed in March 2012, not usually the ideal time of the year to be installing pipe in Iowa,” Gade says. “Fortunately, we had two things going for us: the pipe and unusually mild weather.”
The firm considered materials it had worked with, which included reinforced concrete, ductile iron, fiberglass, and PVC. Gade then met with Advanced Drainage Systems Inc. (ADS) Engineer Jim Merchlewitz and representative Paul Hutton to learn about projects in which SaniTite HP polypropylene pipe, introduced in 2008 specifically for the gravity-flow sanitary sewer market, had been used.
Based on an analysis of the product, Fox determined the pipe was durable enough to stand up to virtually any gas or liquid found in a sanitary sewer. While not part of the original analysis, it also had a price advantage.
“We’ve used a lot of the black ADS corrugated HDPE pipe for stormwater projects, but this was the first time we worked with SaniTite HP,” says Darin Keller, whose firm installed 1,857 feet of 48-inch and 3,000 feet of 36-inch pipe. “I like the product.”
Both diameters are triple-wall construction: an internal, corrugated structural core to minimize deflection surrounded by smooth interior and exteriors walls for enhanced flow. The design meets ASTM F2736 and ASTM F2764 requirements. Each section has dual gaskets and a banded reinforced bell that enable the pipe to exceed ASTM D3212 requirements for water-tightness.
“It doesn’t matter whether it’s summer or not, the installers are going to chip the bells and crack the spigots when they lower concrete into the trench box,” Keller says. “And we would have had to put in the plastic lock liner and welded it in at every joint. Fiberglass has issues in cold weather, and when you cut it there are fibers to contend with; you don’t want to breathe them and they can burn the guy doing the cutting.
“The ADS pipe isn’t heavy. You can drop it from 20 feet and it’s not going to break, even in the winter.”
Merchlewitz and Hutton suggested using A-Lok Products Inc. premium gaskets to ensure a watertight insertion of the pipe into precast concrete manholes. “It fits the triple-wall really tight so there’s no need to grout,” Keller says. “The pipe didn’t need any additional adapters, making installation fast but giving us flexibility. We had no leaks, and no problems.”
Keller used 13-foot instead of 20-foot sticks because the trench box was 25 feet long.
“The shorter pipe gave us room on each side for safety,” he says. “When we’re 20 feet deep, it’s too much of a reach for our excavator (John Deere 450; 110,000-class).”
Backfill consisted of a bed of rock over the top of the pipe, followed by the native soil.
Plan of action
Representatives from the state DOT and Iowa’s Statewide Urban Design and Specifications, the organization that develops common design standards and specifications for public improvements such as sanitary sewers, visited the jobsite to review the use of pipe in other installations.
In addition to vacuum- and pressure-testing, Keller actually went inside and mandrelled the pipe. “I crawled through the run to see if there were any issues,” he says. “Overall, the pipe went in good and tested good. We saw very little deflection. We also tested the joints, all of which were fine.”
Grimes’ population increased 67% from 2000 to 2010, and it remains one the fastest-growing communities in the state and region. To provide for increasing industrial and residential development, Fox Engineering has determined which trunk mains needed to be replaced.
“We developed a plan of action that will enable the city to continue to attract more residents and businesses,” Gade says. “It’s already moving ahead with plans to revitalize Main Street, a project scheduled to be completed in 2014. Our sanitary sewer system plan will handle all these and other future demands.”