Minnesota City’s Water Main Replacement Succeeds Below Tight Street Populated With Underground Utilities

Duluth Standardized on HDPE PE 4710 Pipe

DULUTH, Minn. – A new potable water pipeline that had to be installed under a narrow, one-way downtown street here challenged the logistics and coordination abilities of the city’s utility department.  Surrounded by buildings, the use of large equipment was restricted and a well-mapped out plan had to be created and followed.  In total, more than 900 feet of 20-inch DIPS diameter pipe was installed under Michigan Street in a project that spanned five  months and was completed in October 2016.

“The project basically came about because the corridor through our downtown area is very, very full of other utilities, and one of those lines is the local power company’s duct bank which they needed to replace,” explained Howard Smith, P.E., project coordinator for the Engineering Division of the Duluth Public Works and Utilities Department.

“What the power company  couldn’t do was take the old line out of service or relocate and remove it, then put the new one in its place, so they had to put the new one right next to it.  The only problem was that our water main was in that spot.  This existing water main was from 1888.  It was 16-inch cast iron with leaded joints, which actually had a pretty reliable history of service for us.  One reason for this was the sand backfill that was brought to the site – it was nice and clean.”

Smith and the project team developed a plan that would replace the old water line.  The new pipe would be 20-inch diameter DIPS, high-density polyethylene (HDPE) PE 4710 SDR 11 that would have sections heat fused to produce a leak-free, 950-foot run.

“The first project I worked on with the city using HDPE was in 2003 for another water main for a whole neighborhood.  I think HDPE pipe has been in use here since the mid-1990s.  Basically all we have in our spec right now for water main material is HDPE and we’ll be using it a lot in the future.

“We had a lot of confidence using HDPE pipe for this water main that we would not have any water leaks or issues in the future,” he stated.  “We’ve had very good track record with butt-fused HDPE pipe for not having leaks.  And we figured that using it in this area would give us a pipe that we would never have to go back down to and work on.”

According to Eric Shaffer, P.E., chief engineer for utilities, City of Duluth,  “We use HDPE extensively here in Duluth.  It is good down to minus 40 degrees F without damage to the pipe.  Specifically, we tested a piece.  We fused on a saddle, tapped it, pressurized the pipe with water to about 100 psi and threw it in a freezer for a couple months.  Pulled it out, let it thaw and it was still holding pressure with no damage.”

According to the Plastics Pipe Institute, Inc. (PPI), the major North American trade association representing all segments of the plastics pipe industry, PE 4710 is the highest performance classification of HDPE piping material for water applications.  “PE 4710 HDPE pipe is tough, durable and flexible, meeting AWWA C906 and ASTM F714 standards,” stated Camille George Rubeiz, P. E., F. ASCE, senior director of engineering for the Municipal and Industrial Division of PPI.

“PE 4710 compounds offer an excellent level of performance.  This means PE 4710 HDPE pipe can be used with increased flow capacities plus increased resistance to surge pressure, fatigue and slow crack growth.  The ANSI/AWWA C906-15 standard includes        PE 4710 for sizes up to 65 inches and recognizes the increased durability and reliability of HDPE pressure pipe used in water systems.”

“There are high operating pressures here in town and that’s what contributed to our standard of SDR 11 pretty much across the board for all sizes,” Smith stated.  “We actually have about 130 pounds of operating pressure available on those mains.  Duluth is a city on the hill.  I like to think of it as a mid-west San Francisco.  We have a lot of streets that run parallel to the hillside, and our avenues run up anywhere from a 10 to 26 percent grade in some areas, so the elevation changes really fast.  The area where we were replacing pipe happens to be pretty much at the bottom of the zone, that’s why the pressure is so high.”

The project called for Michigan Street to be dug up for the utility line replacement and add the new water line.  Pipe bursting  frequently used in tight areas, was not an option due to the large number of bell clamps that keep the lead packing material in the joints in the old cast iron pipe.  These steel clamps would require many man-hours to remove each one that are spaced every 12 feet or so.

On each side of the street are high-rise buildings, parking ramps and other areas that could not have access shut off.  Smith’s team divided the project into four phases.  For the two block replacement, the project was broken into four phases, replacing roughly a half of a block at a time.    “This would allow access into each block and keep all the businesses around there functioning.  The first segment was 100 feet, the second was about 290, the third was about 320, and the fourth was about 240 feet,” he explained.

The crew from George Bougalis & Sons, Co. (Hibbing, MN) open cut the whole segment, removed the 16-inch cast iron pipe and connected buildings to a temporary water system.  Pipe for each segment was assembled at grade on the other side of the street, disinfected and in some cases pressure testing above grade also.  Three pieces of equipment were used to lower the pipe into the trench, thread it under the other utilities and reconnect the services with tapping tees from Georg Fischer Central Plastics (Shawnee, Okla.).  The PPI member company also fabricated the large diameter, heavy duty fittings.  Another PPI member company manufactured the HDPE PE 4710 pipe.

Heat fusing the 40-foot-long pipe segments was done by Bougalis.  “Duluth has a certified fuser program,” Smith explained, “which Bougalis went through at the beginning of the project.  Two fusers did all the pipe section butt fusing and electrofusing the side saddles and tapping tees.

“We did an hour-long, 150 psi test on all of those side saddle fusions,” he said.  “Testing for the main butt fused pipeline underwent an eight hour, 150 psi test in accordance with our standard protocol.  We chose that pressure, which some people would probably think is quite high but we do have some pretty high pressures here in town.”  There are 22 taps and side saddles along the 950 feet of pipe.

The new line is seven to eight feet below grade to allow for the frost line and other utilities.  “Now everybody else can run their utilities above us,” Smith said.  “The pipe also enabled the crew to thread it under the existing utilities.  Michigan Street has a sanitary sewer line deeper and then storm water above that, then all the power, communications and steam lines.  Steam lines are for these buildings from our community steam heat system.  Those are always a challenge because of the extreme heat they generate.  We needed to have protective barriers and insulate the pipe lines from each other.  The separation between the lines, which was only about three feet where we crossed them with the HDPE water line, was calculated by  the local steam company .”

For Smith and his team, the next project will be Superior Street, the other main street in downtown Duluth.  “We’ve been planning to redo Superior Street for three or four years.  Michigan Street was a prototype project to see if it was possible to do this large diameter pipe in a very developed, congested area.  We’re proceeding with planning that project which will probably start in 2018 to replace the same 1888-1889 cast iron water main with HDPE pipe.

“I foresee us doing a lot of water main pipe bursting.  That really seems to be the way to budget for replacing cast iron in an entire neighborhood that was installed in 1930, 40’s, 50’s, 60’s.  It’s mostly six-inch pipe and a lot of those we’re going to be pipe bursting with HDPE for the rest of my career.

“I’m sure we’re going to have well over a hundred-year life span on PE 4710 HDPE water mains were putting in town.  And probably 200-years-plus without a doubt is my opinion.  And that’s what we needed here.  This is the one-time solution.”

For additional information, go to the Plastics Pipe Institute’s website at:  www.plasticpipe.org.

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About PPI:

The Plastics Pipe Institute Inc. (PPI) is the major North American trade association representing all segments of the plastic pipe industry and is dedicated to promoting plastics as the material of choice for pipe applications.  PPI is the premier technical, engineering and industry knowledge resource publishing data for use in development and design of plastic pipe systems.  Additionally, PPI collaborates with industry organizations that set standards for manufacturing practices and installation methods.

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