Salt Lake City Uses Trenchless Pipe Repair Technology

Changing technology eliminates the need to excavate streets in the repair of sewer infrastructure.

Chuck Call P.E.

February 8, 2001

Figure 1.  CIPP project being done between two sewer manholes in downtown Salt Lake City, Utah. The work is accomplished without excavating into the street.

(Salt Lake City, Utah) -- Last year Salt Lake City Public Utilities (PU) was authorized by the City Council to increase the repair and replacement of the City’s aging sewer pipes.  Sewer rates were increased and PU set about restoring 20,000 feet of old sewer pipe each year. Part of a 6-year $57.5 million capital improvements program, the City recognized that it needed to have an on-going program of up-grading its sewer system.

Problem

Many of the City's sewerlines are in the advanced stages of structural deterioration—especially the 100-year-old pipes beneath the city's busy downtown area.  Some of the sewer system has suffered structural damage due to hydrogen sulfide corrosion and joint deterioration.

The sewer system consists of vitrified clay and concrete pipes and part brick and mortar pipes. The City looked at new construction methods that would provide a long-term and cost-effective solution and at the same time minimize inconvenience to the public.

Numerous other construction projects were occurring in the downtown area, and additional congestion would have further inconvenienced area residents, workers, and businesses. 

Solution

PU was reluctant to burden business owners and residents with the disruptions that would be caused by digging up the streets to replace sewer pipes.  The cured-in-place-pipe (CIPP) process was selected to fix many of these old sewerlines. This pipe rehabilitation method provides a structurally sound pipe that is also resistant to the corrosive environment that exists in sewerlines.

Process

The CIPP process uses custom-manufactured felt tubes that are impregnated with a polyester thermosetting resin. The tubes are inserted into the damaged section of the pipe, usually through an existing manhole. Water pressure is used to propel the tube through the pipe, turning the tube inside out while pressing it outward against the walls of the existing pipe. When the tube reaches a termination point, the water inside the tube is heated causing the resin to cure. The result is a sturdy, jointless new pipe within the old one.

A robotic cutter reinstates service connections. Once in place, the robotic cutter cuts through the wall of the new pipe, reinstating the connection.

To verify successful completion, video cameras are used and City staff inspects the work throughout the course of the project.

Benefits

This process results in significant savings when compared to a conventional open-cut pipe replacement. In addition to cost savings, a cured-in-place-pipe repair can be done in much less time without large excavations. For example, a conventional open-cut sewerline replacement takes about two weeks per block with significant disruption to traffic and service; whereas, a cured-in-place-pipe project takes only two days and is completed without any excavation.

Figure 2.  Fiber sock being inserted into the sewerline.

Streets weren't closed.  James McDonald, PU inspector explains, "To minimize concerns, customers are notified in advance of the project about what is going to take place in their area." 

"Because these projects involve underground facilities," Rick Reinhold, PU sewer design team leader insists, "most people never know this work is underway. But quietly and with little disruption to the public, we are repairing our sewer system."

The rehabilitated pipe is both sturdy and efficient. This process restores the original life to the pipe at about half the cost of new pipe.  This saves the cost of digging and service line rerouting costs.  By eliminating digging, the trenchless alternative minimizes damage to pavement, thereby reducing long-term street repair costs.

Sewerline rehabilitation has other benefits. Exfiltration from cracks in old sewer lines can contaminate groundwater. Water flowing into the pipe through leaks (infiltration) can erode the surrounding soil. Sealing off these leaks helps maintain the soil support for the pipe.  Also, in cases of more common infilration, when ground water is leaking into the sewerlines, sealing sewer pipes reduces these flows to the wastewater treatment plant and extends the capacity life of this facility.