Tunnel Lining Project Takes Pumping To Great Lengths
A dual phase project to repair aging sewer tunnels in the Los Angeles area has incorporated the use of SCC (self consolidating concrete) in the largest use of this material in North America. Matching the impressive quantities of SCC being used on the project are the lengthy distances the material is being pumped using a standard Schwing concrete pump.
Phase One of the project calls for rehabilitating 30,000 lineal feet of the North Outfall Sewer line which runs from downtown L.A. to the Hyperion Treatment Plant in Playa del Rey. This 1920s vintage line is a semi- elliptical shape measuring 12-feet wide by 10-feet tall. The original concrete pipe was lined with clay tiles which have mostly fallen away due to sulfuric gases that form above the level of sewage and are corrosive to concrete. In some cases the original 12 to14-inches thickness of the old pipe has been reduced to three inches. In the worst cases, other unrehabilitated concrete pipeline has collapsed swallowing property and automobiles in the process.
After constructing a parallel line, the city has set out to rehabilitate the line by contracting with joint venture Colich & Sons/J.R. Pipeline, Los Angeles who was the low bidder on the project. A second phase of 20,000 lineal feet will follow. The city recognized in the bidding process, that “there remain elements of risk and work difficulties that will only be known fully once the work begins and the sewer is opened for full examination.” Most of the work would take place underground to lessen the impact on neighborhoods. Because access would be limited, one of the greatest challenges would be getting the concrete to the repair area.
“Because of the odd shape of the old sewer line, we had special forms constructed, “according to Linda Renteria, Project Engineer for JR Pipeline. The steel forms that slide into place with a specialized carrier expand to allow an outside void of 9-inches on the sides and 12-inches on the top between the form and the excavation left behind after removing the deteriorated concrete pipe. Forty-eight of these specialized bolt-together form sections were supplied as straight pieces and six radial sections allowed the crew to negotiate curves where necessary. Two crews work simultaneously assembling 100-feet of the formwork with the goal to form and pour 500-feet of line per week.
Only three main access points in the entire 6-mile portion of Phase One allowed entry for installation of the forms. As crews assemble the forms, sections with grout ports are inserted at 24-foot intervals. Four-inch hardpipe is laid inside the forms with 4-inch flexible sections hooking into the pumping ports. Because the pump had to be stationed at street level for ready-mix access, more than 1800-feet of slickline was required in some cases.
The SCC specified by the city engineering department required a highly flowable mix design that could meet the long-distance pumping requirements and still flow within the narrow void between the forms and excavation. Cemex in collaboration with Grace Construction products determined the best performing mix for the 50,000 cubic yards required on the job would be a low shrink, self-consolidating concrete. This highly specialized mix needed to meet a shrinkage specification of just .03-percent – the equivalent of 100th of an inch or about the thickness of a piece of paper. The SCC mix was tested at an 11-inch slump or a 23 to 24-inch spread with a J-Ring.
Using a Schwing WP 1250-18X, the crew is pumping the SCC using only about half of the pump’s available 942 psi material pressure and 150 horsepower to maintain volume over the extraordinary distances. With seven-inch diameter pumping cylinders operating through a 55-inch stroke, the result is continuous flow through the pipeline. “We have to maintain the pressure to squeeze the SCC through the voids, “according to Juan Renteria, owner of JR Pipeline, “The 1250 does it effortlessly and smoothly.” A nine cubic yard truck load of SCC is usually emptied in 10 to 15 minutes operating the pump at 15 strokes per minute. His crew reports easy clean-up of the Rock Valve when they shuttle the pump between crews which are made-up of six men – four underground and two on top. The men stay in touch via radio. As one crew is stripping the forms – usually after a 12-14 hour setting time – the other crew is pouring. The two-axle trailer pump is easily towed between access points.
In order to extend the life of the new sewer, poly-vinyl chloride (PVC) sheets are installed on the forms to protect the concrete from the corrosive gases. The pre-cut plastic liners have T-locks, which when embedded in the SCC, become a permanent part of the installation. Because the PVC becomes the exposed portion of the pipeline interior, there is no finishing of the concrete.
The cost prohibitive methods of cut and cover in existing urban areas and the amount of utilities now buried in many streets, make this method of sewer rehabilitation attractive for older cities. The relative ease of pumping these projects with the right equipment and the advances in SCC suggests opportunities for those contractors willing to venture into the underground world of America’s infrastructure.
Project: Lower North Outfall Sewer Rehabilitation, Los Angeles California
Owner: Public Works Department, City of Los Angeles
Contractor: Joint venture Colich & Sons/J.R. Pipeline, Los Angeles
Equipment; Schwing WP 1250-18X stationary concrete pump