PCL Civil Constructors Inc., San Diego, Calif., currently is constructing approximately five miles of the Rose Canyon Trunk Sewer in San Diego, Calif., under a $23.8 million contract with the City of San Diego.  The line is a combination of open-cut sections and six underground drives.  The underground drives, which account for one-sixth of the total length, are required because in several areas the line passes under Caltrans or railroad right-of-way property where an open cut is not possible.  In several other areas the depth of the new sewer pipeline, as much as 45 feet, makes an open-cut trench impractical, according to Rick Selensky, resident engineer for the City of San Diego.

   Of the 24,000 feet of pipeline, 20,000 feet is open-cut with an average depth of 16 feet.  The work is confined within a specific corridor to protect the environment in Rose Canyon.  This corridor will be revegetated at the end of the project. The Ameron reinforced concrete pipe ranged in inside-diameter size from 48 inches to 60 inches, with all of the pipe made with PVC lining.  A ring of PVC is welded to seal the joints afer the pipe is in place.

     One of the challenging aspects of this project is making the live tie-ins to the existing sewer using temporary bypasses, explains PCL Project Manager Mike McKinney.  There are existing 18-in.-diameter and 33-in.-diameter sewer lines in the project area that are already at near-capacity.  In one of the open-cut areas a section of the 18-in. line must be removed to make room for the new, larger pipe.  This will involve a temporary bypass to shift the 18-in. pipe volume into the 33-in. pipeline.  Since this is a combined sanitary sewer and strom drain, this portion of the work is scheduled for after the end of the wet season to avoid the chances of a spill.  Concrete Contractors Interstate, San Diego, Calif., is handling the concrete tie-in structures.

     Westcom Microtunneling, Pleasant Grove, Utah, which is handling the six underground drive sections as a subcontractor to PCL, is using a Lovat tunnel boring machine (TBM) in conjenction with a pipe jacking system. seventeen shafts, 37 feet long and 14 feet wide, were eccavated to provide jacking pits and receiving pits.  PCL excavated the pits while Anderson Drilling, Lakeside, Calif., installed the soldier beams and lagging to shore the walls.  Trebor trench boxes were used for shoring in the shallower pits.  PCL lowered either a backhoe or a mini-excavator into the deeper pits to load out a muck bucket.

     The TBM sits on the front joint of pipe and advances as additional joints are jacked forward in five-foot increments. The unit overcuts the tunnel by only one inch for the 86-in.-outside-diameter pipe.  A double O-ring gasket system prevents the bentonie slurry from escaping into the jacking shaft.  The jacking equipment generates 1,200 tons of force to advance the pipe.

    A muck train rides on tracks placed within the pipe and hauls a 1-1/2 -cu.-yd. muck tub back from the rear of the TBM to the pit, where a crane hoists it out for off-site disposal.  Carpet has been placed under the tracks to protect the PVC lining of the pipe.

     Each 86-in.-outside-diameter joint of pipe is 20 feet long, has eight-in.-thick walls and weighs 53,000 lbs.  The pipe is being manufactured at Ameron's Rancho Cucamonga, Calif., facility, 2-1/2 hours away by truck.  The joints are hauled to the site and stored temporarily on specially modified trailers.  As the joints are needed, PCL moves the trailers adjacent to the pits, where the railroad tracks are placed inside prior to eac joint being lowered into the jacking frame.

   The two longest underground drives are 1,600 feet long and utilize midpoint pits to reduce the jacking length.  The remaining four drives will average just under 200 feet long.  A tunnel laser is being used for guidance of the TBM.

     Other key members of the PCL team in addition to McKinney are General Superintendant Dale Jones and Project Engineer Chris Stack.

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