A Fix at Last

A tunneling machine enables replacement of 450 feet of sewer pipe where persistent discharges had plagued residents of a mobile home park
A Fix at Last

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For years, sewage discharging from cleanouts affected as many as 40 properties at the 650-unit Millpond mobile home park in San Jose, Calif.

 

The 6-inch terra cotta clay sewer lines had bellies packed with grease and debris. Pipe bursting and lining systems did not work because they followed the host pipe without correcting the problem. Repairs involved moving the modular homes off their concrete foundations, then open-cutting to replace the bad sections.

 

The scenario changed when the park owner hired Rod Herrick of H&H Co. in San Martin. After the next sewage eruption, Herrick used a UB-40 tunneling machine from Roddie Inc. in San Martin to install three lines paralleling the old sewer. The equipment maintained a 0.3 percent grade and replaced 450 feet of pipe. Most residents of the retirement community were not even aware that the work was being done.

 

Sound planning

Herrick televised the lines using a GatorCam3+ from Radiodetection. “The sewers looked like a roller coaster,” he says. “Our challenge was to achieve an even percent of fall with the 6-inch HDPE pipe.”

 

Locating utilities and marking their elevations was the touchiest part of the job. Herrick used a tape measure and drafting board to plot the tunneling route before entering the final coordinates into a computer.

 

The sewers ran under and behind homes, making them inaccessible to vehicles. Herrick hired Biggie Crane in Scotts Valley to lift the tunneling machine’s hydraulic power pack and pallets with pilot tubes over a home and into the backyard. He set them down on the three-foot-wide walkway between the house and a toolshed.

 

To excavate a 7-foot-diameter, 9.5-foot-deep shaft for the tunneling machine, John Mahana maneuvered an excavator onto the concrete patio slab next to the home, barely fitting under the patio roof and between its support columns. The crane operator then lifted and lowered the 2,600-pound tunneling machine into the hole. Meanwhile, Jim Voudy dug entry pits under the foundations of three homes using a custom-built hydroexcavator from Keith Huber Inc.

 

“The tunneling shaft was near the middle of a 450-foot run and intersected with a 200-footer coming in at 90 degrees,” says Herrick. “By boring two feet away from these lines, we maintained sewer service until we tied in the new laterals.”

 

To prevent the shaft walls from collapsing and to form a thrust block for the tunneling machine, Mahana and Voudy lowered a round corrugated steel liner into the excavation, then poured two-sack slurry in the annular space. (The mix was soft enough to be hand-excavated later if necessary.)

 

Once the tunneling machine was in the shaft, Mahana set the height, grade and direction using jacking screws that secured the unit to the liner. Releasing the screws allowed him to rotate the machine with the help of a hydraulic winch chained to an I-beam across the top of the shaft.

 

Guidance system

A camera on a surveying transit focused on an LED target behind the boring head. Mahana mounted the transit on an independent adjustable support, setting it to the required height, grade and direction.

 

“We began by pushing in hollow pilot tubes,” says Herrick. “The camera looked through the rods to see the LED target, which was visible on John’s monitor. Crosshairs on the monitor indicated the correct alignment. By rotating the rods using a joystick, John could turn the boring head to correct any course deviations.”

 

Using the forward-reverse joystick, Mahana slid back the machine’s dual hydraulic rams, inserted the first tube with boring head, then advanced the rams. “Provided the soil displaces easily, we don’t need a boring auger,” says Herrick. “The unit’s 40 tons of forward thrust is sufficient to push the tube through a flexible seal in the liner and into the soil.”

 

Mahana then inserted a 3-foot-long pilot tube into the machine, coupled it to the end of the boring head tube, and jacked it forward. He repeated the process until the head appeared in the entry pit, where Voudy disconnected and removed it.

 

“Had a deviation occurred, John would have stopped jacking and rotated the rod to make a course correction,” says Herrick. “We hit some rocky areas that slowed us down a little, but the matrix around them was compressible. The process just pushed them out of the way.”

 

Smooth transition

Voudy mechanically coupled a 7-inch bell-shaped steel expander to the end of the protruding pilot tube. A pulling eye behind the expander was coupled to the first 40-foot length of pipe. “To avoid blocking off the street to stretch out 200 feet of pipe, my guys fused another 40-foot stick each time the machine pulled back 40 feet,” says Herrick.

 

Bushes, privacy fences and hedges in the path of the pipe were aids rather than hindrances. The crew draped the flexible pipe over them, providing increased elevation that helped smooth the transition into the entry pit. The pullback worked like a pipe-bursting system. Mahana extracted the steel tubes as they returned to the shaft.

 

“Mobile home parks don’t have the standard 1 percent grade for 6-inch sewers,” says Herrick. “The average grade is 0.5 percent, but we can maintain as little as 0.10 percent with this machine.”

 

Voudy had the most arduous part of the job, lowering and retrieving the pilot tubes. A canopy over the shaft provided shade and protection from the elements. On hot days, a ventilation system down the shaft kept Mahana comfortable.

 

To reinstate the laterals, Voudy core-drilled the HDPE pipe or cut in a wye, while Mahana hydroexcavated 3-foot-diameter, 4-foot-deep shafts and connected the laterals to the new main. The men installed a cleanout at the three new upstream risers before abandoning the old sewer. The project took two-and-a-half weeks.



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