Perfect Storm

A tunneling machine enables replacement of 200 feet of storm sewer where erosion threatened a home
Perfect Storm
John Mahana of H&H Co. uses a forward-reverse joystick to retract the dual hydraulic rams on the UB-40 tunneling machine, as the hand of Jim Voudy prepares to lower another 3-foot-long hollow pilot tube.

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For years, a storm sewer in Holiday Lake Estates in Morgan Hill, Calif., backed up, flooding a road and the driveway at the end of it. Discharge from the outfall into Anderson Reservoir eroded the steep bank of the homeowner’s lakeside property, placing the yard and house in jeopardy.

An inspection revealed that the 8-inch reinforced concrete storm sewer had 3-inch offsets and 10-inch-deep bellies. Pipe bursting and lining systems would not work because they followed the host pipe without correcting the problems. Open cutting would be difficult and deep.

Access, the homeowners’ association, called Rod Herrick, a previous service provider and owner of H&H Co. in San Martin, Calif. He used a UB-40 tunneling machine from RODDIE in San Martin to install a new storm sewer paralleling the old pipe and maintaining the same 0.30 percent grade.


Detailed survey and planning

The pipe’s condition was due to 40 years of ground movement and root intrusion.

“That area is always sliding toward the reservoir,” says Herrick. “If developers tried building there today, they probably could not get permits.”

Herrick televised the pipe using a GatorCam3+ from Radiodetection and discovered that it fell 7 inches over 200 feet.

“Our challenge was to achieve that even percent of fall with 8-inch SDR17 HDPE pipe,” he says.

Locating utilities and marking their elevations was the touchiest part of the job and took two days. Herrick also mapped trees and pavement sections, using a tape measure and drafting board to plot the tunneling route before entering the final coordinates into a computer.

Meanwhile, John Mahana used a TBO15 Takeuchi mini excavator to dig a 7-foot-diameter, 6-foot-deep shaft at the catch basin, then set the 2,600-pound tunneling machine in the hole.

“We didn’t need an entry pit because the storm sewer daylighted in the bank of the reservoir,” says Herrick.

To prevent the shaft walls from collapsing and to form a thrust block for the UB-40, Mahana and Jim 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.) Mahana then set the height, grade and direction using jacking screws that secured the unit to the liner.


In the cross hairs

The tunneling machine first jacked 3-foot-long hollow pilot tubes along the programmed route to exit near the outfall. The guidance system used a camera on a surveyor’s level focused on an LED target visible inside the lead tube. Mahana mounted the level on an independent adjustable support, setting it to the required height, grade and direction.

The camera sent the image of the target to a monitor. Using the image and a joystick to steer the slanted tip of the lead tube, Mahana maintained the target in the level’s cross hairs, which represented the desired line and grade. He made approximately one course correction per foot.

“The pilot tube is accurate to 1/8 inch for 200 feet,” says Herrick.

A forward-reverse joystick advanced or retracted the machine’s dual hydraulic rams. After Mahana loaded the lead tube, 40 tons of thrust pushed it through a flexible seal in the liner and into the soil.

“The soil displaced easily, so we didn’t need a boring auger,” says Herrick. Mahana repeated the jacking process until the pilot tube reached its destination.


Thar she goes!

As boring neared completion, workers fused the 40-foot sticks of HDPE pipe and attached the pulling head.

“The ideal place to do it was along the shore, but there was no room,” says Herrick. “We had to work on the road, but the last block or so dropped steeply to the shore.”

Realizing that gravity would pull the 700-pound length of pipe down the hill once the descent began, Mahana tethered the pulling head to the excavator with a cable before pushing the pipe on its way. Voudy, driving a Bobcat skid-steer loader on tracks, helped guide it to the waterfront.

“We had some rollers under it, but most of the time it skidded on the asphalt,” says Herrick. “The pipe walls are almost one-half inch thick, so they weren’t hurt.”

As the second hundred feet of pipe edged over the hill, the pipe took off. Mahana supplied braking action to control the descent as Voudy tried to hold it on course. Inertia carried the pipe the last 40 feet across the beach and into the water, as the area was inaccessible to machinery.

The pipe filled almost with water before reaching neutral buoyancy, and floated with just the crown exposed close to the outfall.

“Any time you can float a pipe, it’s easier to pull in,” says Herrick. “The water runs out during installation.”


Reverse procedure

With the pipe in place, Mahana removed the pilot tube from the ram and attached the pulling head. The machine pulled the pipe through the tunnel at three minutes per foot. As the tube sections emerged, Mahana disconnected then and handed them up to Voudy.

The 10-day job concluded with decommissioning the old storm sewer, connecting the new pipe to the catch basin, and tying in the other two storm drain lines. The crew also added rock under the outfall to disperse the discharge and stop erosion.

“We saved the association about $30,000 over open cutting,” says Herrick. “But the neat thing is when the ground moves again, the HDPE pipe will flex without cracking or offsetting.”


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