Complaints from motorists of water ponding on the two-lane John Muir Boulevard in Daly City, Calif., brought a public works crew to investigate.
They found the 8-inch corrugated steel culvert that collects runoff from a golf course and the road badly corroded. Portions of the invert were missing, and several sections were collapsed.
With the golf course on one side of the road and the lake on the other, open cutting was not an option, as it would close the highway. Having worked with TRIC Tools Inc. in Alameda, Calif., Tom Piccolotti, the city’s water and wastewater field director, asked TRIC technical field director John Rafferty if he could burst the 85-foot-long corrugated culvert and upsize it by two inches.
TRIC had never done such work before. “We were concerned that the pipe around the collapsed areas could shred, and the bursting head would compact the shards into an impenetrable glob,” says Rafferty. “Our other concern was the accordion factor, where the head folds the pipe against itself. Either way, the pull is over, and it’s an open cut to retrieve the equipment and replace the pipe.”
The company had tried a combination of mostly pull with some pneumatic hammer assist on a recent job, and believed that was the best formula for corrugated pipe. Using a new 8-inch fluted Unified Force head with a dorsal blade, a standard 4-inch piercing tool from Footage Tools Inc., and TRIC’s HFHD-100 ram, Rafferty upsized the culvert without affecting traffic.
Working partners
City workers jetted the culvert, then inserted a temporary 6-inch PVC pipe to drain the water into Lake Merced. They also strung a line through the pipe so Rafferty could tie it to the pulling cable and drag it to the entry pit. In addition, the men excavated three feet down for the entry pit, on the golf course side, and made a dirt ramp for the 10-inch SDR-17 HDPE pipe, which they provided and fused.
The 85-foot-long culvert discharged 10 feet below the road. Workers cut the exit pit into the side of the hill, then set up a trench box rated for 30 tons of shield capacity (side pressure). Before the job began, the city brought an air compressor to power the pneumatic hammer and fed the air hose into the HDPE pipe. Workers also closed the bike-pedestrian path on the lake side and routed people around the vehicles and equipment. The traffic on John Muir Boulevard continued uninterrupted.
When Rafferty and assistant Ian Hom arrived, they attached the 1 1/8-inch threaded stud cable rated at 93 tons to the 5-foot-long, 4-inch piercing tool.
“The piercing tool is the pulling core and simultaneous hammer assist for our Unified Force head,” says Rafferty. The tool fits inside the fluted head, which also has a 4-inch-tall dorsal blade 12 inches long. A 10-inch skirt expander fits into the back of the head and screws to the HDPE pipe. The expander creates the annular space for the larger pipe. A cable adapter connects the head to the cable.
“For this job, the primary force is a hard static pull,” says Rafferty. “However, we expected the hammer to make a cleaner opening and keep the pull on track. We knew we had enough force to do damage, but didn’t know what effect the hammer would have.”
Beefed up
To prevent the trench box from collapsing against the force of the pull, Rafferty set railroad ties vertically across the enclosure’s steel crossbeams. “Our goal was to be as close to the edge of the box as possible to use the compressive strength of the sidewalls,” says Rafferty. The city’s backhoe lowered the 3-by-3-foot by 1-inch-thick steel resistance plate with 15-inch-diameter aluminum pulling wheel.
As the plate dangled, Rafferty stacked horizontal railroad ties from the ground up to elevate the plate to the level of the culvert. “The bottom of the wheel is the center of the pipe, so it must line up with the center of the hole,” he says. After the backhoe lowered the ram, Rafferty threaded the cable through the wheel.
“The HFHD-100 (high-flow heavy-duty) ram is rated at 60 to 100 tons and has two 5-inch cylinders,” says Rafferty. “That certainly is overkill, but we wanted the extra power to save us should the pipe fold up.” The pull required 40 tons of force. A 20-gpm/4,000-psi pump powered the hydraulics.
The only pause in the pull was when Rafferty grabbed a retainer O-ring that flew off the ram and stretched it back over the grippers. “We stopped pulling for 30 seconds, and when we started again, I realized that the head had moved forward 12 inches,” he says. “I never expected the hammer to take up any more tension than what was already on the cable. That was impressive, because we’re going through corrugated steel and upsizing it by two inches.”
Moving at 6 to 8 feet per minute, the pull was done in less than 15 minutes. The head caught the temporary PVC pipe and dragged it into the exit pit. “It never interfered with the expansion and jacket zipping of the corrugated pipe,” says Rafferty. “Provided the trench box could support the pulling load, anything else in that pipe was gone.”
