A sliplining project overcomes torrential challenges to keep a stone quarry and asphalt plant in operation.
A collapsing 72-inch corrugated metal pipe jeopardized operations at a stone quarry in Stafford, Va. The 800-foot pipe was the main dewatering system for the pit, and the quarry dewatered every other day. Beginning in a 20-foot-deep swale, the pipe also collected stormwater and carried a continuous flow of groundwater.
Large rocks and 3- to 5-foot-diameter boulders rolling into the sinkhole compressed the downstream pipe, backing up runoff. Workers who discovered the situation cut back 50 feet into an 85-foot-high sand hill to expose and remove the bad section, but the attempt failed. Open-cutting without shoring and frequent rains accelerated erosion.
Fearing erosion would undermine and collapse an asphalt plant 40 feet away on the hill, quarry officials called Aaron Enterprises, a boring, tunneling and directional drilling company in York, Pa. When engineers arrived, they found the downstream invert missing, the badly rusted walls curled up, and the pipe compressed to 2 feet by the weight of soil and debris. They decided to encase and remove the bad section, then slipline the entire pipe.
"Sand shifted down constantly and undermined the occasional pine tree," says Superintendent Glenn Grove. "Furthermore, everything in the pit drained to this area, making working conditions like living in a fish bowl."
Significant rainfalls twice a week, difficult wooded and overgrown terrain, and steep drop-offs challenged the crew, but the asphalt plant continued to operate throughout the two-and-a-half-month rehabilitation.
Before workers arrived, in-house mechanical engineer Robert McDowell, P.E., walked the pipeline, plotting coordinates with a handheld Trimble global navigation satellite system. "The coordinates enabled us to keep the 10-foot-long, 96-inch tunneling shield centered and on grade," says Grove. "We hit the good section of CMP dead center."
Controlling water was the team's first challenge. When it rained, runoff in the pipe backed up and filled the swale, which took two to three days to drain. The quarry tried to postpone dewatering until weekends to avoid inundating Grove's workers, but the effort wasn't always successful.
To reach the downstream work site, crews widened the steep road to accommodate the Case 9050B tracked excavator and other full-size equipment. After open-cutting 50 feet back from the compressed pipe, they excavated a 14- by 40-foot-long jacking pit that was overcut several feet and filled in with large stone to allow a constant flow of water.
Behind the jacking pit, workers laid and backfilled 50 feet of 48-inch casing for drainage, and toed in jacking plates backfilled with large stone to support the thrusting forces. "We also sunk two I-beams and set steel resistance plates against them," says Grove. "One plate rested atop the drain casing while the other two reached the bottom of the trench."
The team excavated a pulling pit 150 feet further downstream, then poured a concrete pad reinforced with steel plates to withstand pressures greater than 400,000 pounds from the 48-900 auger boring machine (American Augers) and platform sled (Michael Byrne Mfg.). They parked a Komatsu mini excavator on the slab to handle 15-foot-long DD50 directional drill steels.
Meanwhile, another team built a runway from the base of a 35-foot cliff to the upstream pipe, bulldozing trees along 350 feet of rough, steep terrain through the swale. "We needed 800 feet for the fused 63-inch HDPE pipe, and that's just what we had before reaching the mouth of the pipe," says Grove. They also built a headwall there.
The crew cleared enough rocks and boulders from the open-cut section to set the tunneling shield over the end of the pipe. To advance the shield, workers welded 20-foot sticks of 66-inch, 750-wall (3/4-inch thick) steel casing to it, then butted a push block against the casing. The boring machine jacked the assembly forward with 200,000 pounds of thrust.
"We welded eight gussets from the first casing to the shield and built a temporary masonry bulkhead between them to fill the annular space," says Grove. Advancing the shield also relieved some pressure on the pipe.
Protected under the shield, workers used pneumatic cutoff saws to slice through the compressed metal, tunneling spades to chip through soil and relieve the edges of the shield as it advanced, and rivet busters to break off pieces of rock in its way. A buggy system removed the material.
To handle boulders, the team bored 1.5-inch holes in them with a pneumatic hammer drill, inserted a hydraulic rock splitter, and broke off a slab. "We'd tap the piece, insert an anchor bolt, and extract it with the winch on the boring machine," says Grove. "Splitting boulders went on relentlessly."
After hand-mining 75 feet in four-and-a-half weeks, workers reached the designated good section. "We relied heavily on dayshift foreman Richard Emanuel's construction background and customer relations skills," says Grove. "They were key to the project's success."
Quarry personnel helped build a gravel work area for a subcontractor's field technician to fuse the HDPE pipe using a 21,000-pound McElroy 1600 fusing machine with 3,000 psi and a 25 hp motor. "It's the largest butt fusion machine made, and there are only a few of them," says Grove.
His team set the 50-foot sticks of HDPE pipe with the Case excavator. Workers wearing welding gloves used putty knives to cut off the 1-inch-thick exterior fusion beads, enabling the pipe to slip through the 64.5-inch ID casings. Fusing a joint took 60 minutes.
Fabrication engineer Jeffrey Smith designed a metal pulling eye and mechanism that attached to the lead pipe. Fabricators Jed Lucabaugh and Danny "Bud" Witmern turned his sketch into eight equidistant metal straps radiating back from the eye. "We cut eight equidistance slits in one end of the lead pipe, bent down the divisions to form a nose, and slipped on the assembly," says Grove. Four bolts held each strap to the pipe.
After welding a heavy-duty steel line to the eye and fusing the first stick to the lead pipe, the excavator set it on a support with rollers just before the edge of the cliff. An excavator at the bottom of the cliff used the line to control the pipe's descent, then pulled it downstream.
"We stabilized one steep slope with a large stone bed where we cut trees," says Grove. "To avoid damaging the pipe, we lay 8-inch casing for rollers. We also positioned casings at 50-foot increments through the swale."
At one point, workers believed the steep slope and weight of the pipe would be more than the excavator could handle. "The pipe was at its teetering point and a man could have pulled it," says Grove. "We had restraint precautions in place, but thankfully we didn't need them."
Crank up the power
Smith modified the platform sled to accommodate a beetle motor (Gill Rock Drill Co.) to thread the 5-inch directional drill steels as workers strung them from the pulling pit. The platform pushed the steels more than 1,000 feet upstream through the casing and pipe for attachment to the pulling eye. "It took every ton of thrust the platform had to pull in the 208,000-pound pipe 30 inches at a time," says Grove.
The pull continued nonstop into the night. When the nose emerged from the casing, workers mucked out and bedded the pipe's final resting place where the mini excavator had worked. Then they pulled the pipe into position and backfilled with stone per the quarry's specifications.
After removing the temporary masonry bulkhead, the crew filled the space with a grout mix from Cardinal Concrete Co. To fill the 1.5-inch-wide annular space between the HDPE pipe and casing, workers tapped the pipe, threaded 2-inch couplers on the downstream end, attached the pump to a 2-inch hose, and pumped in the grout.
"The entire job took 100 cubic yards of product," Grove says. "It was the last step in restoring the pipe's structural integrity."