Contractors tell us their most productive, efficient trenching methods.
Underground contractors deal with some of the most difficult digging conditions in North America, yet they find a way to meet those challenges and beat the best Mother Nature can offer. These contractors drill and shoot rock. They drill wells to dewater an area. They excavate trenches up the sides of mountains. Here we have collected five of their stories that tell how it’s done, from Texas to Alaska and from North Carolina to Oregon.
Tough Conditions in Texas
To meet future water demand in northern Texas, the Tarrant Regional Water District (TRWD) recently completed construction of the $140-million Eagle Mountain Connection Pipeline Project in Tarrant County. The 19.5-mile pipeline will move water from two East Texas reservoirs to Eagle Mountain Lake. With the new pipeline, the TRWD will significantly increase its capacity to replenish Eagle Mountain Lake during drought conditions.
Last spring, Garney Construction, of Kansas City, MO, finished construction on a $43.6-million section of the Eagle Mountain Connection. It was a 78-inch and 84-inch water pipe that represented one of the company’s largest pipeline projects to date. Under the contract, Garney installed 47,430 lineal feet (lf) of 84-inch mortar-lined, polyurethane-coated steel pipe supplied by Northwest Pipe Co.
The entire project has a wide variety of conditions. It involves 23,000 lineal feet of rock trenching, tunneling for more than 650 lineal feet under existing roadways, the installation of approximately 700 lineal feet of 78-inch Hobas pipe and an outlet structure in Eagle Mountain Lake in water depths up to 30 feet.
One challenging portion of the 84-inch pipeline came to light when Garney encountered 405 lineal feet of soil that had absolutely no structural capacity. “So the TRWD contracted with us to build an underground bridge to support the pipe,” says Scott Parrish, senior project manager for Garney.
Underground Bridge
To build the bridge, the contractor first drove H-piles on 5-foot centers in two rows every 15 feet under the proposed pipe installation alignment. The piles averaged 65 feet in depth. Next, crews drove sheet piling just inside the H-piles. Then a Komatsu PC 1250 excavated down 16 or 17 feet between the rows of sheet piling. “We’d excavate down to a point 1 or 2 feet below the point where the vertical piles had to be cut off,” says Parrish.
Next the contractor set a laser, marked and cut the vertical piles to the correct elevation so that the bridge deck would follow the proposed slope of the pipeline. That operation was followed by welders, who fastened I-beam rails along the rows of H-piles. Then Garney brought in aggregate backfill for the shallow trench, and graded it to the top of the welded rail.
A concrete bridge deck followed. Garney crews cast big concrete panels onsite, each 1 foot, 9 inches thick by 15 feet in length and 10 feet in width, to span between the rails. Each panel weighed 30,000 pounds. Garney would then swing the 84-inch pipe into place and rest it on 1-foot-thick sandbags atop the concrete deck. Next, crews formed and poured 1,500-psi lean concrete around the pipe. Once the encasement had set up, the contractor pulled the temporary sheet piles and backfilled the trench, and the job was complete.
Some 24,000 lineal feet of the Tarrant project were simply a mater of “dig, and lay pipe,” Parrish says. Portions of it were through high-dollar neighborhoods, while other portions passed through farm fields. The Komatsu PC 1250 dug an average of 13 feet deep, but sections ranged to depths of 30 feet. For those sections, Garney used a combination of double-high, stacked trench boxes and sloping to provide a safe and OSHA-compliant working environment for the employees.
A John Deere 450D excavator handled the backfilling chores. Other equipment used for the job included a Caterpillar 966H loader, a Cat 950H loader, and a John Deere 850D dozer.
Working for Garney, subcontractor H.L. Chapman Pipeline Construction cut the big rock trenches required for the project. Rock trenches averaged about 13 feet deep. The full trench needed to be 10.5 feet wide, so Chapman first used one of its big rock trenchers to cut a pass 4 feet wide. Garney then backfilled that first trench with the ground rock. Next, Chapman made another pass with the big rock trencher, running parallel to the first, cutting 4 feet in width but leaving a 2.5-foot-wide plug between the two trenches. Again, Garney backfilled the second trench to allow for bedding and pipe deliveries in preparation for installation.
Garney filled the trenches in before coming back to lay pipe because, says Parrish, “We didn’t want our productivity to be dictated by the speed of trenching.” Plus, the first pass trench needed to be backfilled to give the trencher footing to cut the second trench.
Chapman started trenching on October 12, 2006, and finished on February 12, 2007. Production ranged from 150 feet to 500 feet per day, depending on rock conditions.
Two Crews Lay Parallel Pipelines
“Dealing with the rock, groundwater, and soils presented a major challenge on this job,” says Jason Koon, project manager for Garney Construction, Kansas City, MO. He is referring to a $9 million wastewater pipeline project in Gwinnett County, GA. Located northeast of Atlanta, the project entails twin parallel pipelines that stretch for 10,900 lineal feet from a booster pump station to an existing lift station.
One is a 48-inch ductile iron gravity sewer and the other is a 36-inch ductile iron force main.
The gravity sewer had cuts ranging up to 38 feet deep; the force main ranged from 9 to 24 feet deep. Under at least a 14-foot layer of variable soils, including running sand and muck, lay rock that had to be drilled and shot, Koon says. In fact, the project involved excavating between 65,000 and 80,000 cubic yards of shot rock.
Blasting was the first step. Crews drilled holes as deep as 35 feet, loaded them with explosives, and shot the soil and rock. Deep wells came next, spaced at about 500-foot intervals.
Boosting Productivity
Garney laid the two parallel pipelines with two crews. One crew worked on both pipelines in the northern half of the job; the other crew worked both pipelines on the southern half. Each crew had three John Deere excavators: a 650D, a 350D, and a 270D.
In deep trenching, the Deere 650D first benched down to the point at which double-stacked trench boxes, from Efficiency Production, could shield a trench 18 feet deep. The trench boxes, each 28 feet long, were specially designed and rated for the depth of the cut. “We worked with Efficiency on the design of those boxes,” says Koon. “They were engineered for cuts in excess of 30 feet in Type C soil.”
After the big Deere 650D excavator had benched down for the deep gravity sewer, dug out the trench, and laid a stick of pipe, the other two smaller excavators would backfill that trench. Then, while the 650D started digging ahead and loading shot rock, the other two excavators and the crew laid the parallel force main pipeline. It was generally not as deep as the gravity sewer.
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Photo: Hobas |
| An excavator lowers a section of Hobas pipe during the Deep River Outfall Segment 2 project. |
Garney figures that productivity was greatly improved by having one crew work both pipelines. That way, the crew was not idle while the 650D benched down and dug out the deep rock. What’s more, Garney doubled that productivity by working two crews that way—each worked in a different place on the two pipelines.
“We viewed it as one laying operation, not two,” says Koon. “And we used that scheme to bid the job. We eliminated the sitting around and watching the 650D load shot rock. We took a seven-man gravity sewer crew and made it a big nine-man crew that laid the force main and the gravity sewer at the same time.”
Koon says that the conventional way to lay the two pipelines would be to work on them one at a time. One crew would install the gravity sewer, followed by another crew to lay the force main.
“That would take twice the men and equipment, compared to one of our crews,” says Koon. “To equal the pace we’re doing, and lay one pipe at a time, would take an extra 10 men and a minimum of four more pieces of equipment.” Garney used the two crews, Koon says, to finish the job in the 180 calendar days the owner allowed.
Blasting Rock in North Carolina
Much of the Deep River Outfall Segment 2 in High Point, NC, required blasting rock, because the sanitary sewer ran through granite in a rock quarry. For the project, Thalle Construction of Hillsborough, NC, installed 13,700 feet of Hobas 54- to 66-inch sewer pipe at depths of 35 to 60 feet.
The deep sewer construction included more than 200,000 cubic yards of benching and restoration, 74,000 cubic yards of rock excavation, 1,427 feet of 96- and 90-inch diameter tunnels and the removal of 4,500 feet of the existing aerial sewer crossings.
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Photo: Efficiency Production |
| An inverted spreader arches downward on the near end of this 30-foot-long trench box. |
The 60-foot burial depth for the outfall is very deep for a sewer. “It was definitely a tough job, up to 60 feet in the ground with an average of 40 feet,” says Ed Kuehnel, project manager for Thalle Construction. “We had one 800-foot run that was 60 feet deep from end to end.
“We had to excavate and build ourselves a bench at 20 feet,” says Kuehnel. “We had to get the earth down to a 20-foot cut. The biggest challenges were the depth and the rock. We had to drill and blast to get the mass rock and trench rock out.”
Kuehnel liked the Hobas pipe. “We were very pleased with the pipe,” he says. “It was the first time that I personally had worked with that pipe. It’s easy to install. And it’s lighter, so your rigging and equipment can be lighter.”
Hobas was the only pipe named in the specs. The centrifugally cast, fiberglass-reinforced polymer mortar pipe fit the city’s requirements without any add-on coatings or linings.
“We wanted a pipe that would be inert to the hydrogen sulfide gases in the sewer, and Hobas fit the bill,” says Ed Powell, president of the High Point, NC–based consulting engineering firm Davis-Martin-Powell & Associates. “That Hobas pipe can be fabricated into a variety of fittings, such as elbows, wyes, and tee-base manhole risers, so the city could have the required qualities throughout.”
Trenching Up a Steep Mountain
Trenching up a 60-degree slope was a primary challenge for the pipeline contractor at the Fernhill Reservoir Project near Forest Grove, OR. Working under an $8.2 million contract, Kerr Contractors Inc. of Woodburn, OR, built two 3,000-lineal-foot pipelines reaching from a water treatment plant up the mountain to a newly constructed 20-million-gallon reservoir. The $26 million project will increase the potable water capacity of the Joint Water Commission, which supplies water to Greater Washington County, one of three counties covering the Portland, OR, metropolitan area.
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Photo: Garney Construction |
| A trio of tractors works on a large-scale trench-digging job. Sections ranged up to 30 feet in depth. |
One of the two lines, a 72-inch welded steel pipe, supplies water to the reservoir. The other line, a 42-inch high-density polyethylene (HDPE) pipe, is an overflow pipe to carry water back down the mountain from the reservoir to settling ponds near the treatment plant.
Starting at the water treatment plant, the steel pipeline extended 800 feet across a wetland, says Bill Chisholm, an area manager for Kerr Contractors. The contractor used dewatering wells to dry out the soil before trenching to a depth of 12 feet with a Hitachi EX 800 excavator. A Volvo L90 loader moved 20-foot pipe joints into place for installation.
Chisholm says Kerr used 20-foot sticks instead of 40-footers because the ground was so wet. “It takes twice as much time to dig out 40 feet of trench as it does 20 feet, and we wanted to get the trench dug and pipe laid as quickly as possible,” Chisholm says.
Next, the contractor launched the Hitachi EX 800 into trenching 500 feet up the mountain at 60 degrees. It was slow going. Kerr had to reach up the mountain with the excavator, pull down soil for a bench, then climb the excavator up to the bench, sit, and dig trench on the downhill side.
“We were digging about 18 feet deep going up the mountain, and we averaged about two 20-foot sticks a day,” he says. After the steepest grade, the slope moderated to about 30 degrees uphill, which is still pretty steep, Chisholm says. Working on the moderate slope for some 1,500 to 1,700 feet, Kerr could place four to five sticks a day.
Other contractors, Chisholm said, might have dug the steep trench from the top down. “You could take a dozer and push dirt downhill,” he says. “You could cut the trench with a dozer or dig it with an excavator from the top down and set pipe with a crane.
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Photo: Garney Construction |
| A John Deere 650D was the primary digging machine for this project. |
“But we dug it from the bottom up with an excavator,” he says. “Using a dozer and crane would have taken a lot longer and cost a lot more money.” Kerr bid the job to dig it from the bottom up with an excavator, Chisholm says.
The HDPE line tracked a different alignment to reach up to the reservoir. Using a Kobelco 480 and the Hitachi EX 800, Kerr trenched up a 30-degree slope with the HDPE pipeline. About 600 feet of the HDPE line was 40 feet deep. For that portion, Kerr used stacked trench boxes for the 20-foot-deep trench and benched the upper 20 feet.
Kerr used trench boxes to grade the trench for the HDPE pipe. Then the box was removed. For a portion of the alignment, the contractor fused together 13 or 14 pieces of HDPE pipe, each 40 lineal feet long, to create sections of up to 600 feet long that lay alongside the trench. Next, three or four excavators would pick up the long sections and lay them into the graded trench.
When trenching 10–15 feet deep, Kerr could average 200–250 feet per day, Chisholm says. While digging the 40-foot-deep portion, progress of the Hitachi EX 800 and the crew slowed to about 100 lineal feet a day.
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Photo: Garney Construction |
| This Komatsu PC1250 dug an average depth of 13 feet while working on the Eagle Mountain Connection Pipeline project in Tarrant County, TX. |
The risk of the HDPE pipeline was that a trench wall would cave in when the boxes were not in place. “We got lucky, and nothing caved in,” Chisholm observes. “We had good ground up there.”
Special Trench Boxes for Alaska Project
“We started this project late in our winter, so we had to fight the weather, and we had to fight high groundwater,” says Lee Darbous, project superintendent for Pruhs Construction Inc. of Anchorage, AK.
The project is a $7 million contract to install 5,300 feet of 48-inch steel spiral water main pipe—in 40-foot lengths. Some 20 dewatering wells, each 30–32 feet in depth, proved the solution to the groundwater problem, Darbous says. The wells were drilled with a large auger. A perforated pipe was installed in the hole, with an electric submersible pump at the bottom.
Most of Anchorage’s water pipe consists of ductile iron pipe laid in 18-foot joints, Darbous says. But the 40-foot joints require less welding, and fewer joints, which means a lowered risk of leakage. On this Tudor East project, half the joints were welded and half were Carnegie joints, which use a rubber O-ring.
Trench-Box Solution
The 40-foot joints presented a challenge for the trench-box configuration. “Normally, we use trench boxes, but with the extra-long sticks, just one box wasn’t going to work; it would be too heavy for our equipment,” Darbous says. “So we called Efficiency Production, and they suggested two joined boxes with abutted arches.”
Mike West, Efficiency’s vice president of engineering, came up with a plan for a long in-line trench shielding system that met the soil and pipe requirements. “We finalized a custom-designed system that combines a 30-foot box with a 24-foot box into one 54-foot modular unit,” says West.
The 30-foot Efficiency box worked closest to the Hitachi Zaxis 800 excavator. At the near end, the spreader pipe arched downward to admit the pipe swinging into place over the box. At the junction between the two boxes, pin-in-place spreader arches with 60-inch cross-beams arch upward at the adjoining ends of each box to permit the pipe to swing in under the arches. Because one box is longer than the other, the junction arches are off-center from the center point of the 40-foot steel pipe joint. That way, the rigging for the pipe lift does not conflict with the arches at the adjoining ends of the two boxes.
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Photo: Hobas |
| On the Deep River Outfall Segment 2, this stretch of trench reached a depth of 60 feet. |
The contractor dragged the two boxes by means of two excavators, the Hitachi Zaxis 800 and a Hitachi 550. The 800 would reach inside the 30-foot box and pull it by the front spreader, while the other excavator pulled the back box. The larger Hitachi dug out 20 feet at a time. Then the two excavators pulled the boxes ahead, and the big excavator repeated the process. With 40 feet of trench excavated, the pipe could be swung into place.
Digging depths averaged 15 feet, with some areas more than 20 feet deep. Above the trench boxes, Pruhs laid the slope back at 1.5:1 for safety purposes. “Where your slope hits the box has to be 2 feet below the box’s top,” says Darbous.
“It worked really well, exactly how we had planned it,” says Darbous. “It definitely met our expectations. Though it was heavy, our machines were able to handle the two shields without a problem.”