October 2012, Vol. 67 No. 10
Features
Rock-Water-Sewers
Vincent Rice has considerable experience planning underground installation jobs and is proud of his company’s successful pre-planning project track record. Rice, founder and president of Aaron Enterprises, York, PA, is extremely knowledgeable in the various disciplines of trenchless technology, and has been providing personalized trenchless installation services since 1976. The company focuses on auger boring, tunneling and directional drilling, and is known within their south central Pennsylvania trade territory as “rock specialists.”
With more than 60 employees currently on staff, Aaron Enterprises has enjoyed steady growth over the years and earned the respect of numerous engineering firms that often turn to Rice and his team of installation professionals to determine the feasibility and most effective approach for completing trenchless projects. Having encountered nearly every challenge imaginable within the realm of the underground world, even Rice could not have anticipated the additional influx of water created by an angry Hurricane Irene as she invaded the inland Pennsylvania jobsite where his crews were tackling an intricate sewer installation project.
Aaron Enterprises served as general contractor for the gravity sewer job that was initiated as a precursor to a road construction project that required existing sewer mains be relocated due to a design change in the approach and exit ramps of Benjamin Franklin Hwy. The project — situated on the outskirts of Pottstown, Montgomery County, PA — had its fair share of inherent challenges from the onset. At .22 percent minimum slope, grade tolerance was critical. Additionally, the lines were to be positioned at a relatively aggressive depth, often exceeding 20 feet. Groundwater, originating from the shallow water table of the nearby Schuylkill River, was also a concern.
“Maintaining the gradient was absolutely critical,” says Rice. “The carrier pipe of the sewer was specified as 24-inch iron mainline and the casing pipe was only 36-inches in diameter, so we didn’t have much tolerance. The other concern was the close proximity of the existing sewer. Alignment was critical because of the new highway changes, At many points along the bore path, the new line was within five inches of the existing sewer.”
And then, there was the rock.
“I started Aaron Enterprises because I felt there was a great opportunity and need for a trenchless installation contractor to serve the south-central region of Pennsylvania,” says Rice. “The area is rich with rock so rock boring and drilling was destined to become our specialty. The Pottstown project is certainly no exception. I was prepared for the ground conditions, and expected that we would likely have to deal with some groundwater issues since the jobsite was so close a major river. But when we started the project in May, an inland-bound hurricane certainly wasn’t on my radar.”
Irene dumped more than eight inches of rain on the area when it came roaring inland in late August 2011. With calculations estimating the damage in excess of $15 billion, the storm ranks among the costliest hurricanes on record in the Northeastern United States.
Staying on target
Auger boring had been specified by the team of engineers hired by Pottstown municipal officials as the installation approach to complete the sewer relocation project. After encountering time-consuming challenges with the specified method on the first two bores, Rice employed the ON Target auger boring system — developed and manufactured by McLaughlin, Greenville, SC — to navigate the strict, narrow gradient tolerances required for the two remaining bores. According to Dave Gasmovic, president of McLaughlin, the ON Target system allows contractors to control both horizontal and lateral directional changes within the bore path.
“Contractors were limited to a steering head that offered only horizontal (up and down) directional changes during a bore,” says Gasmovic. “The ability to control the boring direction in a lateral, left to right movement provides greater accuracy for difficult on-grade bores. The ON Target system provides contractors with a significant upgrade from the traditional manual knuckle technology that has been used by the industry for many years.”
The cutting path — grade and lateral movement — of the ON Target steering head is controlled by hydraulic actuated panels that open and close to keep the head on the intended path. A control station featuring a hydraulic power pack to control the movement of the steering head, along with a built-in water level, helps monitor grade throughout the bore. Two halogen lights in the control station indicate lateral (left-to-right) steering head movements. The system can accommodate bores and subsequent steel casing installations of between 12 and 70 inches.
The Pottstown sewer relocation was composed of four separate bores of 300, 100, 158 and 185 linear feet respectively. The licensed professional engineer on Rice’s staff staked-out the location for each bore, and launch pits were excavated with consideration for the strictly prescribed alignments. Launch pits were over-excavated to accommodate crushed rock that was placed in the bottom of each pit to serve as a means for dewatering a continuous flow of groundwater that seeped into the pits as a result of the proximity to the nearby river. The rock foundation also doubled as a stable base for Rice’s boring machines. Two-inch submersible electric pumps — operating 24 hours a day — were positioned in well points within each pit to remove the continual flow of groundwater; volumes fluctuated daily depending on what the river was doing. As Irene blew through, however, the water level, as expected, reached unprecedented levels.
Necessary adjustments
Results of soil samples extracted from the 20-plus-foot depths required to comply with the specified cover tolerances of the project plan indicated a clean alluvial silt and alluvial grout with minimal blow counts. The plan was to use a guided pilot system approach; but after excavating the first launch pit, and encountering cobble up to eight inches in diameter, followed by fairly hefty boulders, Rice was concerned.
The amount of rock and cobble encountered on the first 300-foot bore prevented Rice’s crew from pushing the guided pilot beyond the 180-foot mark; forcing them to abandon the guided pilot method. After performing several adaptations and employing a combination of techniques, they were finally able to complete the first bore but the process added five weeks to the amount of time Rice had originally estimated.
“That was the point when I started talking to the folks at McLaughlin about the ON Target system,” says Rice. “I knew I couldn’t depend on the guided pilot system for the remaining crossings. I knew something different would need to be used if we were to be successful completing the remaining bores.”
Rice’s auger boring machine was outfitted with a 24-inch ON Target steering head.
“We completed the remaining bores without issue,” says Rice. “The new steering head made it possible to push our way through the cobble and allowed us to redirect the casing and monitor the gradient and alignment of the bore path as we were going along. Another impressive feature of the system is that it helps minimize any deviations. This was really important given the rock-laden conditions and the fact that we had such strict tolerances for alignment and grade.
“We proceeded cautiously after making the change to the new steering head because of the alignment and grade,” Rice explains. “But our production rates increased dramatically and we were able to maintain that productivity with less than five inches of deviation.”
FOR MORE INFO:
Aaron Enterprises, (717) 854-2641, www.aaronenterprises.com
McLaughlin, (800) 435-9340, www.mightymole.com
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