Third Time's the Charm

A contractor completes a difficult directional bore for a new sewer line through challenging Georgia rock using a laser-guided boring system
Third Time's the Charm
The installation plan had three components, an 8-inch force main of 7,000 feet to a pump station, followed by a 3,500-foot stretch of 10-inch gravity sewer, and a run of 8-inch force main covering nearly 8,000 feet.

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t took a contractor receptive to innovation, a good deal of patience and a lot of determination to conquer a pesky 800-foot stretch of tough Georgia blue granite on a sewer installation project.

Coweta County Water and Sewer Authority put its faith in DeKalb Pipeline Company to emerge victorious after two previous contractors failed the task using drills and air hammers.

The DeKalb firm found the solution in a pit-launched laser-guided boring system, designed to achieve pinpoint on-grade accuracy while eliminating some of the difficult steps involved in other installation techniques.

The contractor completed the installation in two 400-foot bores radiating out from a pit at the site of a manhole at the center of the run. The bore took five days as the drill encountered extremely difficult rock conditions, but the project came off successfully as new 10-inch HDPE pipe was pulled through the bore.


Rough going

Even with rock formations that in many places measured as high as 30,000 psi, the county’s sewer expansion project would have been complicated. The entire project involved installing a new sewer for the town of Newnan, a historic assembly of 27,000, about 40 miles southwest of Atlanta. The authority had approved the sewer expansion to accommodate growth.

The plan had three components:

An 8-inch force main of about 7,000 feet to a pump station.

A 3,500-foot stretch of 10-inch gravity sewer.

Another 8-inch force main covering nearly 8,000 feet to be backed up with a second lift station.

The nearly 3-mile-long route was to parallel the busy four-lane Georgia Highway to provide sewer service for two large commercial development projects.

DeKalb Pipeline, based in Conyers, won the job in an open bid process. Founded in 1960 and in its second generation of family ownership, the firm had built the infrastructure of many communities in DeKalb County and owned a reputation as one of northern Georgia’s premier grading and site utilities contractors. James King, president and operations manager, ranked the Newnan project as among the company’s most challenging.

“Our bid was based on everything being open-cut except some of the driveways, intersections, roads and surface obstacles,” King says. “But once we staked out the gravity line, we realized there was an area where a day care facility, dentist office and some overhead utility lines would make it difficult to open a conventional jack-and-bore pit on either side of the driveways. We also suspected there would be some rocks in that area, so to jack and bore the gravity line in that one little stretch was a concern.”


Change of plans

Foreseeing problems with the plan as bid for that 800-foot gravity stretch and the other highly landscaped areas, King and crew looked for an alternative approach while completing the force main and the gravity sections where open-cut was still doable.

“We discussed the possibility of using a directional drill on both the force main and on the stretch of gravity sewer in front of the day care and dentist office,” King says. The firm secured the necessary change orders and identified a subcontractor to complete the 800-foot section using horizontal directional drilling (HDD).

“This eliminated the need to open up 16-foot-deep, 10-foot-wide, 30-foot-long conventional bore pits in the areas that abutted the parking lot and driveways,” King says.

The next task was to review the installation plan and make adjustments based on the change to HDD. The new design included two 400-foot runs to install the 10-inch gravity line through the area, using an established manhole as an entry and exit point. DeKalb selected two HDD subcontractors; one for the two 400-foot gravity shots and another for about 2,800 linear feet of force main along other portions of the project.

The first 400-foot bore appeared to go according to plan. With the first segment installed successfully and the 10-inch HDPE pipe in place, work began on segment two. This bore required the drill to be repositioned in the opposite direction en route to the downstream manhole. That’s when things turned for the worse.

“After about 30 feet into the gravity bore, the HDD subcontractor began to encounter some serious rock,” King says. “After pulling back and several attempts to get though the rock, they determined the drill didn’t have enough power and wasn’t strong enough. We lost about three to four weeks while the subcontractor tried to devise a solution, and were up against some time completion issues. Something needed to be done.”

Meanwhile, the contractor hired to complete the force main line portion recommended another HDD contractor who had a bigger machine with more power. That firm completed the rock-laden section in three weeks, only to discover another setback.

“When we dug down at the middle manhole location to connect the two bores and set the precast manhole, the elevation of the second bore was one foot too high and didn’t allow for the flow of the sewer line,” King says. “To further complicate things, we discovered that the first bore, while on grade, would not pass a lamp test required by the engineer. This rendered the first bore useless as well.”


Technology to the rescue

“There’s no way to retrieve the pipe that had been installed in either of the tunnels, and the contractor had literally fled the scene,” says King. “So we abandoned both lines with grout and went back to the drawing board.” The subcontractor who attempted the first portion of the 800-foot gravity line suggested contacting the Vermeer Southeast dealership for assistance.

“We looped back with the Vermeer dealer to talk about the possibility that the AXIS guided boring system may be a possible option,” King recalls. “The challenge was that doing a 400-foot run in two opposite directions on line and at a grade of 0.8 percent did not allow for any mistakes. It still seemed like a gamble to me, but at this point, doing something was better than doing nothing at all.”

A major concern was the 10-inch HDPE already in place: The AXIS system would be more likely to succeed boring through virgin material. DeKalb therefore sought the blessing of the project engineer to move the middle manhole about eight feet to change the angle just enough to avoid drilling in the direct path of the failed bore.

The AXIS boring system, developed by Vermeer, can install a pipe with a range of materials and diameters at any grade. It is designed to install 10- to 14-inch pipe in lengths up to 350 feet and can maintain grades of less than 0.5 percent. Basic components include power pack, rack, vacuum pump and storage tank. The system can maintain a strict accuracy tolerance and is also flexible.


Getting to it

A pit was constructed at the site of the middle manhole, and the AXIS system was placed into it. From there, the team bored through soil to the upstream manhole and hit it on center. The entire process from setup to completion of the bore was accomplished in only three days.

The system was rotated 180 degrees, and the second 400-foot shot to the downstream manhole began. Rock sample tests indicated the rock along this bore registered at 25,000 psi, and it was difficult to bore. Drilling at a depth of 14 feet, the team hit rock at about 40 feet into the bore and had to retool at 150 feet with a cutter head better able to handle the difficult rock.

“The density of this rock formation was much greater than any of us had imagined,” King says. “The bore likely would have taken less than five days had the rock not created so much havoc. Once we installed the right cutter head, the system performed beautifully.”

Once the bore was completed, the 10-inch HDPE was pulled through the less-than-1-percent-grade bore, and the two sewer lines were connected using solid sleeves. The entire sewer extension was then complete.

“From beginning to end, this was one of those really, really tough jobs,” King says. “And despite all the frustrations, challenges and uncertainty, I was 100 percent pleased with the AXIS system. I don’t know of any other system that would have been able to deal with the rock as hard as it was, and to complete the bore on line and on grade. After two failed attempts by another contractor, Vermeer and AXIS helped salvage the entire project.”


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