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Installing gravity sewer in tough Georgia rock

Facing challenging ground conditions, DeKalb Pipeline succeeds using laser-guided boring system

Jul-2011

It took a contractor who was receptive to an innovative new installation approach, a good deal of patience and a whole lot of determination — after two previous attempts by other contractors using drills and air hammers were unsuccessful — to finally conquer a pesky 800-foot (243.8 m) stretch of tough Georgia blue granite. Hence, project owner Coweta County Water and Sewer Authority put their faith in DeKalb Pipeline Company to emerge victorious.

Even without the tough rock formations — that in many places along the installation route measured as high as 30,000 psi (2068 bar) — the sewer expansion project was complicated enough. It involved installing a new sewer for the town of Newnan, Ga., a quaint, historic assembly of 27,000 residents located 40 miles (64.4 km) southwest of Atlanta in Coweta County. With the city’s population projected to increase steadily over the next few years, the County Water and Sewer Authority approved an expansion of the existing sewer in advance of the anticipated growth.

The installation plan had three components, beginning with an 8-inch (20.3 cm) force main of approximately 7000 feet (2133.6 m) to a pump station; followed by a 3500-foot (1066.8 m) stretch of 10-inch (25.4 cm) gravity sewer; and concluding with a third run of 8-inch (20.3 cm) force main covering a distance just shy of 8000 feet (2438.4 m) that was to be backed up with a second lift station. The nearly 3-mile (4.8 km) long install route was plotted to run parallel to Georgia Hwy. 34, a major four-lane thoroughfare separated by a grass median, to provide sewer service for two large commercial development projects to accommodate the anticipated growth.

Based in Conyers, Ga., DeKalb Pipeline was awarded the job after participating in an open bid process. Since its incorporation in 1960, and currently in its second generation of family ownership, DeKalb Pipeline played a major role in building the infrastructures of many DeKalb County, Ga., communities and has since earned a reputation as one of Metro Atlanta’s and North Georgia’s premier grading and site utilities contractors. Unafraid to tackle even the most challenging projects, James King, president and operations manager for DeKalb, ranks the Newnan project as among the company’s most challenging.

“Our bid was based on everything being open cut with the exception of driveways, intersections, roads and surface obstacles,” King explains. “But once we initially staked out the gravity line, we realized that parts of the line would be very difficult to construct as designed. On location, in particular, was an area where a day-care facility, dentist’s 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 rock in that area, so open cut excavation and jack and bore the gravity line in that one little stretch was a concern.”

Foreseeing problems using the installation method outlined in the bid proposal for this 800-foot (243.8 m) gravity stretch and the other highly landscaped areas, King and crew set out to identify an alternative approach while moving forward to complete both the force main and gravity sections where open cut was still doable. But a resolution for navigating this pesky little 800-foot (243.8 m) span remained a lingering cloud that continued to cast a shadow of angst over the landscape. Then, about midway of the project — a ray of light began to surface on the horizon — and a possible solution was in sight.

“We discussed the possibility of using a directional drill on portions of the force main where there was extensive landscaping and also on the stretch of gravity in front of the daycare and dentist’s office,” King said. “This approach would require us to get change orders for deviating from the original plan, but ultimately we were able to secure all the permissions needed to alter the approach. We identified a subcontractor we felt was capable of completing this 800-foot (243.8 m) section using horizontal directional drilling (HDD), which eliminated the need to open up 16 foot (4.9 m) deep by 10 foot (3 m) wide by 30 foot (9.1 m) long conventional bore pits in the areas that abutted the parking lot and driveways.”

Solution identified but unforeseen challenges ahead
Having now identified a viable installation solution with all the necessary change orders secured, DeKalb was back in business. The next task was to review the installation plan and make adjustments based on the change to HDD.

The design was changed to include two 400-foot (121.9 m) runs to install the 10-inch (25.4 cm) gravity line through the affected area using an established manhole location as an entry and exit point. DeKalb selected two different HDD subcontractors; one for the two 400-foot (243.8 m) gravity shots and another for about 2800 linear feet (853.4 m) of force main along other portions of the project.

The first 400-foot bore (243.8 m) appeared to go according to plan. With the first segment installed successfully and the 10-inch (25.4) HDPE material in place, it was time to move on to segment two. This bore required the drill to be repositioned in the opposite direction en route to the downstream manhole when things began to take a turn for the worse.
“After about 30 feet (9.1 m) 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 King hired to complete the force mainline portion recommended another HDD contractor he was associated with who had a bigger machine with more power he felt would be capable of getting through the hard rock. So DeKalb contracted with that HDD contractor to come in and tackle the uncompleted 400-foot (121.9 m) rock-laden section. The bore was finally completed after three weeks, but only to discover yet another setback and costly delay.

“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 explained. “To further complicate things, we then 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.”

AXIS® guided boring system to the rescue
After a number of challenges, setbacks and mounting frustration, hope was fading as King and company faced a serious dilemma. “We have two 400-foot (121.9 m) runs with HDPE pipe pulled and because one contractor missed grade and one was not smooth and straight, both bores are useless,” King says. “There’s no way to retrieve the pipe that’s been installed in either of the tunnels and we had one of the contractors who literally fled the scene. So we abandoned both lines with grout and went back to the drawing board.”

King was delighted by the suggestion provided by the HDD subcontractor, Cobalt Telecommunications, who attempted the first portion of the 800-foot (243.8 m) gravity line to contact the Vermeer Southeast dealership for assistance.

“Dekalb and Cobalt looped back with the Monte Erritt with Vermeer Southeast to talk about the possibility that the AXIS™ guided boring system may be a possible option,” King recalls. “Remember to keep this in proper perspective considering the challenge of doing a 400-foot (121.9 m) run in two different directions on line and at a grade of 0.8 percent, which did not allow for any mistakes. I still had my doubts but at this point, doing something was better than doing nothing at all.”

The next step was to get an accurate understanding of exactly what the AXIS was capable of doing. One of the major concerns was the presence of 10-inch (25.4 m) HDPE already in place — obviously not ideal since the AXIS system would be more likely to succeed boring through virgin material. To accomplish this, DeKalb needed the blessing of the project engineer to move the middle manhole approximately 8 feet (2.4 m) in order to change the angle just enough so as to avoid drilling in the direct path of the existing, albeit, failed bore.

The AXIS system, developed by Vermeer Corporation, is a pit-launched trenchless installation method designed to achieve pinpoint, on-grade accuracy while eliminating some of the difficult steps associated with other installation techniques. The system has the capability to install a wide range of product on grade with a range of materials and diameters — designed to install 10- to 14-inch (25.4 to 35.6 cm) pipe at lengths up to 350 feet (106.7 m) — and can maintain grades of less than .5 percent. Basic components of the system include power pack, rack, vacuum pump and storage tank. Not only is the system capable of maintaining a strict tolerance of accuracy, it’s also flexible.

A pit was constructed at the site of the middle manhole and the AXIS was placed into the pit. From there the team bored through dirt to the upstream manhole and hit it on center. The entire process from set-up to completing the bore required only three days.

The AXIS guided boring system was rotated 180 degrees and the second 400-foot (121.9 m) shot began to the downstream manhole. Estimates indicated the rock along this section registered between 25,000 and 30,000 psi (1724.1 and 2068.9 bar) and it was difficult to bore. Drilling at a depth of 14 feet (4.3 m), the team hit rock at about 40 feet (12.2 m) into the bore and had to retool at 150 feet (45.7 m) with a cutter head better capable of handling the difficult rock conditions.

“The density of this rock formation was much greater than any of us had imagined. I estimate it was at least 20,000 psi(1379 bar),” King says. “The bore would likely have taken less than five days had the rock not created so much havoc. Once the right cutter head was installed, the system performed beautifully.”

Once completed, the 10-inch (25.4 cm) HDPE was pulled through the less than 1 percent grade bore and the two 400-foot (121.9 m) sewer lines were connected using solid sleeves and the nearly 3-mile (4.8 km) long sewer extension was complete.

“From beginning to end, this was one of those really, really tough jobs,” King says. “And despite all the frustrations, challenges and moments of uncertainty, I can honestly say that I was 100 percent pleased with the AXIS system. It was a lifesaver in so many ways and with such little disruption to the landscape. I don’t know of any other system that would have dealt with the rock as hard as it was and be able to complete the bore on line and on grade. After two failed attempts by another contractor, Vermeer and the AXIS system helped salvage the entire project.”