Proactive Rehab Heads Off Potential Crisis

Proactive Rehab Heads Off Potential Crisis

Underground Construction

In Clayton County, GA, the sewer line under 40 feet of embankment and eight feet of backfill showed signs of imminent failure and the Clayton County Water Authority (CCWA) opted for a proactive approach. Rather than wait for the line to completely fail, they chose to replace it.

Clayton County has a population of 260,000 and is in the southeast quadrant of metro Atlanta. The county has been concerned about the maintenance of their sewer infrastructure for the last 30 years. The county has approximately 1,100 miles of sanitary sewer piping from 8 to 48-inches in diameter. The county services a portion of the Atlanta Hartsfield International Airport.

In the early 1980's, Interstate 675 was built through a corner of the county. The design included a bridge to accommodate Grant Road as it passed over the new interstate. Because the new elevation of the interstate was at or near the existing grade, Grant Road had to be raised up to the elevation of the bridge. This necessitated the placement of embankment about 40-feet thick on the west side of the new freeway. The county also had an eight-inch sewer that paralleled the interstate right-of-way in the Grant Road area. The existing sewer was about eight-feet deep when the embankment was placed.

During the CCWA's ongoing television inspection program, the 8-inch line in question was flagged for rehabilitation. The video showed that the existing line was vitrified clay pipe and was cracked longitudinally. As most experts will agree, this type of cracking is caused by a collection of conditions including: lack of bedding, poor installation or too great a load placed on the pipe. What damaged this line was unknown. What was apparent from the video inspection was that the line segment under Grant Road required replacement.

The design staff at the CCWA decided to replace the line segment from an existing manhole just north of the Grant Road pavement surface, running south approximately 440 feet under Grant Road and into a low swampy area to a shallow manhole. Because the grade at the surface of the road was close to 50 feet above the grade of the sewer line, the cost and disruption of open-cut construction could not be tolerated.

The CCWA's progressive approach to rehabilitation included the use of trenchless methods over the past three years to deal with the challenges they face in maintaining their collection system infrastructure. The line under Grant Road required replacement and the CCWA chose to use ductile iron pipe and the Tenbusch Insertion Method on this project.

This replacement work was performed under an annual contract with Clayton County by Environmental Consortium, with the help of Garney Companies.

Pipe

Ductile iron pipe was chosen for the project because it possesses tremendous columnar and tensile strength, which makes it an excellent material for trenchless applications. American Ductile Iron Pipe Company offers several joints designed for use in direct jacking, trenchless pipeline replacement (pipebursting) and microtunneling installation. Push-Bar pipe was chosen as the most appropriate joints available for this particular job.

Push-Bar Pipe employs an economical adaptation of the premier American Fastite Joint to transfer jacking loads from the pipe barrel directly to the face of the bell. It consists of a high-strength, alloy steel ring, the "push-bar," shop welded to the standard Fastite pipe spigot. This ring is made of the same material that has been used successfully in American's Lok-Ring and other restrained joints for over 30 years. A cushioning compression ring, made of compressible wood products, fits between the push-bar and the pipe bell for added assurance of load distribution. The pipe comes in short joints, as needed by the job site conditions, as well as the standard 20-foot laying lengths and carries the standard Fastite Joint pressure ratings. During installation, Fastite Joint Push-Bar Pipe is jacked or pushed with the spigots ahead. This allows for any debris remaining from the pipe replacement operation to flow smoothly over the bell of the Fastite Joint.

TIM

The Tenbusch Insertion Method (TIM) was chosen because it required the least amount of surface repairs and taxpayer inconvenience. The heavy traffic on the two-lane highway was able to continue uninterrupted.

This patented trenchless method allows the replacement of old pipe with new rigid pipe from one small pit. The existing manholes were used as receiving pits, taking away the cost of new manholes as well as any traffic headache such a replacement would cause. The work pit was located as close to the embankment as possible. This placed it approximately 240 feet from the shallow downstream manhole and approximately 185 feet from the deep upstream manhole.

Following construction of the small work pit, the jacking equipment was installed in the pit and aligned with the old sewer line. The Tenbusch "lead pieces" were then lowered into the pit and inserted into the old sewer. The new pipe was lowered into the jacking frame and the process of replacing the old line began.

Lubricant was applied at the leading end of the new pipe column to reduce friction. Each pipe segment had a flexible hose, which was connected to the one in the previous pipe segment. This hose supplied lubricant, which was ejected at the rear of the trailing sleeve into the annular space.

As the work progressed, the jacking unit advanced the new ductile iron pipe using hydraulic pressure and no percussion. The lead assembly or "Lead Train" of the TIM System when it's assembled, becomes a single rigid structure that dramatically straightens misaligned joints in the existing line. This ability is enhanced by the use of rigid pipes.

Ultimately, the new ductile iron pipes were installed behind the lead assembly at an average rate of one foot per minute.

The front pieces were removed from the manhole quickly, by only one man and an appropriate sling. All that remained was the removal of the jacking unit, connecting the new line at each end, and closing the hole.

This technology allows for the trenchless replacement of existing pipe, both size on size and upsizing using rigid pipe products.

Conclusion

The CCWA's Herbert Etheridge was pleased with the work, largely because:

• The TIM used no percussion and therefore the likelihood of consolidating the material above the pipe was not an issue;
• Given the ease of construction and the efficiency, the pipe was replaced in two pushes at near the rate of one foot per minute:
• Bypass pumping was necessary for only eight days;
• The new pipe material is one with a proven track record of long design life;
• Grant Road traffic was unaffected by the replacement of the old sewer line;
• The existing manholes were allowed to remain in place, diminishing the overall cost to the taxpayer; and
• The 40-foot deep manhole on the northern end of the line would have been especially expensive.

The new line has been televised and inspected. A decision has been made to use this technology in the near future on another exiting line in poor condition and in a place that will not tolerate anything other than a trenchless replacement with a long design life pipe material.

FOR MORE INFORMATION:

Ductile iron pipe: American Ductile Iron Pipe Company, (205) 325-1965, or circle# 183

Rigid pipe insertion: Tenbusch, (972) 221-2304, or circle# 184