Pipe Bursting Success in Large Diameter Upsize Utility Takes on Difficult Sewer Main
Charleston Water System (CWS) in Charleston, South Carolina, provides wastewater services through 700 miles of collection mains, 187 pump stations, and eight miles of deep tunnels, carrying wastewater to its treatment plant.
Charleston Water System (CWS) in Charleston, South Carolina, provides wastewater services through 700 miles of collection mains, 187 pump stations, and eight miles of deep tunnels, carrying wastewater to its treatment plant. On average, the plant treats 19 million gallons per day and releases clean water into the Charleston Harbor.
According to its website, CWS is in the process of replacing the network of deep tunnels that carry raw sewage to its Plum Island Treatment Plant. At an estimated total cost of $186 million, it is the largest infrastructure project in Charleston Water System’s history.
The tunnel system was built in the 1960s. Through the years, the carrier pipe inside the tunnel and the tunnel structure itself began to deteriorate. In 1998, Charleston Water System began the program to replace the tunnel system. Three sections of the original tunnel system have been replaced so far and a fourth section added. Currently, construction is ongoing for the fifth and final section of the original tunnel system, the West Ashley Tunnel Replacement.
|KRG Utility, Lenoir, North Carolina, used a 1250G static pipe bursting system from TT Technologies, Aurora, Illinois, to replace a 1,200-foot section of 8-inch vitrified clay pipe (VCP) gravity sewer main for Charleston Water System (CWS), Charleston, South Carolina.|
As part of the fifth phase, CWS needed to replace a 1,200-foot section of 8-inch vitrified clay pipe (VCP) gravity sewer main. The main, which ran along Harborview Road, was undersized and not capable of handling the additional flows discharged from the pump station force main. To achieve the required capacity, the line needed to be replaced with 18-inch IPS SDR17 high-density polyethylene (HDPE).
CWS contracted Hussey, Gay, & Bell (HGB) in Mt. Pleasant, South Carolina, to act as the design engineer. HGB evaluated three construction methods in an effort to obtain the additional capacity needed for the main. Open-cut, pipe bursting and micro-tunneling were evaluated with regards to cost, impact to the community, as well as the ability to overcome the large upsize and existing cured-in-place liner issue. HGB recommended the pipe bursting option, which was the most cost effective and provided the benefits of trenchless construction.
KRG Utility in Lenoir, North Carolina, was contracted to perform the pipe bursting work. To add to the difficulty of the already significant upsize, KRG would have to contend with a cured-in-place liner inside the existing main. For the burst, KRG Utility used a 1250G static pipe bursting system from trenchless equipment manufacturer TT Technologies in Aurora, Illinois.
KRG Utility is a family-owned and operated company. Since 1978, it has been one of the leading sewer and water rehabilitation service companies in North Carolina, South Carolina and Virginia. KRG Utility offers a range of trenchless methods, including pipe bursting, sliplining and pipe video inspections.
|The project proved challenging because the existing 8-inch VCP pipe had been lined at one time with a cured-in-place liner. Static pipe bursting was specified to attempt the difficult project.|
The company is regarded as one of the top pipe bursting contractors on the East Coast and owns and operates a range of static and pneumatic pipe bursting tools, including several mid-sized tools and some larger diameter rear expander configured tools.
Static Pipe Bursting Process
The static pipe bursting process has expanded its reach over the past decade, becoming popular among many trenchless contractors for both its ease of use and capability. Hydraulically operated static bursting systems with bladed rollers are able to burst/split and replace ductile iron and steel pipes, in addition to being able to use a range of product pipe materials.
TT Technologies’ pipe bursting specialist, Brian Hunter, said, “We’ve seen the static pipe bursting process continue to gain momentum in the industry. It’s become more versatile now that great progress has been made in using various product pipe materials such as ductile iron, fusible PVC and HDPE. The water and sewer industry have been using HDPE and PVC pipe for years now. They are butt fusion welded and well suited for many trenchless applications, including pipe bursting with static pipe bursting equipment.”
In the static process, exit and launch pits are used in the same way they are for pneumatic bursting. First, the hydraulic bursting unit is positioned in the exit pit. Bursting rods are inserted through host pipe and into the launch pit.
|The existing line was replaced for capacity reasons as part of a multiple phase project to restore and replace the utility’s network of deep tunnels that carry wastewater to the Plum Island Treatment Plant.|
“The bursting rods are an important part of the process,” Hunter said. “Linked bursting rods are the industry standard. They don’t suffer from the inherent twisting and torque that screw-together bursting rods experience. Plus, linking the rods speeds the installation process as well as the breakdown procedure. The rods can be quickly removed one at a time at the exit pit as bursting is in operation.”
The flexible guide rod allows the bursting rods to navigate the typical imperfections found on the inside of the host pipe such as sags, humps, dropped joints, debris and other obstacles. At the launch pit, the flexible guide rod is removed. The bladed rollers, bursting head, expander and new pipe are then attached. The specially designed bladed rollers actually split the host pipe instead of ripping or tearing it.
The entire configuration is pulled back through the host pipe by the hydraulic bursting unit. The bladed rollers split the existing pipe, while the bursting head and expander displaces the fragmented host pipe into the surrounding soil. The new pipe is pulled into place simultaneously.
On the Job
Pipe bursting the VCP gravity sewer with a cured-in-place liner was challenging. The capability of the static pipe bursting method was key to the overall success of the project. Several things contributed to the effectiveness of the method.
First, the specially designed bladed rollers were one important component. The bladed rollers were pulled through the host pipe by a hydraulically powered bursting unit. As they were pulled through, they split the host pipe, and in the case of Charleston Water System, split the installed cured-in-place liner within the existing VCP host pipe. Once the line was split, the expander attached to the rollers forced the fragmented VCP pipe pieces into the soil.
Second, the unit’s power was important to completing the project. The ability to not only split the existing host pipe and liner, but the ability to pull in the more than 1,200 feet of new host pipe at the same time made the method efficient and effective.
Third, the trenchless profile of the bursting method was valuable considering the location of the existing pipe. The trenchless method helped reduce overall social disruption, while limiting restoration and helping keep cost under control.
A total of 1,200 LF of 8-inch VCP gravity sewer with cured-in-place liner was replaced with 18-inch OD HDPE through pipe bursting in one week, total time. The bursting was performed over four sewer segments. Four new manholes were removed and replaced, while one remained functional. The original VCP main had a 368,000 GPD capacity at 4/5 full. The new 18-inch HDPE main has a 2,240,000 GPD capacity at 4/5 full, an increase of 508 percent.
“Involving the owner, design engineer, equipment manufacturer and the contractor early on to discuss options and realistic outcomes was critical to the project’s success,” Hunter said. “This was a very challenging project. It was large upsize with an existing cured-in-place liner. There were groundwater issues from the tidal influence. KRG Utility crews had a 24-inch storm drain to contend with just about the sewer main. Many considerations were addressed by Hussey, Gay and Bell in the design stage. They did an excellent job and the KRG Utility crews are well versed in pipe bursting. They know how to get the job done.”