Underground undertaking: new water main installation balances preservation and progress

When the Charleston Water System (CWS) undertook the installation of a nearly mile-long water transmission line from a coastal island in South Carolina’s low country to a nearby main, they had to contend with a distinct environmental and engineering hurdle — the Ben Sawyer Intracoastal Waterway.

The project, which aimed to fortify the area’s water network in the face of a growing population utilized horizontal directional drilling (HDD) to install a mile-long steel pipe — coated with layers of fusion-bonded epoxy and an abrasion-resistant overcoat — at depths reaching 90 feet beneath sensitive wetlands (Figure 1).

The project prioritized minimizing negative impacts and disruptions to nearby communities. Specialized equipment and meticulous planning ensured the underground boring created minimal disturbance to delicate wetland habitats and ecosystems.

“HDD is not a gentle process, especially on such a long pipe subjected to extreme pullback stress,” said James Wilson, a project engineer with CWS. “We were looking for the maximum amount of protection against corrosion to the steel substrate, including external coatings to stand up to the harsh underground environment.”

The installation of the new pipeline also provided an emergency feed to a nearby populated island in the event of a break on the harbor transmission main, while also enabling local water utilities to begin metering water flow – critical for achieving sustainability goals.

Coating combination

While ductile iron pipe (DIP) was deemed appropriate for certain aspects of the project, the predominant portion of the pipeline — totaling 5,180 linear feet — consisted of an 18-inch steel water main.

Although CWS had a solid track record with DIP, the company’s experience with steel pipe was limited. Given the installation method, particularly the stresses from HDD pullback, DIP was not feasible, prompting the utility provider to choose steel.

The project team faced the challenge of selecting an exterior coating system for the steel pipe that was capable of withstanding the harsh underground environment, resisting corrosion and enduring the inevitable damage during the nearly mile-long pull. Additionally, the coating system had to be compatible with an impressed current cathodic protection (ICCP) system, which would be installed to enhance corrosion resistance.

Post-installation, the steel pipe had to endure unincorporated soils beneath marshes and swamps, notorious for causing aggressive corrosion. Fortunately, most of the pipe was laid in a deeper layer of soil known as Cooper marl, which has relatively benign effects on underground infrastructure.

“We wanted the optimal solution, so we landed on a powder-on-powder coatings system that could perform during the immediate installation and over the pipe’s decades of service,” said Alan Townsend, principal with Hussey Gay Bell, a consulting engineering firm.

The team opted for coatings with installation flexibility, designed to work in tandem with the cathodic protection system. This system would offer supplementary electrical corrosion control, safeguarding the integrity of corrosion protection even in case of coating damage.

MCIP Industrial Enterprises Corp., a Texas-based industrial contractor, undertook the preparation of 40-foot sections of steel pipes for coating applications. Employing an SSPC SP-10 blast on both the inside (ID) and outside diameter (OD) of each pipe, they achieved an angular profile of 2.5 to 3 mils to enhance coating adhesion. Subsequently, the pipes were transferred to the application booth for coating. To facilitate efficient field welding of joints, applicators masked off hold back areas, leaving several inches of exposed steel.

For the exterior coating, the pipes underwent a near-simultaneous spray of a 20-mil dry film thickness (DFT) coat of PipeClad® 2000 FBE (Fusion-Bonded Epoxy), immediately followed by a 40-mil DFT coat of PipeClad 2040 Flex ARO (Abrasion Resistant Overcoat) to achieve a 60-mil homogeneous film (Figure 2). This coating system was selected for its compatibility with ICCP systems, and the ARO coating was also chosen for its flexibility, ensuring the pipe and coatings could withstand temporary deformations during installation.

The quick succession of coating applications, combined with induction heating, created a robust interlocking bond between the layers, guarding against moisture and corrosion throughout storage, transit, construction and service. This dual-layer powder system also expedited curing, ensuring high throughput for the shop.

Despite the exterior coating enduring the highest levels of stress during installation, the interior epoxy lining also faced significant strain from the bending of the steel pipe. With flexibility in mind, Sherplate™ PW liquid epoxy was used for interior coating. This 100% solids epoxy met the minimum 25-mil DFT requirements in a single pass via automated sprayers.

Engineered for immersion service in potable water pipes and storage tanks, the coating complies with ANSI/AWWA C210-15 and updated NSF/ANSI/CAN 61 standards. Quality control measures, including holiday testing, were conducted on interior and exterior surfaces before delivery to the construction site.

Down under

To safeguard the wetland environment, heavy equipment was ruled out, necessitating the use of HDD to pull the pre-assembled pipe through the protected wetland area.

To minimize habitat disruption, the mile-long water main was staged, welded and assembled in four-section increments along a municipal road (Figure 3). Applicators used PipeClad 5000 liquid epoxy, achieving a 60-mil DFT for exterior girth welds, ensuring optimal corrosion resistance and durability. For interior welds, Sherplate PW liquid epoxy — offering a one-pass solution — was applied remotely to maintain a quality finish.

Prior to pullback, the crew conducted comprehensive testing, including mil tests, spark tests and internal camera inspections. Steel casing pipe sections prevented hole collapse during drilling through wet soil. After pullback, hydraulic pressure testing ensured integrity.

“Any damage that would cause leakage would be found during the last pressure test,” said Wilson. “In such a harsh environment for coatings, our system successfully stood up to scouring and other stresses.”

Communities connected

The $8.5 million 10-month project was completed on budget and on time. After the pipeline was installed and the infrastructure was in place, the team undertook an extensive site restoration effort. This included adding new landscaping and streetscape features to further enhance pedestrian- and bike-friendly areas near the picturesque waterway.

“The construction site looked good as new,” said Wilson. “You wouldn’t know we were there — or that millions of gallons of water are flowing below.”