No Longer A Pipe Dream

What is the value of a city’s sewer infrastructure? Consider the situation in downtown Honolulu, Hawaii, in March 2006: a 42-in. pressurized force main in a highly populated downtown area cracked, creating the risk of raw sewage backing up into the many hotels, homes and businesses in the popular resort destination of Waikiki.

Public works officials, engineers and contractors in Honolulu solved the problem by installing via microtunnel the new 72-in. Beachwalk Force Main sewer that includes an S-curve — the first successful S-curve microtunneling installation in the United States.

The project was made more difficult because the line had to traverse underground around two bridge abutments and beneath the Ala Wai Canal in busy Waikiki. The project presented significant engineering challenges, but thanks to thorough research, testing and choosing the right pipe combinations and equipment for the job, the project was completed successfully with minimal surface disruption.

Issues and Obstacles
In 2006, the City responded to the cracked sewer line as quickly as possible and installed a temporary bypass system, which was partially above and partially below ground. That bypass project took about three months to complete. Meanwhile, the Honolulu Department of Design and Construction and the Department of Environmental Services began plans for a permanent solution to the problem.

“The damaged force main was fixed by a carbon fiber reinforced wrap and encasing the cracked pipe repair in the line with steel reinforced concrete in three and half days, and a temporary bypass was installed in approximately 90 days; however, we also knew that we were going to need a new second permanent force main for redundancy,” explained Eldon Franklin, who is head of the Wastewater Division, Department of Design and Construction for the City and County of Honolulu.

At this location installing a new force main of almost 6,000 lf would be a challenging assignment for a number of reasons. The original sewer is located in one of the busiest sections of Waikiki, along Ala Wai Boulevard near the Honolulu Convention Center. Engineers were reluctant to conduct installation work near the existing line for fear of causing further damage. There were also two bridges along the proposed route, as well as protected trees. These and other obstacles meant the new line must be diverted under and across the Ala Wai Canal twice in order to intersect with an existing 69-in. collector line.

Innovative Solutions
The traffic congestion and obstacles along the route led engineers to rule out an open-trench installation. Instead, they opted for microtunneling (remote-controlled pipe-jacking). The plan called for a total of five drives. The proposed route of the fifth drive — under and around two bridges and the Ala Wai Canal — included jacking Meyer polymer concrete pipe (PCP) in an S-curve, a design never before accomplished in a microtunneling installation in the United States. According to Tunnel Business Magazine (TBM) April 2012, “To put this into context, the U.S. record for a microtunnel (straight) drive had been 1,625 ft,” (prior to 2008) and this particular S-curve alone was about 1,250 ft in length.

“When we were doing the design work, we could not find any examples (of curved microtunnels) in Hawaii or the United States,” explained James Kwong, P.E., with Yogi Kwong Engineers in Honolulu, a consultant on the project. “With the help of VMT in Germany and Jack Control in Switzerland, we visited and studied some curved microtunnel projects in Europe, some of them already completed and some in progress. We talked to the designers and contractors of those systems and got a very good understanding of the design and construction considerations before we put our own curved tunnel on the drawing board.”

Previous microtunneling projects in the United States were straight-line installations, from the launch shaft to the reception shaft, with the exception of one single-curve drive completed in Hartford, Conn., in 2010. If the sewer design called for a change of direction, a shaft was dug to receive the boring machine and the line was redirected according to the plans.

For the new Beachwalk Force Main sewer, the line would actually make four slight curves, including a long arch that stretched beneath the Ala Wai Canal.

“Without the S-curve design, we would have needed at least two more shafts, one of them located among some protected trees and the other in the canal, which would have created the need for a lengthy Environmental Impact Study. That would have delayed the project for a long time,” said Kwong.

Jacking Pipes
The S-curve portion of the line, which is approximately 1,250 ft in length, was created using 85-in. outside diameter, 72-in. ID Meyer polymer concrete pipe supplied by U.S. Composite Pipe South, a pipe that is designed by the manufacturer for handling the demands of both pipe jacking and sewer conveyance.
“The pipe specifications for pipe jacking are much different than requirements for an open-trench pipe installation,” explained Kwong. “We were installing an S-shaped sewer line under a canal; we could not afford for a pipe section to fail.”

According to Kwong, the pipe used for the S-curve portion needed to have high axial compression strength — a minimum of 13,000 psi — along with manufacturer provided pressure transmission ring or packer to distribute the jacking loads. The pipe also had a smooth, non-absorbent exterior surface for minimum friction during jacking. Exact parallel pipe faces and pipe uniformity were needed for even pressure transfer, and joints design and material qualities were very important to prevent infiltration and exfiltration, particularly in a deflection. The exceptional compressive strength of the pipe also mitigated risk in the event of any lengthy and unforeseen stoppage during the tunneling process and allowing the push to continue without hindrance.

“The quality and uniformity of the Meyer polymer concrete pipe was crucial to the success of this installation,” stated Kwong.

To help convince project engineers that their pipe was right for the job, Meyer subjected its polymer concrete pipe to a 30-day hydro test to prove its joint water tightness at a manufacturer-allowed maximum deflection angle at the plant. The pipe, which has a manufacturer-projected service life of 100-plus years and is required to be watertight up to 35 psi operating pressure, met the specified testing requirements.

Driving The Curves
According to Kwong, the design of the S-curve has two simple curves to help reduce the eccentric loading the pipes go through during the jacking process. Kwong said the work of MTBM operator Rene Inosanto of Frank Coluccio Construction Co. was instrumental in navigating the machine beneath the canal successfully.

“Rene [Inosanto] was the operator of the boring machine and he was very diligent in his efforts,” noted Kwong. “When he needed to make a directional correction, he made them very carefully and gradually to ensure that we got very smooth curves.

“This project was a good test of the pipes and the boring machine,” said Kwong. “The machine we used, a Rasa, was selected by the contractor. At one location along one of the straight drives, it encountered a full face of basaltic cobbles and went through with very little production loss. And the automatic guidance/tracking system (gyrocompass and hydro-level) performed well with in-tunnel survey calibration. It’s very important to have all three — prudent design (alignment and ground conditions), good microtunneling (boring machine, operator, lubrication and tracking), and the right type of pipe to avoid creating excessive eccentric loading of the pipes as you go through the curves.”

The new sewer was designed with foresight; the line is sloped so that it can one day be converted to a gravity system.

“The pipeline is quite large and the intent in the future is to convert that into a gravity sewer from the force main, and overall to eliminate pump stations in the highly populated Waikiki area,” explained Leighton Lum, Ph.D., of R.M. Towill Corp., the prime engineering consultant for the Beachwalk Force Main project.
When the new sewer is fully operational, the bypass line will be removed and the Beachwalk Force Main will go into service. The original 42-in. force main line, now repaired, becomes the redundant backup sewer for the Waikiki area.

“This has turned out to be a very successful effort,” noted Kwong, the project’s geotechnical and trenchless engineering consultant. “We went through a lot of issues that we don’t face in a normal microtunneling project. It really took a team effort, along with great equipment and jacking pipes, to create this new sewer. I think the success of this project will create opportunities for more curved microtunneling projects in the future.”

Steve Gibbs is a freelance writer in Memphis, Tennessee. He has more than 20 years of experience covering civil engineering and public works projects.