Drillers Use HDD to Replace Sewer Line Near Changing Water Table
Boring and excavating may not present the same glamour and beauty as a sunset on the beach, but are necessary for the 700-square-mile island to maintain its splendor. When the city of Maui was in need of assistance to bolster its infrastructure, it turned to horizontal directional drilling (HDD) technology.
The Wailuku/Kahului Force Main Replacement Project was designed to replace an existing sewer line with 2,820 feet of 20-inch pipe, and 1,500 feet of 24-inch pipe for the Kahului Wastewater Treatment Facility and Pump Station. Situated along a narrow yet heavily traveled road, the project was only 100 yards from the shoreline and passed directly in front of a number of businesses.
The project owners had originally planned to use conventional excavation; however, the specifications where changed to HDD because of the impact on area businesses and traffic, as well as ground conditions and an unstable water table.
“The project site is only 100 yards away from the ocean and the water table is just a few feet below the surface, so when the tide changes, the water level would constantly change,” says Pradeep Chand, president and owner of Hawaii Pacific Trenchless (HPT). “HDD was determined to be the best method due to the impact and ground conditions”
Chand’s company is one of only two HDD contractors in Hawaii, and has been specializing in trenchless technology, underground utilities, excavating and septic systems since 2005. Trenchless is not a new concept in Hawaii, but many design engineers and project owners have been slow to adopt the method and are still learning about its advantages.
“HPT is about 60 percent trenchless right now, but that portion is growing. More and more companies are beginning to look at HDD instead of traditional excavating,” says Chand.
The company’s HDD expertise was crucial as they began boring in a new sewer line, always keeping one eye fixed on the water table. Not only was the table incredibly shallow, but the saltwater density from the ocean added another level of complexity. The continual ebb and flow from the ocean current meant that the crew needed to be timely and efficient in their boring and reaming.
“Timing was crucial, due to the various ground formation. The pullback began immediately after the pre-reaming process and, with continuous 24-hour operations, until the pipe was pulled through to avoid any collapse in the borehole,” Chand says.
The unique soil conglomerations of the island include silt, gravel and saltwater, along with boulder-type limestone and coral chunks. This didn’t allow for reclaiming and the crew ended up drying out the mud and using it for backfill.
“This is one of the most challenging aspects of drilling in Hawaii. It may look like sand on top, but if you look at the soil a few feet lower, you could discover a ledge of basalt rock. A soil report can only help so much; you really have to look at the geographic condition around the area and be familiar with the site,” Chand says.
Nature was not the only entity presenting challenges; manmade obstacles also arose. Existing utility lines crisscrossed each other. Apart from the underground maze, the road experienced heavy traffic during the day. This meant the bore path had to be meticulously planned.
“When examining the traffic and businesses of the area, we determined that the drill path needed to be at a depth to accommodate the heavy traffic above and avoid all the utilities,” Chand says.
HPT set to work with just one HDD rig — a Vermeer D100x140 Navigator HDD. The challenge of the confined work area coupled with the changing water table required the crew to drill short and yet deep bores. The bores were made 15 feet below the surface and no less than 5 feet from all existing lines. At this depth, they were able to eliminate apprehension over any vibrations disturbing the water table or collapsing the bore.
To fully satisfy all of the job demands, the project was separated into five different bores. HPT tackled the longest bore first — an 1,100-foot bore of 24-inch fusible PVC pipe. The 300-foot bore using 20-inch fusible PVC pipe was the last completed. To help bore through the difficult rocks and create a clean bore, HPT utilized a Vermeer universal housing unit with a tapered bit.
“We did one pilot bore and three reaming passes. We went through 18-, 24- and eventually a 30-inch ream,” Chand says. “On the longest bore we did, there was a vertical and horizontal bending radius, so you’re not only going down and leveling, but also horizontally steering the bore path.”
Because of the required depth, HPT needed to bore vertically and then level the drill head on the shorter bores. The ability of the D100x140 rig to steer quickly yet maintain control allowed the crew to stay on the targeted path, and as Chand described it, “was just like steering a car.”
Even with the length of the project, the crew was able to maintain a very small footprint. With 320 feet of pipe on board and a crane available, they were able to eliminate the need for an additional boom truck and manpower. The drill boxes were right next to the rig, which meant less loading and unloading, and any supply movements needed were completed efficiently.
“With a smaller footprint, the ability to quickly load and unload the drill pipe along with the pullback capability are some of the reasons why we chose the D100x140,” Chand says.
The project was started in July 2013 and completed in October. Through the combination of Hawaii Pacific Trenchless expertise and the minimally invasive capabilities of HDD boring, the Kahului Wastewater Facility and Pump Station now has the new pipeline needed to help maintain the pristine majesty of “The Valley Isle.” ND
Darrin Cline wrote this on behalf of Two Rivers Marketing.