Battling Rock on the Island of Maui
Hawaii Pacific Trenchless LLC received a contract to install a pressurized sewer system on the Island of Maui via horizontal directional drilling (HDD). As with any HDD bore on the islands of Hawaii, drilling the 6-in. fused pipe presented a multi-dimensional challenge of tough soil conditions, elevated environmental concerns and exceptionally tight bore profile.
Soil
The hard basalt rocks of Maui were formed by two volcanos — the Haleakala volcano to the east and a smaller volcano to the west. The flat area connecting the two volcanoes were formed from the intersection of lava flows from the two volcanos.
The seasoned Hawaii Pacific Trenchless crew knew the areas complex tapestry of soil conditions went beyond the volcanic basalts. According to the U.S. Geological Survey, overlaying the basalts lay a mix of sedimentary rocks consisting of landslide debris, delta deposits and assorted boulders and cinders. Cinder is an igneous rock with bubble-like cavities created when magma is ejected in the air and then solidifies. The crew knew the boulders — ranging from tennis ball to basketball-sized — represented a significant challenge to control.
Environmental
As with most island HDD bores, the crew was required to maintain strict environmental protocols, designed preserve the island’s lush environment. Frack-out mitigation received considerable pre-bore attention by the on-site inspectors — particularly on the stream crossings. A set of fluid control requirements, which included meticulous documentation of fluid volumes used and removed from site, was rigorously enforced.
Bore Profile
Finally, the profile required the crew to maintain a tight bore profile of 6 in. (+/-), while simultaneously negotiating a stream crossing on each of the two bores. The bore profile also paralleled an important AT&T fiber installation.
The Outcome
PART I: Hawaii Pacific Trenchless initially chose the company’s All-Terrain rock drill for the job. Over the next several weeks, the AT generated acceptable production — when in solid rock. It was in the unconsolidated boulders and cinder deposits that control emerged as a significant issue.
Requiring forward pressure and rotation to operate the AT created conditions where the bit wandered when encountering voids in the unconsolidated rock. As weeks turned into months, the crew began to weigh alternative methods.
PART II: Having heard of the tremendous potential of hammers, the decision was made to “bench” the AT in favor of an air hammer. The first demo yielded mixed results. Improved production was mitigated by a déjà vu situation regarding control. Falling under the hammer head, cuttings exerted upward pressure on the head. Without the ability to push fluids in sufficient volume, the crew found themselves fighting the tendency of the hammer to pitch up. The lack of fluid flow also led to unacceptable wear on the bit.
PART III: Still facing nearly 1,000 ft of tough boring ahead, the crew scheduled a StraightLine HDD RockEye 5.0 Hammer System demo. The Hawaii Pacific Trenchless crew used a Vermeer D100x140 HDD coupled to an 1,100 cfm X 350 psi compressor for this demo. Arriving on-site, the RockEye successfully pounded out the shorter of the two bores (300 ft) in less than two days, maintaining tight 6-in. +/-profile window.
Despite the impressive victory, the cinders presented similar challenges, with cuttings falling under the hammer, forcing the head up. The difference maker, in this case, was the RockEye’s ability to push fluids. The crew placed six to eight gallons of 50 viscosity bentonite fluid mix per minute.
The air/bentonite mixture, combined with a slight modification of the carving technique, carried returns and eliminated the control issues that plagued previous attempts.
Using what was learned on the 300-ft shot, taking on the 600-ft bore the following day generated similar results. At approximately the 200-ft mark, spoils volumes — and production rates — fell precipitously. The crew restored cuttings flow by pushing the same Bentonite fluid mix/rate as the previous day. With cuttings again flowing freely, the RockEye resumed its torrid pace — finishing the 600-ft shot in just over three days.
With average production rates clocking nearly 200 ft per day, everyone — including state and local site inspectors — were impressed with the speed and the control of the RockEye, which maintained a tight window of 12-in. +/- throughout the entire bore.
