Erosion, debris and time damaged a 14-inch diameter water pipeline in a canyon (locally known as a barranca) running through the hills near La Conchita, Calif. Originally installed in the 1960s, by 2005 the water line was sitting 12 feet above ground instead of underneath it. The pipe-often patched and repaired-that serves parts of the Ventura beach community, weakened by corrosion and debris loading, needed complete replacement.
A feasibility study was initiated in 2006 to determine the best solution. Options included a full-span bridge across the top of the canyon, a partial-span bridge near the bottom, suspension cables to elevate the pipe and re-routing the pipe around the canyon using a conventional dig-and-install method, says Neil Cole, Casitas Municipal Water District (CMWD) project manager.
Environmental concerns ruled out trenching, which also had limitations on how deep the pipe could be installed, and cable, due to susceptibility to ecological influences as it crossed the deep coastal canyon featuring 70-degree slopes, unstable soil and an intermittent waterway.
A bridge would have cost $5-7 million, estimates Mohammed Hasan, a licensed professional engineer and registered environmental assessor who served as principal consulting engineer for the project. His firm, Hasan Consultants, located in Ventura, Calif., specializes in money-saving innovative solutions.
Ultimately, horizontal directional drilling under the ravine appeared to be the most economical long-term solution at a cost of $620,000. “We are a small district,” Cole explains, “so economic considerations were big.”
The original pipeline was installed using a traditional burial method no longer allowed by current regulations, Cole indicates. Cutting into the canyon slope created “irreparable damage to the land,” Hasan says. “It’s still bald. Environmental regulations won’t let us cut like this today.”
This time around, CMWD not only saved millions of dollars by choosing a less environmentally invasive directional drilling technique to embed 1,120 feet of HDPE pipeline at an angle, but they also saved the adjacent environment. “No flora or fauna was disturbed,” Hasan states. “[The California Department of] Fish & Game was happy.”
Environmental requirements were numerous. One survey addressed the unknown geology in the canyon. CMWD also performed a complete biological survey to ease concerns about the potential for frac-out (the unintentional release of drilling fluid) during construction.
“When drilling with a diamond bit, you have to use mud,” Hasan says. “It’s a very important part of the process for lubrication.” However, because it can get pushed through cracks under the pressure of drilling, general contractor Toro Enterprises from Oxnard, Calif., and drilling subcontractor Ventura Directional Drilling had to submit it for approval along with all the chemicals used. The use of environmentally friendly mud consisting mostly of bentonite eased the approval process.
Materials weren’t all that needed approval. Because both sides of the canyon are the property of a private avocado and lemon ranch, the district also needed to obtain an easement from the property owner for the new alignment, which was required because the existing pipe had to remain in service until the new pipe was connected to the existing system.
“The ravine that the pipe needed to go under is around 180 feet deep and 500 feet across at its widest,” Cole says. “Making sure that it was possible to drill and pull a pipe under this ravine was the biggest geographical hurdle.”
Another hurdle was overcoming the hesitation of Casitas’ elected board members, the primary decision makers who approved funding for the project. They had concerns about the large-diameter pipe, recalls Hasan. “It was the first time directional drilling was used to build a large pipeline for potable water in the western U.S.”
Cole also divulged initial skepticism about using the innovative drilling technology to replace the damaged pipeline because of problems on a previous project. “There was some concern on my part,” he admits, “but it did make the most sense in this case.”
Conversely, Ventura Directional Drilling president and owner John Fields was optimistic. Already familiar with the area after a decade of projects throughout Southern California, he considered this project a good fit for his company, which has successfully completed other challenging trenchless projects.
The Drill Bit
The project, which took Hasan a year to design, was scheduled for completion in four months. It was finished in three. Construction began Feb. 1, 2012, and was completed on April 23-but it wasn’t easy.
Access to the jobsite was complicated by a partially paved one-way road, with bends and an 18-percent grade that was tricky for a semi-trailer to navigate. Two open-top Adler frac tanks, with their rear wheels on the tank, struggled even more.
The canyon is 1,000 feet across, with a creek in the bottom. A geotechnical investigation by Earth Systems of South-ern California uncovered bedrock-type material at 35 feet below the flow line. This rock was classified as a Monterey formation, the top of which, per design, marked the deepest point of the bore.
Using a Ditchwitch JT100, rated at 100,000-pounds pullback capacity, with a jetting drill head assembly that featured steering tools and pilot bore guidance provided by SlimDril International, Fields drilled a 6-inch diameter pilot hole the entire length of the bore at an 18-degree continuous angle. The drill was compact enough to be positioned without the need to remove more than a few avocado trees, in accordance with contractual agreement.
“The biggest challenge was hitting hard material much sooner than anticipated,” Cole recollects. Due to rugged terrain in the ravine, which consists of big boulders and pebbles, more than half the bore was drilled without the usual wire line grid. “The geology was different than anticipated,” Fields agrees. “We changed to rock tooling and mud pumps.”
The bend radius of the bore was calculated at 440 feet-which exceeded the maximum recommended radius of the steering tooling by 60 feet-and consisted of three separate turns. The launch angle was 32 percent and the exit angle was 45 percent. The exit side elevation was 24 feet higher than the entry side elevation, with the total elevation change from entry to exit about 185 feet.
“It was unlike any other project,” Fields says. “We were turning and steering the entire length.” Only 75 feet of the bore was drilled without any steering input; the rest of the bore required an average of 2 degrees steering per rod.
Several drill bits were “lost” (damaged), Hasan explains, and more mud was used than on a typical job. Drilling was stopped at a depth of 130 feet about 350 feet into the pilot bore when the Monterey formation was encountered because the jetting assembly couldn’t penetrate it. When a Melfred Borzall-supplied 4-inch mud motor with a 6-inch chisel style bit was installed, drilling resumed.
Threading the Needle
The pilot bore was completed in five days, with the drill head exiting just 1 foot off line. Once the pilot hole was completed, stacked 12-inch and 18-inch roller cone reamers were passed through the borehole to ream down to 200 feet.
When the production rate slowed, the reamers were retracted. Heavy marine-type clay had balled up both of them, although the drill mud weight had not increased markedly during the pilot bore.
The rock reamers were replaced by an 18-inch fly cutter reamer. A 28-inch fly cutter reamer was then attached to the 18-inch reamer to complete the second pass. A 26-inch barrel reamer completed the final pass. Fused, pressure-tested and disinfected 18-inch DR11 HDPE was then attached to a 24-inch reamer and swivel assembly, which was pulled through by the constantly moving drill rig.
Because the project site is in one of two mega slide areas in southern California, materials were selected to endure shaking from an earthquake, Hasan indicates. They have a design life of 50-100 years, requiring no maintenance.
Once the pipe was installed, Toro Enterprises executed the final flushing and testing, installed the valves and fittings and transferred the water supply from the original line.
The only delays were due to restrictions imposed by the ranch owners during rainfall because of concern about the transfer of root-rotting fungus by equipment. “We had to wash and bleach the tires,” Fields says.
Hasan considered it a small concession, applauding the landowners for “significant cooperation” that included access to the work site and the provision of a staging area for the pipe. That cooperation contributed to on-time completion.
Casitas is satisfied with the solution, says Cole. “We’re very pleased with the results. [We have had] no issues with the new pipe.” In fact, the project’s success has convinced him to incorporate directional drilling on future projects. ND