Described as a “true Texas size sewer bore,” John Wright Construction undertook a job in Buffalo, Texas, that was initially specified to be done using auger boring. However, the contractor came up with a different plan to get the job done.
John Wright Construction tried a little-used method where they drilled the pilot bore on grade using a Vermeer D24x40 Series II Drill, along with the recently released DigiTrak F2 locator, and would pull the open end steel pipe with a reamer as far as possible, and then support the installation using a push rig (auger rig) in the exit pit.
John Wright Construction relied on the accuracy of the new DigiTrak F2 locator from Digital Control Inc. (DCI) to drill 800 ft beneath a busy stretch of Interstate 45 running between Dallas and Houston, with depths to 44 ft and a difficult .34 percent grade. Using a 22-in. reamer to pull 18-in. OD steel pipe with a very rigid 0.375 wall, Wright said, “I know the pilot bore was very straight on line and grade because the push force on my dry bore rig was only 500 psi on a max force possible of 6,000 psi. That 0.375-in. wall pipe is very stiff and if there were any bellies or curves, the dry bore rig would have reacted.”
Wright was able to pull the product pipe 200 ft with just the drill rig and then added the extra coordinated push from the push rig. According to Vermeer’s Brian Van Arman, they made it to the last 40 ft before the drill rig could pull no more and they lost natural returns through the pipe, never installing any augers. They were able to dig up the pipe at the edge of the service road, disconnect the reamer and drill rig and complete the installation by pushing the pipe in. They then cleaned out the pipe using the augers.
This particular bore is worth noting because while the sewer work for horizontal directional drilling (HDD) is increasing, at the same time, there is debate as to whether HDD is accurate enough. Based on the success of Wright’s bore in Buffalo, Texas, the driller is confident in his plan to attempt two more difficult grade bores beneath highways.
DCI has been promoting the use of HDD for sewer bores for more than 15 years, and the DigiTrak F2 locating system, newly released in 2009, is the latest validation of improved tracking capabilities specifically for sewer grade bores.
According to DCI regional field manager Craig Caswell, there are three critical components for conducting successful sewer bores: first, accurate pitch; second, precise locates for line; and the third is extremely accurate depths. DCI recommends that sewer bores be controlled initially based on pitch, but then depth along the bore determines the need for slight adjustments to the pitch to maintain perfect grade.
“The claimed accuracy of DCI locators of +/-5 percent absolute would bring to question the ability to conduct an accurate sewer bore,” says Caswell, “but using specific techniques and safeguards, one can be assured the accuracy can be spot on.”
Surveying an exact depth directly off the drill head at the first grade station, and matching this depth to the locator is a “first safe guard.” It is necessary to maintain the specified and confirmed pitch reading along the bore and verify that the depths match surveyed depths along the bore path. If the pitch is maintained and the depths are matching to the inch, it is a confirmation that everything is perfect.
While one can typically expect with HDD that maintaining perfect grade at all times is difficult, therefore, once a depth begins to vary from the initially prescribed depth, slight adjustments to pitch must be made to maintain the surveyed depths — accurate depths and locates are critical.
The Buffalo, Texas, bore beneath Interstate 45 was originally intended to be below and parallel to a large diameter reinforced box culvert drain. This would have been impossible to get accurate locates or depths due to the metal interference. John Wright Construction was able to arrange for the bore line to be moved 20 ft to the side of the culvert, allowing accurate depths and locate findings. Crossing the reinforced concrete freeway also caused some inaccurate depths and locates, but since these were predictable and shorter distances, it was possible to maintain grade based merely on pitch and make corrections upon crossing the concrete.
“With years of HDD experience, locating the bore path can be the easy part,” said Caswell. “Pulling the larger diameter product over long distances to precise grades approaching level is the part where contractor experience like John Wright is a necessity that cannot be overlooked.”
Diane Galante is a technical writer for Digital Control Inc.