| July 01, 2010 |
Laser straight HDD
Ditch Witch’s OnGrade system cuts the costs and complexity of gravity flow installations
By Tom Jackson
Horizontal directional drilling’s advantages are well known, but the one job that’s proven difficult is gravity flow installations. Utility companies require the slope of these bores to track to within 1 to 2 percent of the design grade so after the pipes are installed water will flow unimpeded downhill. Even the best HDD operators can rarely thread a line that straight.
To achieve that level of accuracy in the past, contractors had several non-HDD options. They could open cut a trench from the surface or use a laser guided, pit launched tunnel boring machine. With HDD some contractors had success digging multiple sight holes down to the drill stem for a visual survey and/or physical measurement of the bore path. All of these, however, were more expensive and time consuming than conventional HDD. “There is a tremendous amount of planning that has to be done using sight relief holes,” says Richard Levings, HDD product manager for Ditch Witch. “And even then accuracy is only so-so, because you don’t know where the drill head is between sight holes.”
Ditch Witch has come up with a solution that preserves all the benefits of HDD operation and gives you a bore path straight enough to meet the demands of gravity flow installations.
At the heart of the Ditch Witch OnGrade HDD system are two components you’d be familiar with, a beacon and a tracker. But this beacon and tracker have been enhanced with new electronics that enable you to measure not just the depth of the drill head, but its depth relative to the design grade.
“The 8500TK tracker and the 850G beacon work much the same as other HDD equipment,” says John Bieberdorf, Ditch Witch electronics products manager. The difference is the tracker is mounted to a grade pole, which includes a laser receiver. The tracker measures the depth of the beacon like a conventional tracker but it also adds that distance to the height of a rotating grade laser plane set up on the surface of the jobsite. The tracker processes all this data and then sends a radio signal to the drilling unit telling the operator exactly how high or low his drill head is as compared to the laser-prescribed design grade. “The beauty of this system is that you don’t have to wait for sight relief holes or the survey points to know where you are,” he says.
With OnGrade, the frequency of your checks depends on the soil conditions and how flat the bore is, Levings says. “Some people may want to check every 2 to 3 feet, especially if they’ve made a steer of any kind, they may want to rotate a bit and then check it. But on average, you would check probably every half a rod length, every 5 feet, reading at the half rod point and then at the end of the rod.”
The 850BG beacon also has a few features that make it unique. The antenna is mounted in the center of the beacon, which is mounted in the center of the housing, instead of off to one side, says Dr. Floyd Gunsaulis, R&D project manager for Ditch Witch. “It’s sensitive down to 1/10th of one percent of pitch,” he says. “It also has 60 roll positions so the operator can maintain accuracy on roll settings.”
Another new feature with the OnGrade system is the ability to track the progress of the drill head off to the side of the bore in the event of obstacles along the bore path.
The system also includes a calibration fixture that’s used before the start of each bore. “This allows us to be perpendicular and establish an exact distance between the tracker and the beacon,” Gunsaulis says. “It also gives you the ability to zero out the pitch of the drill head housing. That helps insure the accuracy.”
The tracker can read the beacon at depths up to 30 feet, barring any metallic elements in the soil. A “drill through” feature allows you to move the tracker out ahead of the bore head, says Bieberdorf. It’s designed to cross roadways and around obstacles and things you can’t walk over. It will give you left-right steering indication and projected depth based on the incline. This left-right steering indication can also be used to assist the driller in making a slight curve from a straight bore path. EW
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