What to Think About When Drilling in Rock
Completing bores in rock can be one of the most satisfying accomplishments of a horizontal directional drilling contractor.
It makes no difference if you are the contractor with the 10,000- or 12,000-lb drill or the company with the 1 million plus-lb drills. Both sizes and everything in between have the same opportunity to bore in rock. We have all heard about the 3,000- and 4,000-plus ft bores that have been completed in conditions that, according to the experts, could not be drilled. These projects were successfully completed because the contractor spent the time to learn about everything he could about the job including site and soil conditions. The successful contractor spent time planning every part of the bore. He eliminated every strike he could before he ever turned a key on the truck to head to the site.
Eliminate strikes by doing the obvious. Walk the bore path, if possible. Look and learn about the soil conditions and topography. Pay attention to the conditions around the site. The rock wall surrounding the pasture and the stone foundation on the 100-year-old barn were not carried very far. This rock was most likely natural to the area. Talk to the locals. An elderly gentleman sitting on his front porch once told me: “The last time a contractor did any work around this road, he started with dynamite.” This one statement explained why no paddle bit, rock bit or roller cone was going to go through the rock encountered on the bore. The job was completed using a rock saw.
Many bid packages include soil tests and core samples. Pay attention to the frequency of the bore samples. Is there any consistency to the results? Or was the project engineered with a soil probe? Large projects may warrant independent soil testing if given the opportunity.
Not every contractor has the luxury of owning every tool or machine that may be needed to ensure the successful completion of the project. You may need to include some specialized tooling, mud system or personnel to work on some jobs. Projects on some federal facilities require escorts or guards just to gain access to the site where the project is to be done. Rail crossings and airport work are also sites that require specialized personnel, training and equipment. The successful contractor is sure to include these costs in the final estimate. Enlist the help of colleges or trusted experts for advice and suggestions in areas where you may not be as adept.
Now that you have the right drill and tooling, mud system and staff to complete the project, it is time to get to work. Be aware of changing ground conditions as you work through the bore. Keep accurate records of the conditions and bore path as you complete the pilot bore. This information could prove invaluable should you encounter conditions that warrant change orders, not to mention the utility that was not located properly.
If there is any one thing that every contractor working in rock should have, it is patience. Rock does not allow many of the practices used in softer soil drilling. In fact, some shots are completed using only the path the soil will allow. Drilling is typically slower and corrections need to be minimized as they can be tedious to achieve. Forcing the tooling to try to complete a bore can be a fatal mistake. You must allow the tool to do the work, just as you cannot force a chainsaw through a log. The cut will happen only as fast as the tool and soil will allow.
Safety is always a concern and a requirement on every construction site. A safe site does not end with the safety meeting in the superintendent’s office or shop. It must be carried over to the project area. By setting up the equipment and supplies so as to eliminate hazards, you will find the job will be completed faster and with more profit. You will eliminate steps to accomplish the necessary tasks by paying attention to detail in this area.
No one tool is always the right choice in drilling. I had the opportunity to work with EBI from Duluth, Minn., on a project near White Pine, Mich. The project was in February 2002. The job called for an 8-in. water line to be installed under the Iron River. The bore path started at the edge of a hotel parking lot and ended in a city park. There were no existing utilities crossing or parallel to the bore path. It was a relatively short shot. The shot was just a few hundred feet from start to finish. The elevation was level from one side to the other.
We knew the conditions were rock. The entry and exit pits showed some solid rock and with some softer areas that had cobble mixed in. EBI had its Ditch Witch 7020 onsite with an air hammer system that had been successful in these types of soil conditions. The plan was to do the pilot bore with a hammer system incorporating air to remove the spoils, as well as providing the force to drill the rock. The “back ream” was to be done using a push reamer. If you noticed earlier, I mentioned the date was February 2002. The Iron River emptied into Lake Superior about 50 yds from the bore path. We were not only dealing with tough cobble and solid rock conditions, we were also working in the dead of winter in the Michigan Upper peninsula. The cold weather caused the small foam lines to freeze, and the wind was kicking up some “Lake Effect” snow about every hour. The snow was heavy enough at some times to blind us from seeing the other side of the jobsite. The cobble turned out to be much deeper than earlier believed. One side of the bore had cobble at 8 ft deep while the other side was more than 10 ft deep. Because the crew had worked through many of the possibilities of this job, they brought in a Railhead EXTReam reamer for the cobble. By mixing the tooling on the back ream, they were able to cut the cobble and maintain a hole to a depth that would allow the push reamer to work in the solid rock.
By careful planning of conditions, equipment, tooling and technique, this project was completed successfully.
Dave Helgeson is director of sales and marketing for Railhead Underground Products, based in Weatherford, Texas.