Drilling Dynamics: Ground Conditions Affect Auger Equipment and Tool Selection
When it comes to production drilling and setting poles, electrical utilities and utility contractors often must make decisions on site about the best equipment and tool for the job.
By Dale Putman
When it comes to production drilling and setting poles, electrical utilities and utility contractors often must make decisions on site about the best equipment and tool for the job. Boring reports provide some insight into the geological makeup of the ground, but the reality is that the conditions can vary dramatically between locations that are just a few feet apart. For this reason, utility crews often rely on two important pieces of equipment—digger derricks and auger drills. While the equipment performs similar tasks, they are best used in combination.
Auger drills deliver more than double the torque over digger derricks, making it possible for them to achieve more downforce on auger tools. Generally speaking, auger drills are capable of 30,000 ft. lbs. to 80,000 ft. lbs., while digger derricks have 12,000 ft. lbs. to 14,000 ft. lbs. of torque. That makes auger drills more suitable for drilling through harder material and for creating larger and deeper holes, up to six feet in diameter and 95 feet deep. While digger derricks are used for drilling, they may be limited to softer ground conditions and holes with smaller diameter and less depth requirements. Typically, digger derricks can drill to 10 feet deep at diameters up to 42 inches. With better pole handling capabilities, digger derricks are ideal to follow behind auger drills, setting poles in the holes prepared by the auger drills.
Choosing the Right Auger Tool for the Job
Equally important is selecting the right auger tool. Selection factors include auger style tools or barrel tools, various types of teeth, and multiple tool sizes.
Remember, you can cut dirt with a rock auger or barrel tool, but you can’t cut rock efficiently with a dirt auger. While this is an over simplification of the selection process, it’s a good rule of thumb. Augers have flights to lift the spoils that are loosened by the teeth and a pilot bit that stabilizes the drilling process for a straight hole. Core barrels cut a single track, applying more pressure per tooth, removing rock materials by lifting out the material as individual plugs. In most ground conditions, it’s best to start with an auger tool first until you reach a point where it is not efficient or it meets refusal to advance because the strata is too hard. At that point, it may be necessary to switch to a core barrel tool for better production. If starting with a core barrel tool, on a digger derrick, it may be necessary to use a pilot bit to hold the tool straight while starting the hole.
Be sure to match the tool with the ground conditions. Core barrels are used when material cannot be effectively drilled with conventional flighted rock auger tools, including conditions such as fractural and non-fractural rock, and non-reinforced and reinforced concrete.
The type of teeth on the tool’s pilot bit is directly related to the application it’s designed to work in. The pilot bit and the flighting teeth should be compatible, with the same strength and cutting characteristics. Other important specifications are auger length, flight length, flight thickness and flight pitch. Various auger lengths are available to allow operators to fit the tool to the available tool clearance on the specific auger drill device or the digger derrick configuration.
Flight length is the auger’s total spiral length. The longer the flight length, the more material can be lifted out of the ground. Long flight length is good for loose or sandy soil. Flight thickness impacts the strength of the tool. The thicker the tool, the heavier, so it’s beneficial to choose only what is needed to maximize payload on the truck and material lifting capacity of the boom.
Flight pitch is the distance between each spiral of the flighting. Too steep of a flight pitch, with loose soil, will allow the material to slide back into the hole. In that situation, a flatter pitch would be more effective. But a steeper pitch will get the job done more quickly when the material is denser. A steep pitch auger tool is good for wet, muddy or sticky clay conditions because it’s easier to remove the material from the auger once lifted out of the hole.
At any time when the auger tool meets refusal, it’s a good time to switch to a core barrel style instead. By design, a core barrel single track cuts through hard surfaces better than multiple tracks produced by a flighted tool. When drilling through hard rock, such as granite or basalt, slow and easy is the best approach. You must be patient and let the tool do the work.
In the most extreme conditions, use a core barrel on an auger drill. In some hard rock conditions, however, a digger derrick with the right tool can also get the job done if the hole required is a smaller diameter. Terex recently introduced a Stand Alone Core Barrel for digger derricks, which can increase productivity when drilling hard rock, such as limestone material. For applications requiring drilling to begin at ground level, a removable pilot bit can be used to stabilize the Stand Alone Core Barrel to start a hole.
Some conditions, such as ground water, warrant specialized tools like drill buckets—often called mud buckets. Another often-overlooked condition is frozen ground and permafrost, which is abrasive. In this situation, a spiral rock auger is able to work efficiently.
Tips for Safe and Productive Drilling
- Call before you dig to know what hazards are above or below the dig location.
- Conduct a job site inspection that includes inspection of the digger derrick, auger drill and tools you plan to use.
- Always follow manufacturer instructions for auger tool repairs. Remember to grease teeth upon installation.
- Always use outrigger pads underneath the stabilizer footing to prevent one side of the machine from sinking and becoming out of level, which can cause your hole to not be plumb.
- Conduct tailgate safety meetings with reminders for personnel to stand at least 15 feet away from drilling operations, to be aware of moving parts and open holes, and to wear proper PPE—including gloves, goggles, hard hats, hearing protection and hi-vis clothing. UP
Prepared by Tracy Bennett, Mighty Mo Media Partners LLC, on behalf of Dale Putman, Auger Tooling Product Support Manager, Terex Utilities. Dale Putman has 43 years of experience working for manufacturers of drilling equipment and auger tools. During that time, he has worked with utilities performing drilling operations worldwide. His thorough understanding of the applications and challenges of drilling operations amplifies his ability to serve Terex Utilities customers.