Drilling dynamics: ground conditions affect auger equipment and tool selection
February 14, 2017 | By Dale Putman
February 14, 2017 – When it comes to production drilling and setting poles, electric utilities and their contractors frequently make decisions onsite regarding the best equipment and tool for the job. Boring reports provide some insight into the geological make-up of the ground, but the reality is conditions can vary dramatically between locations… even those 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 due to differing ground conditions.
Auger drills deliver more than double the torque over digger derricks, making it possible for them to achieve more downward force on auger tools. Generally speaking, they are capable of 30,000 to 80,000 ft-lb of torque, while digger derricks deliver 12,000 to 14,000 ft-lb of torque. That makes auger drills more suitable for drilling through harder material and creating larger and deeper holes—up to 6 feet in diameter and 95-ft deep.
While digger derricks are used for drilling, they may be limited to softer ground conditions and holes with smaller diameter and depth requirements. Typically, they can drill to 10-ft deep at diameters up to 42 in. With better pole-handling capabilities, digger derricks ideally follow behind auger drills, setting poles in the holes prepared by the auger drills.
For example, a job requiring a 20-ft deep hole with 36 in. diameter is best suited to the auger drill because of the depth required. Were the same hole required to be only 10-ft deep, then a digger derrick may be suitable for the job.
Choosing the right auger tool for the job
Equally important to selecting the right machine for the job is selecting the right auger tool. Tools with a hex coupler attachment are used by digger derricks, while those with a square box coupler are used by auger drills. Tools are not specific to the OEM, but that doesn’t mean all tools are created equal. When choosing the right tool for the job, selection criteria include auger-style or barrel tools, types of teeth, and multiple 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 that maxim is an over-simplification of the selection process, it’s a good rule of thumb. Augers have flights to lift the soil that is loosened by the teeth, as well as 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 until you reach a point where it is not efficient or it refuses 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 you must start with a core barrel tool (on a digger derrick), you may need to use a pilot bit to hold the tool straight while starting the hole.
Be sure to match the tool with ground conditions. Most tool specifications include a description of the type of applications for which the auger tool or barrel is designed. For example, some digger derrick augers are designed for compacted soil, stiff clay and soft shale conditions, while digger derrick carbide rock augers tackle medium limestone, sandstone and frozen materials.
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 teeth on the tool’s pilot bit are directly related to the application in which they are designed to work. The pilot bit and flighting teeth should be compatible, with the same strength and cutting characteristics. Other specs that are important in selecting the tool are auger length, and flight length, thickness and pitch. Various auger lengths are available to allow operators to fit the tool to the available tool clearance on your specific auger drill device or digger derrick configuration.
Flight length is the auger’s total spiral length. The longer the flight length, the more material you can lift 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 you need to maximize payload on the truck and material lifting capacity of the boom. I recommend a thicker flight at the bottom of an auger for heavy-duty applications.
Flight pitch is the distance between each spiral of the flighting. In loose soil, too steep of a flight pitch will allow the material to slide right 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. I recommend a steep-pitch auger tool for wet, muddy or sticky clay conditions, as it’s easier to remove the material from the auger once lifted out of the hole.
When the auger tool meets refusal, it’s a good time to switch to a core barrel-style. 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’ve got to be patient and let the tool do the work.
In the most extreme conditions, use a core barrel on an auger drill. However, a digger derrick with the right tool can also get the job done in some hard rock conditions when a smaller-diameter hole is required.
Some conditions, such as ground water, warrant specialized tools like drill buckets (a.k.a. mud buckets). These tools remove fluid/semi-fluid material from the drilled shaft when material does not adhere to auger flighting. At least two styles are available—spin bottom or dump bottom. Both are efficient methods for removing wet soil. Another oft-overlooked condition is frozen ground and permafrost, which is very abrasive. In this situation, a spiral rock auger is able to work efficiently.
Tips for safe and productive drilling
So you’ve selected the machine and tool for the job but, before you begin, always know what’s below and above the dig location. Most (if not all) jurisdictions in Canada and the States have some kind of “Call before you dig” protocol to help protect you and others from unintentional contact with existing underground utilities. Also, always inspect the work area for overhead lines to prevent powerline contact and electrocution.
The jobsite inspection should also include inspection of the digger derrick, auger drill and tools you plan to use. Follow manufacturer instructions for daily pre-shift equipment and tool inspections. It’s important to check teeth to ensure they are in good condition. For example, if rock teeth don’t turn freely, they may wear flat on one side, decreasing life and efficiency. Also look for wear in the teeth pockets.
If the carbide on a bullet tooth has worn away, it’s time to replace the tooth. Not changing worn teeth can severely damage the tooth pocket, which can be costly to repair. Also check the hard-face edges of auger flighting and barrel tools for wear; otherwise, the diameter of the hole may be affected. Re-hard facing the edges prevents a reduction in the hole diameter, and can often be done in the field.
Always follow manufacturer instructions for any auger tool repairs. Follow correct tooth installation and removal procedures, using the proper tools. Many tools are designed to make tooth replacement easy, but it can be a dangerous task if not done right. For example, never strike the carbide face with a hammer. Any time you strike a hardened surface there is risk of metal shattering off, which can cause bodily injury. Finally, remember to grease teeth upon installation. This is very important for maintaining free movement during operation and it makes it easier to remove the teeth when replacing them.
Digger derricks and auger drills use a variety of stabilizers (e.g. A-frame, out-and-down, straight down) but, regardless of type, always use outrigger pads underneath the stabilizer footing. This prevents one side of the machine from sinking into the ground. When the machine is out of level, it can cause your hole to not be plumb. For auger drills, rely on the level indicator to maintain the correct drill angle. For digger derricks, operators must continuously monitor the boom position to ensure the auger remains vertical by extending or retracting and rotating as needed.
Finally, tailgate safety meetings should include 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. When work continues around open holes, either cover the holes or wear fall protection and tie off to an approved permanent structure.
Utility crews must make many decisions about the ground conditions when performing drilling operations. Understanding the ground conditions, the condition of the equipment, capabilities of digger derricks, auger drills, the many tool attachments available and following the manufacturer’s instructions makes the job more efficient and can help prevent incidents.
Dale Putman serves as auger tooling product support manager with Terex Utilities. He has 43 years of experience working for manufacturers of drilling equipment and auger tools, during which time he has worked with utilities performing drilling operations all over the world. Visit www.terex.com/utilities.
* This article also appears in the February 2017 edition of Electrical Business Magazine. Check out our ARCHIVE page for back issues.
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