Drilling Between Rock and Hard Places

Of the many types of geological matter that compose the Earth’s surface, none is more abundant than rock. This naturally formed aggregate of minerals constitutes 89.4 percent of the Earth’s surface; by far the most prevalent geological component of our planet. And while the hard property of rock provides a stable, firm foundation, it is also this characteristic that makes it the most challenging and time-consuming aspect facing underground construction contractors. 

Of all the phrases used to describe petrified geological matter, none is more accurate, relevant or universally understood than “hard as a rock” — and with good reason. Yet for horizontal directional drilling (HDD) contractors, it’s not just the hardness, but rather the varying densities and unpredictable inconsistencies of rock formations that make it so difficult. 

“Understanding what kinds of rock and knowledge of the geographical characteristics and topographical layout of the ground specific to the area is first and foremost,” says Chris Fontana, sales manager, Vermeer Cutting Edge trenchless tooling division. “Having a thorough understanding of the type of rock that contractors will encounter on specific jobsites is critical for making tooling selections that will be most productive and efficient for completing bores through rock using trenchless technology.”

Tod Michael, underground product manager with Vermeer, echoes Fontana’s sentiment. “The sheer toughness and hardness of drilling through rock is challenging enough,” says Michael. “What complicates the process even more is the variation and inconsistency of different rock formations. Tooling technology today is quite advanced and very specific; the result of sound science, advancements in engineering and years of experience based on tooling design and material fabrications that have proven most effective in boring through the many different types of rock. Careful consideration of all these variations is a critical component of making the right tooling selections, and consequently, drilling through rock successfully and efficiently.”
There are literally hundreds of different varieties within the three major types of rock formations — sedimentary, metamorphic and igneous — and of those, vast variation in compositional characteristics relative to density and abrasiveness. And while most contractors are not geologists, it’s important to have a basic understanding of how HDD equipment will perform in rock if HDD jobs are to be completed in an efficient, cost-effective manner.

“HDD manufacturers have done a great job engineering and designing equipment to tackle rock,” says Michael. “The primary challenge facing contractors is selecting tooling best suited for specific conditions. There are a lot of different tools available. Identifying the right tooling, and then mobilizing all equipment that will be required using different tooling selections based on drilling conditions is imperative for achieving efficient production rates, and ultimately, completing HDD jobs profitably.”

Rock Solid Planning
According to Michael, completing bores efficiently and achieving maximum production rates in steering, i.e., the ability for drill operators to maintain a straight bore path as much as possible — is largely dependent on tooling selections. To accomplish this, he reiterates the importance of formulating a thorough drill plan in advance, based on gathering critical information about the type(s) of rock most common to the specific geographic location of the job.

“Diligent and thorough planning bores in advance is probably the biggest piece of the puzzle,” says Michael. “Having a good understanding of the type of rock and anticipating inconsistencies and variations is imperative for making the right tooling choices.”      

Gathering as much information as possible about drilling conditions is also an important component of formulating an effective pre-drill plan. The diameter of the bore, terrain and parameters of specific job specifications are all factors that should be considered when mapping out a bore. The plan should also include strategies for securing and mobilizing all the equipment and supporting components required by tooling and equipment selections, including duplicate backup equipment to minimize delays in the event of unanticipated wear or breakdowns.

Achieving Rock Star Tooling Performance
While boring through rock will likely remain the biggest challenge for underground installation contractors, advancements in tooling engineering and HDD equipment technology is allowing contractors to approach the process with a greater sense of confidence. Experience and observing how equipment performs in these changing conditions has provided HDD and tooling manufacturers with information that has led to the development of tooling that performs best in specific types of rock. A general knowledge of different rock formations is valuable for contractors in making prudent equipment and tooling choices.

Despite the variations presented by the hundreds of different rock formations, there are some basic properties of rock that are fairly consistent and contractors can benefit from having a general understanding of the basic groupings and classifications in anticipating different challenges presented by each. Softer rock like shale, limestone and sandstone will present lesser challenges than the more dense formations like granite and mylonite. And then there’s cobble, caliche and glacial till that all present unique challenges. Consideration for the type of rock is an important component of planning a bore as is choosing the most effective approach for removing cuttings. Hence, knowledge of drilling fluid mixtures most effective different rock conditions is also an important part of the planning equation.

“The bit choice on the front of a rock tool is essential,” says Fontana. “Matching the bit that’s been engineered for a specific formation will help in maximizing cutting efficiency. It’s a good strategy to select tooling that provides the flexibility to adjust tooling as conditions change. Other options may be changing rock cutting technologies, deploying an air hammer or mud motor. The option to change tooling to match changing rock types can increase productivity and minimize premature wear to tooling.”

Different Rock Types Affect Tooling Differently
After identifying the type(s) of rock that present on a specific bore, it’s beneficial to understand how different formations are likely to affect tooling performance. It’s also important to recognize that although each of the most common rock types contractors face — i.e., shale, granite, caliche, glacial till, cobble and river rock — have a lot of the same compositional characteristics, density and abrasive properties of rock varies a great deal. Gathering rock samples to gauge psi and abrasive properties is an important component of preparing bore plans and tooling selections with the capability to stand up to the different challenges of each.

“The inconsistency in rock formations is what makes tooling selection so important,” says Michael. “The degree of difficulty in effectively navigating the resistance to tooling, and the ability of drill operators to make adjustments and corrections to maintain the integrity of the bore path is largely dependent on the capabilities of the tooling and drill. There are effective tools available for accomplishing this, but with so many variables, drilling through rock effectively requires intuitive operator skills and some good luck, as well.”

In addition to a multitude of specialized drill bit options, each designed to perform more effectively in specific types of rock formations; contractors often use an air hammer or mud motor to conquer rock more efficiently. Air hammers are used most often when boring through solid formations and noted for their capability to achieve higher production rates in solid rock conditions. Typically, an air hammer can be swapped out for roller cone or tri-cone bits in mid-bore as drilling conditions warrant with minimal adjustments to most drills. But contractors should anticipate and secure the additional equipment required when using an air hammer, including access to an air compressor and an adequate drilling fluid delivery system, as the composition of fluid used when employing an air hammer will likely differ from the mixture used with conventional bit tooling.

Cobble vs. Rock
While drilling through solid rock is somewhat predictable, boring through cobble is an entirely different story. Cobble presents many challenges, most notably, because of the variation in composition. Cobble is composed largely of stones, pebbles and rocks ranging from marble to larger than basketball size diameter. Void of tooling designed to minimize the deflection of the drill bit resulting from an inconsistent and unstable composition, drill operators often struggle to maintain a straight path. “Drilling in cobble is more a matter of steering the tool where you want it to go than it is the hardness and resistance of a hard, solid drilling surface,” says Michael. “It’s that variation that creates the potential for something to go amiss.”

The role of drilling fluid is also an important consideration when boring through rock. It’s important to monitor cuttings returns continuously. The color and consistency of returns says a lot about the success of bore and the effectiveness of the tooling. Based on the appearance, adjustments to drilling fluid mixtures — like adding polymers to facilitate efficient removal of cuttings and reducing the tendency of cuttings adhering to tooling — can be made accordingly.

“When drilling in solid rock, using a water and bentonite mixture will likely be sufficient to remove cuttings efficiently,” says Michael. “Drilling fluid also plays an important role in cooling the drill bit and helps keep the face of the cutting surface clean. Achieving the right drilling fluid mixture can also minimize wear and extend the life of tooling.

“The best advice is to use the data generated by the drill — things like rotational torque, thrust and the condition of the drilling fluid returns — to gauge the effectiveness of the tooling. Proper drill mixtures also play an important role in keeping mud motors and pumps in good operating condition that may otherwise lead to equipment damage and premature wear, and impede the overall productivity of completing bores through rock.”

Randy Happel is a features writer at Two Rivers Marketing, Des Moines, Iowa.