Selecting the Best Tool for the Rock You’re Drilling In

In the world of tools, there is no “one tool for all.” Just walk into any hardware store and you will see the overabundance of options that exist. In the world of trenchless technology, the same analogy exists. There is no one tool to handle every formation that can be encountered underground.

Isaac Newton calculated what he thought lied beneath the earth’s surface three centuries ago and even though his estimate of density remains the same today, our knowledge has vastly improved. Trenchless technology may not reach down to the levels of density that Newton predicted but it does cut through the rocky surface layer of the earth, which is called the crust.

It is this layer that the contractor for a horizontal directional drilling (HDD) bore needs to be most concerned with. Unlike vertical drilling where the rock formation is likely to be more consistent, in HDD the rock formations encountered can vary greatly in density and consistency. There are three specific rock types that comprise the crust:  Sedimentary Rocks, Igneous Rocks and Metamorphic Rocks.

Know your soil conditions. Hole Openers, such the Osprey SD (above) and Osprey XD (below), are used in alluvial soils.

Know your soil conditions. Hole Openers, such the Osprey SD (above) and Osprey XD (below), are used in alluvial soils.

Sedimentary Rocks are made up of broken pieces of rock, as well as deposits from minerals and calcium from debris such as shells, bones and teeth. It is the sands, mud and pebbles, as well as shale, dolomite and limestone. Igneous Rocks form from the cooling of magma which in turn forms crystalline solids. They can cool below the earth’s surface which results in large crystals to form or above the surface, which results in smaller crystals to form. Granite, diorite, peridotite, basalts and rhyolite are examples of Igneous rocks. Metamorphic Rocks are basically rocks in which minerals have changed due to heat, pressure or chemical reactions below the earth’s surface. Slate, schist, quartzite, gneiss and marble are examples of metamorphic rocks.

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What does this all mean to the driller?  Within each rock type is hundreds of different geological formations that have varying degrees of density, abrasiveness and hardness, as well as inconsistencies that present challenges to the underground contractor. Rates of penetration are adversely impacted by the unpredictable formations that can and will be encountered once a borehole is begun. Geotechnical site investigations should be the first line of defense when taking on an HDD job. The results of the core samples can assist the contractor in the proper tool selection for the geological conditions to be encountered.

Just like the proper screwdriver for the type of screw used, it is important to select the proper reamer or hole opener for the formations to be encountered on an HDD job. The advancements in the HDD industry and the engineering that has gone into the tooling offers underground contractors with exceptional options to get the job done. It all comes down to knowing what works best in each type of rock formation to be encountered.

Alluvial Soil

Unconsolidated aggregate of loose material comprised of silt, sand, clay and gravel deposited by streams or rivers.


Know your soil conditions. Hole Openers, such the Osprey SD (at top) and Osprey XD (above), are used in alluvial soils.

When drilling in alluvial soil conditions Fly Cutters would be the tool of choice. Fly Cutters are built in a variety of styles and shapes. Some exemplify more jetting abilities while others achieve more robust cutting capabilities. Both serve a niche in the HDD market.

RELATED: Smaller Rigs Take on Hard Rock

Soft situations, like the silts and sands, will most likely be better served with a Fly Cutter that has enhanced jetting capabilities since the tool is performing more as a mixer rather than a true cutter.

Fly Cutters used in harder situations, such as some clays and dense alluvials, must accomplish both jetting capabilities with a robust cutting structure. Fly Cutters in this class should also have the ability to cut their way back out of a hole. This means they rotate in the same direction, either push or pull to retrieve the reamer. This ability will serve contractors well in the event they need to inspect the teeth or, if the unthinkable happens, recover the tool due to a drill rod break.

Soft Rock   Up To 7,000 PSI

Softer rock formations include the sedimentary rocks such as shales, coal and sandstone.
With soft rock formations a Milled Tooth Profile Hole Opener provides a much larger tooth surface compared to their Tungsten Carbide Insert (TCI) counter parts. Thus, Milled Tooth becomes an ideal choice when encountering harder alluvial soils and softer rock conditions.

This tooth surface is covered with a harder alloy usually made with tungsten carbide for added wear resistance. The tooth provides a scraping or gouging action that removes larger pieces of soil or rock which increases overall penetration rates.

Medium Rock 7,000 – 18, 000 PSI

Consolidated metamorphic rock that has been subjected to high temperatures and high pressures. This includes Dolomite, Limestone, Marble, Granite and other igneous rock.

Here is where some overlap occurs based on the type of rock, the PSI (compressive strength), the Moe’s Hardness Scale and whether the rock is consolidated or unconsolidated.

In this category a contractor may opt for a TCI Hole Opener with a Chisel Tooth Profile. This will allow faster penetration speeds while also providing some insurance in case the rock becomes a bit more robust than anticipated. A Chisel Tooth Profile offers the contractor a carbide that mimics a Milled Tooth Hole Opener, but provides additional protection up to 18 to  20,000 psi formations.

Contractors may also opt for a TCI Hole Opener with a Conical Tooth Profile when encountering rock formations toward the upper range of this category. This profile has a smaller contact point compared to the chisel profile,which will result in smaller fragments which could reduce penetration rates but add protection against breakage.

Hard/Very Hard Rock 18,000 PSI and Up

Consolidated crystalline structures caused by the movement of the earth’s lithospheric plates where the heating and squeezing conditions form metamorphic rocks. This includes Basalt, Quartz, Taconite and some Granite.
The best tool to tackle the hard to very hard rock north of 18,000 psi is the Carbide Conical Tooth Profile Hole Openers. With hard rock formations the object is to exceed the rocks compressive strength in order to fracture and create small fragments that may be floated from the bore hole. Conical profile inserts offer the best insurance against breakage due to heavy loads applied to the Hole Openers while reaming.

RELATED: Rock Drilling: The Right Tools for a Solid Bore

In addition, Hole Openers designed with conical inserts typically have higher gage and gage protection counts, which is the number of inserts in the outer most two rows of the cone. This heavier gage helps ensure your bore hole doesn’t become under gaged or undersized (cone shaped) which leads to subsequent hole opener failures by point loading the outer bearings.


As an HDD contractor, these guidelines for the proper tool selection can assist when boring in areas where the lithology has been identified by an onsite Geotechnical Site Investigation. The trenchless technology world enters the unknown each time a bore hole is begun. When bottom line performance is what you are looking for, it is important to maximize penetration rates. That’s where tool selection is crucial. Your best defensive is having the best tool for the ROCK!
By Todd Bielawa is president and Jeff Beste is HDD manager at Century Products.
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