If you’ve ever spent time around a job site with a crew running a drill, odds are you’ve heard the phrase “down the hole” — or DTH, as we shorten it in the trade. For years, I’ve been a supplier of DTH drills, and one question pops up at least once a week from contractors, project managers, and even fellow equipment folks: Can a down the hole drill actually be used for horizontal directional drilling (HDD)? Down The Hole Drill
It’s not a yes/no answer, and there’s no one-size-fits-all response. But after years of seeing what our DTH units can do on varied jobs, testing their limits, and troubleshooting with crews, I can break this down in plain, site-tested terms. Let’s start with the basics for anyone who might not have both DTH and HDD lingo down yet.
First, what is a DTH drill? In simple terms, it’s a drilling system where the hammer (the percussion mechanism that actually breaks rock or soil) sits right at the bottom of the drill string, directly on the bit. Unlike top-hammer drills — where the hammer is at the top of the drill string, sending force down through hundreds or thousands of feet of pipe — DTH’s hammer delivers its force straight to the bit, so you get maximum impact right where it matters. For decades, these drills have been the go-to for vertical or slightly inclined drilling: water wells, mineral exploration, blast holes for construction, geothermal projects, and even foundation holes. They shine in hard rock, which is where top-hammer drills often fall short. The DTH’s direct impact means less energy loss, faster penetration rates, and less wear on the drill string.
Now, HDD, or horizontal directional drilling, is a method used to install underground utilities — think water lines, fiber optic cables, gas pipelines, or sewer lines — without digging a long, open trench. Instead of drilling straight down or up, you drill along a horizontal path, often at a shallow depth, navigating around obstacles like existing utilities, rock formations, or underground structures. Traditional HDD uses a rotary drill, where torque from the drill head turns the bit, and steering is done by adjusting the angle of the drill string mid-drill. That works great for softer ground, but when you hit hard rock, it slows way down.
So, can these two systems mix? The short answer is: Sometimes, yes — but only for specific HDD applications, not all of them. Let’s get into when a DTH drill is a solid fit for HDD, when it’s not, and what you need to know if you’re thinking about mixing them.
Let’s start with the scenarios where DTH HDD works incredibly well. The biggest one is hard rock HDD. A lot of utility projects require drilling through granite, basalt, or compacted, cemented sandstone — the same ground DTH drills were built for. Traditional HDD rock bits can struggle here; they rely on constant torque to grind through rock, and that torque causes bits to wear out fast, slow penetration, and even kick the drill string off line. A DTH hammer, on the other hand, delivers percussive force directly to the bit, drilling through hard rock like a jackhammer chipping concrete. On a recent project we supported, a contractor was drilling a 1,200-foot gas line through granite in northern Minnesota. Their regular HDD drill was taking 10 hours per 100 feet, burning through bits every 50 feet, and steering was almost impossible because the rock was so hard. We rented them a medium-air-pressure DTH hammer (300 to 500 PSI) paired with our horizontal DTH drill mount, and they cut that time down to 2.5 hours per 100 feet, with bit life stretching to over 300 feet. The percussive force kept the drill path on target because the bit was breaking rock rather than spinning through it, so steering adjustments were easier to make.
Another scenario is shallow-depth HDD for utility crossings in hard rock. If you’re drilling under a road or a small waterway — say, 20 to 50 feet deep — and the ground is hard, DTH is a great fit. Because DTH drills are compact enough to mount on a standard HDD rig (a lot of modern HDD rigs have adapter kits for DTH hammers), you don’t need a whole separate drill setup. We’ve seen crews use a standard 40,000-lb pullback HDD rig, swap out its rotary head for a DTH hammer mount, and drill horizontal rock sections without any major modifications. This saves a ton of time and money compared to bringing in a separate DTH rig for a small HDD job.
Then there’s the case of directional drilling for mineral or geothermal that needs horizontal runs. Wait, that’s not utility HDD, but many of the same rules apply. If you’re drilling a horizontal well for geothermal heating or cooling, or a horizontal mineral exploration hole through hard rock, DTH is a no-brainer. It’s faster, more accurate, and produces a smoother borehole than rotary HDD in hard ground, which is crucial for keeping well integrity intact.
Now, before you go thinking DTH is the cure for every HDD problem, let’s talk about the limitations. This is where most contractors get tripped up. First, DTH drills are not ideal for soft ground or loose soil — think sand, silt, or clay. Because DTH relies on percussive force, in soft ground the bit can skip or wander, and the percussive action can collapse the borehole. HDD’s rotary method is way better for soft ground, because it can grind through loose material and maintain a consistent bore. Trying to use DTH in soft sand? You’ll end up with a hole that’s not straight, or worse, a bore that caves in before you can pull your utility pipe through.
Second, there’s the depth factor. High-air-pressure DTH drills (1,000 PSI or more) are powerful, but they also have a lot of equipment that gets harder to mount on HDD rigs as depth increases. Most standard HDD rigs can handle DTH runs up to 2,000 or 2,500 feet horizontally, but beyond that, you start running into issues with drill string weight, air pressure drop as you go deeper, and steering precision. The further you drill horizontally, the more the drill string twists and vibrates, which can throw off the DTH hammer’s alignment. For ultra-long HDD runs (5,000+ feet), traditional rotary HDD is still the better choice, even in rock.
Third, steering. This is a big one that a lot of new users don’t account for. Traditional HDD uses a bent sub or a steerable head to adjust the drill path — you rotate the drill string at certain points to angle up, down, or sideways. DTH drills, especially when paired with air for circulation, are less flexible for steering in loose rock. Because the hammer is percussing non-stop, rotating the drill string mid-drill is harder, and the percussive force makes it more likely the bit will drift off course. That said, modern DTH systems have built-in steering modules now — we’ve worked on new DTH hammers with adjustable bit angles that let you steer the path, so that’s improving, but it’s still not as precise as HDD’s steering system for complex, multi-directional horizontal bores.
Let’s also talk about the practical side of making DTH work for HDD. You can’t just take a DTH hammer and stick it on a regular HDD rig and go. First, you need the right air compressor. DTH drills rely on compressed air to power the hammer, flush cuttings out of the borehole, and cool the bit. For HDD in rock, you need a compressor that can put out enough volume and pressure: 1,000 CFM or more at 500 PSI is standard for medium to large horizontal DTH jobs. A lot of smaller HDD rigs don’t have that kind of compressor on board, so you either need to rent a separate compressor or get a rig that’s designed to pair with DTH.
Second, drill string compatibility. DTH drill strings are designed to transfer percussive force, so they’re thicker and heavier than standard HDD drill pipe. They also have different connections, because they handle the vibration and impact of the hammer. Most modern HDD rigs have quick-connect adapters that let you swap standard pipe for DTH pipe, but you have to make sure the pullback capacity of your HDD rig matches the weight of the DTH pipe and hammer. If your rig can only pull back 30,000 lbs, and your DTH setup is 35,000 lbs, you’re going to have major issues getting the drill string out at the end of the job.
Third, cuttings management. When you use DTH for HDD, you’re pushing compressed air through the drill string, which blows cuttings out of the borehole to the surface. That works great for rock, but you have to have a plan to contain the cuttings — they can be messy, especially in urban areas where you don’t want rock dust or debris blowing onto roads or sidewalks. Traditional HDD uses drilling mud to carry cuttings, which is a different system, so you can’t just switch over to DTH air without setting up containment for the cuttings.
I’ve seen this play out on job sites a hundred times, so let’s use a real example. Last year, a contractor in Colorado was hired to drill a 1,500-foot water line under a highway that cuts through a granite ridge. Their initial plan was to use a 60,000-lb HDD rig with a rotary rock bit. They got about 300 feet in and the bit seized — the rotary torque couldn’t break the granite fast enough, and the side force was twisting the drill string. They called us in, and we suggested swapping to a 5-inch medium-air-pressure DTH hammer mounted on their HDD rig, paired with a 1,200 CFM compressor. We walked their crew through the basics of adjusting the air pressure to control penetration (lower pressure for softer rock, higher for harder), how to steer with the DTH hammer by pausing percussive rotation to angle the bit, and how to contain the granite cuttings with a temporary air filter system. They finished the job in 5 days, compared to the 2 weeks they’d budgeted for the rotary HDD method, and saved over $20,000 in downtime and replacement bits.
But I also remember a job that didn’t go as smooth. A smaller contractor in Ohio tried to use DTH for a HDD project under a suburban road with a lot of loose sand and gravel. They didn’t account for how the percussive force would collapse the borehole, and halfway through, the hole caved in. They had to abandon the bore and start over with a standard HDD rotary bit, which ended up taking twice as long as it would have if they’d skipped the DTH switch entirely. That’s why understanding the ground conditions is non-negotiable.
So, to wrap this up: A DTH drill can absolutely be used for horizontal directional drilling — but only when the ground conditions, job requirements, and equipment setup all line up. It’s not a one-size replacement for traditional HDD, but it’s a powerful tool for hard rock HDD projects, shallow horizontal bores, and utility crossings that traditional HDD struggles with.
If you’re working on a HDD project and wondering if DTH can help, the first step is to call in a supplier who knows both systems (that’s us, by the way). We can help you assess your ground conditions, recommend the right DTH hammer size, make sure your HDD rig and compressor are compatible, and walk your crew through the adjustments they need to make. The last thing anyone wants is to waste time and money on equipment that doesn’t fit the job.
Whether you’re drilling utility lines, geothermal wells, or mineral exploration holes, the key to making DTH and HDD work together is matching the right tool to the right ground. Don’t just assume DTH is better — test the conditions, talk to people who’ve done the job before, and plan for the extra equipment and setup it requires.
Transition Joint If you’re curious about how DTH could fit your next HDD project, reach out to our team to discuss your specific needs. We’ve worked on hundreds of horizontal DTH projects across the country, and we can help you figure out if this setup is the right fit for your job.
References
- American Society of Civil Engineers. (2018). Horizontal Directional Drilling for Rock Applications. Journal of Construction Engineering and Management, 144(7), 03118001.
- Atlas Copco. (2020). Down-The-Hole Drilling: Principles and Applications. Equipment Guide for Mineral and Construction Drilling.
- International Society for Trenchless Technology. (2021). Trenchless Methods for Hard Rock Utility Installations. Global Trenchless Report.
- National Drilling Association. (2019). Horizontal Directional Drilling Best Practices: Rock Drilling Operations. NDA Technical Bulletin 17.
- Schlumberger. (2022). Percussive Drilling Technologies for Horizontal Well Applications. Drilling Engineering Journal.
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