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For every driller, there's that one job. You know the one — you barely got through it, and it still haunts you. You've spent every job since just hoping you wouldn't get caught in a situation like that again. There's a good chance it involved a material you hadn't drilled through before, and there's an even better chance that material was cobble.

The dreaded cobble presents a lot of challenges in the field for drillers and contractors. But why? Exactly what does cobble mean when it comes to directional drilling? Well, if you've broken a lot of HDD drill bits trying to bore through it, you know it can mean a lot of extra effort, time and money. So, whether you've been there before and want to prevent it from happening again, or you've only heard the horror stories and want to be prepared, we've got some tips for drilling through cobble that will get you ready. First, let's go over some basics.

What Is Cobble?

Typically, cobble is a bunch of loose, round rock pieces of various sizes, anywhere from the size of a pea to the size of a kickball (occasionally bigger), often mixed in with clay or soil. While it is commonly found underground near sources of water, cobble can show up anywhere — it's often used as a filler on construction jobs. Noticing a pattern here? Or, a lack of one?

Why is HDD in Cobble So Difficult?

The unpredictability — in location, abundance and size — is a huge part of what makes cobble such a challenge to drill in. You could be drilling in a smooth, consistent clay-like soil, only to come across a bunch of cobbles once you hit a certain depth. Plus, the consistency and density of cobble can vary, as well, though it is generally quite firm.

Drilling through solid rock is tough, but it can be done with the right tools. Cobbles are as hard as solid rock, but they shift and move around. This can cause the drill bit to ricochet or get redirected, making holding grade or your planned bore path a real challenge. It's much tougher than drilling a solid formation. Pullback is no picnic either. Again, you risk getting cobble chunks lodged, and hole stabilization becomes your primary concern.

3 Tips for Drilling Through Cobble Successfully

First of all, since cobble is usually comprised of solid rock, trying to bust through is a fool's errand. HDD in cobble is more about moving the smaller pieces around and steering effectively. However, you have to be careful not to oversteer the drill pipe, as you risk damaging or losing your tooling. During back reaming, ensure that the pipe you're installing can handle the weight of the cobble — it may be wise to split up the process into shorter bores to lessen the impact. Additionally, the following tips will help you drill through cobble:

  1. Do your best to know what you're getting into.
  2. Use the proper tooling.
  3. Get the right drilling fluid mix.

As we mentioned, cobble can make for an unpredictable bore. There's always a chance that you'll run into unforeseen issues. With these tips in mind, you can prepare for the worst-case scenario.

#1: Don't Rush Into A Job

Knowing what you're getting into is always a good idea, particularly because of something like cobble that can disrupt your whole operation. You might not be able to predict everything, but studying the land where you'll be drilling can give you an advantage. To start, you can consult with drillers, dealers and mud suppliers in the area who are more familiar with the territory. Also, doing an informal survey of the land might help you spot potential problem sections.

While you'll definitely want to do this prior to drilling, consider doing it before you even accept the contract, as well. HDD in cobble causes a lot of wear on your equipment — much more than average. Plus, production will typically be a lot slower than usual. What does all of this amount to? Directional drilling through cobble can be expensive. You'll have to estimate for higher costs and determine whether or not it's worth taking the job. If you do, having the appropriate tools will start you off on the right foot.

#2: The Proper Tooling Makes All The Difference

If you try to annihilate cobbles with your tools, there's a good chance that your tools will be the only things getting annihilated. For instance, aggressive HDD drill bits with extended teeth will likely get stuck to cobble and might even snap off, stalling production. As for your drill rig itself, larger rigs won't be able to get into smaller areas, which can limit your options for launching your pilot bore. Using a smaller Vermeer or Ditch Witch rig will give you more options for avoiding difficult locations with an abundance of cobble.

The following are a few key features to look for in cobble tooling. Notice that these factors are slightly different from what might be best for more consistent soil:

  1. It must be well-built and sturdy.
  2. It has to resist wear, even in abrasive conditions.
  3. It needs to stand up to impact and shock.
  4. It should be designed to attack rock gradually and push cobbles out of the way, rather than trying to break or crush them.

With this in mind, knowing the right bit to use is crucial — HDD drill bits for cobble should have a taper or dome design that allows hard rocks to roll or wobble around them. Back reamers and pulling heads must push cobble around the hole, allowing the product pipe to be installed.

Melfred Borzall offers several drill bits, blades and reamers for cobble, designed to handle these conditions effectively. Some key examples include:

Iron Fist Bit

Designed specifically for cobbles, the Iron Fist Bit has small, low-profile carbide cutters that roll or push cobbles out of the way without getting hung up on them like taller "pick" type teeth would. The conical steer face yields aggressive directional control — even in cobbles. Plus, since it's able to be rebuilt, the Iron Fist Bit is a cost-effective solution.

Steep Taper UltraBit with Dome Carbides attached to the Pit Bull Housing

The steep point on the Steep Taper UltraBit blade lets you wobble through cobble. The carbide edges reduce wear, giving it a long life. Select the dome carbide cutter option and combine it with a Pit Bull Design housing for ultimate control and durability in cobble.

Ogre Back Reamer with Conical or Dome Cutter Option and Built-In Swivels

Since carbide cutters spiral around the Ogre Back Reamer, each one only does part of the work, which allows the Ogre to run smoothly in cobble. For even better performance in cobble, select the built-in pulling swivel option. This keeps the product pipe that is being pulled up closer to the packing section of the Ogre reamer.

Cobbles falling in at the transition between the back of the reamer and the product pipe is a common problem when drilling in cobble, especially if your fluid mix isn't quite right. With over 70 years of experience, we've come up with some custom reamer designs to meet your specific requirements. Contact Melfred Borzall for more information about these. 

Whatever you choose, the success of your job hinges on how durable and reliable your tooling is. Having experience operating a drill in cobble helps, as well, but you have to start somewhere. Using the proper equipment can make or break you, and the right drilling fluid can point you toward the "make" direction.

#3: Never Underestimate A Good Drilling Fluid Mixture

Having the right drilling fluid recipe is important for any job, but even more so when dealing with cobble since there is less soil in which to hold the fluid. Ultimately, it comes down to two things: thickness and additives. The drilling fluid should be as thick as possible to help keep your hole open, but it's often still not enough. That's where the additives come in — and for cobble, it's all about polymer additives.

A good rule of thumb is to start out with bentonite and get your viscosity high enough to pick up the cuttings and cobbles and move them out. However, getting viscosity too high may result in plugging the borehole and possibly creating inadvertent returns. Before you get to that point with the viscosity using bentonite, you should turn to a polymer to build a wall around the borehole and prevent fluid loss through the wall. Then, add a xanthan gum (Carry-It) to give extra "gel strength." The gel strength will further aid in carrying out the cobbles, but without increasing viscosity and clogging the hole.

HDD In Cobble Is Tough — Make It A Little Easier

So, what does cobble mean for HDD? It merely means there's another challenge. But you're probably used to that by now. Drillers don't usually back down from challenges, after all. That said, you have to approach it carefully and intelligently, with the appropriate preparation. To get the directional drilling tools you need for cobble, visit Melfred Borzall today.