[Note that this article is a transcript of the video embedded above.]

In December 2019, the City of Fort Lauderdale, Florida experienced a series of catastrophic ruptures in a critical wastewater transmission line, releasing raw sewage into local waterways and neighborhoods. Recognizing the need for improvements to their aging infrastructure, the City embarked on a plan to install a new pipeline to carry sewage from the Coral Ridge Country Club pumping station across 7 miles (or 12 kilometers) to the Lohmeyer Wastewater Treatment Plant. But just drawing a line on the map hides the enormous complexity of a project like this. Installing an underground pipeline through the heart of a major urban area while crossing three rivers is not a simple task.

Underground utilities are usually installed by a technique known as trenching. In other words, we excavate a trench down from the surface, place the line, backfill the soil, and repair whatever damage to the streets and sidewalks remains. That type of construction is profoundly disruptive, requiring road closures, detours, and pavement repairs that never quite seem as nice as the original. Trenches are also dangerous for the workers inside, so they have to be supported to prevent collapse. Beyond the human risk, in sensitive environmental areas like rivers and streams, trenching is not only technically challenging but practically unachievable because of the permits required. In fact, trenching in urban areas to install pipelines these days is for the birds. When the commotion of construction must be minimized, there are many trenchless technologies for installing pipes below the ground. One of those methods helped Fort Lauderdale get a 7-mile-long sewer built in less than a year and half, and is used across the world to get utility lines across busy roadways and sensitive watercourses. I’m Grady and this is Practical Engineering. On today’s episode, we’re talking about horizontal directional drilling.

If you’ve ever seen one of these machines on the side of the road, you’ve seen a trenchless technology in action. Although there are quite a few ways to install subsurface pipelines, telecommunication cables, power lines, and sewers without excavating a trench, only one launches lines from the surface. That means you’re much more likely to catch a glimpse. Like laparoscopic surgery for the earth, horizontal directional drilling (or HDD) doesn’t require digging open a large area like a shaft or a bore pit to get started. Instead, the drill can plunge directly into the earth’s surface. From there, horizontal directional drilling is pretty straightforward, but it’s not necessarily straight. In fact, HDD necessarily uses a curved alignment to enter the earth, travel below a roadway or river, and exit at the surface on the other side. Let me show you how it works and at the end, we’ll talk about a few of the things that can go wrong.

The first step in an HDD installation is to drill a pilot hole, a small diameter borehole that will guide the rest of the project. A drill rig at the surface has all the tools and controls that are needed. These rigs can be tiny machines used to get a small fiber-optic line under a roadway or colossal contraptions capable of drilling large-diameter boreholes for thousands of feet at a time. As such, many of the details of HDD vary across projects, but the basic steps and equipment are all the same.

As the drill bit advances through the earth, the rig adds more and more segments of pipe to lengthen the drill string. Through this pipe, drilling fluid is pumped to the end of the string. Drilling fluid, also known as mud or slurry, serves several purposes in an HDD project. First, it helps keep the drill bit lubricated and cool, reducing wear and tear on equipment and minimizing the chances of a tool breaking and getting stuck in the hole. Next, drilling fluid helps carry away the excavated soil or rock, called the cuttings, and clear them from the hole. Finally, drilling fluid stabilizes and seals the borehole, reducing the chance of a collapse.

I have here an acrylic box partly full of sand, a setup you’re probably quite familiar with if you follow my channel. Turns out a box of sand can show a lot of different phenomena in construction and civil engineering. Compared to soils that hold together like clay, sand is the worst case scenario when it comes trying to keep a borehole from collapsing. If I pull away this support, the simulated borehole face caves in no time. If I add groundwater to the mix, the problem is even worse. Pulling away the support, the wall of my borehole doesn’t stand a chance. Let me show you how drilling fluid solves this problem.

I’m mixing up a handcrafted artisanal batch of drilling mud, a slurry of water and bentonite powder. This is a type of clay created by volcanic ash that swells and stays suspended when mixed with water. It’s pretty gloopy stuff, so it gets used in cosmetics and even winemaking, but it’s also the most common constituent in drilling fluids. If I add the slurry to one side of the demo, you can see how the denser fluid displaces the groundwater. It’s not the most appetizing thing I’ve ever put on camera, but watch what happens when I remove the rigid wall. The drilling fluid is able to support the face of the sand, preventing it from collapsing. In addition to supporting the sand, the drilling fluid seals the surface of the borehole to reduce migration of water into or out of the interface. In most HDD operations, the drilling fluid flows in through the drill string and back out of the borehole, carrying the cuttings along toward the entry location where it is stored in a tank or containment pit for later disposal or reuse.

So far HDD follows essentially the same steps as any other drilling into the earth, but that first ‘D’ is important. Horizontal directional drilling means we have to steer the bit. The drill string has to enter the subsurface from above, travel far enough below a river or road to avoid impacts, evade other subsurface utilities or obstacles below the ground, and exit the subsurface on the other side in the correct location. I don’t know if you’ve ever tried to drill something, but so far when I do it, I’ve never been able to curve the bit around objects. So how is it possible in horizontal directional drilling?

There are really two parts to steering a drill string. Before you can correct the course, you need to know where you are in the first place, and there are a few ways to do it. One option is a walkover locating device that can read the position and depth of a drill bit from the surface. A transmitter behind the bit in the drill string sends a radio signal that can be picked up by a handheld receiver. Other options include wire-line or gyro systems that use magnetic fields or gyroscopes to keep track of the bit's location as it travels below the surface. Once you know where the bit is, you can steer it to where you want it to go.

I’ve made up a batch of agar, which is a translucent gel made from the cell walls of algae. I tried this first in the same acrylic box, but the piping hot jelly busted a seam and came pouring out into my bathtub, creating a huge mess. So, you’ll have to excuse the smaller glassware demo. My simulated drill string is just a length of wire. There are two things to keep in mind about directional drilling: (1) Although they seem quite rigid, drill pipes are somewhat flexible at length. If I take a short length of this wire and try to bend it, it’s pretty difficult, but a longer segment deflects with no problem. And, (2) you don’t have to continuously rotate the drill string in order to advance the borehole. You can just push on it, forcing the bit through the soil.

My wire pushes through the agar without much force at all, and a drill string can be advanced through the soil in a similar way, especially when lubricated with water or drilling fluid. The real trick for steering a drill string is the asymmetric bit. Watch what happens when I put a bend on the end of my wire and advance it through the agar. It takes a curved path, following the direction of the bend. If I rotate the wire and continue advancing, I can change the direction of the curve. The model drill string is biased in one direction because of the asymmetry, and I can take advantage of that bias to steer the bit. I can steer the string left, then rotate and advance again to steer the bit to the right. I’m a little bit clumsy at this, but with enough control and practice, I could steer this wire to any location within the agar, avoid obstacles, and even have it exit at the surface wherever I wanted.

This is exactly how many horizontal directional drills work. The controls on the rig show the operator which way the bit is facing. The drill string can be rotated to any angle (called clocking), then advanced to change the direction of the borehole. Sometimes a jet nozzle at the tip of the bit sprays drilling fluid to help with drilling progress. If the nozzle is offset from the center, it can help create a steering bias like the asymmetric bit. Just like the Hulk’s secret is that he’s always angry, a directional drill string’s secret is that it’s always curving. The rig operator’s only steering control is the direction the drill string curves. My friend Daniel at the Coding Train channel built a 2D simulator so you can try steering one of these rigs for yourself. Here's me trying to navigate the drill string around an obstacle and exit on the other side. He’s got a video all about the programming of this simulator, so go check out his video after this one.

DUB: And hey, if that sounds like something you’d like to try for yourself, my friend Dan Shiffman over at the Coding Train YouTube channel built a 2D horizontal directional drilling simulator. This is an open-source project, so you can contribute features yourself, but it’s also really fun if you just want to play a few rounds. If you’re into coding or you're wanting to get started, there is no better way than working through all the incredible and artistic examples Dan comes up with for his coding challenges. Go check him out his video on HDD after this one.

Once the drill string is headed in the right direction, it can just be continuously rotated to keep the bit moving in a relatively straight line. The pilot hole for an HDD project is just an exercise in checking the location and adjusting the clock position of the drill string over and over until the drill string exits on the other side, hopefully in exactly the location you intended. But, not all soil conditions allow for a drill string to simply be pushed through the subsurface.

Rocky conditions, in particular, make steering a drill rig challenging. An alternative to simply ramming the bit through the soil is to use a downhole hydraulic motor. Also known as mud motors, these devices convert the hydraulic energy from the drilling fluid being pumped through the string to rotate a drill bit that chews through soil and rock. This allows for faster, more efficient drilling without having to rotate the whole drill string. The housing of the mud motor is bent to provide steering bias, and the drill string can be clocked to change the direction of the borehole.

Once the pilot hole exits on the other side, it has to be enlarged to accommodate the pipe or duct. That process is called reaming. A reamer is attached to the drill string from the exit hole and pulled through the pilot toward the drill rig to widen the hole. Depending on the size of the pipe to be installed, contractors may ream a hole in multiple steps. The final reaming is combined with the installation of the pipeline. This step is called the pull back. The pipe to be installed in the borehole is lined up on rollers behind the exit pit. The end of the pipe is attached to the remaining assembly, and the whole mess is pulled with tremendous force through the borehole toward the rig. Finally, it can be connected at both ends and placed into service.

That’s how things work when everything goes right, but there are plenty of things that can go wrong with horizontal directional drilling too. Parts of the drill string can break and get stuck in the pilot hole. Drilling can inadvertently impact other subsurface utilities or underground structures. The pipeline can get stuck or damaged on pullback. Or, the borehole can collapse.

The controversial Mariner East II pipeline in Pennsylvania experienced a litany of environmental problems during its construction between 2017 and 2022. Most of those problems happened on HDD segments of the line and involved inadvertent returns of drilling fluid. That’s the technical term for the situation when drilling fluid exits a borehole at the surface instead of circulating back to the entrance pit. The inadvertent returns in the Mariner East II line created water quality issues in nearby wells, led to sinkholes in some areas, and spilled drilling fluid into sensitive environmental areas. The pipeline owner was fined more than $20 million over the course of construction due to violations of their permits, and they are still mired in legal battles and extreme public opposition to the project to date.

In the case of Mariner East II, most of the drilling fluid spills were partially related to the difficult geology in Pennsylvania. Clearly HDD isn’t appropriate for every project. But in most cases, trenchless technologies are the environmentally-superior way to install subsurface utilities because they minimize disruptions on the surface to the people in urban areas and sensitive habitat around rivers and wetlands.