HDD in Tough Conditions: Drilling Between a Rock and a Hard Place

By Phil Kendon
Published: March 2, 2021 | Last updated: July 5, 2023
Key Takeaways

Ensuring you have the right mud and tools is even more essential to your project when it comes to running HDD in tough conditions.

Trenchless construction projects suffer from the same pressures as any construction project – those that make you fall behind schedule and increase cost. However, there is always a higher degree of uncertainty about factors affecting project execution when working underground and running a horizontal directional drilling (HDD) project. This uncertainty makes it even more critical to manage the variables within our control, like using the right materials and tools for the right job.

Using the wrong tool can cause anything from minor slowdowns in drilling speed to complete shutdowns or even rework, for example:

  • Abrasive conditions can wear a drilling head down, making it less effective and slowing down progress.

  • The wrong shape drilling head can lose enough teeth to make the bore narrower over time.

  • The wrong tool could get stuck in the bore, wasting time and money to release it.

Soil or rock conditions vary dramatically from one site to another. It is imperative to understand the underground conditions for each project and to choose the best cutting tool for those conditions. The drilling fluid also plays a crucial role in protecting drilling equipment to minimize downtime and maximize effectiveness.

The Importance of Geotechnical Surveys

Geotechnical surveys discover the type of ground material through a variety of tests on soil samples. Mineralogical composition analysis indicates the abrasiveness of the soil. X-ray diffraction is also used, but it is more expensive and less common.

Engineers can map the proposed routing and see how the conditions change along the drilling path. In some extreme conditions, it may be advisable to change this path to avoid a section that will be difficult to drill through.

It is never a good idea to make too many assumptions about the conditions. Once engineers gain an accurate assessment of ground conditions, they can evaluate the cutting head options. The more thorough the geotechnical survey, the more confident the drilling team can be that they have the right materials for the job.

Read: Geotechnical Engineering Factors to Consider in Underground Pipeline Design and Installation

An Introduction to Soil and Rock Properties

The earth’s crust consists of different types of rock and soil. Each type has its own characteristics that influence which trenchless method will be most effective. The table below summarizes the different types of rock and soil.




Igneous rocks

Formed under high temperatures and pressures – includes granite and marble.

Medium hardness with compressive strength of 7,000 psi to 18,000 psi.

Sedimentary rocks

Rocks forming layers like shale, sandstone, and coal.

Soft rock with compressive strength less than 7,000 psi.

Metamorphic rocks

Consolidated crystalline structures formed under extreme heat and pressure.

Very hard rock with compressive strength of 18,000 psi and above.

Silt / Sand

Loose collection of particles that do not stick together.

Minimal resistance to drilling although borehole stability can be an issue.

Clay / Dense alluvial soil

Particles stick together and swell when hydrated.

Cuttings can stick together and bridge. Cutting tool can get stuck. Formation can swell.

The Right Tool at the Rock Face

There are several types of drilling head designed for different conditions.

  1. Fly cutters are suitable for most types of soil. They have an open structure allowing soil particles to fall into the center. In clay conditions, fly cutters should be designed to drill their way out of the hole. Fly cutters are often used with jetting capabilities so that the pressure of drilling fluid accelerates the drilling progress as well as removing cuttings from the bore.

  2. A milled tooth drilling head is ideal for harder soil and soft rocks. This design has a larger tooth surface compared to other designs making it more efficient. The gouging effect of milled teeth removes large pieces of rock or soil.

  3. A chisel tool profile is best for medium rock. The shape is similar to the milled tooth but much smaller. Chisel tooth drilling heads can accomplish fast drilling speeds and are more resistant to breakages in harder rocks.

  4. Conical tooth profiles give the best protection for hard rock. The conical shape means there is some compromise on drilling speed. It is also possible to increase the gauge protection counts for drilling heads using this profile. This means more teeth in the outer rows of the cone, which prevents the hole from becoming undersized when some teeth break off.

The Role of Drilling Fluid

Drilling fluid is a significant component of many trenchless construction projects. No matter what the ground formation (clay, sand or rock), all cuttings must be removed and displaced with fluid to create and maintain the borehole, increase the rate of penetration (ROP), and reduce wear on downhole tooling. Drilling fluid fulfils several purposes, each of which is essential to the success of a trenchless construction project:

  • Removes cuttings from the bore.

  • Suspends cuttings so that they do not lay down on the bore surface.

  • Provides support for the bore.

  • Protects the formation.

  • Cools and lubricates downhole tools and drill rods

  • Supports clay Inhibition

  • Identifies the formation

Drilling fluid has the potential to create the best conditions for drilling and thus minimize downtime and wear on equipment. When it comes to drilling through different ground conditions, the drilling fluid composition can make a substantial difference to the cost and schedule of the project.

Drilling fluid properties

The first step in making an efficient and cost-effective drilling fluid is to control the quality of the make-up water. Use soda ash controls the pH of the water between 8 to 9.5 and minimize the hardness as low as possible. This allows maximum effectiveness of the bentonite and other additives. It is worth retesting the make-up water each time to ensure the optimum properties for drilling fluid performance.

Bentonite builds the viscosity of the drilling fluid, which is measured using a second marsh funnel test. Clay conditions need drilling fluid with a viscosity of around 35 but rock or cobble conditions need a viscosity of about 65. Other formations require viscosities between this range. Viscosity is an important property for moving cuttings out of the bore and providing support to the bore.

Polymers and other additives are added last to control the bore and improve specific properties of the drilling fluid. PAC polymers improve the fluid loss control by forming a filter cake, which lines the bore, while PHPA polymers help with clay inhibition. Bio-polymers improve gel strengths, which is important for suspending cuttings in the drilling fluid and providing support to the bore.

Lubricants reduce downhole torque and pull back pressures, which helps to protect downhole tools and rods. Foam additives help carry cuttings to the surface while also penetration in some formations.

Formulating the best drilling fluid for the project is dependent on the ground formation. This includes setting the viscosity and choosing which additives to use. Once again, it is the geotechnical survey that provides the information necessary to make this selection.

Drilling fluid pressure

The rate of cuttings removal is also important for trenchless construction. Highly abrasive pieces of rock or coarse sand must be quickly removed from the drilling head to prevent wear and damage to the tool However, pumping too much fluid into the bore can also cause problems. Pumping high pressure drilling fluid into sandy formations can wash out the bore, creating void areas. Cobble formation is often held in place by fine particles. Applying high pressure drilling fluid to a cobble formation can wash these particles out, causing the cobble to fall into the bore.

The ideal pumping rate for a trenchless construction project depends on the type of ground formation. As a general rule, the higher the clay content of the formation, the greater the drilling pressure required. This is because clay sticks together, and it requires a reasonable amount of pressure to clear the clay cuttings out of the bore. Sandy or gravel formations typically run at 1 X the hole volume, whereas reactive clay formations can run as high as 5 X the hole volume.

One application of high-pressure drilling fluid is known as jetting. This is where the drilling fluid actually cuts into the formation ahead of the drilling head to speed up progress. Jetting is used in unique situations and is not recommended as a common practice. It can create downhole issues in sand and cobble formations.

Read: An In-Depth Look at the Role of Drilling Fluid Systems in Trenchless Construction


Every trenchless construction project has unique ground conditions. A geological survey uncovers the specific characteristics and helps engineers to identify the best tools for the job at hand.

Fly cutters and large milled tooth drilling heads work well for soil and soft rock. Chisel tooth profiles suit rocks with medium hardness and conical tooth profiles work best in hard rock.

Besides the drilling head itself, drilling fluid also plays a large role in protecting trenchless construction tools by removing cuttings from the borehole, so it is imperative you have a high quality mud made from the right stuff for your project.

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Written by Phil Kendon | Technical Writer @ Trenchlesspedia

Phil Kendon

Phil Kendon has an undergraduate degree in engineering along with a masters in vocational practice. He has ten years of manufacturing experience in the oil and gas sector along with ten years of experience with non profits. Phil lives on the idyllic paradise island of Mauritius with his wife, Leigh, and 3 children, Timothy, Hannah and Luke. Here he pursues his work with non profits as well as his passion for writing.

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