How Impact Moling Works

By Phil Kendon
Published: March 18, 2021 | Last updated: July 5, 2023
Key Takeaways

Impact moling has remained a top trenchless method despite being invented over 50 years ago.

Impact moling is one of the oldest trenchless methods and remains one of the most common, cheapest and most efficient. It is especially suitable for short length and small diameter piping.


One of the pioneers of impact moling technology was Hans-Jürgen Essig, who introduced the impact mole and soil displacement machine to the German market in 1966. This development was right at the beginning of the trenchless revolution, which has radically changed the construction industry. For the first time, trenchless technology enabled contractors to lay underground pipes and cables and even execute repairs without digging open trenches. (Read also: Who Invented Trenchless Technology?)

Over time other trenchless methods like horizontal directional drilling (HDD), auger boring, and microtunneling were developed and grew in popularity. But the simplicity of impact moling ensured that it maintained its place in the trenchless construction marketplace.


In fact, impact moling has benefitted from advances in other trenchless methods. The technological strides in steering HDD drilling rigs have transferred over to impact moling, for example. Better steering means that the tried and tested method of impact moling can now be used in a broader range of applications.

How Impact Moling Works

Impact moling is like driving a torpedo-shaped metal tube through the ground with a hammer to create a tunnel under the surface. Once the tunnel is complete, a pipe or cable is pulled into the space that has been left behind. Pneumatic power is the driving source for most impact moling machines, and the mole head differs according to the ground conditions.

1. The Equipment

Impact moling equipment consists of three main components:

  • The pneumatic compressor and hoses that supply the source of power.
  • The mole itself, which is a cylindrical sheath with a pointed tip and an inner piston mechanism.
  • The aiming sight to set up the initial direction of the bore.

Some systems use hydraulic rather than pneumatic power, but the principles of both systems are the same.

2. Setting It Up

Impact moling requires both entry pits and exit pits. This arrangement allows for setting up the impact mole at the right alignment and gradient for the installation. The longer the impact mole, the bigger the entry and exit pits will have to be. (Read also: It's the Pits: Pits and Excavations in Trenchless Construction.)


There is a tradeoff here because a longer impact mole improves the bore's accuracy, but a shorter impact mole lowers the environmental impact of the entry and exit pits. Nevertheless, the footprint is still low compared to pipe jacking and other large trenchless equipment.

Once the pits are set up, contractors lay the launch cradle on the floor of the entry pit with the impact mole resting on top. They attach an aiming sight to the impact mole, which looks something like a land surveyor's theodolite. One operator looks through the aiming site, while another person stands in the exit pit with a marked rod.

The rod makes a target for the aiming sight. The operator adjusts the launch cradle until the impact mole perfectly aligns with the desired route. Once they have set the exact line and gradient for the impact mole, the process can begin.

3. Operating the Impact Mole

Pneumatic power supplies the force for the impact force. The machine works with a hammering action. Each successive blow pushes the mole deeper into the ground, thus making the bore. The impact mole head assembly is conical with a pointed tip. This geometry allows the mole to penetrate the soil and break up small stones in its way.

Some impact moles have a two-stroke design. The first stroke applies all the force directly to the head so that it maximises penetration. The second stroke applies the force to the body of the mole, which pushes the whole mole further into the ground. Modern impact moles can operate in reverse, too, so that you can extract the tool if it gets stuck or there is an impassable obstruction in the way.

Operators can determine how far the mole has moved towards the destination by checking the pneumatic hose markings. Once the mole has fully exited the ground at the exit pit, the bore is complete. Operators can remove the mole and attach a pipe or cable to pull back into the bore from the exit pit to the entrance pit.

Impact moles can tunnel through the ground at a rate of up to 1.5 m/s in good conditions. However, this rate could also reduce significantly if the soil is hard or obstacles prevent the mole from passing.

4. How to Steer

Impact moling has been a non-steerable trenchless method for most of its history. However, there are steerable versions that are becoming more readily available.

Even the non-steerable versions can be set up to create an accurate bore. The initial setup is critical for any impact moling project. Getting the alignment in terms of line and gradient right at the entrance pit will ensure that the mole reaches the destination at the correct location. The aiming sight helps operators position the mole accurately in the entrance pit, and the launch cradle provides support so that the mole does not move out of alignment once the impact action begins.

More recent devices that have steering capabilities use angled heads similar to those found in microtunneling. Rotating the head so that the angle faces in a different direction changes the mole's direction as the hammer pushes it further into the ground. Operators can observe the angled head's location using handheld devices on the surface above the impact mole.

5. Pulling the Pipe

Most impact moling installations use the pullback method of installing pipes. This means the pipe is pulled back through the bore from the exit pit to the entrance pit after the impact moling is complete. Pullback installation is common practice for other trenchless methods also, including horizontal directional drilling (HDD).

The mole is typically bigger than the pipe by at least 10%, so the bore is slightly oversized. This margin makes it easier to pull the pipe back through the bore without putting it under too much tension. The fact that impact moling uses straight lines and short distances also contributes to making it easier to pull back the pipe.

Ideal Ground Conditions for Impact Moling

One of the challenges with trenchless construction is keeping the bore open until the pipe is in position. Some other trenchless methods use drilling mud to stabilize the bore, while others involve the insertion of a casing during drilling. Impact moling does not have any of those advantages.

However, a benefit that impact moling does have is the length of the impact mole itself. This metal cylinder with the impact piston inside is a solid tube that moves through the ground. It supports the bore along the entire length of the mole. The longer the mole, the higher the percentage of the bore supported at any time.

Nevertheless, ground that tends to subside is not suitable for impact moling. There should be a level of natural cohesion in the soil that holds the bore open until the pipe is pulled into position. Clay, silt and peat are all soil conditions well matched to impact moling. Loose gravel or sandy conditions are less suitable and more prone to experiencing problems with subsidence. (Read also: Soil Types and How They Affect Trenchless Construction.)

Most Suitable Applications for Impact Moling

Historically, impact moling was used for short bores with relatively small diameters. While the diameter limits remain somewhat similar today, longer bores are possible now because of advances in steering.

Diameter limits for impact moling range from 20 mm on the low end to 180 mm on the upper end. These figures are much smaller than the maximum HDD bore, for example, which can go up to 1,200 mm. An average impact moling tunnel length is about 15 m, but this can extend to about 60 m and beyond with steering.

The diameter limits of impact moling make it suitable for installing water, sewer or gas lines in residential and commercial areas. It can also be used for installing underground cables.

Recent Advances in Impact Moling

As with all trenchless methods, there are continuous technological advances that improve their cost and efficiency. Some of these are aimed at specific limitations of the method, while others simply copy the advantages of other methods.

Impact mole steering has benefitted from advances in other trenchless methods like HDD. HDD steering uses a slanted face on the drilling head to adjust the direction of the bore. One side of the head has an angled face. When the drilling head is pushed forward without rotation, the drill will move in a direction away from the slanted face. Turning the drill head so that the slanted face points in a different direction will, therefore, change the direction of the drill. This exact principle has now been applied to impact moling to provide a level of steering that was not available before. Steering functionality opens up impact moling to longer bores than were previously accessible.

Another technology advance for impact moling is in the selection of impact heads. New designs are more effective for breaking through boulders and rocky ground because of the shape of the head and the materials of construction.

Case Studies

Impact moling is often used for small projects. An example of how it can be used in a residential setting is the installation of a small garden observatory that needed an electrical connection from the main house. A contractor used impact moling for the cable installation of 30 m, meeting a tight schedule and leaving the garden untouched.

In Europe, contractors use impact moling for installing utilities alongside residential roads. Each time the pipe or cable crosses a driveway, there is the potential for major impact and inconvenience. Impact moling allows for crossing each of these small obstructions without interfering with the homeowner's access to their property.

Another instance of where impact moling was successfully employed was a dairy farm that needed a water supply pipe and drainage pipes installed under their access road. The contractor used impact moling to complete the whole project in a few hours. The lack of obstruction on their main access road and the speed of installation made this project successful.


Impact moling bores should have a minimum depth of 10 x the pipe diameter or 1 m, whichever is greater. This limit prevents the severe consequences of subsidence at ground level due to a bore collapse. Impact moling projects usually traverse a road or other surface-level obstacle. Any subsidence can, therefore, have catastrophic consequences.

As with all trenchless methods, one of the most significant risks is striking other underground pipes and utilities during the installation. Unfortunately, this is exactly what happened when an impact mole hit an underground electrical cable carrying 11,000 volts. The impact mole operator suffered major burns due to the incident and died nine days later.

Conducting surveys of underground installations before executing an impact moling project is essential for worker safety. This survey should include examining old drawings of the area, but realizing that is not a perfect method so electromagnetic equipment to identify any undocumented utilities should also be used. (Read also: Identifying and Managing Dangers Associated WIth Subsurface Utilities.)

It is important to consider that pneumatic equipment is also very high powered and can cause severe injury or even death. Only trained workers should operate pneumatic equipment. Contractors should also provide up to date procedures and all necessary personal protective equipment.


Impact moling is a trenchless method that has been in existence for more than 50 years. Having its roots in Germany, it is a method that has spread throughout the globe and has an established place in the world of trenchless construction.

Impact moling is best used for short projects with a small diameter (< 180 mm). These installations tend to run on a straight line between an entry and exit pit. However, advances in steering are making it possible for impact moling to be used in a broader range of applications in future. The ideal ground conditions for impact moling is soil with a natural cohesion. This property keeps the integrity of the bore intact once the mole has passed through.

Pneumatic machinery delivers significant power to an impact mole. Operators should be well trained so that they do not expose themselves to undue risk and potential industry. Care must also be taken to avoid other underground utilities while creating a bore using impact moling.

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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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