Impact Moling

Published: | Updated: June 24, 2021

Definition - What does Impact Moling mean?

Impact moling is a trenchless method used for the installation of small diameter pipes or cables using a pneumatic hammering tool. It is the simplest form of trenchless installation of pipelines for water, sewer, and gas under roads, sidewalks, embankments and other small crossings less than 150 feet. Impact moling is easy to operate, is a low-cost method and requires little to no excavation for the connection and termination pits. However, the diameter and length of the bore are limited.


Impact moling works best in compressible soils and comes in both steerable and non-steerable types. It works on the simple principle of displacing soil while installing the pipe. The impact mole consists of a steel tube powered by a compressor attached to a piston with a pneumatic hammer placed at the head of the casing. The impact mole is first positioned at the correct line and grade after which the compressor is activated, pushing the mole forward by the hammering action at the head.


The mole is long which helps maintain the integrity of the borehole as it advances. Once the borehole is completed, the product pipe is pulled in at the same time that the mole is extracted. Service connections like water and gas pipelines that have to pass under sidewalks, driveways, roads, and other such crossings can be installed using this method. These connections are usually short distances and hence impact moling proves to be the cheapest and fastest method for installing them.

Impact moles are also known by other names such as soil displacement hammers, pneumatic impact moles, or piercing tools

Trenchlesspedia explains Impact Moling

The method is simple and straightforward once pre-installation ground and route investigation have been assessed. The impact mole displaces the soil through which it passes and creates a void through which cables or pipes can be pulled over lengths up to 30 meters.

The Installation Process

As with all trenchless methods, two pits, one at the launching site and one at the receiving end are excavated. The impact mole is carefully aligned at the launch pit and consists of an enclosed steel tube with an air-powered piston. This piston strikes the nose of the impact mole and drives it forward through the soil. The friction between the mole and the ground prevents it from rebounding.

The hammering action is either a simple striking of the piston or a two-stage action of a specially designed moving head. Non-steerable moles are carefully aligned at the launching pit and the hammering action advances the mole in a straight line. The operation can be handled by a single person. The steerable mole can be launched from the surface or from a launching pit and requires a two-man crew.

One operator walks the route of the bore with a walkover locator device and monitors the position of the tool in the ground. The other operator, using the guidance controls ensures that the mole is following the right course. The product pipe or cable duct is towed into the borehole during the procedure or inserted after the borehole is completed.

Impact Moling Applications

Impact moling typically installs pipes with diameters ranging from ½ inch to 4 inches but can be used for pipes up to 10 inches in diameter. Depending on soil conditions, non-steerable moles can bore up to 25 feet in one run but can range anywhere between 10 to 100 feet. Steerable systems can bore up to 200 feet in good soil conditions.

Pipe materials used for this method are usually made of high-density polyethylene (HDPE), polyvinyl chloride (PVC), or steel. Other than water and gas pipelines, impact moling can be used to install cable ducts, garden irrigation, landscape lighting, etc. Ground conditions most suitable for impact moling include soft cohesive soil, clay, silt, and peat.

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