Impact moling is 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 45 m.
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.
Once the borehole is completed, the product pipe is pulled in from the entrance shaft at the same time that the mole is extracted from the exit shaft.
One single drilling stage can open boreholes of diameters from 30 to 180 mm but with multiple stages this method can reach 200 to 250 mm diameters. This method requires, besides the corresponding updated drawings of existing services, some indirect location techniques such as georadar (GPR) and the electromagnetic services detector in order to avoid affecting them during drilling.
Impact moling works best in compressible soils conditions. There are both steerable and non-steerable types wich have different length ranges being able to achieve up to 60 m with steerable rigs. This technique is also known by other names such as soil displacement hammers, pneumatic impact moles, or piercing tools.