(using Impact moles – also known soil displacement hammers, pneumatic impact moles or piercing tools as they may also be known in some parts of the world.)
New installation or replacement of pipes and ducts or cable laying pressure networks.
Applicable to gas, water and cable. Depending on the equipment manufacturers’ ranges, diameters available run from 45 mm to 300 mm with the capacity to install product over lengths of up to ±30 m. The mole diameter is usually significantly larger than the product diameter (15 to 20%) to ensure that the product will fit into the bore and minimise any friction/drag that might impede the advance of the Impact hammer.
Limitations of the system include they must be used in compressible soils as the technique is a soil displacement operation. In most cases impact moles are not steerable although some steerable options do exist but are rare in use. This means that set up and implementation must be carried out very precisely before the impact mole is launched into the ground. The direction of the mole may also be seriously affected by obstacles in the ground such as boulders, unknown utilities etc.
Impact moles offer a quick relatively easy option for the installation of shorter, smaller diameter pipeline and ducts where the ground conditions are not expected to be problematic and may be useful in short road or rail crossings or under waterways.
The Impact Mole is one of the original and simplest of the trenchless technologies. Assuming the pre-installation ground and route investigation has been assessed correctly, the installation of a product pipe using an impact mole is relatively simple and straightforward.
Impact moling is frequently used to install gas and water service pipes to properties from an access chamber or excavation thus eliminating the need to dig a trench across an obstruction such as a road. Cables can also be installed in this manner with or without ducting, although the provision of a duct is generally regarded as a safer method of cable installation and protection.
Impact Moles displace the soil through which they pass to create a void into which cables or socket-less short or long plastic pipes can be pulled over lengths generally up to ±30 m.
If the soil is sufficiently compressible the technology enables the crossing of highways, rail track etc. and installation of house connections for gas, water and other pipe or duct based utilities. Over short distances to ±30 m, with the right ground conditions, impact moles can be accurate and on target. The Impact Mole is placed on a starting cradle and launched from a pit and, by means of a telescopic sight, aim is taken and the height and sides of the machine are adjusted to provide the line and level for the installation.
The mole is moved forward by a compressed air or hydraulically driven piston – the forward movement relies on external friction, consequently if this is missing, such as in loose and/or soft soils, external static support may be required to provide a thrust. Current practice is that the product pipe or cable is attached via a swivel directly to the rear of the mole prior to commencing the bore. This means that as the mole advances the product pipe or cable is laid directly into the ground so no further stage is required to complete the installation.
Most moles are non-steerable with the mole head creating the bore hole and the length of the body of the mole helping to keep the mole on its line and level as it advances through the bore hole. The distance travelled through the ground is usually monitored by marking the chainage on the on the power hose as it advances.
There are two basic non-steerable systems available:
The Rigid System: The piston applies impact directly on to the mole casing and drives the complete hammer and the attached pipe/cable forward with one blow.
The 2-Stroke Method: The piston first strikes a floating head located at the front of the mole with a shaft passing through the mole shell advancing this head into the ground. The piston then strikes the casing so that the body of the mole moves forward, a two-step action.
The 2-stroke-method claims to improve the penetration of the mole into the ground and maintain better directional stability.
Whichever head configuration is used it is vital that Impact Moles are used at a depth sufficient to negate any potential for ground arching or heave at the surface. The ground cover should be at least ten times the diameter of the casing (i.e. for every 100mm diameter of mole – 1 m depth should be allowed). Moles are also equipped with a reversing gear to reverse the mole from the bore should it be necessary such as encountering an obstacle.
On some moles it is also possible to install a locating sonde that can be tracked from surface. However this is not normally used for steering purposes just monitoring.
Steerable impact moles are also available and use a similar method of steering as used for auger bore pilot bores and horizontal directional drilling (HDD). A steerable mole has a slant face rather than a chisel-tip face and the direction of the mole is managed by the positioning of the slant face head. The operator steers the mole by rotating the face in the desired direction, again similar to HDD technology.
A sonde located in the transmitter housing close to the front of the mole allows it to be tracked at ground level using a walkover tracking system.
Use of these moles has however been somewhat limited over the years as the drive lengths tend to be short so any realistic need for steering is negated.