Category: Impact Moling
The impact mole is one of the original and simplest of the trenchless technologies and was developed for the installation of small diameter pipes in compressible soils over short distances. Assuming the pre-installation study 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 proprieties from an access chamber or excavation in a highway for instance thus making road crossing, for instance, possible without digging a trench across the 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.
Soil displacement hammers (pneumatic impact moles) displace the soil to create a void into which socket-less short or long plastic pipes (PE, PVC or PE-X) up to OD 160mm or cables (depending on the soil type) up to 25m in length can be pulled.
If the soil is sufficiently displaceable, this technology enables the crossing of highways and to install house connections for gas, water, etc. and be accurately on target and is a complete trenchless technology. The soil displacement hammer 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 drive.
The soil displacement hammer is moved forward by a compressed air 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.
The chisel-tip head creates a bore hole and the long body length of the mole helps the mole hold its line and level as it advance through the bore hole. The distance travelled through the ground is usually monitored by marking the chainage on the on the air hose as it advances.
There are two basic non-steerable systems available:
1. Rigid System: The piston applies impact to the casing and drives the complete displacement hammer and the attached pipe string forward with one blow.
2. 2-Stroke Method: The piston first strikes the mobile multi-cutter cone and then the casing, so that the soil displacement hammer moves forward in two steps.
For the 2-stroke-method, the soil displacement hammers achieve an installation speed of 15m/h, depending on the soil conditions. The ground cover should be at least ten times as thick as the diameter of the casing to avoid arching or heaving at ground level. The soil displacement machines are also equipped with a reversing gear - the soil displacement hammer with its mobile multi-cutter cone is simply reversed with a lever. A plug-on head with integrated transmitter or a built-in transmitter in the hose makes locating possible but it is not intended for steering the mole.
Once a bore hole has been completed, the product pipe is pulled in as the mole is extracted. A bore hole is typically 15-25 percent bigger than the product pipe.
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 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.
Last Updated on Wednesday, 13 August 2014 11:38