Impact Moling
Impact Hammers were probably one of the earliest forms of trenchless technology to be developed. Initially thought of as weapons delivery system in WWI, the technology has gone on to become almost a standard piece of kit in many utility service vehicles around the world. Impact hammers are used in one of three ways. These include: Moling (Earth Piercing), Ramming and Piling.
Impact hammers are very simple devices comprising an outer, normally steel, shell within which there is a heavy reciprocating piston that is either compressed air or hydraulic fluid driven. The piston operation is designed so that on the forward stroke the force of the piston is applied to the shell of the unit driving it forward. The reverse stroke positions it for the next forward stroke.
Moling
Moling utilises the impact force generated by an impact hammer unit to push the mole unit into compactable soils, displacing the soil around the shell and compacting it to create a void of the required size. The forward progress of the unit allows any pipe or cable attached to its rear end to be pulled into this void as it advances. Two access pits or points are required for the operation to take place, normally with a line of sight between the two so that the mole can be aimed correctly. The system is nominally non-steered, which means that the original set up has to be accurate to ensure that the mole follows the course required. Some companies have developed steerable moles but generally these require intermediate access pits to adjust the steering mechanism.
The disadvantage of these units is that they are easily deflected by obstacle in the ground and can stray off course without the operator being aware of the change. To overcome this, location system manufacturers have developed radio sondes that can be placed in the body of the mole for tracking using a walkover system. These systems have to be extremely robust however due to the impact energy they experience as the unit operates.
To minimise the potential for course change during operation, mole manufacturers have developed a variety of head designs that claim to be enable the units to hammer through harder ground and to break up smaller boulders etc that may lay in the path. Details are best sought from individual manufacturers.
Impact moles are limited somewhat in their application because of the need to displace soils as they progress. A current rule of thumb says that for every 100 mm diameter of the impact mole tool, operators should allow 1 m in depth from surface to avoid surface heave. Given the type of work these units are used for mainly this means that most impact hammers for direct installation work have an upper diameter limit of around 200 mm. In the right conditions however they can be a very cost-effective tool and are normally relatively simple to use and to maintain.