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 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.
Animation of Impact Moling where the product pipe is installed after the bore is completed
Animation courtesy of ISTT
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.
Whilst basically being the same type of machine, the ramming hammer is used in different way altogether. As opposed to using the impact energy to displace soil directly around the outer shell of the unit, a ramming hammer’s energy is used to push a steel pipe section into the ground from a start pit. The pipe installed can be used either as a pipe in itself or as a carrier conduit for separately installed services.
On a typical installation the first section of steel pipe is placed in the start pit on the correct line and level for the installation. The ramming hammer is attached, via a series of size adaption collets, to the rear of this pipe. The hammer is operated and the reciprocating piston drives the pipe section forward directly into the ground. Ground conditions on such a project are vital to the success of such work and should be very well understood before attempting such an installation.
Normally, installations are achieved using an open-ended pipe so that the cutting edge is only that of the pipe (which is often fitted with a cutting edge before commencing the ram). The ground into which the pipe is rammed then forms a plug inside the pipe. When ramming over the required distance is complete this soil is removed using either comapressed air pressure, water jets or scrapers, leaving the pipe in place as required. As with impact molng the ground conditions and unexpected obstacles can cause the pipe to deflect from the required course. So, the system is often used only where precise line and level tolerences are not required. Installations of up to 3,000 mm have been rammed using this technique. Depending on the final use of the pipe, various supporting installation devices have been developed. Details should be obtained from the manufacturers.
Ramming can also be achieved using closed end pipes where ground conditions may not be condusive to open ended techniques
By turning an impact or ramming hammer to the vertical or to an angle has enabled the systems to be utilised in the ramming of ground support piles. Whilst this is a relatively new development, it does seems to be gaining in popularity for small diameter, shorter length piles.
Impact hammer technology has also been adopted by another of the trenchless installation technologies, that of HDD. Here the impact hammer has been adpated to assist in the recovery of stuck drill rods, by adding impetus to the pulling power of the drill rig to realease drill rod strings stuck in the ground; or as an additional power source when trying to bore through harder ground formations that purely rotational drilling power could not overcome.
Ramming hammers have also been utilised on the ends of pipe being pulled into a bore by an HDD rig to add impetus to the pull-in power of the rig.
As mentioned elsewhere in this website, impact hammers are also utilised as the basis for a form of pipe bursting.
Another area in which ramming hammers has been utilised successfully is the installation of a ‘Pipe Arch’. Pipe Arches are utilised where a series of casing pipe are rammed into the ground, often below road ways or rail tracks, to form a preliminary support mechanism separating the ground inside the arch from that surrounding it. Once the pipe arch is competed, the soils within the pipe arch are excavated within the confines and safety zone created by the arch, minimising the potential for ground movement during excavation and reducing the potential for subsidence at surface and therefore in the infrastructure beneath which the new tunnel will pass.