The impact mole or piercing tool is one of the oldest and simplest of the trenchless technologies. It is ideally suited for installation of small diameter PE100 pipes in compressible soils over short distances. Under suitable conditions direct installation of a product pipe using an impact mole is simple and straightforward. [In this Guide we use the term impact mole. In the USA the tools are known as earth piercing tools.]
Image courtesy: INTERNATIONAL SOCIETY FOR TRENCHLESS TECHNOLOGY
Impact moling is used for installation of pipes up to 160mm in diameter over distances up to 60metres. It is one of the most widely used trenchless installation methods.
Impact moling should be carried out at a depth of at least 10 times the diameter of the product pipe or 1 metre, whichever is greater, to avoid surface damage. The speed of moling can affect accuracy of the bore, and the advance rate is on average about 0.3 – 1.5 m/minute for non-steerable moling, and about 0.3 m/minute for steerable moling. The product pipe is usually installed after the bore is completed, but can also be pulled into place while the bore is being moled. This is advantageous in loose soils where the unsupported borehole can cave in.
- Water services.
- Gas services.
- Water mains.
- Gas mains.
- Cable ducts.
- New installation.
- Offline replacement
Non-steerable moles typically involve the excavation of two pits: a launch pit and a reception pit. After the careful alignment of the mole in the insertion pit, the tool is expected to advance through the ground in a straight line. A single person can operate the mole. The impact mole is positioned on a skid or cradle at the desired line and level determined by a sighting level positioned on the mole. Once the mole is in the desired position, the compressor is activated advancing the mole through the hammering action of the reciprocating head.
The cone-shaped reciprocating or chisel-tip head creates a bore hole. The long body length of the mole helps it to hold line and level as it advances through the ground. Marking on the air hose allows the operator to track the mole’s progress as it advances through the ground. Once a bore hole has been completed, the product pipe is pulled in as the mole is extracted, generally by being reversed. A bore hole is typically 15-25% larger in diameter than the product pipe.
Some newer impact moles are steerable. The method of steering is similar to that used in pilot bores and directional drilling with a slant face on the reciprocating head. The direction of the mole is altered through the positioning of the slant face head. The operator steers the mole by rotating the face in the desired direction. A sonde located in a transmitter-housing near the front of the mole allows the mole to be tracked using a walkover tracking system and transmits the ‘clock face’ position of the head to the operator for steering purposes. Steerable moles allow longer lengths to be installed because there is less risk of deviation from the required alignment becoming excessive.
Images courtesy: TRACTO-TECHNIK GmbH & Co KG
The mole consists of a strong abrasion resistant steel ‘torpedo like’ cylinder, with a cone shaped sliding head of very tough steel. Inside the cylinder is a heavy piston with an arrangement of valves so that when compressed air is applied, the piston hammers against the back of the head, thus imparting a forward momentum to the mole, and sufficient impact to the head to break up small stones that might otherwise deflect the mole from its course.
The piston mechanism is reversible, generally by twisting the air hose, so that the mole can be reversed, either to recover it if it goes off course or gets stuck, or to pull back the pipe being installed.
There is a wide range of moles available, a variety of sizes, and a variety of different heads, and the actual performance of these moles can vary considerably. Directional stability is generally better with a longer body, but this requires a longer launch excavation, and most contractors prefer the shortest moles for this reason.
Most impact moles are non-steerable and most bores are planned as straight bores. Non-steerable moles can be designed with fixed or moving heads. Various head designs are available for moles including replaceable heads to adjust to the ground conditions.
There are steerable moles, on which the head is angled, and the operator ‘steers’ by rotating the head, shown below before launching for steering to the right. Steerable systems are now available enabling curved trajectories and bores with multiple direction changes, as well as alignment corrections during the moling process, if needed, but the steering ability is somewhat limited.
Image courtesy: TRACTO-TECHNIK GmbH & Co KG
The mole incorporates a radio sonde transmitting the angled head ‘clock face’ position and is tracked from the surface using a special receiver. Such moles are inherently directionally unstable – stability would inhibit the ability to steer – and anyway the degree of steering available is very limited, the principle advantage being the ability to better stay upon a straight course in difficult ground.
Most moles are pneumatically powered and require a compressor at the surface adjacent to the entry pit. There are also hydraulically powered moles available and these require a hydraulic power pack instead of the compressor.
Recent development has been of moles for use in rock or boulders. These use a modified impact head to break rock and maintain alignment.
Images courtesy: TRACTO-TECHNIK GmbH & Co KG
Lubricant for the mole is fed through an in-line lubricator by adjusting a lever on the in-line lubricator, the mole can be reversed, during the boring operation.
Non-steerable moles must be carefully aligned in the insertion pit because a correct starting alignment is one of the essential conditions for an accurate bore. The launch pit excavation must be cut square and flat to enable a correct alignment at launch. Once in the ground, non-steerable moles are intended to advance in a straight line and the operator can control the maintenance of alignment only through the tool’s air supply. Non-steerable moles can also be provided with a tracking and depth measurement feature. If the bore deviates too much from the intended alignment or depth, the tool is reversed and backed out of the hole and the bore restarted.
Range of Soil Types
Clay. Silt. Sand. Fine gravel. Coarse gravel.
In order to ensure successful completion of an impact moling installation it is essential to know the ground conditions. The method uses a compaction principle to create a bore and thus is most appropriate for compressible soils. Ground conditions suitable for impact moling include clays, silt, peat and generally soft cohesive material. Sands and gravel are considered less appropriate, especially if they are densely packed, while solid rock and boulders have until recently been considered entirely unsuitable for this technique (see above).
Diameter, pressure and Length Range
Impact moling is best suited to installing PE100 pipe in diameters up to 160mm. Length of a single installation is dependent on the soil conditions. Non-steerable moles in ideal conditions can install up to 30 metres in a single shot and steerable moles in ideal condition up to 60 metres, although 10-25 metres is the normal range for most works.
Pressure capability is dependent upon pipe SDR and is not a function of the method.
DESIGN, SPECIFICATION & PLANNING
It is essential to know the depth and location of all existing utilities and other underground objects nearby.
Minimum depth should be 10 times the pipe diameter or 1 metre, whichever is greater.
This method applies tension to the PE100 pipe during installation. The tensile load on the pipe during installation must be calculated to establish whether it exceeds the maximum allowable load. Pipe manufacturers can advise on the allowable loads. If the calculation shows that the maximum permissible load may be exceeded then it may be necessary to increase the capacity of the PE100 pipe by increasing its thickness. This will require a check of ID and OD to ensure that flow capacity and external clearances are still adequate.
HEALTH, SAFETY & ENVIRONMENT
Operators need to be in the launch and arrival pits so these must be supported to allow safe access. The mole must always be launched from its cradle, and never be hand launched, in case it hits an undetected electricity cable whilst the operator is still holding it. Once the mole is correctly aligned the operator should not be in the launch pit when the mole is launched. Similarly there should be no personnel in the receiving pit until the mole has fully emerged into it.
Where cables and other utilities are identified they should be exposed so that progress of the mole can be monitored and stopped if required. In congested launch and reception pits, operatives should always launch the mole away from cables.
With unsteerable moles there is a risk that the mole will deviate from its intended alignment and either emerge at the surface or damage adjacent utilities or underground objects with consequent safety risk especially if either gas pipes or electric cables are present. Also some pipes, such as asbestos cement and clay, can be very sensitive to the sudden dynamic loading introduced with impact moling.
The accuracy of unsteerable moles can be quite high in stable ground conditions where, with proper initial alignment, the accuracy in both line and level may be within 1% of the length of the bore.[ For example if the bore is 20 metres long the mole should be within 200mm (vertically and horizontally) of its intended location at the reception pit.]
The accuracy of steerable moles depends on the accuracy of tracking system in the mole and the skill of the operator. The stated accuracy of the tracking system is usually 2-5 % of the depth, and because the cover depth in impact moling is typically of the order of 1m. it is expected that the accuracy would be within 50-100mm from designed bore path. This makes them applicable in closer proximity to existing utilities and the empirical recommendation for the minimum distance from adjacent utilities may be no more than 0.4-0.6m in ideal conditions.
A common rule of thumb with unsteered moles is that the minimum clearance to other utilities should be 6 times the diameter of the pipe being installed. If impact moling is to be undertaken very close to existing utilities, it is recommended that the existing utility be exposed at the crossing point so that the clearance of the moling head can be visually confirmed. This applies equally to steered and unsteered moles.
STANDARDS & CODES OF PRACTICE
See also Standards and Codes of Practice
- Pipe assembly & handling
- Installation Manual
- End fittings
- Testing & inspection
- Piecing up
- Excavations & pit sizes