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The name Impact Roller or Impact Compaction Roller can be misleading in that it implies a rolling cylinder as used in conventional vibrating and static road rollers. Impact rollers are characterised by non-cylindrical multisided geometrical Drum or Module. The number of sides can vary from three to five. The other defining characteristic is that Impact Rollers derive their energy by turning on their corner (major radius) and falling to the flat side (minor radius). There being no motorised form of energy such as in a vibrating roller.
The purpose of an Impact roller or Impact Compaction Roller is to introduce energy to the ground surface in quick succession, around two impacts per second. By doing this in a rolling action at around 10kph large areas can be treated quickly and a large amount of energy applied to an area in a comparatively short period of time.
Impact rollers and Impact Compaction Rollers differ from conventional rollers in their ability to compact material at depth- depth of influence. Conventional vibrating rollers have a depth of influence of around 300mm in cohesive soils whereas Impact rollers have a depth of influence of 700 to 2500 mm in cohesive soils. In non-cohesive materials depth of influence have been recorded at 3 to 4 m.
The economic advantage of using Impact Rollers and Impact Comapction Rollers stems directly from their depth of influence in in situ materials. By way of example: one hectare compacted to a depth of one meter in situ using a medium energy Impact Roller would take 19.2 hours (assuming 20 passes). The same process by conventional excavate and re-compact method would require the following process.
The depth of influence of an impact roller and Impact Comapction Roller is a product of its energy rating -in Kilojoules. Rollers currently available vary from 12kJ to 30kJ (Potential Energy rating). The higher the energy rating the greater the depth of influence. The number of sides of an Impact Roller has an influence on the energy rating. Since an Impact Roller turns on its major radius and falls to its minor radius, the greater the difference of these radii the greater the lift height. Three sided rollers produce the maximum lift while five sided rollers produce the least lift.
Bulk earth works. Compacting in 1m lifts at 10kph one machine can compact 18,000 BCM per shift.
Ground Improvement, uncontrolled fill sites. Compacting to 2-3m depth in situ.
Induced settlement. The application of a large amount of energy greatly in excess of that imposed in service can bring forward projected settlement.
Sanitary Landfill. Old landfills can be put to higher land use such as sporting fields and residential areas.
Problem sites, high foundation costs. With a combination of ground treatment and engineered footing and structures, piling or stone columns and other high cost foundation treatments can be avoided.
Individual manufactures of Impact rollers and Impact Compaction Rollers may make vested interest claims to support their particular design and some go to extraordinary lengths, even presenting such claims as scientific papers. The technology behind Impact Rollers and Impact Compaction Rollers is basic physics, and manufacturers can and should provide prospective users with the basic data to validate claims.
It should be noted that from a Geotechnical perspective that all information is site specific and the transposing of site information from one site to another is not without hazards.
An Impact Roller / Impact Compaction Roller is to provide compaction at depth i.e. Depth of Influence. The depth of influence is directly proportional to the energy generated by an Impact Roller / Impact Compaction Roller. It therefore follows that if a comparison is made of the energy rating of various Impact Rollers / Impact Compaction Rollers an effective indication of performance can be determined.
Over the years manufacturers have made elusive claims for their products and have avoided making any direct comparisons between rollers. They have certainly not shown a willingness to offer their products for independent testing or evaluation.
Gravity is the most significant and active force acting on an Impact Roller / Impact Compaction Roller. The other force is rotational velocity. Since all Impact Rollers / Impact Compaction Rollers operate at similar travel speed of 10kph, by education we can therefore assume that rotational velocity, being a product of travel and speed, is largely consistent for all forms of Impact Rollers / Impact Compaction Rollers. No manufacturer has simply and effectively provided a means of arriving at rotational velocity. One manufacturer of four sided rollers claims a spring energy advantage but does not provide details. This is commented on further in these notes.
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