There is a wide variety of compaction equipment available. Compaction is usually achieved using a vibrating or non-vibrating steel drum roller, a pneumatic-tyre roller, a sheep-foot roller, or a grid or cleated roller. Each is best suited to specific applications. A large steel drum vibratory roller is often considered the best for general purpose use. The range of material that can normally be compacted economically with each type of roller is shown below. Compaction in cohesive soils are best achieved by impact and weight to increase the soil density. Impact is required to break down the cohesive bonds to enable the soil to become denser.
Kneading compactors with high point loads, such as sheep and pad foot rollers, are required for cohesive soils. Compactions in granular soils are best achieved by vibration. The options are static compactors that simply apply weight and tend to compact from the bottom of the layer up, vibratory compactors that use a mechanical action to consolidate soil particles, and impact compactors that use a high-amplitude whack to compact material. In addition to their normal compaction applications, smooth drum-vibrating rollers and pneumatic-tyre rollers are used as finishing rollers for clay and clayey sand subgrades, as sheep-foot and tamping rollers do not produce a smooth surface. A pneumatic-tyre roller may be used to seal off earthworks from rainwater.
The range of compaction equipment available for subgrades and pavements, and the situations for which they are best suited, is described below.
Compactor selection depends on the material
Used for the compaction of crushed rock, gravels, sands and other granular materials. In general, they’re not best suited to silts or clays but they can be used for compaction of these materials. The performance of smooth-wheeled rollers depends on the mass of the roller, and the width and diameter of the rolls. The compaction depth of the layer for satisfactory results depends on the mass of the roller, but can be up to 450 mm for embankments, and 150 mm for subgrades. Safety issues need to be considered when using these rollers on wet roads with crossfall.
Multi-tyre, pneumatic-tyre rollers
These rollers are usually self-propelled, with smooth tyres on two axles in an offset arrangement, so that the wheels on one axle track are in the gaps between the wheels on the other axle. The mass of the roller can be increased by attaching ballast, and the tyre pressure is sometimes variable. Fine soils with little or no cohesion, for example silts, sandy silts, well-graded sands, and clay soils, compact well when using these rollers. The layer thicknesses should not exceed 230 mm when compacted, and the performance of the roller is a function of tyre pressure, tyre contact area and weight.
Heavy pneumatic-tyre rollers
These rollers have four equally-spaced independently suspended tyres and can be ballast loaded up to a total mass of 50 tonnes. They are usually towed, and are suitable for similar soils to the multi-tyre pneumatic-tyre rollers, as well as for gravels and finer silts. The difference between these and multi-tyre pneumatic-tyre rollers is that the heavy pneumatic-tyre rollers can compact deeper layers, and the surface density achieved is greater.
The sheep-foot name comes from the tapered prong feet on the steel drum of these rollers. They can be either towed or self-propelled, and their mass can usually be increased by filling the drum with water, or with sand and water. Performance is best on cohesive soils at, or drier than OMC. During use these rollers will tend to ‘walk’ out of the fill as compaction occurs and the feet ride on the compacted surface.
These are similar to sheep-foot rollers however the feet are wider, shorter and closer than sheep-foot feet. They are also often diamond-shaped. They can be self-propelled or towed and will compact a wider range of soils than sheep-foot rollers, including silts, and rock fragments, but not uniform sands.
The rolls on grid rollers are manufactured from a mesh of usually 20 mm diameter bars spaced 100 to 150 mm in both directions. Alternatively, they may be smooth drums with a pattern of square holes formed in the surface. They are particularly useful for scoria-type fill (random mixtures of large and small particles, usually angular and fairly soft). Their particular use is in breaking down oversized stones and forcing them below the compacted surface.
These rollers have a rotating eccentric weight to produce a vertical acceleration, which helps to compact the material. They have various types of drum, such as smooth wheel and sheep-foot. The force applied to the soil is proportional to the acceleration in the vertical direction, so they have a better performance than static rollers. Vibrating rollers are suitable for compacting non-cohesive soils, compacting from the base up towards the surface, which always leaves a layer of looser material at the surface. This is caused by the bouncing effect among the particles of that layer as the vibratory effect is transferred downwards. This can be easily overcome by using a compactor with medium to low weight and low amplitude, or by turning off the vibration.
The primary characteristics of vibratory compacters are the weight of the vibrating component (drum or plate), the weight applied through the component to the ground, and the frequency and normal amplitude of the vibration. Heavier weight rollers with high amplitude and relatively low frequency (200-1800 vibrations per minute) can compact very thick lifts (0.5 m) in some granular materials, and up to 300 mm in clay materials.
With less weight, lower amplitudes and generally higher frequencies, a reduced layer thickness can be compacted. Frequency is not generally critical, except that the higher it is, the fewer passes are required, and the faster the compactor can travel to obtain optimum compaction per pass.
When using a vibratory compactor, a pattern must be adhered to, to avoid gaps in the pass coverage. It is also important not to over compact the material. When the drum bounces on the hardened surface, a distinct ‘ringing’ noise can be heard. This can cause damage to the roller, and it will also reduce the density of the material. Therefore, for very thin layers (30 mm), particularly of granular or sandy materials, two passes may be sufficient, and four passes may be too many.
Large dual vibrating drum rollers
Granular materials of up to 250 mm can be successfully compacted in four to 10 passes with these rollers, providing they are correctly calibrated to obtain the most appropriate weight, frequency and amplitude. To complete the compaction, it is generally necessary to operate the roller without vibration for two to four passes to consolidate the surface.
Earth vibratory compacters
These are generally of the large single vibratory drum type with large rubber driving wheels. They are used for difficult terrain and thick layers of earth. Some rollers of this type are fitted with rubber coated drums, which have proved effective for the compaction of chips in chip seal construction.
Powered static rollers
Powered static rollers compact material by pressure only. Loose layer thicknesses of 100 to 150 mm of material are compacted at any one time, and a large number of passes are generally required to obtain full-depth compaction. These rollers compact from the top down. Only the first 50 mm of material is usually compacted by the first four or five passes, and a further 20 or 30 passes may be necessary to complete compaction. One major disadvantage in the use of these rollers is the crushing effect on the upper compacted material, which can produce excessive fine material, affecting the quality of the base course. Various types of rollers are available, including tandem and three steel wheeled 8 to 12 tonne rollers, sheep-foot and grid rollers which are used for bulk material compaction.
Track rolling is commonly used by forest road contractors to compact road and landing surfaces. However, the tracks for construction equipment are designed for low ground pressure. By definition, this means that tracked equipment is poor for compaction. Time and money spent track rolling would be better invested into dedicated compaction equipment. However, track rolling is probably better than nothing and may reduce surface scour.
The range of compaction equipment for hand-held, localised applications – such as culvert installation – are described below.
Power rammer (oscillating foot compactors)
These small compactors, usually operated by hand, have a relatively low output in terms of volume of compacted soil/hour. Generally, these compactors are used for compacting material which has been backfilled into excavations in confined spaces, such as in trenches and around culverts. Compaction of material using a power rammer should be completed in layers. For granular soils, layers should not be more than 230 mm thick, whereas for cohesive soils, layers should be no more than 200 mm thick. Power rammers have a vertical movement ranging from 12 mm to 150 mm at frequencies of 20 to 200 blows per minute.
Vibrating plate compactors
These compacters are available in weights ranging from 50 to 150 kg and have an operating frequency of 400 to 10,000 vibrations per minute, with low amplitude. Vibrating plate compactors are used to compact layers of 75 mm-110 mm compacted thickness of most materials, and are ideal for small areas (up to 20m2) requiring 4-6 passes for optimum compaction.
Single drum vibratory rollers
These compactors are hand-operated, and are ideal for granular materials with compacted layer depths of up to 120 mm with four to six passes. They operate in the high frequency, low amplitude mode.
6.6.1 Compaction process
Typical compaction requirements for fills
To be effective, compaction needs to be done correctly. The layer thickness should be appropriate to the type of material being placed, and the type and capacity of the compaction equipment being used. The table above provides indicative compaction requirements for various types of fill material. Compaction becomes ineffective over 300-400 cm.
The following will help provide additional operational guidance:
- The subgrade should be even and have the required cross-section shape with crossfall, and where necessary, superelevation on corners. Re-shape with a grader or excavator if compaction changes the subgrade shape, and then re-compact. Do not use the aggregate layers to create crossfall and to even out variations in the subgrade shape. It is an inefficient method, and aggregate is usually the most expensive component of the road construction
- Dispose of non-compactable, unsuitable or excess road construction material where there is a low risk of movement
- All culverts and ditches should be in place. Installation of any culverts after the placement of pavement aggregate should be avoided. Typical subgrade compaction consists of six passes of a sheep-foot or steel drum vibrating roller of at least six tonne static weight. Effective compaction is not possible if the moisture content of the material is too far from the optimum moisture content. That is, too dry is as bad as too wet
- Rolling should commence at the outer (lower) edge of the pavement and progress towards the centreline, or upper edge, if superelevated. Rolling with the passes progressing towards the lower edge will cause material to move downhill, resulting in loss of shape
- Fill batters should be overfilled to support earthmoving equipment and to allow the compaction plant to compact the full width of the design cross section, and then trim back to the design batter slope as the fill is built up. This will ensure the full width, including the outer edge of the fill, is effectively compacted
- A forward and reverse pass is made over the same section of pavement before moving to the adjacent section. It is important to check that this is done at the edges of the pavement. When changing direction, the roller should be on the previously compacted section
- An overlap of up to 500 mm over the previous pass to ensure complete coverage should be completed
- A space of 200 to 300 mm should be left on the outside edge of the pavement if this is unsupported. Rolling of this section should take place later with a lighter roller.
- Vibrating rollers should have the vibrator turned off when the roller is stopped or turning
- Rollers jolting during reversing can encourage surface roughness, as can sharp turns or changes in direction, and therefore this should be avoided
- Static drum rollers should have the drive wheels leading on the initial pass to avoid pushing material ahead of the drum
- Best compaction can be achieved with a vibrating roller by using a sequence of a non-vibrating pass, followed by several high amplitude passes, and finishing with low amplitude passes.
Subgrade strength needs to be identified to make sure that the compaction is effective, consistent across the area, and to design standards. Decide on the specific tests and testing frequency during the planning and design phase. These should reflect the scale and complexity of the earthworks, and the consequence of the fill failing.
The following field-testing equipment is considered suitable for compaction testing on most forest earthworks projects:
- Scala penetrometer – strength testing in cohesionless soils. Results are converted into an ‘inferred CBR’
- Shear vane – strength testing in cohesive soils. The results are expressed in kPa
- Clegg Impact Test – testing surface hardness or stiffness. The result (impact value) can be used as an indication of compaction but is not a direct measurement. Impact values can be converted into an inferred CBR (Inferred CBR = 0.07 x (IV)2)
- Nuclear densometer – testing of water content and percentage compaction (if MDD target is provided).
The table below shows the recognised methods for testing the quality of fill materials and construction.
Recognised methods for testing the quality of fill materials and construction