Put a smooth drum on wet clay and it polishes the top of the lift while the rest stays soft. Put a padfoot on clean sand and it churns up the surface it has just compacted. Both rollers are doing what they were built to do, on the wrong material.
This is a guide to the types of rollers in road construction, what each one suits, and the limits the MoRTH Specifications for Road and Bridge Works (Fifth Revision) place on lift thickness and speed. The other source is IRC:SP:97, the Indian Roads Congress guideline on compaction equipment.
Types of rollers in road construction, and how each compacts
A roller compacts by one or more of four actions: static pressure from its weight, kneading that shears and rearranges the particles, impact, and vibration that sets particles moving so they settle into a tighter packing. Each type of roller is a different mix. A static smooth wheeled roller has pressure only. A smooth vibratory drum adds impact and vibration. A padfoot or sheepsfoot drum works by high contact pressure and kneading, and a pneumatic tyred roller by pressure and kneading with no impact at all.
Granular material responds to vibration. Clay does not. Its particles are held together by cohesion and water, and it has to be squeezed and kneaded. That one distinction explains most of the table below.
The table: roller type, weight class and what it suits
| Roller type | Typical weight class | Soil or layer it suits | What it is bad at |
|---|---|---|---|
| Static smooth wheeled, three-wheeled | 8–10 t, the 80–100 kN roller MoRTH names | Thin granular lifts, finishing a sub-grade or formation, breakdown rolling of bituminous mix | Depth. It compacts from the top and bridges soft spots |
| Single-drum vibratory, smooth shell | 8–12 t dead weight is the common class; heavier machines exist | Sand, gravel, moorum, GSB, WMM and rockfill | Wet plastic clay; work close to structures |
| Single-drum vibratory, padfoot shell | The same machine with a pad shell; purpose-built padfoot rollers from about 11 t | Clay and silty soils; breaking down clods | Clean sand and gravel; leaves a dimpled surface that holds rain |
| Tandem vibratory | 8–10 t for paving; light tandems of 900–1,500 kg for shoulders and paths | Bituminous courses | Bulk earthwork on loose or uneven fill |
| Pneumatic tyred (PTR) | 12–15 t on bituminous work; ballasted from about 11 t to 25 t or more | Intermediate rolling of bituminous mix, sealing granular surfaces, uniformly graded soils | Thick clay lifts, rockfill, anything that needs impact |
| Vibratory plate compactor | Light below 400 kg, heavy above | Trenches, edges, behind walls, patching | Area and depth |
| Rammer | Hand-guided; 500 to 750 blows a minute | Cohesive soil in narrow trenches and around pipes | Granular material; output |
Weight classes are typical, taken from IRC:SP:97 and from the rollers MoRTH names. Manufacturers' ranges are wider. What a given machine achieves on a given soil is settled by a compaction trial.
Vibratory roller vs static roller
A static roller compacts by weight alone, so its effect fades quickly with depth. MoRTH's lift limits put numbers on that.
| Layer | Static smooth wheeled roller, 80 to 100 kN | Vibratory roller, 80 to 100 kN static weight | Clause |
|---|---|---|---|
| Embankment and sub-grade | 200 mm | 250 mm | 305.3.5.1 |
| Granular sub-base (GSB) | 100 mm | 200 mm | 401.3.2 |
| Wet mix macadam (WMM) | 100 mm | 200 mm | 406.3.5 |
| Rockfill embankment | Not named | 500 mm, with at least 5 passes | 313.3 |
Fifth Revision; check the edition your contract cites. For rockfill the clause gives the roller as 8 to 10 tonnes static weight.
A vibratory roller reaches density through a thicker lift, which is why the single-drum vibratory compactor does most bulk earthwork. The static roller keeps particular jobs. Clause 310 has the finished formation compacted with an 80 to 100 kN smooth wheeled roller. Clause 401 has the sub-grade rolled with two passes of one immediately before GSB is laid. And it does the breakdown rolling on bituminous courses.
Padfoot roller or smooth drum
For embankment and sub-grade, Clause 305.3.6 names a self-propelled single-drum vibratory roller or a padfoot vibratory roller of 80 to 100 kN static weight, or a heavy pneumatic tyred roller, and leaves the choice to a compaction trial. The soil makes that choice.
A padfoot drum concentrates the weight of the machine on the faces of its pads. They push into the loose lift, compact it from the bottom up, and climb out as the layer tightens, until the drum is riding on the pad tips. Operators call that walking out. The pads also break clods, which matters when the specification wants them under 75 mm in embankment and 50 mm in sub-grade. The older sheepsfoot roller works the same way with smaller feet that fluff the surface as they leave it. The tapered pad was the improvement.
The price is the surface. A padfoot leaves a dimpled finish that collects water, so the top lift, and any lift left open before rain, is sealed with a smooth drum. On sand, gravel or moorum with little clay in it, leave the pads off altogether. IRC:SP:97 puts the smooth vibratory drum on granular material and on mixed soils with up to about 10 per cent cohesive content.
Pneumatic tyred roller use
A PTR is a ballasted body on two axles of smooth tyres, set so the rear tyres cover the gaps left by the front. It is rated by ground contact pressure, which is changed with ballast and tyre inflation. The tyres flex around the particles and knead the layer, closing the surface in a way a steel drum cannot.
Its fixed place in the MoRTH sequence is on bituminous courses. Clause 501.6 sets breakdown rolling with an 8 to 10 tonne static smooth wheeled roller, then intermediate rolling with an 8 to 10 tonne static or vibratory roller or a 12 to 15 tonne pneumatic tyred roller at a tyre pressure of at least 0.56 MPa, then finish rolling with a 6 to 8 tonne smooth wheeled tandem. On earthwork a heavy PTR is permitted and is useful for sealing a finished sub-grade or WMM surface. It delivers no impact, so it is not the machine for a thick lift.
Lift thickness, number of passes, speed, amplitude and frequency
- Lift thickness. The table above is the ceiling. A layer thicker than the roller can reach passes a density test in its top half and fails in its bottom half, and the test hole has to go to full depth to find out.
- Passes. MoRTH fixes a number only for rockfill. For everything else the contractor proves the equipment in a compaction trial, and the count that reached density on the trial strip becomes the rolling pattern. Each pass overlaps the last by at least one-third of the track.
- Speed. Not more than 5 km/h on GSB, WMM and bituminous layers; the clauses for all three say so. Clause 305 sets no figure for soil, and IRC:SP:97 gives 2 to 6 km/h as the working range for vibratory compactors. A faster roller spaces the blows of the drum further apart, so each metre of the layer receives fewer of them.
- Amplitude. How far the drum moves up and down on each cycle, which sets how hard it strikes. High amplitude reaches deeper.
- Frequency. How many times a second it strikes. High frequency puts the blows close together.
The two settings work against each other. High amplitude with low frequency is for thick lifts, soft material and the first passes. Low amplitude with high frequency is for thin lifts, stiff material and finishing. The WMM clause asks specifically for a vibratory roller on which both can be adjusted. A drum left on high amplitude over a layer that is already tight adds nothing: it bounces, loosens the surface and can break the aggregate. Rolling runs from the lower edge to the upper on a one-way crossfall or superelevation, and from the edges to the crown on a camber.
Confined areas and proof rolling
A 10 tonne roller cannot work against an abutment or inside a trench. For fill around structures and for widening strips too narrow for a normal roller, Clause 305 names the alternatives: small vibratory rollers, double-drum walk-behind rollers, plate compactors, power rammers and vibratory tampers. The layer has to shrink with the machine. For backfill to drains Clause 301 sets a maximum of 150 mm compacted. Light plates suit thin layers of sand and gravel. Rammers work by impact and suit cohesive soil in narrow spaces.
Proof rolling is a check, not a method of compaction. A heavy pneumatic tyred roller or a loaded tipper is driven slowly over the finished sub-grade while someone walks alongside watching for the surface deflecting, rutting or weaving under the wheels. It finds the soft pocket that the density holes missed. MoRTH lays down no proof rolling procedure, so where a contract or consultant asks for one, the load and the pass criteria are theirs to fix. What Clause 305 does say is what happens next: soft areas get further compaction, and if that fails the material comes out and is replaced.
Before the roller is ordered
Two facts pick the machine: the classification and plasticity of each fill source, and the compacted layer thickness the programme assumes. What stands next to the work decides the rest. Send those with the contract's compaction table and the answer is usually short. We own the rollers and graders we put on earthwork; they are not hired by the day. What we take on is set out under earthwork and excavation, and the wider plant list is on the equipment page. The density those rollers work to is explained in embankment construction as per MoRTH Clause 305, and how it is measured in OMC, MDD and the field density test.