The short answer
MoRTH Table 300-2 requires embankment to be compacted to at least 95 per cent of the laboratory maximum dry density, and subgrade and earthen shoulders to at least 97 per cent, both against the heavy compaction test of IS 2720 Part 8. A compacted layer may be up to 250 mm thick with a vibratory roller, and moisture at rolling must lie between 1 per cent above and 2 per cent below optimum.
Compaction is the one item of road work that cannot be judged by looking. A layer at 92 per cent and a layer at 97 per cent are the same colour, the same level and equally firm under a boot. The difference appears two monsoons later as rutting, and by then the pavement on top has to come up to fix it.
01Two numbers from the laboratory
Every soil has a moisture content at which a given amount of compactive effort packs it tightest. That moisture is the optimum moisture content, and the density reached there is the maximum dry density. Both come from the heavy compaction test: soil is rammed into a mould in five layers with a 4.9 kg rammer falling 450 mm, at a series of moisture contents, and the dry density is plotted against moisture. The peak of the curve gives the two numbers.
Field compaction is then expressed as a percentage of that laboratory maximum. The percentage is called relative compaction.
02What MoRTH requires
| Part of the work | Minimum relative compaction |
|---|---|
| Subgrade and earthen shoulders | 97% |
| Embankment | 95% |
| Expansive clay, in the subgrade and the 500 mm below it | Not allowed |
| Expansive clay, in the remaining embankment | 90 to 95% |
Against maximum dry density by IS 2720 Part 8.
Before a soil is even accepted as fill, it has to reach a minimum density in the laboratory. This is a test of the material, not of the work.
| Use | Minimum |
|---|---|
| Embankment up to 3 m high, not subject to extensive flooding | 15.2 kN/m3 |
| Embankment over 3 m, or subject to long periods of inundation | 16.0 kN/m3 |
| Subgrade, earthen shoulders, verges and backfill | 17.5 kN/m3 |
03Layer thickness and moisture
A roller compacts from the top down, and its effect fades with depth. The specification therefore limits the compacted thickness of a layer to 250 mm where a vibratory roller is used and 200 mm with a static roller of 80 to 100 kN. A thicker layer may pass a test taken near its surface and be loose at the bottom.
Moisture at the time of rolling must be within 1 per cent above and 2 per cent below the optimum. Too dry, and friction between particles resists packing. Too wet, and the water carries the load and the soil moves under the roller instead of densifying. Soil from a borrow area is rarely at the right moisture when it arrives. It is either watered from a tanker and mixed in, or spread and left to dry.
04Testing in the field
The usual field method is sand replacement. A hole is dug in the compacted layer, the soil removed is weighed and its moisture found, and the hole is filled with calibrated sand to measure its volume. Weight over volume, corrected for moisture, gives the dry density. A nuclear gauge gives a reading in minutes without a hole, and is accepted where it has been calibrated against sand replacement.
- Laboratory maximum dry density
- 1.92 g/cc
- Field wet density from the hole
- 2.07 g/cc
- Field moisture content
- 12%
- Field dry density: 2.07 / 1.12
- 1.85 g/cc
- Relative compaction
- 96.3%
That result passes as embankment and fails as subgrade. The same number, in the same hole, is acceptable 600 mm down and not in the top layer.
05How often
Clause 903 sets the frequency. For embankment, one set of ten density measurements is taken for every 3,000 square metres of each compacted layer. For subgrade and earthen shoulders, one set of ten for every 2,000 square metres. Where a non-destructive method such as a nuclear gauge is used, the number of tests is doubled. Acceptance is on the set as a whole, so one good result does not carry a layer.
06Why layers fail
- Too thick. A 400 mm lift rolled as if it were 250.
- Wrong moisture. Usually too dry in the hot months and too wet after rain.
- Clods. Lumps of clay that were never broken down leave voids no roller will close.
- Too few passes, or the wrong roller. Granular soils respond to vibration. Clays need weight and kneading.
- Edges. The outer metre of an embankment is the hardest to compact and the first place to look.
- A stale laboratory value. The borrow soil changed and the maximum dry density being used belongs to last month's material.
07Subgrade strength
Compaction is the means. The end is a subgrade strong enough to design a pavement on. IRC:37-2018 treats the top 500 mm as the subgrade, compacted to at least 97 per cent, and asks for an effective CBR above 5 per cent on roads expected to carry more than 450 commercial vehicles a day. A stronger subgrade allows a thinner pavement, which is why select soil for the top layers is worth hauling. How the layers above sit on it is in layers of a flexible pavement.