The short answer
MoRTH Table 400-1 gives six gradings for granular sub-base. I and II are close-graded materials for general sub-base, with top sizes of 75 mm and 53 mm. III and IV are coarse, and the specification prefers them for lower sub-base. V and VI are open gradings with very few fines, used as a combined sub-base and drainage layer. All six need a CBR of at least 30 and are compacted to 98 per cent of maximum dry density.
The table below is the one a site laboratory tapes to its wall. Figures are per cent passing each sieve, by weight.
01The six gradings
| IS sieve | I | II | III | IV | V | VI |
|---|---|---|---|---|---|---|
| 75.0 mm | 100 | 100 | ||||
| 53.0 mm | 80 - 100 | 100 | 100 | 100 | 80 - 100 | 100 |
| 26.5 mm | 55 - 90 | 70 - 100 | 55 - 75 | 50 - 80 | 55 - 90 | 75 - 100 |
| 9.50 mm | 35 - 65 | 50 - 80 | 35 - 65 | 55 - 75 | ||
| 4.75 mm | 25 - 55 | 40 - 65 | 10 - 30 | 15 - 35 | 25 - 50 | 30 - 55 |
| 2.36 mm | 20 - 40 | 30 - 50 | 10 - 20 | 10 - 25 | ||
| 0.85 mm | 2 - 10 | |||||
| 0.425 mm | 10 - 15 | 10 - 15 | 0 - 5 | 0 - 8 | ||
| 0.075 mm | under 5 | under 5 | under 5 | under 5 | 0 - 3 |
A blank cell means no limit is set for that sieve in that grading.
02What each grading is for
| Grading | Top size | Character | Use under Clause 401 |
|---|---|---|---|
| I | 75 mm | Close-graded, 10 to 15% passing 0.425 mm | General sub-base |
| II | 53 mm | Close-graded, finer than I | General sub-base |
| III | 53 mm | Coarse, 10 to 30% passing 4.75 mm | Preferably lower sub-base |
| IV | 53 mm | Coarse, 15 to 35% passing 4.75 mm | Preferably lower sub-base |
| V | 75 mm | Open, 0 to 5% passing 0.425 mm | Sub-base cum drainage layer |
| VI | 53 mm | Open, 0 to 3% passing 0.075 mm | Sub-base cum drainage layer |
The pavement design guideline reads the same table a little differently. IRC:37-2018 recommends Gradings III and IV for the drainage layer and treats the others as the filter or separation layer beneath it. Designers follow one or the other, so do not assume. The drawing or the contract's technical schedule will name the grading for each sub-layer, and that is the one to produce.
03The other requirements
| Property | Requirement | Test |
|---|---|---|
| Aggregate impact value | 40 maximum | IS 2386 Part 4, or IS 5640 |
| Liquid limit | 25 maximum | IS 2720 Part 5 |
| Plasticity index | 6 maximum | IS 2720 Part 5 |
| CBR at 98% dry density | 30 minimum | IS 2720 Part 16 |
Where water absorption of the coarse fraction exceeds 2 per cent, the wet aggregate impact value is determined under IS 5640.
Liquid limit and plasticity index are measured on the fraction passing 0.425 mm. They are the tests that catch clay, and clay is what turns a draining layer into one that holds water.
04How a plant hits a grading
GSB is not crushed to a grading, it is blended to one. The plant produces a coarse fraction, one or two intermediate fractions and fines. A sieve analysis of each tells the operator what proportions will land inside the band, and the feeders are set accordingly. The close gradings, I and II, need a steady supply of fines. The open ones, V and VI, need the fines held back, which usually means scalping them off before the blend.
Aim for the middle of the band, not the edge. Material that sits on a limit at the plant will be outside it at the site after one loading, one haul and one tip, because handling breaks stone down and shakes fines to the bottom.
05Why loads fail
- Fines over the limit. The usual source is overburden coming down with the rock. The cure is at the face, covered in overburden removal.
- Plasticity index over 6. Clay again. Screening out the natural fines and replacing them with crusher dust normally fixes it.
- Segregation. The sample was taken from the coarse skirt of a stockpile, or the load separated in transit. Sample from several points and depths, and remix at the stockpile with a loader.
- Wrong top size. A 75 mm grading supplied where a 53 mm one was specified, or oversize slipping past a worn screen.
- Impact value. Weathered rock from the top benches. This belongs to the source and no plant setting will change it.
06Laying it
Clause 401 asks for the material to be spread, watered and compacted to at least 98 per cent of maximum dry density from the heavy compaction test, at a moisture content 1 to 2 per cent below optimum. A single compacted layer may be up to 200 mm where a vibratory roller is used. The same test and density apply to the layer above, and the differences are set out in GSB vs WMM.