GSB material: gradings I to VI, tests and laying under MoRTH Clause 401

A purchase order that reads GSB and a tonnage leaves out the one thing the plant needs to know. Granular sub-base under MoRTH Clause 401 is six different gradings, and material blended for one will not necessarily pass another.

GSB stands for granular sub-base. It is the first granular layer of a road, laid on the compacted sub-grade and under the wet mix macadam, and it does two jobs: it spreads load onto the soil and lets water out of the pavement. The MoRTH Specifications for Road and Bridge Works, Fifth Revision (2013), give six gradings in Table 400-1 and the physical tests in Table 400-2. Your contract says which grading applies.

What GSB is made of and where it sits

Clause 401 allows natural sand, crushed gravel, crushed stone, crushed slag, or a combination. Brick metal, kankar and crushed concrete are permitted in the lower sub-base only. On a highway package fed by a crusher it is crushed stone: coarse fractions and crusher fines blended until the mix sits inside the specified envelope. The same material goes into haul roads, yards and working platforms, as the materials page lists. The layers of a highway pavement shows the whole stack.

The six gradings in Table 400-1

Each figure is percent by weight passing that IS sieve. Read down a column, not across.

Gradings I and V are identical from 75 mm down to 9.5 mm and then part company: I wants 10 to 15 percent passing 0.425 mm, V allows 0 to 5. Gradings III and IV are limited on four sieves only, so they run coarse and open.

Which grading goes where

Two documents speak to this, in different words. MoRTH Clause 401.2.1 says Gradings III and IV should preferably be used in the lower sub-base, and Gradings V and VI as a sub-base-cum-drainage layer. IRC:37-2018 says in Clause 7.2.1 that the drainage layer is Grading III or IV and the filter or separation layer is Grading I, II, V or VI. Where the GSB is 200 mm or less, a single drainage-cum-filter layer of Grading V or VI serves.

Physical requirements in Table 400-2

Where water absorption (IS 2386 Part 3) is above 2 percent, Clause 401.2.2 asks for the wet aggregate impact value to IS 5640, and soft aggregates such as kankar, brick ballast and laterite are tested wet as well. Older site notes quote a ten percent fines value for GSB. It is not in the Fifth Revision's table. Aggregate tests for highway work explains what each test measures.

Laying and compaction

• Moisture. At compaction the material should be 1 to 2 percent below OMC.
• Roller. Up to 100 mm compacted thickness may be rolled with a smooth-wheeled roller of 80 to 100 kN. A single compacted layer of up to 200 mm needs a vibratory roller of 80 to 100 kN static weight. Roller speed is not to exceed 5 km/h.
• Density. At least 98% of maximum dry density by IS 2720 Part 8.
• Two layers. Where GSB is laid as upper and lower sub-base, each layer is not less than 150 mm (Clause 401.2.1). IRC:37-2018 Clause 7.2.2 puts separate drainage and filter layers at a minimum of 100 mm each and a single filter-cum-drainage layer at 150 mm.

GSB is measured in cubic metres, finished in position, and the plant sells it in tonnes. The quantity of GSB and WMM in a kilometre of road shows the arithmetic between the two.

Test frequency and level tolerance

What goes wrong at the plant

Nearly always, fines. Too many and the layer holds water, the opposite of what a sub-base is for. They usually arrive with the feed: weathered seams, overburden not stripped clean, a wet face. Clay in that fraction pushes the plasticity index past 6 before the grading looks wrong. Too few fines and the layer stays bony: it will not close under the roller, and the density test fails.

The control is in two places. Screen deck apertures decide what each fraction contains, and the blend proportion decides what the stack contains. Grading V is not Grading II with less dust added by eye: 0 to 5 percent passing 0.425 mm means the fine fraction has to be screened out, and a lower deck that blinds in damp weather puts it straight back. On our crushing plants a change of specified grading is a change of decks, about a shift of work.

Then the stockpile. A conical stack segregates as it is built, coarse to the toe and fines in the core, so a loader working only the edge sends out something the lab never sampled. Reading a sieve analysis explains how to tell a plant fault from a sampling fault.