Abstract:
Granular Sub Base Course (GSB) provided as one of the structural layer of pavements should also serve as an
effective drainage layer. In India current guidelines recommend using natural sand, crushed gravel, stone or slag or
combination of these, in the GSB layer. While the above combinations may fulfil the structural requirement, it is not
clear whether they meet the minimum drainage requirements of 300 m/day as per AASHTO specifications.
This paper summarizes the laboratory permeability carried out on 1) Crushed stone aggregates and slag in different
combinations with non plastic fines such as quarry dust and 2) Gravel- Aggregate combination in the ratio 60:40.
The first combination was tried for Grade III requirements as per MORT&H specification (5th Revision) for High
volume roads. The second combination was tried for Grade III requirements as per rural roads Manual, IRC SP 20,
used for low volume roads in India.
The objective is to compare the permeability characteristics of GSB gradations prepared with different mixes in
order to assess their ability to drain, based on the permeability criteria. Horizontal and vertical permeability were
tested in the laboratory for these GSB mixes and the results have been reported.
From the study it is observed that while all the combinations of crushed stone – slag mixes meet the minimum
permeability criteria in the horizontal as well as vertical directions, 100% granulated steel slag (GLDS) does not
meet the requirement in the vertical direction. Also while the Gravel – Aggregate combination (60:40) just meets the
minimum permeability requirement in the horizontal direction, there is negligible discharge in the vertical direction.
