This document discusses granular sub-base (GSB) layers used under pavements. GSB acts as both a drainage and load distribution layer. It prevents water from damaging pavement layers and allows water to drain through without harm. Testing showed that GSB Grade III has the highest permeability, making it best for drainage. GSB Grades I and II are too dense for drainage. Combining Grades III and V provides both drainage and structural support. Proper camber is needed for drainage. Grade V can be used for drainage and structure but requires verifying the camber.

1. Prepared by
M.Sreekanth Reddy

2. Act as Drainage Layer
Act as Load Distribution Layer
Load Distribution & Drainage Layer

4.  Weakening of Pavement Layers
 Loss of bond between layers
 Loss of Strength and durability of asphalt mixtures.
 Cause for pavement distresses such as ravelling, rutting
and stripping.

6. It can preventing capillary water to rise to top road
layers (Particle size of drainage layer is planned in such
way that the capillary action stops and can not go
beyond drainage layer)
It has to act as a medium where water can pass through
without damaging other road layers.

8. Grade of GSB Permeability at 2.5%
slope m/day
Permeability at 3.5%
slope m/day
Grade I No Flow was observed
Grade II No Flow was observed
Grade III 3675.66 3056.23
Grade IV 2418.28 1698.74
Grade V 2423.14 1930.83
Grade VI 1293.62 1297.34
Grade III+V 3049.40 2493.53
Grade III+ VI 2432.37 2212.83
Grade IV + V 2366.20 2040.49
Grade IV + VI 1605.64 1489.04
GSB Grade I,II - Densest Graded III,IV – Open Graded
GSB Grade V & VI - Close Graded

9.  AS from the previous table , The GSB Grade III, Grade
V, Grade III+V have maximum hydraulic conductivity
at 2.5 % slope.
 Hence 2.5% camber for GSB is required to prevent
moisture damage to the pavement.
 Grade III - 3675.66 m/day
 Grade V - 2423.14 m/day
 Grade III+V – 3049.40 m/day

10. Densest grades among all grades specified in MoRTH.
Having 65 % of 4.75 mm passing material.
These grades showed good structural stability than
drainage property.
It is used in areas where there is need of structural
support to pavement rather than drainage
requirement.

11. Grade III GSB act as only drainage layer.
Highest coefficient of permeability among all
gradations specified in MoRT&H for GSB at optimum
gradient 2.5%.
So Grade III can be effectively used for heavy monsoon
regions as a Drainage layer.
Structural stability of this grade can be improved by
working on combination grade III+V .

12. It can be used as drainage cum structural layer
Fulfil the drainage requirement with providing full
thickness of 300mm
But while providing these grades camber provision
must be verified.
Grade V is having approximately 50% finer
material i.e 4.75 mm sieve passing , so this grade is
considered as Close graded.