What is the granular sub base?

The granular subbase forms the lowest (bottom) layer of the pavement structure, and acts as the principal foundation for the subsequent road profile, provides drainage for the pavement structure, and protects the structure from frost.

How is GSB calculated?

Gsb = Bulk dry specific gravity of the total aggregate P1,P2,Pn = Percentages by weight of aggregates, 1,2,n; G1,G2,Gn = Bulk specific gravities of aggregates 1,2,n. The bulk specific gravity of mineral filler is difficult to determine accurately at the present time.

What is GSB in road construction?

GSB :- Granular sub base or GSB is a typical layer adopted in highway construction, which is laid above the subgrade and below the crust materials. Naturally available material consisting of various grades may be used for GSB, the largest grain size up to 65mm.

What is MDD of GSB?

According to Indian standards i.e. MoRT&H 5th revision Grade III is open grade to be provided for GSB. As per compaction test conducted and obtained MDD of 2.14gm/cc & OMC of 3.18 % to be achieved in the field.

What is the purpose of GSB?

Function. It acts as a drainage layer for the pavement to avoid excessive wetting and weakening of subgrade. Strength wise GSB is more superior as compared to the subgrade.

Why is GSB used?

INTORDUCTION The Granular Sub Base (GSB) layers are used in the flexible pavements to provide a stress-transmitting medium to spread the surface wheel load to a large area so as to prevent the shear and consolidation deformations.

What is minimum thickness of GSB?

➢The recommended minimum thickness of granular base is 225 mm for traffic upto 2 msa and 250 mm for traffic exceeding 2 msa.

What is compaction factor of GSB?

Specified compaction for GSB – 98 % of MDD (MDD by IS 2720 Part – 8). There is a possibility of getting a higher compaction which results normally upto 104% due to the presence of coarse crushed material.

What is the thickness of GSB?

The material shall be mechanically (viz grader, etc.) spread in uniform layers such that compacted thickness not exceeding 200mm – 250mm with due consideration to loose thickness (compaction factor) of GSB.

What is maximum dry density of GSB?

The maximum dry density of Granular Sub Base used in Road Construction must be 2100 Kg/Cum. The maximum dry density of Wet Mix Macadam used in Road Construction must be 2230 Kg/Cum.

What is CBR value of GSB?

➢ The combination of GSB and sub-grade simulated for the study shows that as GSB/Total thickness ratio increases, CBR increases from 2.5% to 55% for soil to GSB. The angle of internal friction from 120 to 400 and the corresponding shear strength from 0.91 kg/cm2 to 1.75kg/cm2.

What is the CBR value for subgrade?

CBR value is taken at 2.5mm & 5 mm penitration. in subgrade material CBR value should be nearly 15-19% .