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Granular Sub-base Base (GSB) quantity / Compaction Factor value and / Loose thickness of GSB Layer


In this article we shall discuss three important points about granular sub base  (GSB)

1- How can we calculate the required GSB  quantity? 

2 -  What is value of compaction factor for GSB?

3- How much loose thickness of GSB should be taken to get proper compacted layer thickness ? 



1- How can we calculate the required GSB quantity?



Let us consider

Length of GSB bed (L) = 500 Met.
Breadth of GSB bed (B) = 7 Met.
Thickness of GSB Layer (T)  = 150 mm =0.150 Met.

Max. Dry Density (MDD) =2.221 Gram/ Cubic Centimeter = 2221Kg/ Cubic metre

Note -  The value of MDD is calculated by conducting proctor density test as per IS 2720 (part -28) : 1983 (Reaffirmed - May 2015).

Then
Volume of GSB  Layer = ( Length of GSB Bed X Width of GSB Bed X Thickness of GSB Layer)

Volume of GSB  Layer = L X B X T
 Volume of GSB  Layer   = 500 X 7 X 0.150  Volume of GSB Layer    = 525 Cubic Metre

We Know That
Mass = Volume X Density
in this case -

Mass = Volume X MDD

Putting the value in the Formula

Mass = 525 X 2221 Kg
Mass  = 11,66,025Kg

To convert above quantity in Tonn
We Know That

1 Tonn = 1000 KG
Then

Mass =(1166025/ 1000) Tonn
Mass   = 1,166.025Tonn


Hence required GSB quantity is 1,166.025 Tonn. 



2 -  What is value of compaction factor for GSB?

Density of Blended aggregate /Density of GSB Mix = 1.60 gram/cc  to 2.0 gram/cc
                  = 1600 - 2000 KG/ cubic metre

In this case
Let us consider

Density of Blended aggregate /Density of GSB Mix = 1.70 gram/cc = 1700 Kg / cubic metre

Value of Max dry density (MDD) = 2.221 gram/cc = 2221 Kg/ cubic metre

Compaction Factor = (Max.Dry Density/Density of Blended Aggregate )

        =2.221/ 1.70
        =1.307 = 1.30
Hence compaction factor = 1.30

3- How much loose thickness of GSB should be taken to get proper compacted layer thickness ? 

Compaction factor = 1.30
Thickness of GSB Layer  = 150 mm

Hence
 Loose thickness of GSB Layer = ( Compacted layer thickness X Compaction Factor )

 = 150 X 1.30
 = 195 mm

Hence Loose Thickness of GSB Layer is 195 mm. 



Thanks a lot 
Mukesh Kumar
 Gyan of engineering 


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