Group Index method is an empirical method based on the soil properties.Group index formula is used for designing the Flexible Road Pavements and its thickness. For designing the thickness of different section of road pavement, We need the value of Group Index and type of Traffic Volume.
Group Index
Group Index is an arbitrary index assigned to the soil types based on percentage fines, liquid limit and plasticity index. It helps in grading of the soils. Its value ranges from 0 to 20. The higher GI value, Poorer the sub grade soil, more the thickness of pavement. Lesser its value, Higher the grade of soil.
Group Index, GI=0.29+0.005ac+0.01bd
a= value of that portion of the material passing through 0.075mm sieve in the excess of 35% but not exceed 75% ( its value should be whole number and number from 0 to 40).
b= value of that portion of the material passing through 0.075mm sieve in excess of 15% but not exceed 55% ( its value should be whole number and ranges from 0 to 40).
c= liquid limit in excess of 40% and not more than 60%. Its value should be whole numbers and ranges from 0 to 20.
d= plasticity index exceeding 10% and not more than 30% . It ranges from 0 to 20.
Traffic Type
Traffic Volume is divided into 3 groups based on the CVPD (Commercial vehicle per day)
Traffic Type CVPD
Light <50
Medium 50-300
Heavy >300
From the below graph, we can easily calculate the value of thickness of pavement corresponding the value of Group Index and Traffic Type. In the graph, the darked line represent combined thickness of surface, base, sub base course and dotted line represent the thickness of surface and base.
These method is based upon the physical properties of the soil and does not consider soil strength while calculating the thickness of the pavement.
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Group Index
Group Index is an arbitrary index assigned to the soil types based on percentage fines, liquid limit and plasticity index. It helps in grading of the soils. Its value ranges from 0 to 20. The higher GI value, Poorer the sub grade soil, more the thickness of pavement. Lesser its value, Higher the grade of soil.
Group Index, GI=0.29+0.005ac+0.01bd
a= value of that portion of the material passing through 0.075mm sieve in the excess of 35% but not exceed 75% ( its value should be whole number and number from 0 to 40).
b= value of that portion of the material passing through 0.075mm sieve in excess of 15% but not exceed 55% ( its value should be whole number and ranges from 0 to 40).
c= liquid limit in excess of 40% and not more than 60%. Its value should be whole numbers and ranges from 0 to 20.
d= plasticity index exceeding 10% and not more than 30% . It ranges from 0 to 20.
Traffic Type
Traffic Volume is divided into 3 groups based on the CVPD (Commercial vehicle per day)
Traffic Type CVPD
Light <50
Medium 50-300
Heavy >300
From the below graph, we can easily calculate the value of thickness of pavement corresponding the value of Group Index and Traffic Type. In the graph, the darked line represent combined thickness of surface, base, sub base course and dotted line represent the thickness of surface and base.
(Note: These above picture is only for reference purpose)
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| Group index method chart |
These method is based upon the physical properties of the soil and does not consider soil strength while calculating the thickness of the pavement.
YOU MAY ALSO LIKE:
Sight Distance-Highway Engineering
No-Fines Concrete and its Mix Proportion
How its Made- Plastic Road in India
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