Granular Sub base (GSB) specifications, GSB Grading, layer thickness, Water absorption, Physical requirements (like liquid limit, Plasticity index ,AIV), construction procedure or Rolling , measurement and treatment of GSB as per MORTH 5th revision and Indian standard in Civil engineering
Granular sub-base (GSB)
SUB BASE (GSB)– The full form of GSB is granular sub base.it is being laid in road work because it plays two major roles
1- it works as drainage layer
2- it provides a good sub base to laid base layer on it.
How to calculate the quantity, compaction factor value and loose thickness of GSB? To know more information kindly visit on below link 🔗
The material to be used for the work shall be natural sand, crushed gravel, crushed stone, crushed slag, or combination thereof depending upon the grading required use of material like brick metal, kankar and crushed concrete shall be permitted in the lower sub base. The material shall be free from organic or other deleterious constituents.
Grading III and IV shall be used in lower sub base. Grading V and VI shall be used as sub-base-cum-drainage layer.
Where the sub-base is laid in two layers as upper sub-base and lower sub-base, the thickness of each layer shall not be less than 150mm.
If the water absorption of the aggregates determined as per IS: 2386(part-3) is greater than 2 %, the aggregates shall be tested for wet aggregate impact value (AIV) (IS: 5640). Soft aggregates like kankar, brick ballast and laterite shall also be tested for wet AIV.
Physical requirements of Granular sub base-
CONSTRUCTION OPERATION -
Spreading and compacting- the sub base material of the specified grading shall be mixed homogenously with the help of motor grader .the required water content shall be determined in accordance with IS:2720 (part 8).
Moisture content of the mix shall be checked in accordance with IS: 2720 (part 8) and adjusted so that, at the time of compaction, it is from 1 to 2 % below the OMC.
Immediately spreading the mix, rolling shall be done. If the thickness of the compacted layer does not exceed 100mm, a smooth wheeled roller of 80 to 100 KN weight may be used. For a compacted single layer up to 200mm the compaction shall be done with the help of a vibratory roller of min. 80 to 100KN.Rolling shall commence at the lower edge and proceed towards the upper edge longitudinally for portion having unidirectional cross fall or on super elevation. For carriage way having cross fall on both sides, Rolling shall commence at the edges and progress towards the crown.
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Each pass of the roller shall uniformly overlap not less than one –third of the track made in the preceding pass. The speed of roller shall not exceed 5 KM per hour.
Rolling shall be continued till the density achieved is at least 98% of the max. Dry density for the material determined as per IS: 2720 (part8).
All loose, segregated or other defective areas shall be made good to the full thickness and re-compacted.
Measurement - Granular sub –base shall be measured as finished work in position in cubic metres.
Types of treatment for soil to make suitable for sub-base
(I) Lime treated soil for improved sub-grade/sub-base
(II) Cement treated soil and cement-fly ash treated sub-base/base
To know more about cement treated soil and cement flyash treated sub-base, kindly click on below link 🔗
(III) Crushed cement concrete sub-base
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