Granular Sub Base
Granular Sub-base (GSB) consists of laying and compacting well graded material on prepared sub-grade in accordance with the specifications.: GSB layer is intermediate layer in between Sub-grade and Wet mix macadam. GSB layer is laid in roads and higway works due to its properties .
It acts as a drainage layer for the pavement to avoid excessive wetting and weakening of subgrade.
- Permeability requirements of Granular Sub-base (GSB) is 300m/day as per IRC 37:2012 Flexible Pavement
- Strength wise GSB is more superior as compared to the subgrade.
Material Requirement for Granular Sub-base (GSB)
- River/Natural sand
- Crushed Gravel
- Crushed Stone
- Blast Furnace slag metal
- Brick metal, kankar and crushed concrete is permitted in lower sub-base.
- Granular Sub-base (GSB) materials shall be free from organic or other deleterious constituents
- It shall confirm the grading requirements as per MORTH V Table 400-1
Grading I & II – Filter Layer
Gradng III, IV, V, VI- Drainage Layer
Grading Requirements-Morth V Table 400-1:
|IS Sieve (mm)||Grading I||Grading II||Grading III||Grading IV||Grading V||Grading VI|
Types of Gradation:
- Few points of contact
- Poor interlock (shape dependent)
- High permeability
- Good interlock
- Low permeability
- Only limited sizes
- Good interlock
- Low permeability
Quality Tests and its Frequency as per MORTH V:
|Test Performed||Relevant IS Code||Frequency of Test||Limits|
|Sieve Analysis||IS 2720 (Part4)||1 test per 400 cum||As Per Morth Table 400-1|
|Moisture Content||IS 2720 (Part2)||1 test per 400 cum||-2 to +1% of OMC|
|Aggregate Impact Value||IS 2720 (Part4)||1 test per source or as required||<40%|
|Wet Aggregate Impact Value||IS 5640||As required (If WA > 2%)||<40%|
|Liquid Limit||IS 2720 (Part5)||1 test per 400 cum||<25%|
|Plasticity Index||IS 2720 (Part5)||1 test per 400 cum||<6%|
|MDD & OMC||IS 2720 (Part8)||1 test per 400 cum||–|
|CBR at 98% Dry Density||IS 2720 (Part16)||1 test per 400 cum||>30%|
|Water Absorption||IS 2386 (Part3)||1 test per 400 cum||<2% (if WA>2%, AIV is required)|
|Field Dry Density||IS 2720 (Part28)||1 test per 400 cum||>98% of MDD|
Plant and Machinery Required
GSB Construction Operation
Toe Line Marking:
- The toe line/fixing of pegs to mark the limits of Granular Sub Base(GSB) shall be done before the commencement of work.
- The width of pavement for toe line shall be calculated by referring the drawings with due considerations to formation width plus the offsets and slopes of different layer’s depth the current layer from the formation road level.
Hauling and Dumping:
- The required quantity of GSB shall be calculated and informed to GSB area mate.
- Dumping pattern, such as to ensure uniform spread shall be determined and explained to the dumping mate.
- The GSB material shall be hauled to site by tippers and dumped on the bed in predetermine pattern.
Spreading and Grading:
- 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.
- Control pegs at every 10m along C/L shall be fixed on both sides of the bed in such a manner that they are not disturbed by grader movement. (5m pegs are suggested on curves).
- Required level of pegs shall be marked on the pegs with suitable vertical offset.
- After rough initial spreading, a cotton/nylon string (line dori) shall be held at the marks on the control pegs at both sides.
- String’s height from the bed top is measured at fixed horizontal offset from C/L.
- The grader operator shall be directed according to measurement in the above step to cut and fill at every point.
- Each grader run shall overlap 1/3rd of blade length.
- The moisture content shall be checked for OMC an if it is less than required water may be added by sprinkling and if it is more than required, shall be exposed to sun.
- Calculate the water required,
Water(L)= 1000*MDD*(OMC-NMC) *Volume of GSB
- The moisture content shall be adjusted with allowance for evaporation losses in a manner to achieve +1 % or -2% below OMC.
- The compaction shall be done mechanically using vibratory rollers of 80-100 KN static weight for entire width of pavement.
- The rolling shall commence from lower edge and proceed to upper side with 1/3rd of overlap.
- Generally, 6-8 number of passes are required for compaction of 98% of Lab MDD.
- Each layer shall be tested by Sand replacement method.
- GSB Productivity : 12000 m³/month/team/shift
- Grader Productivity: 90 m³/hour
- Roller Productivity : 80 m³/hr
Above details are assumed for the front availability and material is not a constraint within a 5 Km approach and 8-10 Hr of working.
you can also read Construction Procedure for Roads and highway works (PART-1)