Construction Methodology of Granular Sub base(GSB)

Granular Sub Base (GSB) shall consist of laying and compacting well-graded material on prepared and approved subgrade in accordance with clause 401 of MORT&H specification. In other words, GSB is laid in between the subgrade and the base course of all highway pavements.

The primary functions of GSB are:

  1. Serves as an effective drainage layer to drain off the water entering into the pavement layers.
  2. In flexible pavement, Distributes the wheel load stresses transmitted through the surface course and granular base course. however, in the case of rigid pavement, the GSB layer only serves as an effective drainage layer.
  1. Dumpers
  2. Motor grader
  3. The vibratory roller of capacity 80-100KN
  4. Water tanker
  5. Survey kit

As per clause 401.2 of MORT&H, the material to be used for the construction of the granular sub-base shall satisfy the following requirements.

  1. The maximum particle size of the material shall be less than 75mm.
  2. If the water absorption of the aggregate determined as per 18:2386 (Part – 3) is greater than 2%. The aggregates shall be tested for Wet Aggregate Impact Value(AIV) (IS: 5640). The aggregate impact value should be <40%.
  3. The 10% fines value should be more than 50KN.
  4. Soaked CBR value should be of a minimum of 30%.
  5. Gradation of the material shall be followed as per approved project design/drawings.
  6. The liquid limit shall not be more than 25 per cent and the plastic index should be less than 6.

Fig. 01 – Granular Sub Base

Initially, on a prepared subgrade, a trial run will be made with the mix to determine the loose spread thickness and water content required to produce the requisite compacted thickness of one layer of 200mm or to suite the required thickness.

  1. The base shall be prepared by removing all vegetation and other extraneous matter; if any, lightly sprinkled with water if necessary and rolled with two passes of 80-100KN smooth wheeled roller.
  2. Marking shall be done by fixing the batter pegs on both sides at regular intervals to the length and depth of the approved subgrade.
  3. Well-graded material shall be transported and dumped at the site by means of end dumpers and dumped in the form of heaps/stockpiles.
  4. The material shall be spread by means of the motor grader to the required slope and grade.
  5. Water shall be sprinkled uniformly and care should be maintained that the moisture content at the time of compaction is in the range of -2 to +1 per cent of OMC.
  6. The material shall be processed by means of mechanical disc harrows and rotavators or ploughed until the layer is uniformly wet and thoroughly mixed with water, immediately thereafter rolling is to be taken up with required compacters.
  7. If there is any segregation after spreading, the same shall be rectified by spreading fines before rolling.
  8. The rolling shall be carried out from lower edge to upper edge longitudinally with the 80-100KN vibratory roller and it should be maintained that the speed of the roller should not be more than 5km/hr. Rolling shall commence from edges towards the center when there is a crossfall on both sides.
  9. Rolling shall be continued until there are no tyre impressions marked on the surface and a specified degree of compaction is achieved.
  10. Depressions and high spots shall be rectified by adding or removal of fresh material with proper compaction.

Quality control for the above work shall be as per table 900-3 of MORT&H specification.

  1. The gradation of the material shall be controlled at the crusher plant by making necessary adjustments.
  2. The compacted surface shall be checked for field density at a frequency of 1 test per 1000sqm. The degree of compaction should not be less than 98% when tested as per IS: 2720 (Part – 8).
  3. The surface shall be checked for levels at as per MORT&H intervals longitudinally and transversely and the same shall be within +10/-20mm.

GSB materials collected from the source are subjected to laboratory tests like liquid limits, plasticity index, and gradation to detect deleterious materials. If the requirements are satisfied, the material will be approved.


Aggregates should be produced or crushed in the crusher plant with care, ensuring the flow of dust, noise and blasting materials are minimized.

Each activity and sub-activity in the current segment’s hazards, risk assessment, and risk control measures must be identified, and the necessary action plan must be initiated.

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Also Read: What is Formwork (Centering & Shuttering) in construction?

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  1. May 18, 2021

    […] aggregate and granular material, pre-mixed with water, to a dense mass on prepared and approved granular sub-base in accordance with clause 406 of MORT&H […]

  2. May 20, 2021

    […] the integrity of Granular Sub Base during […]

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