Granular Sub Base(GSB) is a natural or designed construction material used as a sub-base layer for road construction. Granular Sub Base is a layer in the road foundation just above the compacted sub-grade layer. GSB or granular sub base prevents capillary water from rising; its particle size is so designed that the capillary action stops and cannot go beyond the GSB layer. Secondly, it works like a drainage layer where water can pass without damaging other road layers.

Granular Sub Base material

Also, Read: Construction of Mastic Asphalt for Roads and bridges

Equipment and Machinery for Granular Sub Base:

The following sets of equipment are necessary for the GSB construction work.

  • Grader/Loader/Excavator as required 
  • Vibratory Rollers (80 -100 KN static weight)
  • Water Tanker (10,000 Litres) 
  • Tippers/Dumpers -10T /20T (as per site requirement)

GSB Material:

The GSB material shall be crushed stone aggregate free from organic and other deleterious constituents or natural river bed material having proper gradation. 

  • It shall conform to grading-I of table 400-1 of MORTH specification, with the percentage passing 0.075mm size restricted to 5%. 
  • The material shall have four days of soaked CBR of a minimum of 30%.

Also, Read: Subgrade – Steps for Preparation of Subgrade

GSB material testing

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Physical properties of GSB Material:

  • The portion of the total aggregate passing through a 4.75mm sieve shall have a sand equivalent value of not less than fifty when tested following the requirement of IS 2720 (Part-37). 
  • A mix of different sizes of crushed aggregates from approved sources shall be so proportioned to achieve the specified grading.
  • The Proportioning shall be done by ascertaining the proper gradation of the individual ingredients and the blend determined by trial and error method to achieve the gradation specified. 
  • MDD & OMC shall be established for the material blend, and it will be ensured before the actual execution of that material used in GSB layer has a CBR value of 30% or more when compacted and finished.
  • In case of variation of gradation in the course of work, the proportion shall be suitably modified, and the entire required test shall be carried out per relevant specification.
  • The material shall be blended at source/crusher to achieve the specified gradation and shall be jointly checked at the site for conformance to gradation and other tests as defined in section 900 of MORTH.

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IS Sieve Designation (mm)Percent by weight passing the IS sieve Grading-I
5380 – 100
26.555 – 90
9.535 – 65
2.3620 – 40
0.42510- 25
0.0753 – 10

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Granular Sub Base construction

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Granular Sub Base Construction Method:

Spreading of GSB Layer:

  • Mark the outside lines of the GSB layer with lime powder or pegs on the completed subgrade.
  • Requisite sub-surface drain and drain outlet at the level of sub-grade/sub-base shall be completed before GSB construction.
  • Before the sub-base is laid, the existing subgrade will be finished by removing all vegetation and other Sub-extraneous matter, lightly sprinkled with water if necessary and rolled with two passes of 80 -100 KN smooth wheeled roller.
  • After loading material with the help of a loader on the dumpers, transported it to the GSB location.
  • The required material shall be dumped between the limiting lines previously marked with the help of lime powder.
  • The material of the GSB layer confirming Table 400.1 shall be spread in layers of uniform thickness of 200 mm compacted thickness for new flexible pavement, and 150 mm compacted thickness for Service Roads and rigid pavement. A grader shall be used for this activity.
  • The moisture content of the material should have OMC +1.0% or –2.0%; if not, either a mixing or drying process must be adopted. Water can be added to raise the moisture content to the required limits. After spraying water with the browser, the material shall be properly mixed to obtain a homogeneous mix. The mixing can continue with the help of a grader. The grader shall then carry out the final well-defined grading.
GSB Layer

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Compaction of GSB Layer:

  • Compaction of Granular Sub Base shall start immediately after achieving the required moisture content.
  • The compaction shall be done with a vibratory compactor. 
  • The compaction pattern, including the number of passes required, shall be finalized after the full-scale experiment at the site to achieve 98% of MDD determined as per IS: 2720 (Part -8). The general pattern shall be as follows:
    • Initial rolling: Two static passes with a Vibratory roller
    • Subsequent rolling: Four vibratory pass
  • One roller pass shall include both forward and reverse movement of the roller. The speed of the roller shall not exceed 5.0 Km /Hour.
  • The compaction shall commence from the lower edge and move to the upper edge width by width.
  • Quality control tests shall be carried out prior to the commencement of the next layer.
  • The rain cuts shall be repaired before placing the drainage layer.
  • The compaction behind the structure shall be accomplished with a vibratory roller or plate compactors to achieve 98% of MDD.
  • The surface of GSB layer shall have a suitable cross fall to enable efficient surface drainage.
  • The finished level GSB shall be within the tolerance limits specified in Table 900.1, i.e. +10.0mm to – 20.0mm.
GSB in road

Also, Read: Dense Bituminous Macadam (DBM) Preparation and Laying Methodology

Quality Control Tests for Granular Sub Base:

  • The Quality control tests shall be conducted on the GSB material at the source before transportation.
  •  At the GSB location, after the completion of the compaction operation following tests are pertinent.
Sl noTestCodeFrequency
1GradationTable 400.11 test per 200 cum.
2Atterberg limitsIS:2720 (Part-5)1 test per 200 cum.
3Moisture contentIS:2720 (Part-2)1 test per 250 cum.
4DensityIS:2720 (Part-8)1 test per 500 sqm
5Deliterious contentIS:2720 (Part-27)As required
6CBRIS:2720 (Part-16)As required

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Granular Sub Base Construction and Quality Control
Article Name
Granular Sub Base Construction and Quality Control
Granular Sub Base(GSB) is natural or designed construction material, used for road construction as a sub-base layer. Granular Sub Base is a layer in road foundation just above the compacted sub-grade layer.
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Construction Civil
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