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. GSB or granular sub-base prevents capillary water from rising; its particle size is so designed that the capillary action stops and cant go beyond GSB layer. Secondly, it works like a drainage layer where water can pass without damaging other road layers.
Equipment and Machinery for Granular Sub Base:
The following sets of equipment are necessary for the for GSB construction work.
- Grader /Loader/Excavator
- Vibratory Rollers (80 -100 KN static weight)
- Water Tanker (10,000 Litres)
- Tippers / Dumpers -10T /20T (as per site requirement)
The 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 soaked CBR of a minimum of 30%.
Physical properties of Granular Sub Base Material:
The portion of the total aggregate passing through 4.75mm sieve shall have a sand equivalent value of not less than fifty when tested in accordance with the requirement of IS 2720 (Part-37).
|IS Sieve Designation (mm)||Percent by weight passing the IS sieve Grading-I|
|53||80 – 100|
|26.5||55 – 90|
|9.5||35 – 65|
|2.36||20 – 40|
|0.075||3 – 10|
A mix of different sizes of Crushed aggregates from approved sources shall be so proportioned to achieve the grading specified.
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 so blended. It will be ensured before the actual execution that material used in GSB 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 in accordance with relevant specification.
The material shall be blended at source/crusher to achieve the specified gradation. This shall be jointly checked at the site for conformance to gradation and other tests as defined in section 900 of MORTH.
Granular Sub Base Construction Method:
- Mark the lines of GSB 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 the commencement of GSB construction.
- Now prior to the laying of sub-base, the subgrade already finished shall be made 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.
- The material shall be loaded with the help of loader on the dumpers for transporting to the GSB location.
- The required material shall be dumped between the limiting lines 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,150 mm compacted thickness for Service Roads and rigid pavement at Toll Plazas. Grader shall be used for this activity.
- The moisture content of the material should have of OMC +1.0% or –2.0%, if not, either mixing or drying process must be adopted. Water can be added to raise the moisture content up to the required limits. After spraying water with the help of the browser, the material shall be properly mixed to obtain a homogeneous mix. The mixing can continue with the help of grader. The grader shall then carry out final well-defined grading.
- Compaction of GSB shall start immediately after achieving the required moisture content. The compaction shall be done with a vibratory compactor. The compaction pattern, including no 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:
a) Initial rolling -2 static passes with Vibratory roller
b) Subsequent rolling – 4 vibratory pass
Note: One roller pass shall include both forward and reverse movement of the roller). The speed of the roller shall not topping 5.0 Km /Hour.
- The compaction shall commence from the lower edge and shall move to 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 structure shall be accomplished with the use of vibratory roller or plate compactors to achieve 98% if MDD.
- The surface of GSB 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.
Quality Control Tests for Granular Sub Base:
The Quality control tests shall be conducted on the material at the source prior transportation and at the GSB location after the completion of compaction operation. Following tests are pertinent to GSB.
|1||Gradation||Table 400.1||1 test per 200 cum.|
|2||Atterberg limits||IS:2720 (Part-5)||1 test per 200 cum.|
|3||Moisture content||IS:2720 (Part-2)||1 test per 250 cum.|
|4||Density||IS:2720 (Part-8)||1 test per 500 sqm|
|5||Deliterious content||IS:2720 (Part-27)||As required|
|6||CBR||IS:2720 (Part-16)||As required|