GSB Construction Method – MORTH 401 Guide with Material, CBR & QA (2026)
Role: Structural foundation layer for flexible pavements
GSB Construction Flow
Methodology for Construction of Granular Sub-Base (GSB)
Granular Sub-Base (GSB) is a critical pavement layer constructed above the prepared subgrade to provide structural support, facilitate drainage, and distribute traffic loads to the underlying layers. The construction of GSB shall be carried out in accordance with MoRTH Clause 401, approved drawings, specifications, and project quality requirements.
1. Pre-Construction Requirements
Prior to commencement of GSB works, all technical approvals, resources, equipment, and quality control arrangements shall be verified to ensure smooth execution of the activity.
1.1 Document Review & Approvals
The following documents shall be reviewed and approved before starting construction:
- Approved Construction Drawings
- Approved GSB Gradation / Mix Design
- Approved Quarry Source
- Approved Method Statement
- Quality Assurance Plan (QAP)
- Inspection & Test Plan (ITP)
1.2 Resource Planning
Adequate manpower, equipment, materials, and testing resources shall be arranged before commencement of work.
- Deployment of construction personnel
- Availability of approved materials
- Planning of equipment and machinery
- Survey and quality control arrangements
- Material transportation planning
1.3 Equipment & Testing Instruments
The following equipment and testing instruments shall be available for construction and quality control activities:
- Motor Grader
- Vibratory Roller (10–12 T Capacity)
- Water Tanker
- Tipper Trucks
- Wheel Loader
- Total Station / Auto Level
- Field Density Testing Equipment
- Hand Tools (Hammer, Measuring Tape, String Line, etc.)
Manpower & Responsibilities
Adequate manpower shall be deployed to ensure the successful execution, quality control, and safety of GSB construction activities. The key personnel and their responsibilities are summarized below.
| Personnel | Responsibilities |
|---|---|
| Construction Manager | Overall planning, execution, coordination, resource management, quality compliance, and liaison with the Client and Consultant. |
| Engineer / Supervisor | Day-to-day supervision of construction activities, deployment of resources, quality monitoring, and coordination of inspections. |
| Land Surveyor | Establishment of centerline, reference points, level controls, measurement records, and verification of alignment and levels. |
| Material Engineer | Implementation of QA/QC procedures, material approvals, laboratory and field testing, and maintenance of quality records. |
| Safety Supervisor | Monitoring compliance with HSE requirements, conducting safety inspections, hazard identification, and enforcement of PPE usage. |
| Skilled & Unskilled Helpers | Assistance in material handling, site housekeeping, equipment support, and general construction activities. |
2. Subgrade Readiness Check
Construction of the Granular Sub-Base (GSB) layer shall commence only after the prepared subgrade has been inspected, tested, and formally approved by the Engineer. The subgrade shall conform to the requirements of the approved drawings, technical specifications, and relevant provisions of the MoRTH Specifications.
Before permitting the placement of GSB material, the following quality control checks shall be carried out to ensure that the subgrade is suitable for supporting the overlying pavement layers.
- Formation Level: Verify that the finished formation level is within the specified tolerances and matches the approved longitudinal profile.
- Carriageway Width: Ensure that the required carriageway width, including any additional widening, has been achieved as per the approved drawings.
- Cross Fall/Camber: Confirm that the specified cross fall or camber has been provided to facilitate proper surface drainage.
- Compaction: Check that the subgrade has been compacted to the specified density in accordance with MoRTH requirements and the approved Quality Assurance Plan (QAP).
- Field Density Test: Verify that all field density test results comply with the specified acceptance criteria and have been approved by the Engineer.
- Surface Condition: Ensure that the subgrade surface is firm, even, free from rutting, loose material, standing water, organic matter, and other contaminants.
- Soft Spots and Weak Areas: Identify and rectify all soft spots, yielding areas, depressions, or unstable locations before commencement of GSB work.
- Drainage Arrangement: Confirm that adequate drainage has been provided to prevent water accumulation over the prepared subgrade.
A joint inspection of the completed subgrade shall be carried out by the Contractor, Independent Engineer/Consultant, and Client. GSB laying shall commence only after obtaining formal approval through the Inspection Request (IR) and ensuring that all quality records, field test reports, and survey checks are satisfactory.
3. Setting Out
After obtaining approval of the prepared subgrade, setting out shall be carried out to establish the exact alignment, width, levels, and cross fall of the Granular Sub-Base (GSB) layer. Survey work shall be performed using a Total Station, Auto Level, or other approved surveying instruments by qualified survey personnel.
Accurate setting out is essential to ensure that the GSB layer is constructed in accordance with the approved drawings and specified geometric tolerances.
- Center Line Establishment: Mark the center line of the roadway using approved survey control points.
- Edge Line Marking: Mark the left and right edges of the carriageway to define the limits of GSB construction.
- Reference Pegs: Fix permanent reference pegs outside the construction area to preserve survey control throughout the work.
- Offset Peg Installation: Install offset pegs at suitable intervals to facilitate line and level checking during spreading and compaction.
- Level Verification: Check formation levels, longitudinal profile, and cross fall at specified intervals using an Auto Level or Total Station.
- String Line (Where Required): Provide string lines or grade wires to control finished levels during GSB placement.
All survey records, setting-out calculations, and level sheets shall be verified by the Survey Engineer and approved by the Engineer before commencement of GSB spreading. Any discrepancies in alignment or levels shall be corrected prior to material placement.
4. Material Production and Quality Control
The Granular Sub-Base (GSB) material shall be produced from an approved quarry or crushing plant using naturally occurring gravel, crushed stone, crushed gravel, sand, or a combination of approved granular materials conforming to MoRTH Specifications (Section 400).
Production shall be carried out under controlled conditions to ensure that the material consistently satisfies the specified grading, strength, durability, and cleanliness requirements before dispatch to the construction site.
- Approved Source: Material shall be obtained only from approved quarry sources after inspection and approval by the Engineer.
- Gradation: The combined grading shall comply with the requirements specified in MoRTH Table 400-1 for the approved grading designation.
- Strength Requirement: The Soaked 10% Fines Value shall not be less than 50 kN, unless otherwise specified in the Contract.
- Cleanliness: Material shall be free from clay lumps, organic matter, shale, soft particles, and other deleterious substances.
- Stockpile Management: Approved materials shall be stockpiled separately to prevent contamination and segregation.
- Production Control: Regular sampling and laboratory testing shall be carried out to ensure uniform quality throughout production.
- Approval Before Dispatch: Only materials conforming to the specified requirements shall be transported to the work site.
Routine laboratory and field quality control tests, including grading, Atterberg Limits (where applicable), Aggregate Impact Value, Los Angeles Abrasion Value, Water Absorption, Soaked 10% Fines Value, and other specified tests, shall be carried out in accordance with the approved Quality Assurance Plan (QAP). Details of the testing frequency are provided in Section 9 – Quality Control Tests.
5. Moisture Conditioning
Before spreading and compaction, the moisture content of the GSB material shall be adjusted to achieve the Optimum Moisture Content (OMC) determined from the laboratory compaction test. Proper moisture conditioning is essential for achieving the specified field density and ensuring uniform compaction.
The target moisture content during field compaction shall generally be maintained within OMC ± 2%, unless otherwise specified in the Contract or approved by the Engineer.
- Dry Material: If the moisture content is below OMC, water shall be applied uniformly using a water tanker fitted with a sprinkler system, followed by thorough mixing using a grader or rotavator.
- Wet Material: If the moisture content exceeds the permissible limit, the material shall be aerated, scarified, or spread in thin layers to allow excess moisture to evaporate before compaction.
- Uniform Mixing: Moisture shall be distributed uniformly throughout the full depth of the layer to avoid localized wet or dry pockets.
- Moisture Verification: Field moisture content shall be checked periodically before and during compaction to ensure compliance with the specified limits.
Compaction shall commence only after confirming that the field moisture content is within the specified range. Excessively dry or wet material may result in inadequate density, loss of stability, and premature pavement distress.
6. Transportation of GSB Material
After obtaining laboratory approval, the Granular Sub-Base (GSB) material shall be transported from the approved quarry or crushing plant to the construction site using clean and suitable tipper trucks. Transportation shall be carried out in a manner that preserves the grading, moisture content, and overall quality of the material until it reaches the laying location.
The Contractor shall ensure that transportation operations are properly planned to maintain a continuous supply of material and avoid delays during spreading and compaction.
- Approved Material Only: Only material that has passed all specified quality control tests and has been approved by the Engineer shall be dispatched to the site.
- Clean Transport Vehicles: Tippers shall be clean and free from soil, clay, debris, oil, or any other contaminants before loading the GSB material.
- Prevent Segregation: Loading and unloading shall be carried out carefully to prevent segregation of coarse and fine aggregates during transportation.
- Avoid Contamination: The material shall be protected from contamination by soil, vegetation, construction debris, or other foreign materials throughout transportation and unloading.
- Moisture Preservation: The moisture content shall be maintained as close as possible to the specified Optimum Moisture Content (OMC) to facilitate proper compaction at site.
- Covering of Material: During hot, windy, or long-distance transportation, loaded tippers shall be covered with suitable tarpaulin sheets to minimize moisture loss, dust generation, and material spillage.
- Controlled Unloading: Material shall be unloaded uniformly along the alignment or at designated stockpile locations to facilitate efficient spreading by the motor grader without causing segregation.
- Avoid Overloading: Vehicles shall not be overloaded, and transportation shall comply with statutory axle load limits to prevent damage to existing roads and ensure safe operation.
The Site Engineer shall verify that each load of GSB material originates from the approved source, is free from contamination and segregation, and possesses adequate moisture for placement. Any material found to be contaminated, segregated, or outside the specified moisture range shall be rejected or reconditioned before use.
7. Spreading of Granular Sub-Base (GSB)
After transportation to the work site, the approved Granular Sub-Base (GSB) material shall be deposited in a controlled manner and spread uniformly over the prepared subgrade using a Motor Grader or other approved spreading equipment. The spreading operation shall ensure that the material is evenly distributed over the entire width of the carriageway without causing segregation of coarse and fine particles.
The thickness of the loose layer shall be carefully controlled by considering the anticipated compaction factor, so that the specified compacted thickness is achieved after rolling. Depending on the project specifications and compaction characteristics of the material, the GSB may be constructed in one or more layers.
During spreading, continuous checks shall be carried out to ensure compliance with the approved drawings, line, level, and cross fall.
- Uniform Layer Thickness: Spread the material to the predetermined loose thickness so that the required compacted thickness is achieved after rolling.
- Specified Carriageway Width: Ensure that the full carriageway width, including any widening, is covered without leaving untreated edges.
- Cross Fall/Camber: Maintain the specified cross slope or camber as shown in the approved drawings to facilitate proper drainage.
- Longitudinal Profile: Maintain the required line and level throughout the length of the pavement.
- Uniform Surface: The spread layer shall be free from segregation, depressions, corrugations, ridges, or other surface irregularities.
- Removal of Oversized Material: Any oversized stones, foreign materials, or segregated patches observed during spreading shall be removed and replaced with approved GSB material.
- Moisture Uniformity: Ensure that the moisture content remains within the specified range (generally OMC ± 2%) before commencing compaction.
The Site Engineer shall verify the loose layer thickness, carriageway width, line, level, and cross fall at regular intervals before rolling commences. Any deficiencies in thickness, segregation, or surface profile shall be corrected immediately. Compaction shall not begin until the spread layer has been inspected and approved by the Engineer.
Avoid dumping large heaps of GSB material directly onto the prepared subgrade. Material should be discharged in a controlled manner and spread progressively using the motor grader. This practice minimizes segregation, ensures a uniform layer thickness, and improves the quality of the finished GSB layer.
8. Compaction of Granular Sub-Base (GSB)
Immediately after spreading and final level correction, the Granular Sub-Base (GSB) layer shall be compacted using approved compaction equipment to achieve the specified density, strength, and stability. Compaction shall commence only after confirming that the moisture content of the material is within the specified range and that the spread layer has been inspected and approved by the Engineer.
Rolling shall be carried out using a suitable vibratory roller in accordance with the approved rolling pattern. The objective is to obtain a dense, uniform, and stable layer without causing segregation, displacement, rutting, or crushing of aggregate particles.
- Commencement of Rolling: Begin rolling immediately after moisture conditioning and final grading to prevent loss of moisture.
- Compaction Equipment: Use an approved vibratory roller of suitable capacity capable of achieving the specified field density.
- Rolling Direction: Rolling shall generally be carried out in the longitudinal direction, parallel to the centre line of the road.
- Overlap of Passes: Each successive roller pass shall overlap the preceding pass by at least one-third of the roller drum width to ensure uniform compaction.
- Uniform Rolling Speed: Maintain a slow and uniform rolling speed to avoid displacement or wave formation in the material.
- Correction of Defects: Any segregation, rutting, displacement, loose patches, or surface irregularities observed during rolling shall be corrected immediately before further compaction.
- Edge Compaction: Areas inaccessible to large rollers, including edges and confined locations, shall be compacted using plate compactors or other approved equipment.
- Completion of Compaction: Continue rolling until the specified field density and surface finish are achieved throughout the entire layer.
Unless otherwise specified in the Contract, the compacted GSB layer shall achieve a minimum 98% of the Maximum Dry Density (MDD) determined in accordance with IS:2720 (Part 8) or other approved laboratory compaction method.
9. Rolling Pattern and Compaction Procedure
A systematic rolling pattern shall be followed to achieve uniform compaction over the entire pavement width while maintaining the required line, level, and cross fall. The rolling sequence may be finalized after establishing a trial stretch and approval by the Engineer.
9.1 Rolling on Straight Sections
- Rolling shall commence from both outer edges of the carriageway and gradually progress towards the centre.
- Each successive pass shall overlap the previous pass by at least one-third of the roller width.
- The roller shall travel at a uniform speed, generally not exceeding 5 km/h.
- Rolling shall continue until no further visible movement of the material occurs and the required density is achieved.
9.2 Rolling on Superelevated Curves
- Rolling shall commence from the lower edge of the pavement and progressively move towards the higher edge.
- The rolling sequence shall maintain lateral stability and prevent movement of the material downslope.
- Any lateral displacement, cracking, or segregation shall be rectified immediately before continuing the rolling operation.
9.3 Compaction Equipment
| Equipment | Purpose |
|---|---|
| Vibratory Roller (80–100 kN or approved) | Main compaction of GSB layer |
| Static Roller (where required) | Initial seating and finishing passes |
| Plate Compactor / Walk-behind Roller | Compaction near edges, structures, and inaccessible areas |
The approved rolling pattern shall be maintained throughout construction. The number of roller passes established during the approved trial stretch shall be adopted unless revised with the Engineer's approval.
10. Moisture Control During Compaction
Maintaining the correct moisture content during compaction is essential for achieving the specified field density and ensuring long-term pavement performance. Both insufficient and excessive moisture adversely affect the compaction characteristics of GSB material.
The field moisture content shall generally be maintained within Optimum Moisture Content (OMC) ± 2%, as determined from the laboratory compaction test.
- Low Moisture Content: If the material is drier than the specified limit, clean water shall be uniformly applied using a water tanker fitted with a sprinkler system, followed by thorough mixing.
- High Moisture Content: If the material contains excessive moisture, it shall be scarified, aerated, and allowed to dry before further compaction.
- Uniform Moisture Distribution: Water shall be mixed uniformly throughout the full depth of the layer to eliminate localized wet or dry pockets.
- Field Verification: Moisture content shall be checked periodically during construction to ensure compliance with the specified limits.
- Restriction: Compaction shall not be carried out on excessively dry, saturated, or waterlogged material.
Proper moisture control significantly reduces the number of roller passes required to achieve the specified density, improves layer stability, minimizes aggregate crushing, and enhances the overall quality and durability of the pavement foundation.
11. Quality Control (QC) During GSB Construction
Quality Control (QC) activities shall be carried out throughout the construction of the Granular Sub-Base (GSB) layer to ensure compliance with the approved drawings, MoRTH Specifications, Quality Assurance Plan (QAP), and Inspection & Test Plan (ITP). All laboratory and field tests shall be performed using calibrated equipment by qualified personnel, and the results shall be documented for approval by the Engineer.
11.1 Field Quality Control Tests
The following field and laboratory tests shall be carried out during GSB construction to verify material quality and compaction.
| Test | Purpose |
|---|---|
| Field Density Test (Sand Replacement / Nuclear Density Gauge) | To verify achievement of specified field density. |
| Moisture Content Test | To ensure compaction is carried out within OMC ± 2%. |
| Gradation Analysis | To confirm compliance with MoRTH grading requirements. |
| Thickness Measurement | To verify compliance with the specified compacted layer thickness. |
| Line, Level & Cross Fall | To verify conformity with approved drawings. |
11.2 Frequency of Testing
- Field Density Test shall be carried out as specified in the approved ITP/QAP or at the intervals specified in the Contract.
- Gradation testing shall be performed at least once per day, for every approved source, or whenever there is a noticeable change in material quality.
- Moisture content shall be monitored continuously before and during compaction.
- Thickness, line, level, and cross fall shall be checked at regular intervals during construction.
11.3 Acceptance Criteria
- Field Density ≥ 98% of Maximum Dry Density (MDD) or as specified in the Contract.
- Moisture Content within OMC ± 2%.
- Material grading shall comply with the approved MoRTH grading.
- The compacted surface shall be stable, uniform, and free from segregation, rutting, or loose material.
- Finished line, level, width, and cross fall shall be within the specified construction tolerances.
All test reports shall be reviewed by the QA/QC Engineer before offering the completed GSB layer for inspection. Any non-conforming results shall be rectified and retested prior to proceeding with the next pavement layer.
12. Surface Finish, Final Checks and Approval
After completion of compaction, the finished GSB layer shall be inspected to ensure compliance with the approved drawings, project specifications, and MoRTH requirements before commencement of the subsequent pavement layer.
- The surface shall be smooth, dense, and uniformly compacted.
- No loose particles, segregation, rutting, cracking, corrugation, or depressions shall be present.
- Finished line, level, carriageway width, thickness, and cross fall shall conform to the approved drawings.
- All high spots, low spots, and surface irregularities shall be corrected immediately.
- The completed surface shall provide a firm and stable platform for laying the next pavement layer.
Any defective portion shall be scarified to the required depth, replaced or corrected using approved GSB material, regraded, moisture-conditioned, and recompacted until all specification requirements are satisfied.
The completed GSB layer shall be offered for joint inspection by the Contractor, Consultant, and Engineer. Construction of the subsequent pavement layer shall commence only after obtaining written approval through the Inspection Request (IR).
13. Defect Rectification
Any deficiencies observed during inspection, quality control testing, or construction shall be rectified immediately before proceeding with subsequent pavement layers.
| Defect | Corrective Action |
|---|---|
| Low Density | Scarify, adjust moisture to OMC, and recompact until the specified density is achieved. |
| Segregation | Remove segregated material, replace with approved GSB material, regrade, and recompact. |
| Insufficient Thickness | Add approved material, regrade, and compact to the specified thickness. |
| Excess Thickness | Trim excess material, restore line and cross fall, and recompact. |
| Improper Cross Fall | Scarify the affected area, regrade to the correct profile, and recompact. |
14. Final Survey and Acceptance
Upon completion of the GSB layer, a final survey shall be conducted to verify compliance with the approved drawings and specified construction tolerances.
| Inspection Item | Acceptance Requirement |
|---|---|
| Finished Level | Within contract tolerance (typically ±20 mm or as specified). |
| Layer Thickness | Within specified tolerance. |
| Carriageway Width | As per approved drawings. |
| Cross Fall/Camber | Within specified tolerance. |
| Surface Regularity | Smooth, stable, and free from depressions or undulations. |
15. Layer Approval and Documentation
Before commencing the next pavement layer, the following quality records shall be submitted to the Engineer for review and approval.
- Approved Material Test Reports
- Field Density Test Reports
- Gradation Test Reports
- Moisture Content Records
- Survey Records (Line, Level, Width & Cross Fall)
- Thickness Measurement Records
- Calibration Certificates (where applicable)
- Inspection Request (IR) and Engineer's Approval
16. Practical Site Execution Sequence
The following sequence is typically adopted during construction of the Granular Sub-Base (GSB) layer on highway projects.
- Approval of Prepared Subgrade
- Survey Setting Out
- Approval of GSB Material
- Transportation to Site
- Controlled Dumping of Material
- Spreading by Motor Grader
- Moisture Conditioning (OMC ± 2%)
- Final Grading and Dressing
- Compaction Using Vibratory Roller
- Field Density & Moisture Testing
- Survey Verification (Line, Level, Width & Cross Fall)
- Rectification of Defective Areas
- Final Inspection and Survey
- Consultant/Engineer Approval (IR Clearance)
- Proceed with Wet Mix Macadam (WMM) Construction
Material Property Requirements
The quality and performance of the Granular Sub-Base (GSB) layer largely depend upon the engineering properties of the aggregates used. Therefore, all materials proposed for use in the works shall be sampled, tested, and approved before transportation to the construction site.
The material shall satisfy the minimum physical and engineering property requirements specified below:
| Property | Specified Requirement |
|---|---|
| California Bearing Ratio (CBR), Soaked | ≥ 30% |
| Aggregate Impact Value (AIV) | ≤ 40% |
| Liquid Limit (LL) | ≤ 25% |
| Plasticity Index (PI) | ≤ 6% |
| 10% Fines Value (Soaked) | ≥ 50 kN |
Materials failing to meet the above requirements shall be rejected and removed from the works.
GSB Gradation Requirements
The aggregate grading shall conform to the requirements of MoRTH Table 400-1. The selected grading shall be based on the approved mix design and project requirements. Proper gradation is essential to achieve adequate compaction, stability, load distribution, and drainage characteristics within the GSB layer.
| Sieve Size | Gr-I | Gr-II | Gr-III | Gr-IV | Gr-V | Gr-VI |
|---|---|---|---|---|---|---|
| 75 mm | 100 | - | 100 | - | 100 | - |
| 53 mm | 80–100 | 100 | - | 100 | 80–100 | 100 |
| 26.5 mm | 55–90 | 70–100 | 55–75 | 50–80 | 55–90 | 75–100 |
| 4.75 mm | 25–55 | 40–65 | 10–30 | 15–35 | 25–50 | 30–55 |
| 0.075 mm | <5 | <5 | <5 | <5 | - | 0–3 |
During production, regular sieve analysis shall be carried out to ensure that the material remains within the approved grading envelope. Any deviation from the approved gradation shall be corrected before dispatching the material to the site.
17. Acceptance Criteria
Acceptance of the GSB layer shall be based on compliance with specified requirements for material quality, gradation, moisture content, density, thickness, levels, width, and cross fall. The compacted layer shall achieve not less than 98% MDD unless otherwise specified in the Contract.
Following successful inspection and approval by the Engineer/Consultant, permission shall be granted to proceed with the construction of the overlying pavement layer.
QA/QC TABLE – GRANULAR SUB BASE (GSB)
🔷 Inspection at Source
| Test / Parameter | Standard | Frequency | Acceptance Criteria |
|---|---|---|---|
| Gradation | IS: 2720 (Part 4) | 1 Test / 400 m³ | As per MoRTH Table 400-1 (5th Revision) |
| Atterberg Limits | IS: 2720 (Part 5) | 1 Test / 400 m³ | Max 25% |
| CBR (4 days soaked) | IS: 2720 (Part 16) | As & when required | Min 30% at 98% of MDD |
| Modified Proctor | IS: 2720 (Part 8) | Every Month | Used for MDD & OMC determination |
| Water Absorption | IS: 2386 (Part 3) | As & when required | If >2%, conduct further durability checks |
| Soundness Test | IS: 2386 (Part 5) | As & when required | Max 12% (Na₂SO₄), Max 18% (MgSO₄) |
| Aggregate Impact Value | IS: 2386 (Part 4) / IS: 5640 | As & when required | Max 40% |
| Deleterious Content | IS: 2720 (Part 27) | As & when required | Sodium sulphate ≤ 0.5%, Organic matter ≤ 1.0% |
🔷 In-Process Inspection
| Test / Parameter | Standard | Frequency | Acceptance Criteria |
|---|---|---|---|
| Moisture Content | IS: 2720 (Part 2) | Each & every pit | -2% to +1% of OMC |
| Field Density (Sand Replacement) | IS: 2720 (Part 28) | 0–1000 m² = 1 test (03 pits) | As per specification |
| Field Density (Nuclear Method) | AASHTO T 310 | 0–1000 m² = 2 tests | As per specification |
🔷 Acceptance Criteria
| Parameter | Requirement |
|---|---|
| Compaction Control | Each & every layer |
| Relative Density | As per MoRTH Specification Clause 903.3.1 |
| Finished Level Tolerance | As per project specification |
18. Critical Control Points
The following critical control points shall be strictly monitored during the construction of the Granular Sub-Base (GSB) layer to ensure compliance with quality requirements and prevent construction defects.
- Do not lay GSB on an unapproved subgrade.
- Do not commence compaction without verifying moisture content.
- Prevent segregation during transportation, dumping, and spreading of material.
- Ensure the required field density is achieved before conducting acceptance testing.
- Verify layer thickness, levels, and cross fall before approval.
- Maintain daily survey records, quality control reports, and inspection documentation.
- Rectify all defects immediately before proceeding to the next pavement layer.
- Obtain Engineer/Consultant approval prior to commencement of subsequent works.
Frequently Asked Questions (FAQ)
1. What is the purpose of GSB in highway construction?
Granular Sub-Base (GSB) acts as an intermediate layer between the subgrade and the pavement layers. It distributes wheel loads, improves drainage, minimizes deformation, and provides a stable platform for subsequent construction activities.
2. What are the gradation requirements for GSB as per MoRTH?
MoRTH Section 401 specifies six grading categories (Gr-I to Gr-VI) with sieve-wise percentage passing limits. The selected grading depends on project requirements and approved mix design.
3. What is the minimum compaction requirement for GSB?
GSB is generally required to achieve a minimum field density of 98% of Maximum Dry Density (MDD) determined by the applicable laboratory compaction test, unless otherwise specified in the contract documents.
4. What is the minimum soaked CBR requirement for GSB material?
The soaked California Bearing Ratio (CBR) value for GSB material is generally required to be 30% or greater, subject to project specifications.
5. Why is moisture control important during GSB construction?
Maintaining moisture content near the Optimum Moisture Content (OMC) helps achieve maximum compaction and ensures the required density and structural performance of the GSB layer.
6. What are the common quality control tests conducted on GSB?
Common tests include sieve analysis, moisture content determination, field density testing, Atterberg limits, Aggregate Impact Value (AIV), Water Absorption, and California Bearing Ratio (CBR) testing.
7. Can this methodology be used for site inspections and QA/QC activities?
Yes. This methodology provides practical guidance for site engineers, highway engineers, consultants, and QA/QC personnel involved in inspection, testing, and acceptance of GSB works.
