Granular Sub-Base (GSB) Construction, QC, Tests & Calculations β MoRTH 401 GSB Construction Method β MORTH 401 Guide with Material, CBR & QA (2026)
MoRTH Reference: Section 400 β Clause 401 (5th Revision)
Role: Structural foundation layer for flexible pavements
π£οΈ GSB Layer Construction Flow Chart
Site Clearance & Preparation
β
Subgrade Preparation & Approval
β
Setting Out & Level Marking
β
Material Approval (GSB)
β
Spreading of GSB Material
β
Water Sprinkling
β
Compaction by Roller
β
Field Density Test (FDT)
β
Level & Thickness Check
β
Rectification (if required)
β
Final Approval of GSB Layer
1. Scope of Granular Sub-Base
Granular Sub-Base (GSB) is the foundation of durable highways. This guide provides complete MORTH 401 specifications, material selection, gradation limits, CBR & compaction criteria, layer thickness, and QA procedures. Perfect for site engineers and QA/QC teams, it ensures stable pavements, accurate construction, and long-lasting performance.
2. Materials & Physical Requirements
- Crushed stone, gravel, moorum, sand or approved combinations
- Free from organic matter, clay lumps, and deleterious materials
3. Equipment
- Front Loader
- Tipper Trucks
- Motor Grader
- Water Tanker
- Vibratory Roller (8β10 Ton)
- Auto Level / Total Station
4. Tools & Tackles
- Hammer
- Crowbar
- Spade
- Line Rope
- Pegs
- Lime Powder
- Measuring Tape
5. Manpower & Responsibilities
- Construction Manager
- Engineer / Supervisor
- Land Surveyor
- Material Engineer
- Safety Supervisor
- Helpers
Construction Manager
Responsible for overall planning, execution, and resource management. Ensures compliance with safety and quality requirements, conducts coordination with the Project Manager and Consultant Engineer, and ensures all tests and records are maintained as per contract.
Engineer / Supervisor
Responsible for execution of work as per drawings and specifications. Ensures deployment of resources, maintains quality standards, enforces safety practices, and coordinates with consultant staff for approvals.
Land Surveyor
Responsible for establishing reference points, alignment, and level control. Records levels before and after compaction and assists in joint inspections with consultant staff.
Material Engineer
Implements QA/QC procedures, ensures materials meet specification requirements, conducts laboratory and field tests, and maintains all quality records.
Safety Supervisor
Ensures all activities are carried out safely as per approved project safety plan and monitors compliance with EHS requirements.
6. Materials
- Crushed stone / crushed gravel or combination of materials
- Material shall conform to MoRTH specifications
- 10% Fines Value β₯ 50 kN (soaked condition)
- Material shall meet required grading (Table 400-1)
All materials shall be tested and approved before use in the work.
| Property | Requirement |
|---|
| CBR (Soaked) | β₯ 30% |
| Aggregate Impact Value | β€ 40% |
| Liquid Limit | β€ 25% |
| Plasticity Index | β€ 6% |
7. GSB Gradation Requirements (MoRTH Table 400-1)
| Sieve | Gr-1 | Gr-2 | Gr-3 | Gr-4 | Gr-5 | Gr-6 |
|---|
| 75 mm | 100 | - | 100 | - | 100 | - |
| 53 mm | 80β100 | 100 | - | 100 | 80β100 | 100 |
| 26.5 mm | 55β90 | 70β100 | 55β75 | 50β80 | 55β90 | 75β100 |
| 4.75 mm | 25β55 | 40β65 | 10β30 | 15β35 | 25β50 | 30β55 |
| 0.075 mm | <5 | <5 | <5 | <5 | - | 0β3 |
8. Detailed Construction Procedure
- Preparation of Subgrade:
GSB shall be laid only after approval of the prepared subgrade layer in accordance with Clause 305. The surface shall be clean, free from loose material, and properly compacted to specified density. Any soft spots or irregularities shall be rectified before commencement.
- Setting Out:
Toe lines, centerline, and levels shall be marked using pegs, string line, and lime powder. Alignment, width, and levels shall be verified and approved prior to laying.
- Material Placement:
Calculated quantity of approved GSB material shall be transported and deposited uniformly over the prepared surface. Care shall be taken to avoid segregation during unloading and handling.
- Spreading of Material:
Material shall be spread in uniform layers using a motor grader to the required loose thickness. Any segregation observed shall be corrected immediately by mixing using grader blade.
- Quality Check β Pre-Compaction:
Gradation of material shall be checked at QC laboratory before laying. Visual inspection shall ensure uniformity of mix and absence of oversized or segregated particles.
- Moisture Conditioning:
Water shall be sprinkled uniformly using water tanker to achieve Optimum Moisture Content (OMC). Moisture content shall be maintained within +1% to -2% of OMC for effective compaction.
- Final Grading:
The layer shall be graded carefully to achieve required camber, cross slope, and longitudinal profile. Surface shall be smooth, even, and free from segregation.
- Compaction:
Compaction shall be carried out using a vibratory roller of 8β10 ton capacity. Rolling shall start from the lower edge and progress towards the upper edge for cambered sections.
- Rolling Pattern:
Rolling shall be carried out longitudinally with each pass overlapping at least one-third of the roller drum width to ensure uniform compaction.
- Trial Stretch:
A trial stretch shall be executed to determine the optimum number of passes, roller type, and moisture content required to achieve specified density.
- Correction During Rolling:
All depressions, high spots, and segregated areas shall be identified and corrected during initial rolling itself by adding/removing material and re-compacting.
- Camber & Level Check:
Camber, cross slope, and levels shall be checked using auto level/total station and corrected before final rolling.
- Final Rolling:
A final smooth (static) pass shall be applied over the entire stretch to achieve uniform surface finish.
- Rolling Speed Control:
Speed of roller shall be maintained uniform and shall not exceed 5 km/hr to ensure proper compaction.
- Field Density Testing:
After completion of compaction, field density tests shall be conducted using sand replacement method or nuclear gauge to verify degree of compaction.
- Level Recording:
Finished levels shall be checked and recorded. Any deviation beyond permissible limits shall be corrected before proceeding to next layer.
- Approval:
The completed GSB layer shall be inspected jointly with the Engineer/Consultant and approved before laying subsequent pavement layers.
Note: No subsequent layer shall be laid without approval of the finished GSB layer.
9. Acceptance Criteria
QA/QC Table - GSBQA/QC TABLE β GRANULAR SUB BASE (GSB)
π· Inspection at Source
| Test / Parameter | Standard | Frequency | Acceptance Criteria |
|---|
| Gradation | IS: 2720 (Part 4) | 1 Test / 400 mΒ³ | As per MoRTH Table 400-1 (5th Revision) |
| Atterberg Limits | IS: 2720 (Part 5) | 1 Test / 400 mΒ³ | 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% (NaβSOβ), Max 18% (MgSOβ) |
| 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 β€ 0.5%, Organic matter β€ 1.0% |
π· 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) | 0β1000 mΒ² = 1 test (03 pits) | As per specification |
| Field Density (Nuclear Method) | AASHTO T 310 | 0β1000 mΒ² = 2 tests | As per specification |
π· 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 |
Routine testing shall be carried out as per Clause 900 of MoRTH Specifications.
11. Inspection Plan & Testing
| S.No | Activity | Inspection Required | Tests Required | Remarks |
|---|
| 1 | Approval of Subgrade Top Layer | Yes | Yes | Field density test and levels jointly recorded |
| 2 | Spreading / Fill Layers | Yes | No | Method control |
| 3 | Final Layer | Yes | Yes | Joint survey and record |
| 4 | Dumping Area | Yes | --- | Top soil disposal at instructed locations |
| 5 | Density Checks | Yes | Yes | After completion of compaction |
GSB ITP (Inspection & Test Plan) with Hold & Witness Points | MoRTH 401 Highway QA/QCInspection & Test Plan (ITP) for GSB Works
Granular Sub-Base (GSB) β Hold & Witness Points as per MoRTH 401 / IRC Standards
Scope: This ITP covers material approval, subgrade preparation, spreading, compaction, testing, and final acceptance of Granular Sub-Base (GSB) layer in highway construction projects.
1. Material Approval & Testing
| Activity | Inspection Requirement | Reference | Responsibility | Type |
|---|
| Source approval of GSB material | Quarry / borrow area approval | MoRTH 401 | Engineer | H |
| Gradation test | Compliance with Table 400-1 | IS 2386 | QA/QC | H |
| Atterberg limits | Plasticity Index check | MoRTH Specs | QA/QC | H |
| CBR test | Minimum strength requirement | IRC 37 | QA/QC | H |
| Stockpile inspection | Clean and segregated material | QA Plan | Engineer | W |
2. Subgrade Preparation
| Activity | Inspection Requirement | Reference | Responsibility | Type |
|---|
| Subgrade compaction check | Achieved required density | MoRTH 300 | Engineer | H |
| Proof rolling | No deformation or rutting | IRC Guidelines | Engineer | W |
| RFI clearance | Approval before GSB laying | QA System | Engineer | H |
3. Spreading of GSB Material
| Activity | Inspection Requirement | Reference | Responsibility | Type |
|---|
| Layer thickness check | As per design drawings | GFC Drawings | Engineer | H |
| Moisture content | Within OMC range | IS 2720 | QA/QC | W |
| Uniform spreading | No segregation | MoRTH 401 | Engineer | W |
4. Compaction Works
| Activity | Inspection Requirement | Reference | Responsibility | Type |
|---|
| Rolling sequence approval | Vibratory + static rolling | QA Plan | Engineer | H |
| Field density test (FDT) | β₯ 98% MDD | IS 2720 (Part 28) | QA/QC | H |
| Moisture adjustment | Water control before rolling | Specs | Engineer | W |
5. Final Inspection & Acceptance
| Activity | Inspection Requirement | Reference | Responsibility | Type |
|---|
| Surface finish check | Uniform and compact layer | MoRTH 401 | Engineer | H |
| Level verification | Tolerance compliance | GFC Drawings | Engineer | H |
| Approval for next layer | Clearance for WMM / DLC | QA System | Engineer | H |
Key References: MoRTH Section 401, IRC 37, IS 2720, IS 2386
Purpose: Quality control of Granular Sub-Base layer in highway pavement construction.
12. Safety Measures
- All safety measures shall be followed as per approved EHS Plan.
- Personal Protective Equipment (PPE) is mandatory.
- Proper traffic management shall be ensured at site.
- All equipment shall be inspected before operation.
- Safe working practices shall be maintained at all times.
- Field Dry Density shall be β₯ 98% of Maximum Dry Density (MDD).
- Surface level tolerance shall be +10 mm / -20 mm.
- Top width shall be within +25 mm of the specified drawing.
All works shall comply with MoRTH specifications and approved drawings.
13. Test Calculations
Field Density Calculation (Example)
- Weight of excavated soil = 3.80 kg
- Volume of hole = 0.0024 mΒ³
- MDD = 1.95 g/cc
Bulk Density = 1.58 g/cc
Degree of Compaction = 81% (Rejected)
14. Surface Finish & Tolerances
| Item | Tolerance |
|---|
| Longitudinal Level | Β±20 mm |
| Cross Slope | Β±0.3% |
| Surface Evenness | β€12 mm (3 m straight edge) |
| Layer Thickness | β€200 mm |
15. Engineerβs Inspection Checklist
- β Approved material source
- β Gradation within limits
- β Moisture within OMC range
- β 98% MDD achieved
- β Surface tolerances verified
16. Conclusion
Proper gradation, moisture control, compaction, and systematic quality control testing
are essential for a durable Granular Sub-Base. Compliance with MoRTH 401 ensures long-term
pavement performance and structural stability.
Frequently Asked Questions (FAQ)
1. What is the purpose of GSB in highway construction?
Granular Sub-Base (GSB) provides load distribution, drainage, and a stable foundation between the subgrade and base layers. It improves pavement performance and increases the overall lifespan of the highway.
2. What are the gradation limits for GSB as per MORTH 401?
Gradation limits are defined in MORTH Clause 401 with sieve-wise percentage passing requirements from 75 mm down to 0.075 mm. Proper gradation ensures strength, stability, and adequate drainage.
3. What compaction and CBR criteria should be met for GSB?
GSB must typically achieve a minimum of 98% of Modified Proctor Maximum Dry Density (MDD), and the soaked CBR value should generally be β₯ 30% as per project specifications.
4. Can I use this guide for site inspections?
Yes. This guide is structured for field engineers and QA/QC professionals, making it suitable for practical site inspections and compliance verification.
5. Is this content updated for 2026 standards?
Yes. The information reflects current MORTH specifications and updated quality control practices relevant for 2026 highway projects.
