Granular Sub-Base (GSB) Construction Manual

Granular Sub-Base (GSB) Construction Methodology

Quality Assurance, Material Evaluation & Field Placement Specifications

1. Executive Summary & Structural Function

The Granular Sub-Base (GSB) layer plays a vital role in flexible pavement structures by distributing stress from upper bound layers to the underlying subgrade, providing a drainage medium to prevent water logging, and protecting the subgrade from frost action. Quality control requires rigorous source approval, precise job mix design blending, strict moisture control, and compaction verification to achieve statutory field density limits.

flowchart TD %% --- PRE-CONSTRUCTION STAGE --- A(["Start"]) --> B["Identification of Source by Contractor"] B --> C["Collection of Sample (Quarry Area)"] C --> D["To be witnessed by CSC"] D --> E["Test for Sieve Analysis, Water Absorption, Flakiness & Elongation Index, AIV, Sand Equivalent"] E --> F{"Meets Specification?"} F -- No --> G["Reject the Source"] F -- Yes --> H["Mix Design\n(Sieve Analysis, Blending Proportion as per Grading, At MDD & Soaked CBR)"] H --> I{"Mix Design OK?"} I -- No --> G I -- Yes --> J["Approve the Source for Usage"] J --> K["Collection of Sample from Approved Stockpile"] K --> L["Mixing at Yard Site using Mechanical Equipment"] L --> M["Field Moisture Test"] M --> N["Spreading to required Grade & Watering"] N --> O["Rolling at +1% to -2% below OMC (minimum 8 passes)"] O --> P["Check for Completion of Earthwork (Scarify if needed)"] P --> Q["Field Density Test"] Q --> R{"Meets Required Density?"} R -- No --> S["Scarify, Rewater & Re-roll"] S --> Q R -- Yes --> T["Approved for Next Layer"] T --> U(["End"]) %% --- Alternative Path if Fails in Testing Phase --- J --> V["Send I.T.D. Sample to Lab for Testing"] V --> W{"Test Results OK?"} W -- No --> X["Suggest Suitable Remedy (Screening, Blending, or Stabilization)"] X --> K W -- Yes --> K %% --- Style Definitions --- classDef process fill:#e7f0fd,stroke:#4682b4,stroke-width:1px; classDef decision fill:#fff5b7,stroke:#b8860b,stroke-width:1px; classDef fail fill:#fcd5ce,stroke:#d22b2b,stroke-width:1px; classDef pass fill:#d4f8e8,stroke:#2e8b57,stroke-width:1px; class A,B,C,D,E,H,J,K,L,M,N,O,P,Q,T,U,V process; class F,I,R,W decision; class G,X,S fail;

2. Sequential Execution Guidelines

Phase 1: Source Identification & Laboratory Approval

The contractor collects aggregate samples from prospective quarry sources in the presence of the Construction Supervision Consultant (CSC). Physical testing encompasses:

  • Grain Size & Shape: Sieve Analysis, Flakiness Index, and Elongation Index.
  • Durability & Strength: Aggregate Impact Value (AIV), Water Absorption, and Sand Equivalent value.
  • Mix Design: Proportioning of aggregates to conform to designated grading envelopes while achieving target Maximum Dry Density (MDD) and soaked 4-day California Bearing Ratio (CBR).

Phase 2: Plant Mixing & Moisture Control

Aggregates are mechanically blended at a central batching/mixing plant or yard to prevent segregation. Moisture content is evaluated to ensure laying occurs within the target range of +1% to -2% of Optimum Moisture Content (OMC).

Phase 3: Spreading, Rolling & Compaction Checks

The material is spread uniformly over the prepared subgrade using motor graders or mechanical pavers. Compaction is executed with heavy smooth-wheel vibratory rollers for a minimum of 8 passes.

Field Density Verification Loop: Field dry density (FDD) is tested following compaction. If the layer fails density requirements (typically <98% MDD), it must be scarified, re-watered, mixed, and re-rolled until acceptance criteria are satisfied.

Phase 4: Periodic Testing & Remediation

Independent Testing & Stockpile Remediation: Independent Third-Party (I.T.D.) routine samples are tested periodically. If routine stockpile samples fail specifications, suitable remedies such as mechanical screening, additional aggregate blending, or soil stabilization must be applied before processing continues.

3. Quality Control & Testing Specifications

Quality Assurance Parameter Standard Acceptance Limit Testing Standard / Frequency
Aggregate Impact Value (AIV) ≤ 40% (Sub-base usage) IS: 2386 (Part 4) / 1 per source
Flakiness & Elongation Index Combined total ≤ 35% IS: 2386 (Part 1) / 1 per 500 m³
Water Absorption ≤ 2.0% IS: 2386 (Part 3) / 1 per source
Sand Equivalent Value ≥ 50 IS: 2720 (Part 37) / Routine check
Moisture Content during Laying OMC -2% to +1% Field Moisture Test per lift
Minimum Field Compaction ≥ 98% Maximum Dry Density (MDD) Sand Replacement Method / 1 per 1,000 m²
Minimum Soaked CBR As per design (typically ≥ 20% to 30%) IS: 2720 (Part 16) / 1 per 2,000 m³