Guidelines for Use of Fly Ash in Road Embankments

IRC SP 58: Key Formulas, Tables & Specifications - Introduction

1. Material Characterization (Clause 3.3.1.8)

• Typical Geotechnical Properties of Fly Ash (Table 1):

2. Compaction Specifications (Clause 4.7.5 & Table 2)

• Layer Thickness:

  • Vibratory rollers (80-100 kN): up to 400 mm (after site trials)
  • Static rollers (80-100 kN): max 200 mm
  • Hand-held tampers (near structures): max 100 mm

• Moisture Content: OMC ± 2% (IS:2720 Part 8)

3. Quality Control (Clause 5)

• Testing Frequency:
  • IS Heavy Compaction Test: 2 tests/3000 m³
  • Moisture

Scope and Application: Key Specifications from IRC SP 58

1. Material Characterization (Clause 3.3.1.8 & Table 1)

• Specific Gravity: 1.90 - 2.55
• Maximum Dry Density (MDD): 0.9 - 1.6 gm/cc
• Optimum Moisture Content (OMC): 18% - 38%
• Shear Strength: Angle of internal friction 30° - 40°, cohesion negligible
• Permeability: 8×10⁻⁶ to 7×10⁻⁴ cm/sec
• Particle Size Distribution: Clay 1-10%, Silt 8-85%, Sand 7-90%, Gravel 0-10%
• Compression Index (Cₐ): 0.05 - 0.4

2. Compaction Requirements (Clause 4.7 & Table 2)

• Layer Thickness:
  • Vibratory rollers (10-15 kN): 100-150 mm
  • Medium rollers (60-100 kN): ~250 mm
  • Heavy rollers (80-100 kN vibratory): up to 400 mm (with trials)
  • Static rollers (80-100 kN): max 200 mm
• Compaction passes: 2 static + 5-8 vibratory passes recommended
• Moisture Content: Within OMC ± 2%, adjusted for weather

3. Construction Practices

• Side soil cover must be compacted simultaneously with core.
• Embankment slopes should not be steeper than drawings; benching required if natural slopes > 1:4.
• Cover soil thickness ≥ 500 mm on top of fly ash embankment.
• Handheld tampers for inaccessible areas with max 100 mm layer thickness.

4. Quality Control (Clause 5)

• Density Tests: Minimum 1 per 1000 m² per layer, IS:2720 (Part 28)-1974.
• Moisture Tests: 1 per 250 m³

Key Design Considerations from IRC SP 58 for Fly Ash Embankments

1. Site Investigations (Clause 3.2)

• Topography, hydrology, and subsoil investigations essential.
• Refer IRC:36-1970 for detailed procedures.

2. Material Characterisation (Clause 3.3)

• Particle size, MDD & OMC (IS Heavy Compaction Test, IS:2720 Part 8).
• Shear strength (triaxial/direct shear tests), compressibility, permeability.
• Typical geotechnical properties (Table 1):

• Fly ash with dry density < 0.9 gm/cc generally unsuitable.
• Liquefaction risk low if compacted properly at OMC.

3. Embankment Geometry & Cover (Clause 3.4)

• Side slopes: 2H:1V (horizontal:vertical).
• Earth cover thickness: 1 m for embankments ≤ 3 m height; increase for higher/flood-prone embankments.
• Cover soil plasticity index: 5-9% (IS:2720 Part 5).
• Minimum compacted dry density for cover soil: 1.75 gm/cc (IS:2720 Part 8).

4. Stability Analysis (Clause 3.4.2 & 3.4.3)

• Use Limit Equilibrium Method (Slip Circle method).
• Factor of Safety (FoS) calculated by comparing resisting and driving moments.
• Simplified Bishop Method recommended.
• Computer programs from IRC approved for design.

5. Compaction & Quality Control (Clauses 4.7 & 5)

• Minimum dry density after compaction: 1.75 gm/cc (Clause 1

Key Specifications & Guidelines for Fly Ash Embankments (IRC SP 58)

Material Characterisation (Clause 3.3.1.8)

• Tests as per IS:2720 for:
  • Particle size distribution
  • Maximum Dry Density (MDD) & Optimum Moisture Content (OMC) by Modified Proctor (IS 2720 Part 8)
  • Shear strength (c, ϕ) by triaxial/direct shear (IS 2720 Part 4)
  • Compressibility & consolidation parameters
  • Permeability and capillarity for drainage design

Typical Geotechnical Properties (Table 1)

Design & Construction Notes

• Fly ash density < 0.9 gm/cc generally unsuitable.
• Compaction to MDD at OMC is critical.
• Liquefaction risk is low if compacted properly; mitigate by drainage and layering.
• Fly ash consolidates quickly with minimal post-construction settlement.
• Follow IRC:36-1970 for site investigations.

Compaction Control

• Use Modified Proctor test curve for optimum moisture and MDD.
• Field compaction should match lab MDD and OMC.

flowchart TD
    A[Site Investigation] --> B[Material Characterisation]
    B --> C[Lab Tests (IS:2720)]
    C --> D[Design Parameters: c, ϕ, MDD, OMC, Cv, Cc]
    D -->

Quality Control for Fly Ash Embankments (IRC SP 58)

Key Specifications & Formulas

• Minimum Dry Density after Compaction:

  • General embankments: ≥ 95% of Maximum Dry Density (MDD) (IS:2720 Part 8-1983)
  • Bridge abutments (embankment length = 1.5 × height): 100% of MDD

• Compaction Layer Thickness:

  • Using vibratory rollers (80-100 kN): up to 400 mm (after site trials)
  • Using static rollers (80-100 kN): max 200 mm
  • Hand-held tampers near structures: max 100 mm

• Moisture Content:

  • Fly ash moisture: OMC ± 2% (IS:2720 Part 8-1983)
  • Cover soil moisture: at OMC

Testing Frequency

Compaction Procedure Summary

• Spread material evenly, control moisture.
• Compact fly ash ASAP after spreading.
• Vibratory roller passes: 2 static + 5-8 vibratory.
• Maintain records of density tests.
• Remove & replace soft areas if density not achieved.

Summary Table: Compaction Specifications

Handling and Transportation of Fly Ash (IRC SP 58 - Clause 4.6)

Key Specifications:

• Transport: Pond ash is transported in covered dumper trucks to minimize moisture loss and dusting.
• Moisture Content: Pond ash usually has sufficient moisture to prevent dusting; excess moisture causing spillage requires periodic inspection and selective lifting from drier zones.
• Stockpiling: Should be avoided if possible. If necessary:
  • Spray water regularly to prevent dusting.
  • Cover stockpiles with tarpaulins or a thin soil/granular layer.
  • Restrict traffic to moist areas to avoid airborne ash.

Important Geotechnical Properties of Pond Ash (Table 1)

Handling Best Practices Summary

• Use covered vehicles to avoid moisture loss.
• Avoid stockpiling; if unavoidable, control dust by water spraying or covering.
• Control traffic on stockpiled ash to minimize dust dispersion.

Diagram: Handling & Transportation Flow

flowchart LR
    A[Fly Ash Pond] --> B[Covered Dumper Truck]
    B --> C[Site Delivery]
    C --> D{

Precautions Against Corrosion (IRC SP 58 - Clause 4.8)

Key Points:

• Sulphate Content: Fly ash sulphate content must comply with limits in Section 3.3.1.8 to avoid sulphate attack on adjacent concrete.
• Protection Measures:
  • Paint adjacent concrete surfaces with bitumen or moisture-proof compounds.
  • Pipes (cast iron, lead, copper, PVC, terra cotta) have minimal corrosion risk except aluminium conduits which are vulnerable.
  • Protect pipes with polythene sheeting, bituminous coating, or embed in inert soil with a minimum cushion thickness of 500 mm.
• Drainage Pipes:
  • Use perforated or partially slotted pipes near seepage zones.
  • Preferred materials: PVC or ABC pipes for durability.
  • Pipes must have adequate wall strength to support embankment loads.
  • Provide filter protection around pipes to prevent internal erosion.

Summary Table: Corrosion Protection Measures

Additional Notes:

• Regular monitoring of sulphate content and moisture is essential.
• Ensure embankment compaction and drainage to minimize corrosion risk.

flowchart TD
    A[Fly Ash Embankment] --> B{Sulphate Content within Limits?}
    B -- Yes --> C[No special corrosion risk]
    B -- No --> D[Apply bituminous coating on concrete]
    D --> E[Protect pipes with polythene or inert soil cushion (≥ 500 mm)]
    A --> F[Seepage Zones]
    F --> G[Install perforated PVC/ABC pipes]
    G --> H[Provide filter protection around pipes]

This ensures long-term durability of embankment and adjacent structures against corrosion.

Key Specifications & Formulas for Finishing Operations (IRC SP 58, Clause 4.7.3 & 4.7.5)

Spreading & Compaction:

• Side soil cover: Provided with core, mechanically compacted progressively; no addition after core construction.
• Spreading: Mechanical means; finishing by motor grader with hydraulic blade control for slope & grade.
• Lift thickness & roller weight:

• Moisture content: Fly ash at OMC ± 2%; cover soil at OMC (IS:2720 Part 8).
• Compaction passes (vibratory roller): 2 passes without vibration + 5-8 passes with vibration.
• Roller specs: Mass per meter width = 2300-2900 kg/m; Frequency = 1800-2200 rpm.
• Layer thickness with handheld tampers: Max 100 mm.

Compaction Density Requirements (Table 2):

Additional Notes:

• Break clods > 50 mm in cover soil.
• Simultaneous laying of cover soil and fly ash before compaction.
• Finished layers must be parallel to final embankment cross-section.
• Maintain uniform construction over full width; bench natural slopes steeper than 1:4 before fill.
• Top capping layer thickness ≥ 500 mm for pavement subgrade.

flowchart TD
    A[Spread fill material mechanically] --> B[Check moisture content (OMC ± 2%)]
    B --> C[Compact layer]
    C -->

IRC SP 58 — Key References, Formulas, and Specifications for Fly Ash Embankments

1. Compaction Specifications (Table 2)

2. Compaction Layer Thickness & Roller Details (Clause 4.7.3 - 4.7.4)

• Layer thickness:
  • Vibratory rollers (10-15 kN): 100-150 mm
  • Medium vibratory rollers (60-100 kN): ~250 mm
  • Heavy vibratory rollers (80-100 kN): up to 400 mm (with site trials)
  • Static rollers (80-100 kN): max 200 mm
• Roller specs: Mass per meter width = 2300-2900 kg/m; frequency = 1800-2200 rpm
• Passes: 2 passes without vibration + 5-8 passes with vibration

3. Moisture Content Control (Clause 4.7.2)

• Maintain moisture near Optimum Moisture Content (OMC) ± 2%
• Avoid over-wetting to prevent liquefaction
• Use sprinklers for uniform water application

4. Quality Control Tests (Clause 5.2 - 5.3)

• IS Heavy Compaction Test (IS:2720 Part 8): 2 tests per 3000 m³ ash
• Moisture Content (IS:2720 Part 2): 1 test per 250 m³ ash
• Field density tests (IS:2720 Part 28): 1 test per 1000 m² compacted area

5. Related Standards

• IS:2720 series (Soil Tests)
• IRC:34, 36, 75, 89, SP:50
• MOST Specifications for Road & Bridge Works (Section 900)

flowchart LR
    A[Fly Ash Embankment Construction] --> B[Layer Spreading]
    B --> C[Moisture Control (OMC ± 2%