Guidelines for the Design of Flexible Pavements for Low-Volume Rural Roads (First Revision)

IRC SP 72: Introduction - Key Formulas, Tables & Specifications

1. Traffic Load Assumptions (Clause 10.2)

• HCV (Heavy Commercial Vehicle) Rear Axle Loads:
  Laden = 10.2 tonnes, Unladen = 5 tonnes
• MCV (Medium Commercial Vehicle) Rear Axle Loads:
  Laden = 6 tonnes, Unladen = 3 tonnes
• Light Vehicles (<3 tonnes) excluded from pavement design.
• HCV proportion: 0-20% of CVPD; MCV >80%.
• 10% of CVPD overloaded by 20%.
• For ADT ≤ 100, HCV = 0.

2. Quick Estimation of CBR (California Bearing Ratio)

• For plastic soils:
  [ \text{CBR} = \frac{75}{(1 + 0.728 \times WPI)} \quad (R^2=0.67) ] where
  [ WPI = P_{0.075} \times PI ]

  • (P_{0.075}): decimal fraction passing 0.075 mm sieve
  • (PI): Plasticity Index (%)

• For non-plastic soils:
  [ \text{CBR} = 28.091 \times (D_{60})^{0.3581} \quad (R^2=0.84) ] where (D_{60}) = grain size diameter at 60% finer (mm).

3. Gravel Base Thickness Design (Clause 6.2)

• Gravel base thickness depends on subgrade class (S1-S5) and traffic category (T1-T3).
• Minimum gravel base thickness: 100 mm with soaked CBR ≥ 80%.
• If gravel base material unavailable, use:
  • WBM Grade-III layer: 75 mm
  • Add remaining thickness to subbase.

4. Conversion Table for Base to Subbase Thickness (Table 4)

IRC SP 72 - The Design Process: Key Points

Though the specific clause text is not provided, the design process in IRC SP 72 typically follows these steps:

1. Traffic Analysis

• Determine Cumulative ESALs (Equivalent Single Axle Loads) over design life.
• Use traffic growth factors and axle load spectra.

2. Subgrade Evaluation

• Assess CBR (California Bearing Ratio) or Resilient Modulus (Mr) of subgrade soil.
• Classify subgrade strength (e.g., weak, medium, strong).

3. Pavement Layer Design

• Select layer thicknesses based on traffic and subgrade.
• Use empirical or mechanistic-empirical design methods.

4. Material Specifications

• Use locally available materials maximizing cost-effectiveness.
• Follow IRC material standards for aggregates, bitumen, etc.

Typical Formula for Design Thickness (Flexible Pavement)

[ D = \frac{Z \times \sigma}{S} ]

Where:

• D = pavement thickness
• Z = load repetitions factor (from traffic)
• σ = allowable stress
• S = soil strength parameter (e.g., CBR)

Reference Table: Typical Subgrade Strength Classification

flowchart TD
    A[Traffic Data] --> B[Calculate ESAL]
    B --> C[Subgrade Evaluation]
    C --> D[Material Selection]
    D --> E[Pavement Thickness Design]
    E --> F[Final Pavement Design]

For detailed design charts and specific formulas, refer to Clause 5: The Design Process (Page 5) and Recommended Pavement Designs (Page 30) in IRC SP 72.

IRC SP 72: Traffic Parameter for Pavement Design

Key Points from IRC SP 72

• Traffic Parameter Definition (Clause 2.2.1):
  Traffic parameter is the cumulative number of commercial vehicle repetitions expected over the design life of the road, based on actual traffic counts during lean and peak seasons.

• Design Traffic Computation (Clause 3.4):

  • Use actual traffic counts on existing roads.
  • Consider seasonal variations (lean and harvesting seasons).
  • Apply growth rate to project traffic over design life.
  • Sum cumulative vehicle repetitions (usually commercial vehicles).

• Vehicle Grouping (Clause 1.3.1):

  • Heavy commercial vehicles (trucks, full-sized buses) and lighter commercial vehicles (tractors, tempos) are grouped together.
  • Laden/unladen/overloaded percentages are not separately considered.

Typical Formula for Cumulative Traffic (N)

[ N = A \times (1 + r)^n \times L ]

Where:

• (N) = cumulative number of commercial vehicles over design life
• (A) = present average daily commercial traffic (vehicles/day)
• (r) = annual growth rate (decimal)
• (n) = number of years (design life)
• (L) = lane distribution factor (if applicable)

Summary Table: Traffic Parameters

flowchart TD
    A[Traffic Counts] --> B[Lean Season Count]
    A --> C[Peak Season Count]
    B & C --> D[Average Daily Traffic (A)]
    D --> E[Apply Growth Rate (r)]
    E --> F[Cumulative Traffic N over Design Life (n)]
    F --> G[Pavement Design]

**Use this

Subgrade Strength Evaluation - IRC SP 72

Key Parameters:

• Subgrade strength is assessed by 4-day soaked CBR (%) values.
• Exceptions: Areas with annual rainfall < 500 mm and very deep water table.

Typical Soaked CBR Values (Table 4.2.1.1)

* Expansive clays (e.g., BC soil) may have CBR < 2%.

Equilibrium Subgrade Moisture Content Estimation (Nomograph Steps):

1. Locate in-situ dry density on axis A-A.
2. Draw perpendicular to intersect water table depth curve → point W.
3. Horizontal from W to vertical axis B-B → point M.
4. Join M to fixed point O, extend to vertical axis C-C → point N.
5. Add soil plasticity index (PI) from N → point R (NR = PI).
6. Join R to average annual rainfall axis D-D → point T.
7. Extend to axis E-E → point U = equilibrium subgrade moisture content (%).

Notes:

• Use IS 2720-Part 40 for Free Swelling Index on expansive clays.
• Moisture content affects CBR; equilibrium moisture content accounts for rainfall, water table depth, and soil plasticity.

flowchart TD
    A[In-situ Dry Density (Scale A-A)] -->|Perpendicular| W[Water Table Depth Curve]
    W -->|Horizontal| M[Vertical Axis B-B]
    M -->|Join to O| N[Vertical Axis C-C]
    N -->|Add Plasticity Index| R[Point R]
    R -->|Join to Rainfall Axis D-D| T[Point T]
    T -->|Extend to Axis E-E| U[Equilibrium Moisture Content]

This procedure ensures realistic subgrade strength values for pavement design under local moisture conditions

Key Points from IRC SP 72 on Pavement Composition & Use of Local Materials

1. Pavement Thickness & Composition (Clause 2.1.3):

• Conduct field survey and lab tests on local materials (including industrial waste).
• Use design traffic parameter and subgrade strength to determine thickness from the Design Catalogue (Para 8).
• Aim to maximize use of local materials by mechanical stabilization or additives (lime, lime-flyash, cement).

2. Categories of Locally Available Materials (Clause 5.2.1):

• Selected granular soil for subgrade.
• Stabilized soils (mechanical, lime, cement, lime-flyash).
• Natural soft aggregates (moorum, kankar, gravel).
• Brick and overburnt brick metal.
• Stone metal.
• Industrial wastes.

3. Design Approach:

• Use suitable blending or stabilization to meet strength requirements.
• Optimize thickness and layer composition economically.

Typical Pavement Layer Thickness Determination (Conceptual)

Stabilization Additives Effect

flowchart TD
    A[Locally Available Materials] --> B[Field Survey & Lab Testing]
    B --> C{Suitability?}
    C -- Yes --> D[Mechanical Stabilization / Additives]
    C -- No --> E[Blending with Other Materials]
    D --> F[Pavement Layer Design]
    E --> F
    F --> G[Thickness & Composition from Design Catalogue]
    G --> H[Construct Pavement]

Summary:
IRC SP 72 emphasizes thorough testing and stabilization

Pavement Design of Gravel/Soil-Aggregate Roads (IRC SP 72)

Key Specifications:

• Traffic Range: Suitable for up to 100,000 ESALs (Equivalent Standard Axle Loads).
• Base Material: Gravel/soil-aggregate as per Clause 402 of MORD Specifications.
• Shoulders: Minimum 100 mm thick compacted subbase quality material; gravelled shoulders (≥1 m wide) recommended for high animal-drawn cart traffic.
• Surface: Non-bitumenised gravel surfacing material must meet MORD Clause 402.
• For >100,000 ESALs: Use crushed stone or soil-cement base with black-topped surface.

Cumulative ESALs for 10-year Design Life (Sample)

ADT = Average Daily Traffic; CVPD = Commercial Vehicles per Day

Important Notes:

• Gravel Base: Must satisfy MORD Clause 402 for gradation and quality.
• Shoulders: Provide lateral support; gravelled shoulders advised for heavy non-motorized traffic.
• Subgrade Evaluation: Essential for thickness design (refer to IRC:SP:72 Clause 15).
• Material Quality: Percent passing IS 0.075 mm in subbase should not exceed 5%.

Summary Formula for ESAL Calculation (Simplified):

[ \text{Cumulative ESAL} = \text{ADT} \times \text{CVPD} \times \text{Load Factor} \times \text{Design Life (days)} ]

where Load Factor depends on vehicle types (HCV/MCV) and their damage equivalence.

flowchart TD
    A[Traffic Data] --> B[Determine ESALs]
    B --> C[Pavement Material Selection]
    C --> D{ESAL ≤ 100,000?}
    D -- Yes --> E[Use

Design of Flexible Pavement for Traffic > 1,00,000 ESAL (IRC SP 72)

Key Specifications:

• Design Life: 10 years.
• Minimum Design Traffic: 1,00,000 ESAL.
• Base Course Thickness: Minimum 150 mm for 1,00,000 to 10,00,000 ESAL.
• Thickness Increments: Multiples of 75 mm or 100 mm for practical construction.
• Frost Areas: Minimum total pavement thickness = 450 mm (300 mm sub-base + 150 mm base).
• For > 20,00,000 ESAL: Refer IRC:37-2012; thick bituminous layers may be replaced by granular layers.

Recommended Pavement Thicknesses (for Gravel Roads, CBR > 5):

Traffic Categories & ESAL Ranges:

Design Catalogue Notes:

• Use Fig. 6 for stabilized soil sub-base and soil cement base thickness.
• Pavements are designed for future strengthening.
• Gravel roads with CBR > 5 can handle up to 1,00,000 ES

IRC SP 72: Recommended Pavement Designs - Key Points

1. Traffic Categories (ESAL Applications)

2. Subgrade Strength Classes (CBR)

3. Thickness Recommendations (mm)

• Gravel Roads:
  • Up to 60,000 ESAL, gravel roads perform well if CBR > 2.
  • If CBR > 5, gravel roads satisfactory up to 1,00,000 ESAL.
• Flexible Pavements:
  • Minimum 150 mm base course for 1,00,000 to 10,00,000 ESAL.
  • Thickness multiples of 75 or 100 mm for practical construction.
  • Minimum 450 mm total thickness in frost-prone areas (300 mm sub-base + 150 mm base).
• Frost Susceptible Areas Gravel Base Thickness (mm):

IRC SP 72: Drainage and Shoulders - Key Points

1. Drainage Design (Clause 9.1)

• Cross-slopes: Provide adequate cross slopes on carriageway and shoulders to shed water quickly.
• Subgrade elevation: Raise top of subgrade/improved subgrade at least 300 mm above ground level (GL) and 600 mm above highest groundwater table (GWT).
• Roadside drainage: Provide properly designed roadside ditches and cross-drainage structures.
• Layer drainage: For low permeability subgrade (e.g., clay), extend at least half the sub-base thickness (minimum 100 mm) across shoulders to improve drainage.
• Sub-base material: Should have less than 5% passing 0.075 mm sieve for good drainage.

2. Shoulders (Clause 9.2)

• Shoulders provide lateral support especially important for unbound pavements.
• Use subbase quality material, compacted to minimum 100 mm thickness.
• For high shoulder traffic (e.g., animal carts), provide gravelled shoulders at least 1 m wide from carriageway edge.
• Where shoulders are not gravelled and traffic is low, grass the shoulder to prevent erosion.
• For black-topped surfaces, shoulder material thickness should be not less than 600 mm.

Summary Table: Shoulder Specifications

Drainage Layer Illustration

graph LR
  A[Surface Layer] --> B[Base Course]
  B --> C[Sub-base Layer]
  C --> D[Subgrade]
  C -.-> E[Extended Sub-base across Shoulders (≥ 50% thickness, min 100 mm)]
  F[Roadside Shoulder] --> E

For detailed pavement thickness and traffic categories, refer to Tables 8 and 9.2 in IRC SP 72 and the

IRC SP 72: Assumptions & Design Parameters (Clause 10.2 & Related)

Vehicle Load Assumptions

• Heavy Commercial Vehicles (HCV):
  • Laden Rear Axle Load = 10.2 tonnes
  • Unladen Rear Axle Load = 5 tonnes
• Medium Commercial Vehicles (MCV):
  • Laden Rear Axle Load = 6 tonnes
  • Unladen Rear Axle Load = 3 tonnes
• Light Vehicles (LV): Gross laden weight < 3 tonnes; excluded from pavement design.

Traffic Composition

• HCV = 0 to 20% of CVPD
• MCV = > 80% of CVPD
• 10% of CVPD are overloaded by 20% of max permissible load.
• For ADT ≤ 100, HCV = 0.
• Laden and unladen vehicles are equal in number.

CBR Estimation Formulas

• For plastic soils: [ \text{CBR} = \frac{75}{1 + 0.728 \times WPI} \quad (R^2 = 0.67) ] where, [ WPI = P_{0.075} \times PI ]

  • (P_{0.075}) = decimal fraction passing 0.075 mm sieve
  • (PI) = Plasticity Index (%)

• For non-plastic soils: [ \text{CBR} = 28.091 \times (D_{60})^{0.3581} \quad (R^2 = 0.84) ]

  • (D_{60}) = grain size diameter at 60% finer (mm)

Base & Subbase Thickness Conversion (Excerpt from Table 4)

• Minimum gravel base thickness = 100 mm
• Gravel base soaked CBR ≥ 80% preferred
• If unavailable, use **