Report Containing Recommendations of IRC Regional Workshops on Rural Road Development
1984 Edition

IRC SP 26: Overview - Essential Formulas & Tables for Subgrade Strength Evaluation (CBR)

Subgrade Moisture Conditions (Clause 1.5)

• Condition 1: Water table more than 1.5 m below subgrade

  • Subgrade assumed at equilibrium moisture level (approximately optimum if not known)

• Condition 2: Water table at or less than 1.5 m below subgrade

  • Subgrade considered fully saturated (after 4 days of soaking)

Calculating Strength Index (CBR) (Table 4)

(a) Condition 1: Equilibrium Moisture Content

(b) Condition 2: Soaked Moisture Content

IRC SP 26 aggregates vital recommendations from four regional workshops focused on rural road development across India. Below is a condensed overview of Section 2 (starting page 9):

Principal Recommendations (Section 2)

• Planning: Advocates inclusive planning engaging local stakeholders and utilizing regional resources.
• Geometric Design: Recommends a minimum carriageway width of 3.5 m for single-lane rural roads; alignment should minimize earthwork requirements.
• Pavement Design: Supports flexible pavement designs incorporating effective drainage; typical thickness ranges from 150 mm to 300 mm depending on traffic.
• Material Usage: Encourages use of locally sourced materials with proper quality checks; granular sub-base and base layers are preferred.
• Cross-Drainage: Design culverts and drainage systems to accommodate peak water flows, preventing water accumulation.
• Construction & Quality Assurance: Emphasizes layer-wise construction with standard compaction and continuous supervision.
• Maintenance: Suggests routine and periodic maintenance plans to extend pavement lifespan.

Indicative Pavement Thickness Table

For detailed charts and design specifications, see Sections 3.2 and 3.3.

flowchart LR
    Planning --> Geometric_Design
    Geometric_Design --> Pavement_Design
    Pavement_Design --> Pavement_Materials
    Pavement_Materials --> Cross_Drainage
    Cross_Drainage --> Construction_QC
    Construction_QC --> Maintenance

This diagram illustrates the integrated methodology advised for rural road projects under IRC SP 26.

IRC SP 26: Supplementary Formulas and Tables

1. Subgrade Moisture Scenarios (Clause 1.5)

• Scenario 1: Water table exceeds 1.5 m below subgrade; moisture near equilibrium (close to optimum).
• Scenario 2: Water table at or within 1.5 m from subgrade; subgrade assumed saturated (4-day soaking).

2. Strength Index (CBR) Computation (Table 4 & Clause 37.5)

Essential Specifications and Formulas for Rural Road Planning (IRC SP 26)

1. Road Land and Carriageway Widths (Clause 7.0)

2. Horizontal Curve Radius & Stopping Sight Distance (SSD) in meters

3. Allowable Gradients (%)

4. Road Network Planning Guidelines

Fundamental Formulas, Tables & Standards for Geometric Design (IRC SP 26)

1. Roadway and Land Widths (Clause 7.0)

• Two-lane roads: 7.0 m carriageway width; alternate 5 m width for reduced capacity.
• Minimum widths above 3 m may be considered based on traffic volume and terrain.

2. Horizontal Curve Radius & Stopping Sight Distance (SSD)

3. Permissible Gradients (%)

4. Subgrade Moisture Conditions (Clause 1.5)

• Condition 1: Groundwater deeper than 1.5 m, subgrade at equilibrium moisture.
• Condition 2: Groundwater at or above 1.5 m, subgrade fully saturated.

5. Strength Index (CBR) Calculation for Condition 1

Principal Formulas and Tables for Pavement Design (IRC SP 26)

1. Subgrade Strength Index (Clause 6.6)

For subgrade under soaked conditions (Condition 2):

[ \text{Strength Index} = 6.6 d + 246.3 \left(\frac{1}{M}\right) + 3.8 \left(\frac{1}{F}\right) + 10.3 R - 1.56 ]

Where:

• (d) = Dry density (gm/cc) post compaction
• (M) = Design moisture content (%)
• (F) = Percentage passing 75 micron sieve
• (R) = Coarse fraction retained on 75 micron sieve (<1)

2. Pavement Thickness for Road Categories I & II

Refer to Fig. 6 in IRC SP 26 for CRRI design curves.

3. Ad-hoc Flexible Pavement Thickness (Table 5)

Key Formulas and Tables Relating to Pavement Materials (IRC SP 26)

1. Dry Density Formula for Subgrade (Clause 6.6)

[ d = 6.6 v_d + 246.3 \left(\frac{1}{M}\right) + 3.8 \left(\frac{1}{F}\right) + 10.3 R - 1.56 ]

Where:

• (d) = Dry density (gm/cc) after compaction
• (M) = Design moisture content (%)
• (F) = % passing 75-micron sieve
• (R) = Coarse fraction retained (<1) on 75-micron sieve

2. Pavement Thickness for Ad-hoc Designs (Table 5)

Note: Thicknesses indicate compacted layer thickness.

3. Recommended Pavement Crossfall (Table 6)

• Steeper crossfall advised in regions of heavy rainfall.
• Shoulders should have crossfall 0.5% greater than pavement but not less than 3%.

4. Compaction Standards

• Top 300 mm of embankment/subgrade: Minimum 95% of Proctor density
• Embankment body: Minimum 90% of Proctor density
• Layer thickness controlled to ensure uniform compaction

Essential Specifications and Calculations for Cross-Drainage Structures (IRC SP 26)

1. Types and Appropriate Applications

2. Loadings (As per IRC 6-1966)

• IRC Class A: For permanent bridges and culverts.
• IRC Class B: For temporary or timber structures.

3. Roadway Widths for Structures

4. Gradient and Profile Design

• Maximum gradient on low-level crossings and approaches: 1 in 12
• Vertical curves must allow safe stopping sight distance
• Minimum vertical curve length: 15 m
• Profile and cross-section should be smooth curves
• Vents to be evenly spaced along causeways

5. Culvert Definition

• Length up to 6 m between supports
• Types include pipe, RCC slab, RCC box, stone slab, masonry arch
• Selection depends on site conditions and flow requirements

Key Construction and Quality Control Guidelines (IRC SP 26)

Construction Details:

• Soil Stabilization:
  • Pulverization to ensure 100% passes 25 mm sieve, 60% passes 4.75 mm sieve
  • Compaction to at least 95% Proctor density using power rollers
  • Lime pre-treatment recommended for heavy clay soils
  • Manual mixing involves 4-6 thorough turnovers after stabilizer application
• Water Bound Macadam (WBM):
  • Requires complete rolling to achieve aggregate interlock
  • Ensure adequate water supply during construction
• Bituminous Surfacing:
  • Includes surface dressing and premix carpet techniques
  • Premix requires heated aggregate and specialized mixing equipment

Quality Control Tests and Frequencies (Table 11):

Essential Equipment (Table 12):

• Standard IS sieves (40 mm down to 75 micron)
• Proctor compaction apparatus
• Aggregate impact testing machine
• Liquid and plastic limit apparatus
• Moisture meters, core cutters, balances, sand baths

Test Procedures Summary (Table 13):

• Moisture content via oven drying (105–110°C) or field alcohol method
• Liquid and plastic limits per IS 2720 Pt V; Plasticity Index = Liquid Limit - Plastic Limit
• Compaction per IS 2720 Pt VII & VIII (light and heavy methods)

Soil Stabilization Process Flowchart

flowchart TD
    Pulverisation --> Moisture_Adjustment
    Moisture_Adjustment --> Stabiliser_Application
    Stabiliser_Application --> Mixing_4_6_Passes
    Mixing_4_6_Passes --> Compaction

Maintenance Guidelines per IRC SP 26

Fundamental Concepts

• Maintenance Definition (3.7.1): Activities aimed at preserving roads in near-original condition to ensure serviceability and prolong life.

Resource Allocation (Table 14):

Periodic Renewal Intervals (Clause 7.5):

• Earth roads: Apply 150 mm moorum layer every 3 years.
• WBM roads: Scarify, reshape and add 75 mm stone metal every 3 years; compact to ≥95% Proctor density.
• Bituminous roads (plains): Surface dressing every 4 years, premix carpet every 6 years for heavy traffic areas.
• Bituminous roads (hilly): Premix carpet every 6 years; surface dressing is generally not recommended.

Maintenance Operations (3.7.3):

• Routine maintenance includes patching and repairs scheduled before, during, and after rainy seasons to prevent major deterioration.
• Maintain berms of 30-50 cm width for lateral support.
• Ensure cleanliness and functionality of roadside and cross drainage systems.
• Conduct tree pruning and vegetation control to avoid road damage.

Maintenance Management:

• Maintain comprehensive road data including geometry, condition, traffic, and maintenance history.
• Implement standards for quality, quantity, and operational efficiency.
• Utilize mobile maintenance units and provide regular staff training.