Rural Roads Manual
2002 Edition

IRC SP 20 advises using locally available soils compacted to optimal moisture content with 8-10 tonne rollers for subgrade and embankment, maintaining a 2% camber for drainage. The sub-base should consist of granular materials like moorum, gravel, or Water Bound Macadam Grade II laid in two layers totaling 150 mm thickness, compacted separately. The base course involves Dry Lean Concrete (1:4:8 mix) with a thickness between 100 and 150 mm, cured for at least 14 days. Surface courses can be either thin bituminous layers over semi-rigid bases or Roller Compacted Concrete (22 cm thick) for axle loads up to 10.2 tonnes. RCC must meet flexural strength ≥ 40 kg/cm², compressive strength about 350 kg/cm², with slab sizes of 5 m by 3.75 m incorporating contraction joints every 5 m sealed properly. Proper compaction and drainage provisions are essential to ensure durability.

The manual defines culverts as structures up to 6 meters long, small bridges as 6 to 30 meters, and minor bridges up to 60 meters, typically with heights not exceeding 8 meters. Standard formation widths are 7.5 meters for typical rural roads and may be reduced to 6 meters for hill or short roads. Geometric standards specify overall and carriageway widths for culverts and bridges based on roadway width. Design loading generally follows IRC Class A with impact considerations. Wearing coats vary depending on surface type, with bituminous roads requiring premix carpets and seal coats, and submersible structures mandating cement concrete wearing surfaces. Hydraulic designs ensure adequate waterway capacity using catchment analysis and Manning's formula. Additional provisions include signage and drainage protections to safeguard structural integrity.

For prime coats, slow-setting bitumen emulsion or medium curing cut-back bitumen (for cold climates) is recommended. The surface must be clean and dry before application using suitable spray equipment. Prime coat viscosity and quantities depend on surface porosity, with curing times of at least 24 hours before subsequent layers. Tack coats use rapid, medium, or slow-setting emulsions or RC-70/MC-70 cutback bitumen applied on cleaned surfaces to bond new bituminous layers to existing ones. Quantities differ based on whether the surface is granular or bituminous. A thin sand layer may be applied over prime coats to prevent pickup. Fresh bituminous surfaces may not require tack coats.

Climatic factors such as temperature and rainfall critically determine material choice and design methodology. Materials must be selected based on their performance under local temperature extremes and precipitation patterns. In high rainfall regions, materials with superior drainage and moisture resistance are preferred. Hot climates require bitumen with higher softening points to prevent deformation, while arid regions need materials resistant to shrink-swell and dust. Cold or temperate zones benefit from frost-resistant aggregates and concrete pavements. Overall, designs must incorporate adequate drainage systems and flexible pavement structures to accommodate climatic stresses and ensure longevity.

The process begins with preparing the subgrade to the required grade and camber, ensuring it is free from dust. Coarse aggregates are spread evenly in layers of 75 mm or 100 mm thickness depending on grading, using templates to maintain profile control. Rolling is performed with 8-10 tonne power rollers starting at edges and moving inward with overlapping passes, stopping when partial compaction with voids remains for screenings. Screenings are applied gradually in thin layers, with dry rolling, watering, brooming, and repeated rolling to achieve a firm surface. For shoulders, earthen materials must be compacted to at least 100% of Standard Proctor dry density, with maximum dry unit weight of 16.5 kN/m³, plasticity index below 6, and liquid limit under 25. Vibratory rollers are recommended for compaction, with typical layer thickness around 15 cm.