Guidelines for the Design of Flexible Pavements for Low-Volume Rural Roads (First Revision)
2015 Edition

The recommended subgrade strength classes according to IRC SP 72 are categorized based on CBR values as follows: Very Poor (S1, ≤ 2%), Poor (S2, 3–4%), Fair (S3, 5–6%), Good (S4, 7–9%), and Very Good (S5, 10–15%). A minimum design CBR of 5% (Fair class S3) is suggested even for low traffic volumes. Evaluation involves conducting detailed soil surveys with IS standard testing, including compaction and CBR measurements from at least three samples per kilometer. The road is segmented based on soil types and groundwater conditions, and average CBR values are used for design. Groundwater depth, rainfall, and environmental impacts are also factored in. If CBR is less than or equal to 2%, replacement or stabilization of the subgrade with better soil is recommended when cost-effective.

Traffic estimation for low-volume rural roads under IRC SP 72 involves analyzing population data of nearby villages, socio-economic activities such as agriculture and industry, traffic counts on comparable existing roads, and origin-destination surveys. Seasonal variations and traffic growth over the design lifespan are also considered. Roads serving smaller populations (below 500) are generally assigned traffic categories below T3. Vehicle groupings combine heavy and medium commercial vehicles without separately accounting for overloads. This approach ensures a rational, data-driven projection of commercial vehicle repetitions used for pavement design.

For challenging subgrade soils such as black cotton soil, IRC SP 72 suggests several stabilization methods: Lime stabilization, effective for silty clays and black cotton soils, reduces plasticity index and enhances strength using lime with at least 70% CaO content; Cement stabilization, suitable where lime is insufficient or quicker strength gain is necessary, targets a 7-day unconfined compressive strength of at least 1.7 MPa with a subbase thickness of 100 mm or more; Controlled compaction at 95% standard proctor density with moisture content slightly above optimum (1-2%) to minimize swelling; and use of granular sub-base materials like natural sand, moorum, gravel, or crushed stone meeting grading requirements.

Overlay thickness determination involves first identifying and rectifying underlying causes of pavement distress such as drainage or shoulder inadequacies. The total required pavement thickness is then calculated based on the subgrade's design CBR and projected traffic volume at the end of the design life, referencing the design catalogues for stabilized bases. Overlay thickness is the difference between this total required thickness and the existing pavement thickness. Limits on additional water-bound macadam (WBM) layers are specified: up to 150 mm (two layers) for traffic up to category T, and up to 225 mm (three layers) for traffic between T and T_ (2 million standard axles). Special considerations include reconstruction for black cotton soils or severely deteriorated pavements and ensuring overlay materials are equal or superior in quality to existing layers.

According to IRC SP 72, effective drainage design involves providing adequate cross-slopes on both carriageways and shoulders to facilitate rapid runoff of rainwater. The subgrade or improved subgrade surface must be elevated at least 300 mm above the natural ground level and 600 mm above the highest groundwater table. Properly designed roadside ditches and cross-drainage structures are essential for channeling water away. For subgrades with low permeability such as clays, the sub-base should extend at least half its thickness (minimum 100 mm) across the shoulders to improve drainage. Additionally, sub-base materials should have less than 5% fines passing the 75-micron sieve to ensure permeability. These measures collectively prevent water accumulation, reduce weakening of the pavement structure, and prolong pavement life.