In modern highway engineering, the quality of the subgrade is considered the most critical factor governing the overall performance and lifespan of the pavement structure. The subgrade acts as the primary load-bearing foundation for all upper pavement layers, and therefore it must be constructed using approved, well-compacted, and stable soil material free from organic content, debris, and weak clay pockets. Ideally, the material should be uniform in nature, properly moisture-conditioned, and capable of achieving the required strength and stiffness under compaction. In many standard highway specifications, subgrade strength is also correlated with California Bearing Ratio (CBR), where a minimum value of around 8% to 10% is generally expected for typical highway applications, with higher requirements for heavily trafficked corridors and expressways.

Compaction of the subgrade is a key performance requirement and is strictly controlled in field operations. For normal highway projects, the subgrade must achieve a minimum of 95% of the Maximum Dry Density (MDD) as determined by the Modified Proctor Test, while for national highways, expressways, and other heavy traffic roads, the requirement is often increased to 97% MDD. In critical structural zones such as formation level or heavily loaded sections, compaction levels may even be pushed closer to 98% MDD to ensure long-term stability. Field compaction is executed in layers, typically within 150 mm to 250 mm thickness, with proper moisture control maintained close to the optimum moisture content to achieve uniform densification. Field density testing through sand replacement or nuclear density methods is used to ensure compliance, as inadequate compaction at this stage can lead to differential settlement, rutting, and premature pavement failure.

Alongside subgrade quality, road shoulders play a vital structural and functional role in pavement design. Shoulders provide lateral support to the pavement edges, help distribute wheel loads, and significantly improve drainage and road safety. Depending on the classification of the road, shoulder widths vary from about 1.0 to 2.5 meters per side in rural highways, 2.5 to 3.5 meters per side in national highways, and up to 3.0 to 3.75 meters per side in expressways and high-speed corridors. Shoulders may be constructed as earthen, granular, or paved systems depending on traffic intensity and design requirements. Earthen shoulders are typically compacted soil extensions of the subgrade, while granular shoulders use crushed aggregates to improve drainage and stability. In high-standard highways and expressways, paved shoulders constructed with bituminous or cement-treated layers are commonly adopted for enhanced durability and safety.

Compaction requirements for shoulders are also strictly defined, with earthen and granular shoulders generally required to achieve at least 95% MDD, while paved shoulders follow compaction standards similar to the adjacent pavement layers, often ranging between 93% and 98% depending on the layer type. Properly designed and compacted shoulders ensure lateral confinement of pavement layers, reduce edge failures, provide emergency stopping space, and assist in effective surface water drainage. In modern pavement systems, subgrade and shoulder structures are no longer treated as secondary components but as integral parts of a continuous load distribution system, where their performance directly influences the structural capacity and service life of the entire roadway.