subgrade

Subgrade refers to the in-situ soil layer that forms the foundation of a pavement structure. It comprises the native soil or an engineered layer prepared beneath the pavement, including any stabilization applied to improve strength. The subgrade supports the pavement’s loads and distributes them to deeper soils, influencing settlement, shear strength, and long-term performance. The stiffness of the subgrade is commonly described by the modulus of subgrade reaction (k) or by bearing capacity, and design aims to ensure adequate support to prevent excessive deformation under traffic.

Subgrade performance depends on soil type, moisture, density, compaction, and temperature. Poorly draining or highly plastic clays can swell with moisture, reducing strength and causing rutting or pumping. Frost-susceptible soils may heave in cold climates. Therefore, proper drainage, moisture control, and compaction are essential. Practices include shaping and proof rolling to identify soft spots, achieving target compaction, and using stabilization or replacement where the native soil is inadequate. In weak soils, lime or cement stabilization, replacing with better material (subbase or select borrow), or reinforcing with geosynthetics can improve load distribution and reduce deformation.

Assessment and design rely on field and laboratory testing. Common methods include California bearing ratio (CBR) tests, plate bearing tests, and field deflection tests such as falling weight deflectometer (FWD) evaluations to estimate subgrade stiffness. Drainage enhancements, such as subdrains or geosynthetic layers, and frost protection measures are often incorporated to maintain performance. Overall, subgrade quality is a primary determinant of pavement life and maintenance requirements.