This study examined the influence of matric suction on the California Bearing Ratio (CBR) performance of compacted subgrade soils through controlled laboratory testing. A series of CBR tests and suction measurements were performed on specimens prepared at varying moisture contents to capture the contribution of suction to load resistance. Measured suction values were linked with corresponding CBR results to establish predictive relationships that yielded unsaturated \(\:{CBR}_{u}\) values for design application. These values were incorporated into flexible pavement design procedures and compared with pavement structures derived from soaked CBR data. The findings indicated that matric suction produced a significant increase in subgrade strength, which led to higher resistance to penetration and greater stiffness within the soil skeleton. The test results showed that soil in a dry condition developed CBR values between one and a half and three times those recorded under soaked conditions, which reduced the likelihood of deformation under traffic loading. Incorporation of suction-based strength in the design process generated pavement thicknesses that aligned with in-service seasonal moisture conditions rather than the overly conservative thicknesses produced with soaked strength. The outcomes demonstrated that matric suction contributed directly to effective stress and improved the mechanical stability of subgrade layers.
