Application and Evaluation of Cone Penetration Testing for Determining Internal Friction Angle in Composite Subgrade Cushion Layer

The internal friction angle is a key parameter in assessing the shear strength of cushion layers in composite subgrades. This study refines the evaluation of the internal friction angle in test pile composite subgrades by utilizing cavity expansion and stress rotation theories. Laboratory calibration tests and field experiments with various cushion layers are conducted to analyze how different thicknesses affect deformation and the internal friction angle during loading. Significant correlations are found between the internal friction angle of sand cushion layers and cone penetration resistance, as well as between the internal friction angle of gravel cushion layers and relative deformation. As the cone penetration resistance increases, so does the internal friction angle of sand cushion layers, which ranges from 27.6° to 34.2° for sand and 32.2° to 42.2° for gravel. For flexible pile composite subgrades, the internal friction angle of sand cushion layers typically falls between 33.0° and 36.6°. In semi-rigid composite subgrades, gravel cushion layers show an internal friction angle ranging from 36.0° to 39.5°, while rigid pile composite subgrades display angles between 37.7° and 40.9°. This research provides both theoretical insights and empirical data to aid in the selection of cushion layers for composite subgrade design.