Cumulative plastic strain characteristics and prediction models og raded coarse-grained soil airport subgrade

To reveal the evolution law of cumulative plastic deformation of coarse-grained soil subgrade under the coupling effect of particle gradation, stress state, and moisture condition, and to solve the differential settlement problem of airport subgrade under long-term cyclic loading, fractal dimension was adopted as a single index to replace the traditional dual indices for characterizing the gradation of coarse-grained soil. The optimal gradation interval of coarse-grained soil was determined based on the Talbot fractal gradation equation, and four groups of typical gradations were selected to carry out repeated loading triaxial tests. The influences of gradation, cyclic dynamic loading, and dry-wet cycles on the cumulative plastic strain of compacted coarse-grained soil were systematically investigated. By comparing typical cumulative deformation prediction models, a basic model suitable for coarse-grained soil subgrade fillers was screened out, the statistical correlations between model fitting parameters and fractal dimension, cyclic stress ratio, and moisture state were analyzed, and a modified prediction model simultaneously considering the coupled effects of the three factors was proposed and its accuracy was verified. The results show that cyclic stress ratio has a significant control effect on the development of cumulative plastic strain, and controlling the ratio of deviator stress to confining pressure at a low level (less than 1.23) can effectively inhibit the development of cumulative plastic deformation. With the increase of fractal dimension, the cumulative plastic strain decreases first and then increases. When the coarse-grained soil reaches the maximum dry density (fractal dimension is 2.49), its cumulative plastic strain is the smallest. Dry-wet cycles significantly aggravate the plastic deformation of coarse-grained soil, and the soil deformation transits from the plastic shakedown stage to the plastic creep stage when the number of cycles is greater than 7. The modified prediction model established based on the Monismith model has a good prediction effect on the cumulative plastic strain of coarse-grained soil subgrade under different cyclic stress ratios, fractal dimensions, and humidity conditions. The research results can provide a theoretical basis and technical support for deformation control, gradation optimization, and long-term service safety evaluation of airport subgrade.