Cumulative plastic strain characteristics and prediction models og raded coarse-grained soil airport subgrade
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摘要: 为揭示颗粒级配、应力状态与湿度条件耦合作用下粗粒土道基累积塑性变形演化规律,解决机场道基长期循环荷载下不均匀沉降问题,采用分形维数单指标替代传统双指标表征粗粒土级配,基于Talbot分形级配方程确定粗粒土最优级配区间,并选取4组典型级配开展重复加载三轴试验,系统研究了级配、循环动荷载及干湿循环作用对压实粗粒土累积塑性应变的影响规律;通过对比典型累积变形预估模型,筛选出适用于粗粒土道基填料的基础模型,分析模型拟合参数与分形维数、循环应力比、湿度状态的统计关联,提出可同时考虑三者耦合影响的修正预估模型并验证精度。研究结果表明:循环应力比对累积塑性应变发展具有显著控制作用,将偏应力与围压的比值控制在较低水平(不大于1.23)可有效抑制累积塑性变形发展;随分形维数增大,累积塑性应变呈先减小后增大的趋势,当粗粒土达到最大干密度(分形维数为2.49)时,其累积塑性应变最小;干湿循环作用显著加剧了粗粒土的塑性变形,当循环次数大于7时,土体变形由塑性安定阶段过渡至塑性蠕变阶段;基于Monismith模型构建的修正预估模型,对不同循环应力比、分形维数及湿度条件下粗粒土道基的累积塑性应变具有良好预测效果。研究成果可为机场道基变形控制、级配优化、长期服役安全评价提供理论依据与技术支撑。Abstract: 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.
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表 1 加载试验方案
Table 1. Loading test scheme
试样
编号分形维数 围压/kPa 循环
应力比初始含水
率/%干湿循
环次数1 2.36 28 1.23 7.00 1 2 2.43 7.60 3 2.49 7.80 4 2.55 8.20 5 2.49 0.50 7.80 1 6 0.86 7 1.23 8 1.78 9 1.23 0 10 1 11 3 12 7 表 2 常用的累积塑性应变预测模型
Table 2. Commonly used predict models for cumulative plastic strain
表 3 预测模型参数应用范围
Table 3. Application range of prediction model parameters
模型参数 N/次 Rc D Cw/次 应用范围 1~1.0×104 0.50~1.78 2.36~2.55 0~7 -
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