Microscopic Damage Evolution of Moraine Soils under Freeze-Thaw Cycles Based on PFC2D Simulation
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摘要: 为探究冻融循环作用下冰碛土性质劣化的微观损伤机制，基于离散元理论提出一种通过水颗粒膨胀实现土体冻融损伤模拟的方法.利用颗粒流软件PFC2D模拟三轴压缩试验，结合室内试验结果对比分析，在模拟冰碛土的力学性质变化方面显示出高度的准确性和可靠性，揭示了冻融冰碛土受载时微裂隙、位移场和力链场的演化过程与破裂特征.结果表明：（1）试样冻融过程中微裂隙呈由四周产生并逐渐向中间扩展的“累计演化”趋势，其中张拉微裂隙占据主导地位，在冻融前期（2~5次）颗粒间以水平挤压为主从而大量发育偏90°倾向张拉微裂隙；（2）冻融作用引起的冰碛土性质劣化在冻融前期（2~5次）尤为明显，粘聚力c随冻融次数N的增加呈负指数函数递减规律，而内摩擦角φ呈小幅波动态势；（3）试样受载过程中剪切微裂隙占据主导地位，微裂隙发育呈“慢→陡→缓”趋势演化，根据其裂隙演化特点，将加载过程的应力-应变曲线划分为4个变形阶段；（4）冻融后试样受载时减速斜率转换点B移动到峰值应力点C之前，说明B点在微裂隙扩展-贯通-形成破坏过程中可以作为“前兆特征”；冻融20次试样受载时破坏程度更剧烈且形成明显剪切破坏带.Abstract: In order to investigate the microscopic damage mechanism of the degradation of the properties of freezing-thaw (F-T) damaged moraine soils, a method of simulating F-T damage of soils through water particle expansion is proposed based on the discrete element theory. Using the particle flow software PFC2D to simulate the triaxial compression test, combined with the comparative analysis of the test results, this method is accurate and reliable in modeling the changes in mechanical properties of moraine soils and reveals the evolution of microcrack; displacement field; force chain field and rupture characteristics of F-T moraine soils during the loading process. The results show follows (1) The microcracks in the F-T process show a trend of "cumulative evolution" that arises from the surrounding area and gradually expands to the middle, and the tensile microcracks are dominant; at 2-5 times of F-T processes, horizontal compression between particles dominated and a large number of tension microcracks inclined at 90° were developed. (2) The deterioration of moraine properties caused by F-T is particularly obvious in the early freeze-thaw period (2-5 times), the cohesion c decreases as a negative exponential function with the number of F-T cycles, while the internal friction angle φ shows a small fluctuation. (3) The specimen loaded process is dominated by shear cleavage, with the trend of "first slow, then steep, and finally slow" evolution, and the stress-strain curve is divided into four deformation stages according to the evolution characteristics. (4) When the sample is loaded after freeze-thaw, the transition point B of deceleration slope moves before the peak stress point C, indicating that point B can be used as a "precursor feature" in the process of microcrack expansion-through-formation failure; The specimen with 20 F-T cycles were more severely damaged when loaded and formed distinct shear zones.
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图 4 山南市近5 a气象资料
Fig. 4. Meteorological information for Shannan City in the last five years
图 8 受载应力-应变曲线及破坏形态对比
Fig. 8. Comparison of loaded stress-strain curves and damage patterns
图 9 最佳关系曲线对比
a.N=0；b. N=2；c. N=5；d. N=10；e.N=15；f. N=20
Fig. 9. Comparison of best relationship curves
图 10 粘聚力与内摩擦角对比(试验与模拟结果)
Fig. 10. Cohesion and angle of internal friction (comparison of experimental and simulation results)
图 12 冰碛土试样不同位置处SEM图像
a.边缘位置；b.中心位置
Fig. 12. SEM images of moraine samples at different positions
图 13 不同冻融次数后微裂隙数量占比
Fig. 13. Percentage of microfracture number after different freeze-thaw times
图 14 不同冻融循环次数后微裂隙玫瑰花图
a. 2次；b. 5次；c. 10次；d. 15次；e.20次
Fig. 14. Rosettes fissures after different numbers of freeze-thaw cycles
图 15 不同冻融次数后力链场与位移场
a.位移场；b.力链场
Fig. 15. Displacement field and force chain field after different numbers of freeze-thaw cycles
图 16 应力-应变与微裂隙关系曲线
a.N = 0；b.N = 2；c.N = 5；d.N = 10；e.N = 15；f.N = 20
Fig. 16. Stress-strain versus microfracture curves
图 17 未冻融试样加载破坏演化全过程
a.微裂隙演化；b.位移场；c.力链场
Fig. 17. The whole process of loading damage evolution of unfrozen-thawed specimens
图 18 冻融20次试样加载破坏演化全过程
a.微裂隙演化；b.位移场；c.力链场
Fig. 18. The whole process of damage evolution of specimens loaded by freezing and thawing for 20 times
表 1 基本物理性质及粒径分布
Table 1. Basic physical properties and particle size distribution
物理性质 干密度
(g/cm3)比重 孔隙率
(%)天然含水率(%) 1.7 2.66 18.3 10.71 粒径分布 d10(mm) d30(mm) d60(mm) Cu Cc 0.58 0.83 1.27 2.2 0.94 
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表 2 试验方案
Table 2. Test scheme
含水率(%) 围压(kPa) 冻融循环次数N 试样个数 10.71 2, 10, 20 0 3 2 3 5 3 10 3 15 3 20 3
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表 3 平行粘结接触模型细观参数
Table 3. Parallel bonded contact model parameters
粘结类型 土-土粘结 水-水
粘结土-水
粘结颗粒接触模量(MPa) 12 12 12 颗粒刚度比 2 2 2 摩擦系数 0.67 0.50 0.50 平行粘结模量(MPa) 12 12 12 平行粘结法向与切向刚度比 1 1 1 平行粘结法向强度(kPa) 105. 3 1 263.6 1 263.6 平行粘结切向强度(kPa) 39 468 468 平行粘结摩擦角(º) 25 0 0
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表 4 不同冻融次数下试样三轴压缩模拟起裂指标统计
Table 4. Statistics analysis of crack initiation indicators from triaxial compression simulations under different numbers of freeze-thaw cycles
冻融次数 剪切起裂应力
σNs (kPa)剪切起裂应变
εs(10-3)张拉起裂应力
σNt (kPa)峰值应力
σf(kPa)0 44.4 1.300 0.296 100.0 2 36.1 1.170 0.256 94.2 5 28.4 1.060 0.247 89.4 10 28.1 1.070 0.190 88.2 15 28.4 0.935 0.148 88.4 20 26.4 0.858 0.125 87.8
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表 5 试样破坏形态对比(试验与数值模拟)
Table 5. Comparison of damage patterns of specimens (experimental and numerical simulation results)
试验试样 数值模拟 σ3 (kPa) 10 10 N=0 
N=20 
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