Directionality of fissures in expansive soils under cyclic action of coupling wetting-drying and freeze-thaw
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摘要: 湿干冻融耦合作用（WDFT）是导致寒区膨胀土地基发生劣化现象的重要因素,而膨胀土地基裂隙发育的方向对工程安全稳定存在长期影响。围绕膨胀土裂隙发育的方向性问题,采用自行设计的单向环境边界加载装置,开展了WDFT边界条件下表面裂隙发育特征的大尺寸试样单元试验,运用数字图像采集与处理方法得到了不同WDFT循环下的表面裂隙发育特征。试验结果表明,冻结过程中膨胀土内部液态水含量对表面裂隙的分布及形态产生重要影响,当临界饱和度较大时,冻结过程对裂隙起到促进作用,反之则为抑制作用；而在后期WDFT下的裂隙率逐渐趋于稳定。在此基础上,以裂隙图像灰度直方图为基础,通过梯度方向法对裂隙发育的局部方向性进行了统计,引入无量纲影响因子对裂隙角度进行了量化归纳,进一步探讨了膨胀土在下的裂隙演化规律。所采用的量化分析方法可为相关研究提供参考。Abstract: The wetting-drying and freeze-thaw(WDFT) coupling effect is an important factor leading to the deterioration of the expansive soil foundation in the cold regions, and the direction of fissure development in the expansive soil foundation has a profound impact on the long-term safety and stability. Focusing on the directionality of crack development in expansive soils, a unit test of the characteristics of surface crack development under WDFT coupling boundary conditions is carried out using the self-designed unidirectional environmental boundary loading device, and the digital image acquisition and processing methods are used to obtain characteristics of surface fissure development under WDFT coupling cycle. The test results show that the liquid water content in the expansive soil during freezing has an important effect on the distribution and morphology of surface cracks. When the critical saturation is large, the freezing process promotes the cracks, and vice versa. The fracture rate gradually stabilizes at later stage of WDFT. Based on the gray histogram of the fissure image, the local directionality of fissure development is calculated by the gradient direction method, and the dimensionless influence factor is introduced to quantify and summarize the fissure angle. The crack evolution laws of expansive soil are further investigated. The quantitative analysis method used can provide reference for related researches.
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Keywords:
- expansive soil /
- coupling wetting-drying and freeze-thaw /
- fissure /
- sample test /
- directionality
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图 5 湿干冻融耦合下试样表面裂隙率随时间分布
Figure 5. Distribution of surface crack rate over time under WDFT
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图 6 干燥过程中不同时刻表面裂隙发育方向统计结果
Figure 6. Statistical results of surface crack development direction at different moments under WD
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图 7 干燥、冻结、融化完成时刻表面裂隙发育方向统计结果
Figure 7. Statistical results of surface crack development direction at different moments under DFT
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图 8 影响因子f随表面裂隙发育方向统计结果
Figure 8. Statistical results with surface crack development direction by f
表 1 膨胀土的基本物理性质
Table 1 Physical properties of sample
基本指标 Gs wp /% wL /% δef /% ρdmax /(g·cm-3) wopt /% 2.67 20.3 65.9 76 1.56 24.1 矿物成分 蒙脱石 石英 长石 方解石 钠长石 61.5% 31.9% 6.1% 0.5% 
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表 2 湿干冻融耦合循环边界条件
Table 2 Details of WDFT in tests
条件 温度/℃ 时间/h 循环次数 施加方式 湿润 室温 — 7 抽气饱和 干燥 40 至 Srcr 为止 7 称重法对 冻结 −20 24 7 试样质量 融化 20 36 7 进行监控 注： “—”表示持续时间以质量稳定为准；温度指的是冷源或热源的温度。
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