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初始裂隙扰动下紫色土的土水特征曲线

Soil-water characteristic curve of sandy viscous purple soil under initial fissure disturbance

  • 摘要: 紫色土结构松散易侵蚀,在降雨-蒸发作用下易产生干缩裂隙,严重影响土体的持水性能。该研究为探究初始裂隙对紫色土持水特性的影响,制备不同初始裂隙参数的环刀试样,采用轴平移法测量试样土-水特征曲线(soil-water characteristic curve,SWCC),并结合Fredlund-Xing模型拟合。通过灰色关联度分析法,定量揭示裂隙特征参数与SWCC特征参数、Fredlund-Xing模型参数间的关系。结果表明:1)初始裂隙明显改变土体持水行为,其影响集中于SWCC的边界效应区,表现为低吸力段(<4 kPa)快速脱水并伴随平台期,而过渡区与残余区持水特性相对稳定;2)灰色关联分析进一步揭示,裂隙率是影响持水性能的首要因素(关联度>0.67),裂隙率与进气值呈明显负相关,证实裂隙网络作为优先导水通道对低吸力段土体水力行为具有主导作用;3)具有初始裂隙紫色土SWCC具有“平台期”现象,其机理可归因于裂隙-基质孔隙双通道系统的协同作用:宏观裂隙空间主导低吸力段快速排水,微观基质孔隙控制中高吸力段持水行为;4)通过低场核磁扫描试验证实孔隙分布变化规律,即裂隙发育程度与孔隙度成正比,与孔隙分布范围成反比。通过 Fredlund-Xing 模型对裂隙性土体水力特性的有效表征,可探究初始裂隙对紫色土持水性能的劣化效应规律,为紫色土地区降雨入渗引发的边坡失稳风险研究、水土保持及灾害防治工程提供理论依据与参考。

     

    Abstract: Purple soil is often characterized by a loose structure and high erodibility, particularly prone to desiccation cracks under the alternating action of the rainfall and evaporation. These cracks can significantly damage the integrity of the soil structure and further degrade its water-holding capacity. This study aims to systematically clarify the influence of the initial cracks on the water-holding capacity of the purple soil. The undisturbed ring knife samples were prepared with the initial crack parameters, including the crack ratio, crack length, average crack width, and crack number. The soil-water characteristic curve (SWCC) of each sample was obtained using the axis translation. The Fredlund-Xing model was adopted to conduct fitting analysis on the measured SWCC data. Meanwhile, the grey correlation analysis was employed to explore the influence of the various crack parameters on the characteristic parameters of the SWCC and the fitting parameters of the Fredlund-Xing model. Additionally, the low-field nuclear magnetic resonance (NMR) scanning tests, including the nuclear magnetic resonance T2 spectra analysis and pore throat distribution analysis, were carried out to verify the correlation between soil pore distribution and crack development. The results indicated that the influence of the initial cracks on the water-holding characteristics of the purple soil was concentrated mainly in the low suction section of the SWCC, indicating the rapid dehydration that was accompanied by the "plateau period". In contrast, the initial cracks had a minimal impact on the transition and residual section of the SWCC, where the water-holding characteristics remained relatively stable. Further analysis showed that the air-entry value of the soil was inversely proportional to the crack ratio and dehumidification rate. The Fredlund-Xing model sand showed an excellent fitting performance for the SWCC of the purple soil with the initial cracks, particularly with the determination coefficient generally higher than 0.85. The parameter, α, which was closely related to the air-entry value, was inversely proportional to the crack ratio and dehumidification rate. While the parameters n and c showed no significant linear correlation with any crack parameters. The grey correlation analysis revealed that the crack ratio was the primary dominant influencing factor on the water-holding capacity of the purple soil (correlation degree > 0.67), followed by the average crack width, whereas the crack number shared the weakest correlation with the water-holding capacity. The initial cracks also destroyed the internal structure of the soil. Thus, a dual-pore system was formed with the interconnected crack space and intact non-cracked soil matrix. The proportion of the large pores was dominated by the macro-cracks, while there was a relative decrease in the proportion of the medium and small pores that were controlled by the soil matrix. The preferential water flow effect of the crack system was reduced in the soil air-entry value at the low suction stage. The "plateau period" also occurred in the SWCC. The NMR scanning tests confirmed that there was a close correlation between pore distribution and crack development: The nuclear magnetic resonance T2 spectra and pore throat distribution verified the negative effect of the cracks on the soil pore distribution. A multi-scale verification on the SWCC was integrated with the macro water-holding, micro pore structure, and model fitting parameters under crack disturbance. The dominant role of the crack network was clarified as a preferential water flow path to regulate the soil hydraulic behavior. The influence mechanism of the initial cracks on the water-holding of the purple soil can provide a solid theoretical basis and key parameter support for soil and water conservation, soil improvement, and accurate thresholds of the landslide disaster early warning in purple soil distribution areas.

     

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