Laboratory model tests on capillary barrier infiltration using actively heated fiber optic method
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摘要: 毛细阻滞效应是多层、不同粒径非饱和土入渗过程中的一个自然现象。为了探究多层土水分入渗的毛细阻滞过程,设计了室内模型试验,采用主动加热光纤法（AHFO法）对多层土的水分迁移进行试验,并结合频域反射法（FDR法）和直接观测法作为验证。试验结果表明：相较于FDR法和直接观测法,AHFO法对降雨入渗所产生的毛细阻滞现象具有较好的观测效果,可观测出水分运移的更多细节；运用FDR法,对AHFO传感器进行原位标定,曲线拟合精度R2均大于0.93,具有较高的体积含水率监测准确度；毛细阻滞层对降雨入渗具有明显的阻滞效应,即存储屏障上部入渗和减少水分向下部水体渗出。相关研究结论为毛细阻滞现象研究以及土壤水分场监测提供了一种新的监测方法。Abstract: The capillary barrier effect is a natural phenomenon during the infiltration of unsaturated soil layers with different particle sizes. In order to test the capillary barrier effect of multi-layer soils, laboratory model tests are designed. Subsequently, the actively heated fiber optic (AHFO) method is used to test the water migration of the model tests, and the direct observation method and the frequency domain reflection (FDR) technology are used for verification. The analysis of test shows that compared with the direct observation method and the FDR method, the AHFO method has a better observation effect on the capillary barrier phenomenon caused by rainfall infiltration, and can observe more details of water movement as well. The FDR method is used to perform the in-situ calibration of the AHFO sensor, and the curve-fitting accuracy R2 is greater than 0.93, indicating a high accuracy of volume water content monitoring. The capillary barrier layer has a significant retarding effect on rainfall infiltration, that is, infiltration water can be saved at the storage barrier which can also reduce seepage to the layer under the barrier. The research results may provide a new method for the research on capillary barrier effect and the monitoring of water content distribution.
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图 1 细土-粗土毛细阻滞系统界面附近的毛细水静水平衡
Figure 1. Hydrostatic equilibrium of capillary water near interface of fine soil-coarse soil capillary barrier system
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图 8 温度特征值与体积含水率关系
Figure 8. Relationship between temperature characteristic value and volume water content
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图 9 入渗不同阶段的影像与含水率分布
Figure 9. Images at different stages of infiltration and vertical distribution of water content
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图 11 湿润锋平均运移速度和时间的变化关系
Figure 11. Relationship between average velocity and time of wet front migration
表 1 两类土含水率监测方法对比
Table 1 Comparison of two types of monitoring methods for water content of soils
对比项目 FDR法 AHFO法 直接测量参数 介电常数 温度变化量 监测方式 点式 分布式 传感器标定 出厂前完成标定,必要时可再次标定 出厂后室内或原位标定 适用土层 低盐度的 粗、细粒土层 粗、细粒土层 现场安装 直接埋置 直接埋置或钻孔埋设 
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表 2 试验土体的基本参数
Table 2 Basic parameters of the test soils
土类 颗粒相对质量密度 目标干密度/(g·cm-3) 饱和渗透系数/(10-2cm·s-1) 细砂 2.69 1.58 1.03 中砂 2.68 1.62 2.30
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表 3 FDR传感器主要技术指标
Table 3 Main technical parameters of FDR sensors
含水率测量范围/(m3·m-3) 传感器尺寸/cm 工作温度/℃ 测量耗时/ms 精度/(m3·m-3) 0~1 8.9×1.8×0.7 -40~60 10 ±0.03
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表 4 FBG解调仪主要技术指标
Table 4 Main technical parameters of FBG demodulator
光纤类型 波长分辨率/pm 解调速率/Hz 动态范围/dB 62.5/125 1 ≥1 35
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表 5 时间-降雨强度表
Table 5 Time-rainfall intensity setting
降雨阶段 降雨时间/h 降雨强度/(mm·h-1) 降雨类型 I 0~2 3~5 暴雨—大暴雨 II 2.5~6 7~14 大暴雨 III 19.33~24.33 6~12 大暴雨
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表 6 湿润锋到达不同高程的时间对比
Table 6 Comparison of time of wet front arriving at different heights
(min) 抵达高程/cm 含水率/% 8 10 12 45 49.5 55 58.7 35 202 204 206 25 295 301 307 15 491 499 510.8 5 1392 1395 1400
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