Spatial pore characteristics of unsaturated soils caused by dielectric geometric factors under permittivity method with 1 GHz frequency range

A dielectric measurement device called a frequency domain reflectometry (FDR) has been designed and constructed for the dielectric measurement of unsaturated soil consisting of a volumetric soil water content of about 0.1, 0.2, and 0.3 m³/m³ with different soil porosity around 0.40–0.45, respectively. The dielectric constant is measured in the frequency range 1 GHz. Soil calibration tests and tracer injection tests on standard sand and river sand are carried out in the laboratory. FDR measurement probes at different soil depths allow volumetric soil water content and dielectric constant measurements. The tracer concentration in the pore water is monitored by determining the dielectric constant, from the soil impedance. From the relationship between volumetric soil water content and dielectric constant, the specific calibration equations for the unsaturated soils were derived, and one can easily estimate the volumetric soil water content from the response of the measured dielectric constant for the soils. In the study of dielectric mixture models using α-value of 0.5 which is dielectric geometric factor, the effective porosity for the soils was computed to a range of 87–92% compared with the soil porosity.     

温度对土壤介电常数的影响规律研究北大核心CSCD

介电常数是频域反射法测定土样含水率的关键参数之一。为获取温度对土壤介电常数的影响规律,利用矢量网络分析仪测得不同含水率、含盐量的试样在不同试验温度下的介电常数。通过分析土样介电常数随不同因素的变化规律,最后建立了介电常数与温度的关系。结果表明:温度对土样介电常数的影响主要表现在冰水剧烈相变阶段。当温度低于某一温度或者高于冻结温度后,介电常数受温度的影响较小。负温条件下,土样介电常数随含盐量增大而增大。引入阶梯函数建立的三阶段模型,可有效描述土样介电常数随温度的变化规律。     

掺石灰黏土电阻率试验研究

利用4电极电阻率测量装置,在室内对各种灰土样的电阻率进行测试对比研究,揭示了不同因素如灰土比、含水率、饱和度、土的结构和土粒粒径等对灰土样电阻率的影响。在不同含水率（15 %、20 %、25 %和30 %）条件下,素土及其与不同石灰掺量（1:10、1:15和1:30）的混合,通过轻型击实试验后,测量其横向及竖向断面上的电阻率,研究电阻率与不同石灰配比击实土样的物理力学性质之间的关系。实验结果表明,当含水率在15 %~20 %之间时,较小的含水率增量引起混合土样电阻率的大幅度下降;当含水率约达到20 %后,其电阻率随含水率的继续增加而减小的幅度很小;在15 %含水率时,掺灰土电阻率比素土样高;在30 %含水率时,其电阻率低于素土样,而且掺量越大,电阻率越低;由于击实导致土体颗粒的结构重新排列,电阻率在横向上大于竖向;电阻率随着无侧限抗压强度的增加而增加,在电阻率大于50 ?•m时,电阻率随无侧限抗压强度的增加而减小;饱和度、干密度与电阻率呈负指数变化规律,孔隙率、孔隙比与电阻率呈指数变化规律;含水率对于电阻率的影响较为敏感。     

Field Measurement of Suction, Water Content, and Water Permeability

This paper presents a review of techniques for field measurement of suction, water content, and water hydraulic conductivity (permeability). Main problems in the use of field tensiometers are addressed and hints on how to improve tensiometer performance are given. Advantages and limitations of instruments for indirect measurement of suction including electrical conductivity sensors, thermal conductivity sensors, dielectric permittivity sensors, filter paper, and psychrometer are discussed. Techniques for water content measurement based on dielectric methods are then presented. These include time and amplitude domain reflectometry and capacitance. Finally, a brief overview of methods for measurement of water permeability in the field is presented.     

LNAPL 污染土壤介电性质的实验研究——-以石英砂为例

为研究轻非水相液体污染土壤的介电性质,在实验室条件下,利用石英砂( 粒径0. 106 ～ 0. 212 mm) 、柴油和水的混合物模拟LNAPL 污染土壤,采用开口同轴探头法利用高频电磁波( 200 MHz ～ 3 GHz) 在污染土壤中的反射获得介电常数,并分析土壤中油水体积含量变化与相对介电常数的变化规律。实验结果表明: 非饱和状态下石英砂的相对介电常数随着体积含水量和含油量的增大而增大,达到饱和状态后体积含油量的增加对相对介电常的影响不明显; 被污染的非饱和状态的砂样在放置过程中,相对介电常数随着放置时间的延长呈现先增大最后趋于稳定的变化趋势。     

高铁路基粗颗粒土水力学参数测试方法研究

高铁路基粗颗粒土的水力学特性对路基内部水分运移及路基的长期累积变形有重要影响。而对这种高压实度的粗颗粒土,常规非饱和土试验仪器存在试样尺寸小、制样难度大的缺点,难以应用。介绍了一种用于测试高压实度路基粗颗粒土-水力学参数的试验装置,利用张力计和时域反射计量器,分别测量路基粗颗粒土在浸湿、干燥阶段不同高度处土体基质吸力、介电常数的变化情况,获得高压实度下高铁路基粗颗粒土土-水特征曲线;并通过瞬态剖面法获得路基粗颗粒土非饱和渗透系数与基质吸力的关系。试验结果表明,该套装置能够适用于最大粒径为20 mm的粗颗粒土,试样压实度最大可达到0.95。通过对一组试验结果分析,并结合Ekblad等试验结果,发现随着填料细颗粒含量增大,?（与进气值有关的参数）值逐渐减小,土体进气值增加;粒径越大,细颗粒含量越低,土体储水能力越低,对应n（与排水程度有关的参数）值越大。为路基粗颗粒土-水力学参数的测定提供了方法。     

广西北部湾人造陆域吹填土电冲击特性研究

吹填土陆域的固有特性是造成其频受雷击的重要因素之一。针对广西北部湾吹填土的电冲击特性,结合理论分析、室内试验和数值模拟的方法对其展开研究,探究其与外电场的响应规律。首先从Archie电阻率模型理论出发,结合土体的介电常数公式,提出了吹填土的电冲击模型,认为影响其与外电场之间的响应规律的主要因素是干密度、含水率和含盐量。电冲击试验表明,吹填土击穿场强随含水率对数和干密度对数的升高呈线性降低,随含盐量的升高呈指数降低。再结合数值模拟结果发现,含水率、干密度和含盐量越高,吹填土内部场强越大,受冲击电场作用越显著,对外电场的响应越强烈,其中又以含水率和含盐量的影响为主。     

基于电阻率的非饱和土基质吸力预测

针对非饱和土吸力测量与计算的困难,提出了一种利用土的电阻率结构参数预测基质吸力的方法。通过滤纸法测试重塑膨胀土的基质吸力,分析了非饱和土的基质吸力与体积含水率的关系,即土-水特征曲线的变化规律。在测试非饱和土横向与竖向电阻率的基础上,探讨了非饱和土的电阻率与其体积含水率的关系,并进一步研究了土的电阻率结构性参数 、 和 随体积含水率的变化规律。发现基质吸力与电阻率结构参数 之间存在良好的抛物线关系,利用这种关系可以准确预测非饱和土的基质吸力。利用土的电阻率结构参数预测吸力,避免了吸力量测与计算的困难,简单实用。     

Response characteristics and tomographic monitoring of the electric capacitance after irrigation in the soil–plant (ginkgo tree) continuum in a laboratory

The temporal and spatial variations of electrical capacitance of the soil–plant continuum in a laboratory were investigated. Based on a potted ginkgo tree with six irrigations, the electrical capacitance of the soil, trunk, and soil–trunk electrode pairs was measured, corresponding soil water content was computed, and spatial imaging was conducted. The soil electrical capacitance increases to a local maximum value immediately after irrigation, and then decreases gradually with time. During the late period of each drying procedure, there is a decussation area of high water content at the middle part of the circular region in the spatial imaging, which indicates root distribution in soil to some degree. Upon irrigation under a dry soil condition, the trunk electrical capacitance gradually increases for a period, and then begins to fluctuate with time; whereas, under a wet soil condition, it always fluctuates with time. Under a stable condition of overall change, the trunk electrical capacitance changes saltatorially and daily periodically. The temporal variation of the soil–trunk electrical capacitance exhibits both the character of soil and trunk.     

岩石电容——电介质电容之一

岩石是电介质之一.发现了岩石具有电容性,就应该探讨电介质是否具有电容性.本文根据岩石充放电一次电压和二次电压得到的岩石电容性给出了电介质电性模型.依据介质为电容电阻串联的新的模型,利用跨学科的前人对黄铜矿、黄铁矿、玻璃、云母、琥珀以及聚乙烯胶囊的时间域和频率域实验数据进行验证,并进一步与介电常数经典理论对比验证.验证结果表明,新模型的电性参数关系与黄铜矿、黄铁矿、玻璃、云母、琥珀及聚乙烯胶囊实测电性参数关系非常吻合;新模型适用于各种时间的弛豫以及直流和交流环境;新模型也完全能表达各种介电常数和电损耗的关系.主要结论有,介质是漏电的含有内阻的电容;它可以看作若干含内阻电容和漏电电阻的并联电路;介质的阻抗及等效电容取决于介质的电容、电容内电阻、介质电阻及频率;岩石电容是电介质电容的一种.     

用NMR法验证在冻土中TDR标定曲线的可靠性

TDR技术是一种较新的测定冻土中未冻水含量的方法, 它有很多优点, 但是由于TDR技术应用于冻土中测定未冻水含量起步较晚, 其可靠性有待于进一步验证. NMR技术是一种较成熟的测定冻土中未冻水含量的方法. 通过应用NMR和TDR两种方法对不同初始含水率下的棕壤土冻融特征曲线的测定, 对TDR法的标定曲线的可靠性进行了验证. 结果表明: 采用TDR法与NMR法测得的冻融特征曲线明显不同, 在相同负温下前者的测定值高于后者, 且其差异随初始含水率增高而加大; NMR法的测定结果表明, 初始含水率对冻融特征曲线的影响很小; 而TDR法的测定结果表明, 由TDR法测得的冻融特征曲线, 受初始含水率影响很大, 随初始含水率的升高而明显加大. 这表明含冰率和温度对冻土的介电常数影响很大, 因此, 采用TDR法测定冻土的未冻水含量时不宜采用原有的标定曲线.     

LNAPL污染物及水含量对石英砂介电常数的影响

为研究LNAPL污染物及水含量对石英砂介电常数的影响,首先以石英砂模拟单一土壤环境,利用柴油和水的混合物来模拟轻非水相液体(LNAPL)污染物污染土壤,然后通过矢量网络分析仪和开口同轴探头,分析高频超宽带电磁波在土壤中的反射并计算介电常数。实验结果显示,石英砂介电常数随着体积含水量和含油量的增加而增加,直到达到饱和状态,此时石英砂介电常数趋于稳定;石英砂介电常数实测值处于极化模型和De Loor模型之间,结果与De Loor模型接近。     

时域反射技术在地学研究中的应用

综合介绍了时域反射技术在地学研究中的应用。包括时域反射仪的工作原理、测量参数(特征阻抗、介电常数、土壤含水量、土壤电导率)、时域反射探针技术以及时域反射技术在农业地质、水文地质、灾害地质等研究中的应用与进展。     

Experimental investigation of dry density effects on dielectric properties of soil–water mixtures with different specific surface areas

The dielectric constant of soil is used to estimate its water content in a range of applications. Unlike the widely known effect of water content on the soil dielectric constant (consistent direct proportionality), only a limited number of studies have reported the effects of soil dry density, however, with equivocal results. This paper, therefore, investigates the effects of dry density or degree of compaction on the dielectric constant of five different soil types. The results of the experimental work for the soils ranging from sand to Bentonite clay with distinct specific surface areas were evaluated based on the use of two simple mixture models (De Loor and Birchak). The effects of dry density on the soil dielectric constant were found to be soil type dependent. This is demonstrated by the experimental data and further proven by the modified De Loor model. The behavior is shown to be defined by the changes in the free water, bound water, and solid particle volume fractions, ultimately controlled by the soil specific surface area. The dielectric constant changes from being directly proportional to dry density to inversely proportional at a threshold specific surface area of between 122 and 147 m²/g. Supported by the experimental observations, parametric analysis has revealed that the range for the dielectric constant of bound water was found to be 9–37, while the geometrical parameter α in the Birchak model was found to be 0.4–0.8.

     

地球物理学方法在区分多年冻土区咸水层和淡水融化层中的应用

Direct current resistivity and ground penetrating radar surveys were employed to obtain the value of the resistivity and dielectric constant in the brine near the Barrow, Alaska. The geophysical surveys were undertaken together with the permafrost drilling program for the measuring of the ground temperature regime and for the core sampling. The sampled cores were measured for their physical and chemical properties in the laboratory under different temperature conditions (-60 to 20 ℃). Laboratory results support field observations and led to the development of a technique for distinguishing freshwater taliks and brine layers in permafrost. These methods were also employed in freshwater taliks near Council,Alaska. The electrical resistivity is a powerful and sensitive parameter for brine detection. However, the resistivity is a less sensitive indicator of the soil type or water content under highly saline conditions.High frequency dielectric constant is an ideal second parameter for the indication of the soil type, liquid water content and other physical properties. The imaginary part of the dielectric constant and resistivity have a significant dependence upon salinity, i.e. upon freezing temperature. The ground temperature regime and the freezing point of the brine layer are important parameters for studying the electric properties of permafrost terrain.     

土的电阻率室内测试技术研究

土的电阻率可反映土的组成和结构特征。室内测试是研究土电阻率的主要方法之一。本文在分析影响电阻率因素的基础上,总结了国内外现有土电阻率测试的原理与方法,介绍了作者研制的一种实用土电阻率测试仪,并通过典型试验,论证了该测试仪器的可行性,可以在室内土电阻率测试试验中应用。     

基于粒径分布曲线的非饱和砂土土水特征曲线概率预测模型

土水特征曲线定义了非饱和土的基质吸力和含水量之间的关系,与非饱和土的渗流和强度等特征有密切关系.本文通过对100组砂土的粒径分布曲线和土水特征曲线进行分析,结合常用的VG模型提出了基于粒径分布曲线的非饱和砂土土水特征曲线概率预测方法,并基于另外30组数据对提出的模型进行了验证.研究表明,基于粒径分布曲线无法唯一确定土体...     

Correlation of unsaturated soil and dielectric property for monitoring of subsurface characteristics: development of unsaturated dielectric mixing models and its application

The behavior due to rainfall infiltrating the ground plays a role in landslides, groundwater recharge and various other ground responses. Most of these geotechnical behaviors have a correlation between soil pore space and soil volumetric water content in the unsaturated and saturated soil porous media. Therefore, the soil porosity associated with soil pores and the distribution of volumetric water content are significantly important hydrological characteristics. In the case of shallow slope failure such as landslide, the infiltration activity due to the connectivity of soil pore spaces in a porous media is induced. Slope failure may be attributed to the effect of a wetting front with the slope due to liquid infiltration, which changes the volumetric water content, soil matric suction and shear strength of the slope. This study was performed with an unsaturated injection test using a frequency domain reflectometry (FDR) dielectric device which measures the dielectric constant of unsaturated soil and the study then proposed the unsaturated dielectric mixing models to calculate soil porosity and effective porosity of unsaturated soils. From the experimental results the ratio of effective porosity to porosity of soils are measured in a range of 70–85%. These experimental results show a decrease of about 5–10% for unsaturated soil compared to the ratio of effective porosity to porosity of saturated soil. The infiltration passages of tracer material are restricted within the pore connectivity in the unsaturated soil which is caused by dead-pores in the soil. Using the FDR device and the unsaturated dielectric mixing models, we can consider the acquisition of physical properties to detect the infiltration activity, the response of the dielectric constant along with the injected tracer and hydrological parameters for the unsaturated soil porous media.     

Factors affecting the complex permittivity spectrum of soil at a low frequency range of 1 kHz–10 MHz

To establish the factors affecting the permittivity spectrum of soil in the low frequency range, the complex permittivities of soils were measured at the frequency range of 1 kHz–10 MHz. The effects of frequency, water content, soil types and heavy metal contamination on the dielectric response were evaluated and their theoretical mechanisms were discussed. Measurement distortions such as electrode polarization in the low frequency measurements were identified. The permittivity of the soil was analyzed at frequencies above 100 kHz, which was experimentally determined to be the limiting lower frequency. The real permittivities of the soils were strongly related to the volumetric water content, since it is determined by the polarizability of the free water. The effective imaginary permittivities of soils increased with volumetric water content due to the increase in conduction losses. The spatial polarization and conduction loss were found to be the main mechanism in the real and effective imaginary permittivity, respectively. Since such mechanisms are influenced by the specific surface area of the soil particles, the presence of particles with high specific surface area in the soil matrix was found to affect the permittivity of soil. Contamination of saturated soil by cationic species resulted in decreases in the real permittivity due to the decreased orientation polarization of water molecules caused by hydration of ions, but increases in the effective imaginary permittivity due to ionic conduction.