TDR探头设计及含水量和干密度的联合监测技术
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摘要
含水量和干密度是岩土工程中两个重要的物理参数,常常被用于路基、边坡、堤坝及地基处理等施工过程的质量控制。常规的测试方法及装置需对含水量及干密度分别进行测试,并且无法进行长期监测。由于TDR(时域反射法)能快速、方便、准确地测试含水量和干密度,已经被越来越多的工程实际应用。ASTM标准提供了测试的方法和探头。但是,该标准中的探头只能测试地表土体的含水量和干密度,而无法埋置于深层土体中进行长期监测。传统的TDR探头无法满足多尺度含水量与干密度的要求,故需设计新的探头。
土体在外加电磁场作用下会表现出电化学特性,该特性与土体中各组分的体积比、土体类型以及土中水有很多大关系。介电常数是反映介质在交变电场作用下极化程度的物理量,反映储存于材料中能量的程度。土体的介电常数受含水量的影响。电导率是反映了电荷在电场作用下的活动能的一个物理量。土体的电导率主要受到固体颗粒的电导率、土体的孔隙率、饱和度和孔隙液体的电导率影响。时域反射法(TDR)技术可以测试土体的介电常数和电导率。
从TDR系统的组成出发,对探头的构造形式、探针的间距、直径和探头的影响区域等进行系统的分析,利用有限元软件HFSS和Matlab程序进行计算,并提出了合理设计TDR探头的方案。当探针长度大于20cm时,可以将仪器造成的测试误差控制在1%以内,比较理想。探针的间距和探针的直径之比不应该大于10,以降低探针附近的能量集中。研制出了模型试验探头和现场试验探头。
介绍了几种土体介电常数和含水量关系的模型,可以建立土体介电常数和含水量的关系,通过TDR测试得到的介电常数来计算土体的含水量。含水量和干密度的联合测定方法主要有两种：两步法和一步法。两步法和一步法测试之前都需要进行参数的标定,本文对4类不同的土进行了参数标定,并建立了参数标定的数据库。
利用模型试验探头来监测模型地基制备和饱和过程。地基制备过程中不同深度各层填土含水量和干密度的测试结果表明：含水量测试绝对误差在3%之内,干密度测试相对误差在3%之内,测试结果有很高的精度。地基制备完成后,从模型槽底部向上加压注水饱和。通过埋设在不同深度的TDR探头,监测整个饱和的过程。试验结果表明：试验结束时土体未达到完全饱和,饱和度约为94%。利用现场试验探头来测试勘察工程现场不同深度土体含水量和干密度,与传统的方法比较,误差在5%以内,能满足工程实际的需要。
Soil water content and dry density are important parameters for quality control in the process of construction of Embankment, slope, dam and foundation treatment in geotechnical engineering. Soil water content and dry density are needed to be measured by the traditonal method and intruments and couldn't be monitored in a long time. Since TDR(Time domain reflectometry) could measure the soil water content and dry density quickly, easily and accurtately, it had been greatly applied in engineerings. The standard of ASTM provided the method of measurement and the probe. However, the probe of the standard could only measure the surface soil water content and dry density and couldn't be embedded in deep soil to monitor the soil water content and dry density in a long time. The traditional probe couldn't meet the demand of measurement of water content and dry density in different scales. Therefore a new probe needed to be designed.
Soil would show the electrochemical property in an external electromagnetic field. This property had a great relationship with the soil volume ration of each component, the soil type and water. The dielectric constant reflected the physical parameter of the degree of polarization in an alternating field and the degree of energy stored in the material. The dielectric constant of soil water content was affected by water content. The conductivity reflected the physical quantity of the charge of activities in the electric field. The conductivity of soil were mainly affected by the conductivity of solid particles, soil porosity, saturation, and the conductivity of the pore fluid. The technology of Time domain reflectometry (TDR) could be used to measure the dielectric constant and the conductivity.
According to the compositon of the TDR system, the form of the probe, the spacing of the probe and the diameter of the probe were analyzed in system. We used the finite element software HFSS and Matlab program to calculate and propse reasonable method of the probe design. When the probe was longer than 20cm, the error due to the instrument were less than 1%. The ratio of probe spacings to probe diameter should not be greater than about 10, which would reduce the energy concentration around the probe. Finally, a model test probe and field test probe were developed.
Several relationships between the dielectric constant and water content were introduced. We could obtain the water content by the dielectric constant measured by TDR using the relationship between the dielectric constant and water content. There are two meathods of the joint measurement of water content and dry density:two step method and one step method. Two step method and one step method requied the calibration of parameters before the start of measurement. This paper did the calibration on 4 different types of soil and established the database of paramters.
The model test probe was used to monitor the process of foudation eastablishment and saturation. The results of the water content and the dry density at the process of foudation establishment in different depths showed that:the abosolute error of water content was less than 3% and the relative error of dry density was less than 3%. The results had a great accuracy. When the foudation establishment was completed, water was impoured into soil from the bottom of the model tank with pressure. The TDR probes were embeded to monitor the process of saturation. The results of experiment showed that:the soil didn't reach the full saturation at the end of experiment and the degree of saturation is 94%. The field test probe was used to obtain water content and dry density at different depths in field. The error of water content and dry density was less than 5%, which- would meet the requirements of engineerings.
土体在外加电磁场作用下会表现出电化学特性,该特性与土体中各组分的体积比、土体类型以及土中水有很多大关系。介电常数是反映介质在交变电场作用下极化程度的物理量,反映储存于材料中能量的程度。土体的介电常数受含水量的影响。电导率是反映了电荷在电场作用下的活动能的一个物理量。土体的电导率主要受到固体颗粒的电导率、土体的孔隙率、饱和度和孔隙液体的电导率影响。时域反射法(TDR)技术可以测试土体的介电常数和电导率。
从TDR系统的组成出发,对探头的构造形式、探针的间距、直径和探头的影响区域等进行系统的分析,利用有限元软件HFSS和Matlab程序进行计算,并提出了合理设计TDR探头的方案。当探针长度大于20cm时,可以将仪器造成的测试误差控制在1%以内,比较理想。探针的间距和探针的直径之比不应该大于10,以降低探针附近的能量集中。研制出了模型试验探头和现场试验探头。
介绍了几种土体介电常数和含水量关系的模型,可以建立土体介电常数和含水量的关系,通过TDR测试得到的介电常数来计算土体的含水量。含水量和干密度的联合测定方法主要有两种：两步法和一步法。两步法和一步法测试之前都需要进行参数的标定,本文对4类不同的土进行了参数标定,并建立了参数标定的数据库。
利用模型试验探头来监测模型地基制备和饱和过程。地基制备过程中不同深度各层填土含水量和干密度的测试结果表明：含水量测试绝对误差在3%之内,干密度测试相对误差在3%之内,测试结果有很高的精度。地基制备完成后,从模型槽底部向上加压注水饱和。通过埋设在不同深度的TDR探头,监测整个饱和的过程。试验结果表明：试验结束时土体未达到完全饱和,饱和度约为94%。利用现场试验探头来测试勘察工程现场不同深度土体含水量和干密度,与传统的方法比较,误差在5%以内,能满足工程实际的需要。
Soil water content and dry density are important parameters for quality control in the process of construction of Embankment, slope, dam and foundation treatment in geotechnical engineering. Soil water content and dry density are needed to be measured by the traditonal method and intruments and couldn't be monitored in a long time. Since TDR(Time domain reflectometry) could measure the soil water content and dry density quickly, easily and accurtately, it had been greatly applied in engineerings. The standard of ASTM provided the method of measurement and the probe. However, the probe of the standard could only measure the surface soil water content and dry density and couldn't be embedded in deep soil to monitor the soil water content and dry density in a long time. The traditional probe couldn't meet the demand of measurement of water content and dry density in different scales. Therefore a new probe needed to be designed.
Soil would show the electrochemical property in an external electromagnetic field. This property had a great relationship with the soil volume ration of each component, the soil type and water. The dielectric constant reflected the physical parameter of the degree of polarization in an alternating field and the degree of energy stored in the material. The dielectric constant of soil water content was affected by water content. The conductivity reflected the physical quantity of the charge of activities in the electric field. The conductivity of soil were mainly affected by the conductivity of solid particles, soil porosity, saturation, and the conductivity of the pore fluid. The technology of Time domain reflectometry (TDR) could be used to measure the dielectric constant and the conductivity.
According to the compositon of the TDR system, the form of the probe, the spacing of the probe and the diameter of the probe were analyzed in system. We used the finite element software HFSS and Matlab program to calculate and propse reasonable method of the probe design. When the probe was longer than 20cm, the error due to the instrument were less than 1%. The ratio of probe spacings to probe diameter should not be greater than about 10, which would reduce the energy concentration around the probe. Finally, a model test probe and field test probe were developed.
Several relationships between the dielectric constant and water content were introduced. We could obtain the water content by the dielectric constant measured by TDR using the relationship between the dielectric constant and water content. There are two meathods of the joint measurement of water content and dry density:two step method and one step method. Two step method and one step method requied the calibration of parameters before the start of measurement. This paper did the calibration on 4 different types of soil and established the database of paramters.
The model test probe was used to monitor the process of foudation eastablishment and saturation. The results of the water content and the dry density at the process of foudation establishment in different depths showed that:the abosolute error of water content was less than 3% and the relative error of dry density was less than 3%. The results had a great accuracy. When the foudation establishment was completed, water was impoured into soil from the bottom of the model tank with pressure. The TDR probes were embeded to monitor the process of saturation. The results of experiment showed that:the soil didn't reach the full saturation at the end of experiment and the degree of saturation is 94%. The field test probe was used to obtain water content and dry density at different depths in field. The error of water content and dry density was less than 5%, which- would meet the requirements of engineerings.
引文
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Birchak, J. R., Gardner, C. G., Hipp, J. E., and Victor, J. M. High dielectric constant microwave probes for Sensing soil moisture, Proc. IEEE,1974,62(1):93-98.
Chen, R. P., Daita, R. K., Drnevich, V. P., and Kim, D. H. "Laboratory TDR monitoring of physico-chemical process in lime kiln dust stabilized clayey soil." Chinese Journal of Geotechnical Engineering,2006,28(2):249-255.
Chen, R. P., Drnevich V. P., Yu, X., Robert L.N., and Chen, Y. M. Time Domain Reflectrometry Surface Reflections for Dielectric Constant in Highly Conductive Soils. Journal of Geotechnical and Geoenvironmental Engineering, ASCE,2007,133(12):1597-1608.
Chen R. P., Chen Y. M., Xu W., Liang Z. G., and Feng W. A new equation for dielectric permittivity of saturated soils based on polarization mechanics. Journal of Zhejiang University-Science A,2008,9 (3):293-302
CHEN R P, XU W, CHEN Y M. Measuring dielectric constant in highly conductive soils based on surface reflection coefficients [J]. Journal of Geotechnical and Geoenvironmental Engineering,2010,47:197-206
Dalton, F.N., Herkelrath, W.N., Rawlins, D.S., Rhoades, J.D., Time domain reflectometry:simultaneous measurement of soil water content and electrical conductivity with a single probe. Science,1984,224:989-990.
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Davis, J.L., Chudobiak, W.J., In situ meter for measuring relative permittivity of soils. Geol. Surv. Can.,1975,75(1):75-79.
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