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Water Content Measurement in Expansive Soils Using the Neutron Probe

机译:用中子探针测量膨胀土中的水分

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摘要

Capacitance-type methods for measuring soil water content are known to be unreliable in expansive soils, as cracking disrupts the intimate contact between the soil and the measuring device. The neutron probe, which infers water content from the thermalisation of a cloud of neutrons, is potentially less affected by cracking. The effect of cracking on neutron probe measurements was investigated by a series of numerical simulations using an axisymmetric finite element model based on seven-group neutron-diffusion theory. The simulations employed a consistent soil cracking model based on Maryland clay, in which crack volumes are determined from the changes in void ratio in the shrinking bulk soil. The results show that the presence of cracks in a clay soil affects the inferred water content and that measurements affected by air-filled cracking under-predict not only the water content in the uncracked soil peds but also the average water content in the larger cracked soil mass. The reason for this under-prediction is understood by considering the spatial distribution of the thermalised neutrons in the cracked and uncracked soils. The fast neutrons emitted from the source are seen to diffuse preferentially along air-filled cracks, traveling a large distance from the detector before they become thermalised, thus reducing their likelihood of being back-scattered to the detector where they can be counted. The proximity of the first crack to the probe in the ground also affects the measurement. Water-filled cracks are seen to have the opposite (but lesser) effect to air-filled cracks. A comparison of a simple uniform width crack model to a more realistic model in which crack width varies with changing water content shows that the model is sensitive to crack distribution and that the linear calibration expressions that are typically employed for neutron probes are likely to be unreliable in cracked clay soils.
机译:电容式测量土壤含水量的方法在膨胀土壤中是不可靠的,因为开裂会破坏土壤与测量设备之间的紧密接触。从中子云的热化中推断出水含量的中子探针受裂纹影响的可能性较小。使用基于七族中子扩散理论的轴对称有限元模型,通过一系列数值模拟研究了裂纹对中子探针测量的影响。该模拟采用基于马里兰粘土的一致的土壤开裂模型,其中裂缝体积由收缩的散装土壤中空隙率的变化确定。结果表明,粘土中裂缝的存在会影响推断的水含量,充气裂缝影响的测量不仅会预测未破裂土壤小伙子的含水量,还会预测较大裂缝土壤的平均含水量。质量通过考虑裂化和未裂化土壤中热化中子的空间分布,可以理解这种预测不足的原因。从源头发出的快速中子可以优先沿着充满空气的裂缝扩散,在它们变得热之前,它会从探测器传播很远的距离,从而降低了它们向后散射到探测器中的可能性。地面上的第一个裂纹与探头的接近程度也会影响测量。可以看到充水的裂纹与充气的裂纹具有相反(但较小)的作用。将简单的均一宽度裂缝模型与更现实的模型进行比较,在该模型中裂缝宽度随含水量的变化而变化,表明该模型对裂缝分布敏感,并且通常用于中子探针的线性校准表达式可能不可靠在开裂的粘土中。

著录项

  • 来源
    《Geotechnical testing journal》 |2011年第3期|p.255-264|共10页
  • 作者单位

    Associate Professor, Priority Research Centre for Geotechnical and Materials Modelling, The School of Engineering, The Univ. of Newcastle, Callaghan,Newcastle 2308, Australia;

    Senior Fellow, The School of Engineering, The Univ. of Newcastle, Callaghan, Newcastle 2308, Australia;

    Professor, Dept. of Engineering Mechanics, Hohai University, Nanjing210098, China;

  • 收录信息 美国《科学引文索引》(SCI);美国《工程索引》(EI);
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

    neutron probe; finite element model; cracks;

    机译:中子探针有限元模型裂缝;
  • 入库时间 2022-08-18 00:12:00

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