Permeability is one of the most important engineering properties of clayey soil. However, the traditional method for determining the permeability coefficient is time-consuming. To establish a simple and accurate predictive method to obtain the permeability coefficient of artificial clayey soil based on the double cut-off transverse relaxation times (T₂ cut-offs) using low-field nuclear magnetic resonance (NMR) technology, eight kinds of artificial clayey soil with different mineralogical compositions were prepared in the laboratory. Evaporation tests at 40°C were carried out on the saturated artificial clayey soil samples in an oven. During the evaporation process, NMR tests were also performed on the artificial clayey soil every hour. The results showed that the evaporation process could be divided into three stages according to different evaporation rates: the constant rate stage (CRS), the falling rate stage (FRS), and the residual stage (RS). The water evaporated in the CRS and FRS was defined as the absolute movable water and the partially movable water, respectively. The water that could not evaporate in the RS was defined as the immovable water. Based on the cumulative signal amplitudes in the T₂ spectrum corresponding to different kinds of water, the double T₂ cut-offs were defined. On the basis of the double T₂ cut-offs and T₂ spectrum of the saturated sample, an improved Timur–Coates (TC) model was established. The prediction capability of the improved model was evaluated by finding the determination coefficient (R²), mean absolute error (MAE), and root-mean-square error (RMSE). Compared with the typical TC model, the prediction accuracy of the improved model was much higher. In addition, the relationships between the double T₂ cut-offs and fractal dimension (D) of the T₂ spectrum of saturated artificial clayey soil were also identified.

中文翻译：

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基于双T2临界值的人工黏土渗透系数确定的改进预测模型

渗透性是黏土最重要的工程性质之一。然而，用于确定渗透系数的传统方法是费时的。建立基于双截止横向弛豫时间（T _2）的人工黏土渗透系数的简单准确预测方法使用低场核磁共振（NMR）技术，在实验室中制备了八种具有不同矿物组成的人造粘土。在烤箱中对饱和的人工粘土土壤样品在40°C下进行蒸发测试。在蒸发过程中，每小时还对人造粘土进行NMR测试。结果表明，根据蒸发速率的不同，蒸发过程可分为三个阶段：恒速阶段（CRS），下降阶段（FRS）和残留阶段（RS）。在CRS和FRS中蒸发的水分别定义为绝对活动水和部分活动水。在RS中不能蒸发的水被定义为不可移动的水。T ₂光谱对应于不同种类的水，定义了两个T ₂截止值。基于饱和样品的双重T ₂截止值和T ₂光谱，建立了改进的Timur-Coates（TC）模型。通过找到确定系数（R ²），平均绝对误差（MAE）和均方根误差（RMSE）来评估改进模型的预测能力。与典型的TC模型相比，改进模型的预测精度更高。另外，双重T ₂截止与分形维数（D）之间的关系。还确定了饱和人工粘土的T ₂光谱。