The physical, chemical and engineering properties of clay, particularly for strength, deformation and permeability, largely depend on the characteristics of bound water. The bound water has been a hotspot research focus on the matter in soil mechanics, engineering geology and so on. Despite numerous studies on bound water of clay, a unified theory for the quantitative determination of bound water film thickness has not been proposed yet. This paper put forwards a point of view that the boundary of bound water in clay is located at the position of the water layer closest to the shear plane in the electrical double layer (EDL). Molecular dynamics (MD) was adopted to determine the thickness of bound water in Na-montmorillonite. The thickness of bound water in the pore containing two water layers was determined as about 6 Å regarded as tightly bound water after verifying by experimental data. The physical properties of bound water were also investigated showing that the bound water suffers compressive stress more than 10³ kPa from the clay surface and has shear viscosity several times greater than bulk water. Additionally, an application for predicting the hydraulic conductivity of clay by considering the bound water was introduced. A modified Kozeny-Carman equation with the variable viscosity of the seepage fluid was proposed, satisfying accuracy compared with experimental data. The findings in this study are helpful for revealing the mechanism of bound water on the surface of soil particles.

粘土的物理、化学和工程性质，特别是强度、变形和渗透性，在很大程度上取决于结合水的特性。束缚水一直是土力学、工程地质等领域研究的热点。尽管对粘土结合水进行了大量研究，但尚未提出用于定量测定结合水膜厚度的统一理论。本文提出粘土中结合水的边界位于双电层（EDL）中离剪切面最近的水层位置的观点。采用分子动力学（MD）来确定钠蒙脱石中结合水的厚度。经实验数据验证，含有两层水层的孔隙中结合水的厚度确定为约6 Å，视为紧密结合水。还研究了结合水的物理性质，表明结合水承受的压应力超过 10³  kPa 从粘土表面和剪切粘度比本体水大几倍。此外，还介绍了通过考虑结合水来预测粘土水力传导率的应用。提出了渗流流体黏度可变的修正Kozeny-Carman方程，与实验数据相比精度满足要求。本研究结果有助于揭示土壤颗粒表面结合水的形成机制。