基于核磁共振的土体孔隙水形态与渗透性研究

NMR-Based Study on Pore Water Form and Permeability of Soil

为探究水力梯度及孔隙水形态对土体渗透性的影响,测定重塑土样在0~500 kPa等8级吸力作用下的核磁共振T_(2)曲线;将孔隙水划分为可动水和不动水,定量分析可动水与吸力/水力梯度关系,并将水力梯度、可动水引入Coates渗透模型进行修正。结果表明:随着吸力从0 kPa增加至500 kPa(水力梯度从0增加到50),土体T2曲线面积和T_(2)最大值逐渐减小,T_(2)最小值几乎不变;可动水含量随吸力/水力梯度的增加先迅速增大,随后增速减小,最后趋于稳定,二者之间可用指数函数很好地拟合;不动水含量随吸力/水力梯度增加呈指数减小;可动水与不动水比值(FFI/BVI)和水力梯度呈良好的线性关系;简化Coates模型显示土体渗透率与水力梯度的二次方成正比;孔径是影响孔隙水渗流的关键因素,吸力/水力梯度作用下,土体孔隙水排出过程可用Young-Laplace方程描述,土体孔隙水由大孔隙开始排水,并逐渐向小孔隙发展,大孔隙先完成排水过程;水力梯度的增大会提高可动水占比,导致更多可动水参与渗流作用,导致土体渗透率显著增大;简化Coates模型直观展示了水力梯度-孔隙水形态-渗透率关系,该模型仅含有水力梯度一个未知数,可为路基稳定性分析与防护设计提供理论参考。

In order to explore the effects of hydraulic gradient and pore water form on soil permeability,the NMR T_(2) curves of disturbed soil are measured under 8 kinds of suction such as 0-500 kPa.The pore water is divided into movable water and immobile water,the relationship between movable water and suction/hydraulic gradient is quantitatively analyzed,and the hydraulic gradient and movable water are introduced into Coates permeability model to be corrected.The result shows that(1) the T_(2) curve area of soilmass and the maximum value of T_(2) gradually decrease as the suction increases from 0 kPa to 500 kPa(the hydraulic gradient increases from 0 to 50),and the minimum value of T_(2) is almost unchanged;(2) with the increase of suction/hydraulic gradient,the movable water first increases rapidly,then increases slowly,and tends to be stable finally;(3) these two can be well fitted with an exponential function,and the immobile water content decreases exponentially with the increase of suction/hydraulic gradient;(4) the ratio of movable water to immovable water(FFI/BVI) and hydraulic gradient is fitted with a linear function;(5) the simplified Coates model shows that the permeability of soil is directly proportional to the quadratic hydraulic gradient;(6) pore radius is the key factor affecting seepage of soil,under the influence of suction/hydraulic gradient,the process of water drainage can be described with Young-Laplace equation,the pore water starts to drain from macropores and gradually develops to micro-pores,and the macro-pores complete the drainage process firstly;(7) the increase of hydraulic gradient raises the proportion of movable water,leading to more movable water participating in seepage and a significantly increase of soil permeability.The simplified Coates model visually shows the relationship among hydraulic gradient,pore water form and permeability,containing only an unknown(hydraulic gradient),and providing a theoretical reference for subgrade stability analysis and protection design.