不同渗透系数地层中泥浆渗透成膜试验研究

为了改善泥浆在高渗透性地层中难以成膜的问题,给泥水盾构工程中泥浆的配制提供参考,针对膨润土泥浆在砂土地层中渗透成膜的过程,以某高铁隧道砂卵石地层中泥水盾构施工为背景,开展了泥浆在不同渗透系数地层中的成膜试验。试验采用自行设计的内径为300 mm的泥浆渗透装置,利用分级加压的方式使泥浆压力转化为地层的有效支护压力,分析了地层渗透系数、泥皮厚度、泥浆滤失量三者的关系以及成膜的判断方法。研究结果表明:泥皮厚度随着地层渗透系数的变大表现为先增大后减小,而泥浆滤失量随之不断增大,在泥皮厚度增大阶段厚度与滤失量成正相关,在泥皮厚度减小阶段厚度与滤失量成负相关,泥皮厚度最大值点是泥浆渗透有效成膜的转折点;过泥皮厚度最大值点的k线可为泥浆适应性判断提供参考,k线左侧是泥浆渗透成膜的稳定区域,而在k线右侧区域,泥浆颗粒的消耗量与滞留量呈负相关,泥浆的适应性较差;采用D₁₅/d₈₅的值能大致地划分泥浆的渗透成膜类型,D₁₅/d₈₅在10附近是成膜是否有效的过渡区,该区域对应k线右侧临近区域,泥浆渗透可以形成泥皮,但稳定性较差,故泥皮不能作为有效泥膜形成的标志;增大泥浆中颗粒的粒径能提高泥浆的成膜能力,使泥浆k线向右移动,能有效解决泥浆在高渗透性地层中成膜难的问题。

Experimental Study on Filter Cake for Slurry Infiltration in Different Permeability Strata

To solve the problem of filter cake formation in highly permeable strata and provide a reference for slurry preparation in construction with a slurry shield, in view of the process of bentonite slurry infiltration in the sand stratum, a series of pressure filtration tests was conducted based on the shield construction practice of a certain high-speed railway tunnel. A self-designed apparatus with an inner diameter of 300 mm was used in the test, and the slurry pressure was transformed into the effective support pressure by means of staged pressure. The relationships among ground permeability, thickness of filter cake, and quantity of filtration in slurry infiltration was analyzed, as well as the method of judging filter cake formation. The results indicate that the thickness of the flter cake increases and then decreases as permeability increases, and the amount of discharged water increases accordingly. The thickness of the flter cake is positively correlated with the amount of discharged water in the increasing stage, but negatively correlated in the decreasing stage. The maximum point of thickness of the flter cake is the turning point of valid formation in slurry infiltration. The k-line, exceeding the maximum thickness of the flter cake, can provide a reference for the judgment of slurry adaptability. On the left side of the k-line is the stable area of filter cake formation, while on the right side, the consumption of slurry particles is not directly proportional to the retention, and the adaptability of the slurry is poor. The value of D₁₅/d₈₅ can roughly divide the formation of filter cake. A D₁₅/d₈₅ value of approximately 10 is the transition between valid and invalid formations of the flter cake. This transition area corresponds to the area adjacent to the right side of the k-line, where an unstable flter cake can be built up. Therefore, the presence of a filter cake cannot be taken as a sign of the valid formation of the filter cake. By increasing the particle size of the slurry, the ability of slurry cake formation can be improved, thus moving the k-line to the right, which can effectively solve the problem of slurry cake formation in high-permeability strata.