基于三维纤维孔隙模型的土工织物管袋反滤失效机理研究

The filtration applications of geotextile tubes are typically subjected to biaxial tensile strains, which can cause the variations of pore size and results in the failure of filtration properties. Whereas the filtration design is based on the unstrained pore size，which results in the pore size turn out to be too large or small. The large pore sizes cause the unacceptable loss of soils, and even cause debacles. The small pore sizes can be easily clogged, and influence the dewatering of dam, which lead to landslides. In the previous studies，the fibers of geotextiles are projected onto the plane. The related formula can't explain the phenomenon that the pore sizes sometimes increase, and sometimes decrease with the biaxial tensile strains. In this research, the biaxial tensile strains of geotextile tubes will be tested in situ. The pore size on both plane and thickness direction will be estimated, and the three-dimensional pore structure model will be established. The geotextiles will be taken as orthotropic material. Image analysis will be adopted to evaluate the 3D pore structure of geotextiles under biaxial tensile strains. The mechanics model of strained pore structure of geotextiles will be established. Depending on that, the analytical solutions of several pore size parameters corresponding to biaxial strains will be proposed. The filtration properties under biaxial strains will be tested by gradient ratio test. The regression equation between strained pore sizes characteristics and strained filtration properties will be studied. Then the strained filtration properties can be predicted by the unstrained pore size characteristics. Furthermore，based on the principle of fluid mechanics, the change of flow section in the 3D model will be used to establish the theoretical contact between the pore size characteristics and filtration properties. The results is expected to provide theoretical references for the filtration design of geotextile tubes.

土工织物管袋常处于复杂的双向受拉状态，拉应变引起织物孔径显著变化，但现有设计均基于无应变状态的孔径，导致工程中孔径偏大或偏小。孔径偏大造成风浪吸砂跑砂，引起决堤；孔径偏小造成堤坝内孔隙水不能及时排出，引起滑坡。现有试验测得受拉孔径有时减小、有时增大。目前的理论研究多将纤维投影到平面，所得公式无法解释上述试验矛盾。本项目拟通过原位试验测试土工管袋应变水平，综合考虑织物平面及厚度方向的孔隙变化，建立三维纤维孔隙模型。将土工织物作为正交各向异性材料，通过数字图像法，测试不同应变水平、不同经纬向应变比受拉，土工织物纤维结构的变化，建立力学模型，推导孔径参数随应变的变化公式。运用淤堵试验，测试反滤参数随双向拉应变的变化。采用多元回归分析，建立微观孔径参数与宏观反滤参数变化的回归方程。通过流体力学原理，基于三维模型过流断面的变化，推导孔径与反滤参数的理论联系。研究成果可为土工管袋的反滤设计提供依据。

土工织物管袋在充填压力、上部堆载等复杂条件下，织物表面双向拉应变显著，造成满足反滤设计准则的管袋在工程中产生漏土或淤堵。本项目测试了不等轴双向应变比n分别呈1, 2, 3, 4时，土工织物细观孔径参数及宏观反滤性能参数随双向拉应变增长的变化。通过数字图像法，测试了土工织物孔径分布曲线、纤维排布参数的变化，分别建立了有纺及无纺织物的孔径结构模型，推导了土工织物开孔面积率及等效孔径随双向拉应变的变化公式，该公式可预测工程应变下的孔径参数变化，避免应变造成的反滤失效。.进行了不等轴双向拉应变下的梯度比试验及透水率试验，量化双向拉应变比n及应变水平对土工织物反滤性能的影响。试验结果表明，在指定的n下，长纤及短纤无纺织物梯度比（GR）随双向应变增大而增大（例如n=1时，由0%到30%：30%应变，试样CN100的GR值由1.22增加到5.57，试样CN400的GR值由4.53增加到26.64），应变对于较厚织物的反滤性能影响更明显；系统渗透系数及漏土量随双向拉应变增大而减小。在机织方向应变水平相同时，GR值随应变比n的增大呈现减小趋势，但漏土量随n的增大而减小。说明应变值越大，越容易发生淤堵，而漏土量与孔径形态和大小都有关，n值较大的不等轴应变使孔径窄长，减小了漏土量。当机织方向应变相同时，同种无纺织物n=1组的系统渗透系数和透水率均比n=2,3,4组大，但n=2,3,4组结果之间没有稳定的大小关系，说明等轴双向拉应变比不等轴双向拉应变更能有效地增大孔隙率，而不等轴双向拉伸下，纤维排布存在可变性，使n=2,3,4组渗透系数及透水率不与n值呈现稳定的相关性。单位面积质量相同时，相同应变条件下的长纤织物呈现比短纤织物GR值大，渗透系数小，漏土量少的总体趋势。本项目建立了细观及宏观土工织物参数之间的联系，揭示了土工织物在双向拉应变下的反滤失效机理。