Permeability mechanism of PVC-P geomembrane at peripheral joints of high membrane-faced rockfill dams
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摘要: 高面膜堆石坝坝面PVC-P土工膜防渗结构表现优越，差异位移致使周边缝处防渗结构中PVC-P土工膜处于大延伸率状态，是防渗体系的薄弱环节。针对变形态PVC-P土工膜防渗性能是否满足运行期工程技术要求，选用自主研制的变形态土工膜渗透试验仪展开了多组延伸率试样渗透性能试验研究，利用低场核磁共振技术探究了变形态PVC-P土工膜微观渗透演变机理。结果表明：变形态PVC-P土工膜防渗性能衰减，渗透系数随延伸率的增加而增大；延伸率的增长使得膜内孔喉发育、含量增加及孔喉连通性增强，这是防渗性能衰减的本质原因。尽管研究成果表明变形态PVC-P土工膜仍具有较低的渗透系数，但高面膜堆石坝周边缝处PVC-P土工膜拉伸变形复杂，可能存在局部损伤或破损情况，建议采取工程技术措施降低其延伸率，以延长服役周期。Abstract: The PVC-P geomembrane in the impervious structure of high-membrane faced rockfill dams has excellent impermeability. However, it has large elongation at the peripheral joints due to differential displacement, which is the weak section of the impervious system. To identify whether the permeability of the PVC-P geomembrane in tensile deformation meets the engineering requirements during the operation, a series of permeability tests at several elongations are carried out by a self-developed permeability tester for geomembrane in tensile deformation, and the microscopic permeability of the PVC-P geomembrane in tensile deformation microscopically is explored by the low-field nuclear magnetic resonance (NMR) technology. The results show that the impermeability of the PVC-P geomembrane decreases, and the permeability coefficient increases with the increase of elongation. It further causes the development of pore throats, the increase of content and the enhancement of pore-throat connectivity within the geomembrane, which is the essential reason for the decrease of the impermeability. The PVC-P geomembrane in tensile deformation still has a low permeability coefficient, however, it has complicated tensile deformation at the peripheral joints of the high-membrane faced rockfill dams, which may cause local damages or breakage of the geomembrane. Consequently, it is recommended to take engineering measures to reduce the elongation to prolong the service life.
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图 3 PVC膜母材渗透系数-水压力曲线图
Figure 3. Curve of permeability coefficient and water pressure of PVC geomembrane base metal
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图 4 变形态试样渗透系数-水压力图
Figure 4. Diagram of permeability coefficient and water pressure of specimen in tensile deformation
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图 5 0%延伸率PVC-P低场核磁共振T2特征谱图
Figure 5. T2 characteristic spectra of PVC-P geomembrane of low-field NMR at elongation of 0%
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图 7 变形态试样低场核磁共振T2特征谱
Figure 7. T2 characteristic spectrum of low-field NMR of specimen in tensile deformation
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图 8 变形态膜低场核磁共振T22特征谱
Figure 8. T22 characteristic spectra of low-field NMR of specimen in tensile deformation
表 1 PVC-P土工膜主要物理力学特性表
Table 1 Main physical and mechanical properties of PVC-P geomembrane
技术指标 横向 纵向 厚度/mm 2.0±0.2 2.0±0.2 单位面积质量/(g·cm-2) 1.77 1.77 断裂强度/MPa 9.65 10.10 断裂延伸率/% 310.04 289.99 注：执行技术标准为《SL235—2012土工合成材料测试规程》[18]（以下简称《规程》）。 
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表 2 PVC-P土工膜母材孔喉分布表
Table 2 Distribution of pore throat of PVC-P geomembrane base metal
孔喉半径/μm 孔喉分布/% 1.0 MPa 1.1 MPa 1.2 MPa 1.3 MPa 1.4 MPa 1.5 MPa 0.05~0.15 0.2970 0.2890 0.2969 0.2818 0.2829 0.3506 0.15~0.25 0.1634 0.1487 0.1602 0.1697 0.1807 0.2003 0.25~0.35 0.0194 0.0267 0.0262 0.0374 0.0498 0.0504
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表 3 所有试验延伸率试样孔喉分布表
Table 3 Distribution of pore throat of specimen at all elongations
孔喉半径/μm 孔喉分布 0% 50% 80% 125% 0.03~0.05 0 0 0.1266 0.6039 0.05~0.15 0.2970 1.3989 2.1896 2.6535 0.15~0.25 0.1634 0.4370 0.6491 0.8537 0.25~0.35 0.0194 0.0435 0.0951 0.2187 0.35~0.40 0 0 0 0.0254
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