Behavior of Bituminous Geomembrane Seams Exposed to Tensile Stress
- Eric
- Jan 17
- 2 min read
Presentation at the GeoANZ#2 conference in Melbourne
Geomembranes can be exposed to tensile stresses for a variety of reasons such as drag-down on steep slopes or displacement caused by the settlement of the subgrade. For monolithic geomembrane products such as HDPE or LLDPE, fusion-seaming aims at creating a continuity at the polymer level between the two panels welded together. A well-made seam should theoretically see minor disruptions in the profile of molecular weight when going from one side of the seam to the other, and the seam itself should exhibit mechanical properties in the same range as the properties of the sheet.
On the other hand, bituminous geomembranes are typically made of a polyester nonwoven geotextile supported by a glass fiber mat, which are saturated with bitumen and covered with mineral particles. To seam panels together, the bitumen is melted over a width of approximately 20 centimeters. The performance typically requested for such a seam is a tensile-shear strength similar to the sheet strength, when tested at room temperature. However, there is an important difference between polyolefin and bituminous geomembrane seams: the bituminous geomembrane seam remains an heterogeneous assembly. There is no mechanical connection between the two panels seamed together, i.e., there is no continuity between the two nonwoven geotextiles. The transmission of in-plane stress from one side of the seam to the other solely depends on the shear strength properties of the bitumen in the welded area, where it acts as a binder between the two panels.
Given the dominant role of bitumen in the sealing efficiency as well as for the transmission of stress across a bituminous geomembrane seam, it is important to describe the rheology of bitumen, a viscoelastic product, which properties vary greatly with temperature. Bitumen goes from a crystalline, quasi-brittle structure at temperatures in the range of -20°C to -30°C, to an essentially liquid state, starting at temperatures ranging between +110°C to +130°C. While an elastic behavior can be observed under lower temperatures, a viscous behavior is usually observed under high temperatures.
On the other hand, the glass-fiber reinforcement exhibits a high tensile modulus across the range of potential service temperatures. While this modulus preserves the dimensional stability of the sheet, it also redirects a potential in-plane elongation toward the seams, which could affect their continuity under some circumstances.
To quantify the properties of the bituminous geomembrane and on its seams under a variety of temperatures, a series of tests were conducted using well-accepted ASTM Standards, but using a variety of conditioning temperatures. Properties such as the tensile modulus, the tensile strength and the elongation at break of the sheet as well as the seam shear strength were measured. The results obtained are used to discuss the risks associated with the development of in-plane tensile stresses.
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