Deformation behavior of seepage-control geomembranes in the core wall of a rockfill dam is crucial to the safety of its seepage-control system. However

conventional uniaxial tensile tests are of low accuracy in modeling the multidirectional stress state that dominates the operation of a geomembrane. To clarify the deformation capacity of a geomembrane under its real operating conditions

this study puts forward a solution to the challenges in material selection for a rockfill dam core wall

conducts comparative uniaxial and multiaxial tensile tests on three geomembrane materials－HDPE

LLDPE and PVC commonly used in dam seepage-control systems－and examines the mechanical properties of these geomembranes. The findings reveal traditional uniaxial tests overestimate the deformability of geomembranes－the yield strains of HDPE and LLDPE under multiaxial tension (1.6% - 2.4%) are significantly lower than those under uniaxial tension (13%)

while PVC exhibits no distinct yield point. However

PVC geomembrane demonstrates ultimate tensile strains (92.8% - 112.4%) that far exceed those of PE (by 34.4% - 50.0%). If elastic-stage strain energy density is used to evaluate deformation adaptability

PVC exhibits the highest elastic strain energy density under multiaxial loading

substantially greater than both PE variants

and thicker PVC geomembranes show greater deformation accommodation capacity. This study demonstrates the elastic strain energy density derived from multiaxial tensile tests can serve as a reliable criterion for assessing deformation performance of seepage-control geomembranes.