TIME-DEPENDENT MECHANICAL RESPONSE OF HDPE GEOMEMBRANES

Characterization of the long-term mechanical response of geomembranes used in waste-containment facilities is crucial to designing base line r and cover systems that perform satisfactorily. To investigate the lo ng-term mechanical response, strain-controlled multiaxial tension test ing was performed over a fourfold variation of strain rate using a new ly developed multiaxial tension-test apparatus capable of performing c onstant strain rate and constant stress creep tests. This device subje cts a geomembrane specimen to multiaxial stress states and allows for the development of strain conditions that vary from plane strain at th e clamped edges to balanced biaxial at the center. Results from testin g high-density polyethylene (HDPE) specimens indicate that the secant modulus and strength decreases considerably at strain rates appropriat e for long-term field applications. The strength of HDPE measured at a typical laboratory strain rate of 1% per minute can be more than twic e the strength predicted at a strain rate of 1E-6% per minute, which m ay be representative of field performance for a typical 30-year design life. A hyperbolic model and the Singh-Mitchell creep model (which wa s originally formulated for soils) are shown to capture the time-depen dent mechanical response of HDPE well.