A limitation with HDPE geomembranes is their susceptibility to stress cracking – a brittle failure below the yield point of the geomembrane. Applied stresses (particularly localised stress concentrations) and excessive strain may stimulate the development of stress cracks in HDPE geomembranes. These processes may occur relatively quickly if the antioxidants are depleted. This paper compares HDPE geomembranes for their response to potential stress cracking strains. One of the geomembranes compared was manufactured using Polyethylene of Raised Temperature Resistance (PERT) resin. This resin gives higher stress crack resistance and greater tensile strength/elongation than some other resin formulations. The stress crack resistance for a 1.5 mm (60 mil) thickness of the PERT based HDPE geomembrane – also referred to as new HDPE geomembrane in this paper is three times more than that for a standard HDPE geomembrane that has material properties that meet the minimum GRI GM13 requirements – also referred to as a GRI GM13 spec geomembrane in this paper. Large scale high pressure strain testing was completed on the new HDPE geomembrane and the GRI GM13 spec geomembrane in order to evaluate their response to localized strain from point loading that may be associated with stress cracking. The results show fewer localized strains across the geomembrane in comparison with a standard GRI GM13 spec HDPE geomembrane under similar loading conditions. The geomembranes are also being tested for stress cracking and retention of OIT and HP OIT after aqueous immersion of 180 degree dead-folds in concentrated brine solution. The large scale high pressure strain testing is discussed.