For a safe design of geosynthetic reinforced structures the long term behavior and especial-ly the available resistant tensile force over the time under variable boundary conditions, such as tempera-ture, chemical influences or loading conditions, has to be well known and understood. The known effects on the tensile strength, are considered in the design by applying so called reduction factors, e.g. for creep, installation damage or chemical degradation. Those reduction factors are determined separately without considering a possible interdependency. The main mechanism of the chemical degradation of polyester in high alkaline environment is the so called alkaline or “external” hydrolyses. In contrast to the internal hy-drolyses, which provokes an evenly distributed, very slow degradation of the whole cross-section of the synthetic material, the external degradation generates a much faster “surface corrosion” with creation of fissures or cracks. If this does occur while the material is loaded, the fissures are widened and their prop-agation are dramatically accelerated. This results in much faster loss of strength. The paper reports on the consequence on the long term tensile strength if a PET reinforcement in high alkaline environment is sub-jected to tensile load.