Current practice in the design of reinforced soil is to calculate the long-term strength of a reinforcement damaged during installation by multiplying the two partial safety factors RFID and RFCR. This procedure assumes that there is no synergy between them. To test this hypothesis, stepped isothermal method creep-rupture tests were performed on a polyester geosynthetic in its undamaged and damaged states. The geosynthetic chosen and the method of damage were specifically chosen to yield a 30% reduction in strength with the minimum of scatter. From simple tensile testing, the reduction factor for installation damage, RFID, was measured to be 1.42. Stepped isothermal method tests on the undamaged geosynthetic gave a creep-rupture characteristic with RFCR = 1.70 for 106 hours (114 years). Thus, the predicted reduction factor for the long-term strength of the damaged geosynthetic is (RFCR)(RFID) = 2.41. The measured reduction factor was 2.26, 7% less than predicted. It is recommended that the practice of using (RFCR)(RFID) is upheld in the knowledge that the prediction will be slightly conservative.