High-density polyethylene (HDPE) geomembrane sheets are assembled through polymer fusion, where a quantity of energy is supplied at the sheets’ interface. As long term performanc,e is concerned, one· of the observed consequence of time effect is an increased modulus along with a reduced strain value at break. Either a result of time effect or a consequence of welding condition, this strain reduction must be limited in order to avoid local failure, leak development or brittlenes . Time effect over secant modulus is discussed and the importance of seaming process is enphasised.