According to relevant new regulations in China, a composite liner system involving geosynthetic materials must be installed at the bottom of an expanded landfill. The deformation and integrity of the composite liner under a variety of factors are important issue to be considered in the design of a landfill expansion. In this paper, we investigate the strain distribution in geosynthetic materials within the composite liner system of expanded landfills, including strains in geosynthetic materials resulting from overall settlement and lateral movement of landfills, localized subsidence in landfills, and differential settlement around gas venting wells. The allowable strains of geosynthetic materials are discussed based on the results of tensile tests, and the corresponding design criteria for composite liner systems are proposed. Meanwhile, practical measures allowing strain control in geosynthetic materials used in landfill engineering are proposed.