The viscous properties of a polymer geogrid reinforcement and Toyoura sand were separately evaluated by changing stepwise the strain rate as well as performing sustained loading and load/stress relaxation tests during otherwise monotonic loading in, respectively, tensile loading tests and plane strain compression (PSC) tests. The viscous properties of the two materials were formulated and simulated in the framework of a non-linear three-component rheology model. The viscous properties of geogrid-reinforced Toyoura sand in PSC were evaluated. Strains in the geogrid were evaluated by the photogrametric method. The time history of tensile force in the geogrid during sustained loading at a constant external load obtained by analysing the measured time history of strains showed that the tensile force in the geogrid decreased with time. In that case, the possibility of ultimate creep rupture of the geogrid is very low.