Translational slides in clays are often characterized by long-lasting intermittent movements associated with the fluctuations of pore-water pressures. These are in turn associated with either the rise or fall of the groundwater table for the case where the failure surface develops in the saturated zone below the phreatic surface or the infiltration of rainwater for the case where the failure surface is located above the phreatic surface in the unsaturated zone. Physically-based models designed to support hazard analysis of landslide movements and early warning systems require the integration of time-dependent (viscous) constitutive models for the shear displacements because landslide movements are typically controlled by the viscous behaviour of the clay geo-material. However, little information is available on the creep response of clay geo-materials under unsaturated conditions concerning different water content and matric suction values. This thesis presents an investigation of the viscous response of a clay geo-material under saturated and unsaturated conditions. Creep and relaxation tests have been first carried out on saturated clay samples by means of direct shear box. For the creep tests, the shear force was increased to a target value and maintained constant while monitoring the shear displacements. For the relaxation tests, shear displacement was applied to a target value and maintained constant while monitoring the shear stress decay. To gain a conceptual understanding of the viscous response of the clay in shear, analogue models were developed based on combinations of springs and dashpots. The aim of this modelling simulation was to identify a single mechanical model to simulate both creep and relaxation response using single set of parameters. Tests on unsaturated samples at different water content were finally carried out to gain insight into viscous response of the clay under unsaturated conditions.