The rheological behaviour of a granular material can be investigated by a simple compression test. However, during very slow flow, the centre of the sample becomes drier with a consequent increase in the plastic yield stress. An Eulerian description of this two-phase flow is proposed, with separate mass balances for the solid skeleton and fluid, together with Darcy's law and a constitutive relation for the solid skeleton. Finally, a differential equation linking the spatial and temporal derivatives of the void ratio with the drainage parameters and skeleton characteristics is obtained and solved numerically. Several compression tests on mineral pastes validate the model. (C) 2003 Published by Elsevier B.V.