There are few experimental results on the influence of the intermediate principal stress on the behaviour of unsaturated soils. Particularly, the effect of intermediate stress on the collapse response of a soil subjected to a suction reduction path has not been studied in depth. This paper describes soaking tests performed in a hollow cylinder device, in which specimens of a compacted clayey silt have been saturated under constant mean and deviatoric stresses but at different intermediate stresses. The descriptions of sample preparation, testing device characteristics, test procedures and interpretation methods are presented. Preliminary analyses of the results seem to indicate that collapse is larger when the intermediate stress coincides with the minor one, i.e. under conventional axi-symmetric triaxial stress state.