Lightweight Cemented Soils (LWCS), i.e., materials prepared by mixing natural soil, water and cement with air foam, are characterised by a heterogeneous microstructure, consisting of large foam-induced voids distributed in a cemented porous matrix. The reduced volume weight coupled with a good mechanical strength makes LWCS suitable for many geotechnical applications. However, the role of the microstructure on their macroscopic mechanical behaviour is not completely clear yet. A novel experimental investigation on the mechanical response of LWCS under triaxial loading paths using in-situ x-ray microtomography (XRμCT) is presented here, focusing on the evolution of foam-induced porosity. The tomographies are acquired during the shear phase of multiple triaxial compression tests, performed at different confining stress levels. Image analysis is employed for obtaining porosity maps and incremental strain fields of the samples. At low confining stress, the sample deformation essentially consists in the progressive opening of sub-vertical dilatant fractures connecting the foam-induced voids. At higher confining stresses, localised compactions develop in the cemented matrix along the directions coherent with the deviatoric load path. The results presented above indicate that the deformation and failure mechanisms of LWCS strongly depend on the mean stress level.