Weak layers of snow are thin layers of low density and cohesion that consist of a complex network of sintered ice grains. These layers are very fragile and their failure is considered to be the primary cause of dry slab snow avalanche release. This study reports on the results of modelling the mechanical response of these weak snow layers with discrete element (DEM) simulations, using X-ray microtomographic images of real snow samples as input data. An original method for approximating arbitrary grain shapes in DEM was developed that enabled us to accurately capture the effect of snow microstructure on the mechanical behaviour of varying snow types, including weak snow layers. Three samples of different snow types have been subjected to simple shear simulations with varying normal pressure and a qualitatively similar result was obtained for all three samples. The simulation results permitted us to study the failure as well as post-peak strain softening and residual stress in snow. Failure envelopes of different snow types are presented and the effect of the microstructure on the failure envelope characteristics is analyzed. A detailed investigation of the interplay between shear failure and collapse is conducted and put into perspective by the damage that progressively propagates through the specimen.