Abstract To study the shear behaviour of discontinuities filled with thin sand fillers a series of rotary shear tests was performed and the results were compared with those obtained for unfilled and planar discontinuities. Shear strength for the filled discontinuities was always lower than for an unfilled discontinuity. At the beginning of the shear phase rolling friction was considered to play an important role becoming more relevant than sliding friction. When shear displacement increased, the removal of the asperities of the rock surfaces, the fragmentation of the grains, and abrasion produced a dusty gouge affecting rolling friction and vanishing its influence and sliding friction become more relevant. Strain gauges glued at the lateral surfaces of the rock samples allowed for a better understanding of how stresses changed during the test. When the shear load was applied the stress field changed continuously during the test even for a planar and unfilled discontinuity eventually due to the removal of the asperities from the rock surfaces. For filled discontinuities this reorientation of the stress field became more gently probably because the grains of the filler rolled over the rock surfaces without removing so many asperities as in the case of unfilled discontinuities.