A 10-cm-thick layer of synthetic fault gouge (angular quartz sand with millimetric grains) is sheared over several meters at low confining pressures (0.1--0.5 MPa) using a ring shear apparatus (inner radius of 10 cm). The shear strength of the gouge exhibits a significant slip-weakening behavior active over decimetric slip distances. Tests on the influence of particle size, particle geometry, and boundary conditions are reported. A long-term compaction of the gouge samples is also observed but appears to be decoupled from the frictional slip weakening. At microscale, series of photographs taken during the runs enable us to follow the formation of a shear band in which most of displacements and strains localize. From image analysis we estimate the strain field in the material surrounding the shear band and show that this outer region accounts for most of the global volumetric strain. We argue that the mechanical coupling between this outer region and the shear band is responsible for the frictional slip weakening. Accordingly, the complex structure of the fault zone is shown to be crucial for its frictional behavior. Implications of our results for fault mechanics are discussed in the companion paper.