The mandrel comprises a body having a longitudinal axis together with a radially-expandable annular assembly. The annular assembly presents a peripheral surface for pressing against a borehole and a peripheral surface for pressing against the body. Around the longitudinal axis, the annular assembly comprises a plurality of radial-expansion blocks. Each block has a first bearing surface, a second bearing surface, and two side surfaces disposed respectively facing a side surface of at least one adjacent block. The block are displaceable relative to one another between a radially-contracted configuration, and a radially-expanded, engagement configuration in which each side surface of a block is in contact with the side surface of the adjacent block.