Abstract: The unit comprises a power-driven horizontal rotor including a shaft carrying radially mounted tool sets comprising hammers swivel-mounted at at least an intermediate point of the major dimension thereof. Upstream of the shaft there is a serrated blade subjected to oscillatory movements in a normal direction to the plane of lay thereof, the rotor and blade being carried on a frame the rearward end whereof is mounted on two wheels interconnected by an axle adapted to be raised from and lowered onto the frame.