impact from hammer 52 depending upon its direction of motion a plurality of fins generally indicated at 80 are mounted beneath anvil 72 embedded in ground 82 a typical fin 83 has a surface 84 parallel to impact receiving surface 76 and a second surface 86 parallel to surface 78 the fins are located in a plane parallel to top surface 74 thus it can be seen that each fin has ground engaging surfaces disposed to efficiently transfer the impact motion of the hammer to the ground referring to fig2 a pair of opposed l shaped supports 90 and 92 are mounted on opposite sides of frame 16 a pair of hydraulic actuators 94 and 96 are mounted on supports 90 and 92 respectively a four sided frame 98 is connected to the lower ends of hydraulic actuators 94 and 96 referring to fig3 frame 98 has a pair of sides 100 and 102 for engaging the top of pad 70 guide means 104 and 106 carried by pad 70 are received through appropriate openings in sides 100 and 102 to locate it with respect to the pad actuators 94 and 96 are independently movable to provide the user means for properly aligning pad 70 with respect to the boom referring to fig4 a pair of supports 108 and 110 are mounted on opposite sides of frame 16 a hydraulic actuator 112.