A crew had completed drilling a 4.5' diameter shaft for a micro-pile installation and were placing thread bar reinforcement into the drilled shaft. Two employees used pipe wrenches to hold the bar and lower it into the shaft. The first employee was holding his wrench approximately 6' above the ground and the second employee was holding his at waist height when he lost his grip on the wrench. The full weight of the bar fell on the injured employee's wrench, which pulled his wrench and hand down to the ground. His right middle fingertip was pinched between the bottom of his pipe wrench and the ground, resulting in a partial amputation to the distal phalanx.