An employee was welding two high-density polyethylene (HDPE) pipes together using a fusion machine. As the employee attempted to slip check the pipe, they noticed that the south pipe was slipping out of the fusion machine jaw. Hydraulic pressure was released to reposition the jaw clamp onto the south pipe and the fusion machine jaw clamp slipped off the south pipe. The employee's arm was pinned between the fusion machine and the parallel pipe in the ditch, resulting in a chipped elbow bone and a laceration.