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Hazard Alerts

Footclamp Accident

Drill Safe


Description of the accident

The operator was lowering the rods into the borehole after changing the bit. The rod string was seated in the jaws of the foot clamp and the injured man was assisting in making-up the drillrods. He was handling a 36” Stilson wrench which became jammed onto the drill rod and in an effort to free the wrench, tramped on to the foot clamp pedal. As the pedal depressed it opened the foot clamp and released the drillstring which fell into the hole causing the hoisting plug to hit the top of the clamp. The force with which the hoisting plug hit the foot clamp caused the foot clamp pedal to lift very abruptly. The force of the upward moving pedal sliced through the injured man’s safety boot causing a deep laceration in the sole of the man’s foot and severe contusion of his ankle.

Apparent causes

Wommer clamps (foot clamps) are manufactured with two springs connecting the pedal and the eccentric section which rides against the jaws. These springs are designed to act as shock absorbers in circumstances such as those described above. In this case, the clamp however was modified by the insertion of a bolt which fixed the pedal and the arm of the eccentric sections of the foot clamp. This effectively made the eccentric section and the pedal a single part thereby negating the effect of the springs. It is unclear why the extra bolt was inserted.

Additional factors

Several other factors contributed to this accident:

  1. The injured man was a security guard who was “pressganged” into working as a drill rig assistant because the regular member of the drill crew was sick. The injured man therefore had not undergone any training whatsoever to prepare him for the work being done, he was not made aware of the hazards on the drill site and he was not in possession of all the requisite PPE at the time of the accident.
  2. Wommer clamps are manufactured with a steel tread plate welded at the end of the foot pedal. This is provided to allow the drill rig assistant’s foot to be comfortably placed on to the pedal. The tread plates are normally welded with an upward turned lip to prevent a foot from slipping from it. In this case the lip of the tread plate acted as a knife in slicing through the man’s boot.


  1. It is clear that the injured man did not possess the necessary skills, knowledge or experience to allow him to safely work on an active drill site. It is essential that all operational staff are trained and assessed before being allowed to work on a drill site.
  2. Wommer clamps, as is the case with any piece of manufactured equipment, must not be modified without the approval of the manufacturer.
  3. It is essential that drilling operations do not begin at the start of the shift until and unless the full drill crew complement is present on site and is fit and able to work.
  4. Pre-start checklist should be modified to insert the requirement detailed in 3 above.