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Mine Disasters in the United States


Magma Copper Company
Magma Mine Falling Material Accident

Superior, Pinal County, Arizona
August 10, 1993
No. Killed 4

MSHA Final Investigation Report  (19.6 Mb)  PDF Format

See also:   Magma Mine Fire, Nov. 24, 1927
  Magma Mine Cave-in, May 10, 1982

An ore pass raise collapsed in this mine while four persons were working inside.  The victims had entered the raise to free the hang-up of material in the ore pass side of the structure and the ground support structure failed.


Overview

On August 10, 1993, at about 9:45 p.m., an ore pass raise collapsed at Magma Copper Company's Magma Mine while four persons were working inside.  The fall of materials accident resulted in the deaths of four miners:
  • Jeff S. Christiansen, Operations Tech I
  • John H. Dalton, Jr., Materials Handling Group Leader
  • Alfred D. Edwards, Materials Handling Team Leader
  • Nicholas P. Truett, Support Tech III
The accident occurred at the 865 Raise when the miners climbed into the manway compartment to free a hang-up of material in the ore pass side of the structure.  Armored cribbing, dividing the manway and the adjoining ore pass compartments, dislodged and allowed ore, cribbing, and timber to fall into the manway striking and killing the four victims.

The raise, which had been constructed by an independent contractor, Dynatec Mining Corporation, had been opened for production about six weeks before the accident.  It was subsequently closed to repair damage from structural settlement and blasting, and then placed back in production the night before the accident.

The 865 Raise was designed as a timber-framed ground support structure comprised of two compartments, a manway and an ore pass.  The raise was 364 feet high and framed with 10-inch by 1a-inch timber.  A single bearing set was hitched into the rock and encased in concrete at the bottom of the timber structure.

The 865 and two other raises had been designed to transfer ore, ventilate, and provide a secondary escapeway.  During development of access drifts and a borehole for this project, loose and soft ground was encountered causing Magma to abandon plans for the three raises and to incorporate all three functions into a single raise, the 865.

The poor ground conditions causing the development problems were located near the site selected for the 865 Raise.  Consequently, Magma said they designed the raise for adverse ground conditions.  However, adverse ground was not encountered during development of the raise and the design was not modified to be appropriate for the ground in which the raise was developed.

About a month after the 865 Raise was first used for production and 12 days before the accident, 60 to 100 cubic yards of a water, sand, and cement mix, normally used for backfilling stopes in the mine, was dumped into the raise.  Once the mix was in the raise, muck was not withdrawn from the raise for about 20 hours, allowing the cemented mix to set, forming a plug.  As a result, hang-ups occurred in the ore pass compartment and Magma blasted the constriction in attempts to free the ore.

Six days after the mix was dumped, the 865 was closed to repair damage from the blasting and raise settlement.  Inspections conducted by Magma and Dynatec revealed eight to ten inches of settlement from Set 8 through Set 20.  There was joint separation, a broken divider plate, sheared blocking, loose and broken ladders, displaced landings, movement of the divider wall toward the manway, and divider cribbing and ore in the manway.  Besides settlement, the cribbing in the manway was evidence that the divider posts were moving outward, away from one another.  These conditions indicated that the raise was in a state of impending failure.  The MSHA investigators determined that an imminent danger existed as defined in Section 3{j} of the Federal Mine Safety and Health Act of 1977.

The single bearing set at the bottom of the timbered structure bore the entire load of the raise, about a half million pounds, when the raise was empty.  Although a number of repairs were affected, no effort was made to stop the settlement or correct the outward movement of the timber framework.

Magma decided that the raise be returned to service by midnight shift of August 8, 1993.  The raise was placed back in production at some time during the evening shift of August 9.  Magma began dumping in the raise as soon as it was available.  Ore was pulled for the balance of this shift and through the succeeding shift without unusual incident.  Dumping continued in the raise and ore was not pulled during day shift, August 10.

Ore was pulled during the evening shift and the raise was emptied to about Set 8 where it was reported to be hung-up.  Two of the victims tried unsuccessfully to free the hang-up by blasting.  They sought assistance from two supervisors who joined them later in the shift.  When the four miners entered the raise, the ground support structure failed, fatally injuring them.



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