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MSHA - Fatal Investigation Report


Metal and Nonmetal Mine Safety and Health

Accident Investigation Report
Surface Nonmetal Mine

Fatal Accident (Asphyxia)

Phelps Dodge Morenci, Incorporated
Phelps Dodge Morenci, Incorporated (Mine)
Morenci, Greenlee County, Arizona
I.D. 02-00024

August 9, 1998


Larry L. Weberg
Supervisory Mine Safety and Health Inspector

James E. Eubanks
Mine Safety and Health Inspector

Originating Office
Mine Safety and Health Administration
P.O. Box 25367, DFC
Denver, CO 80225-0367

Robert M. Friend
District Manager


Pete R. Sierra, tailings dam serviceman, age 55, was asphyxiated at about 9:45 a.m., on August 9, 1998, when he aspirated slurry. Sierra had a total of 28 years and 46 weeks mining experience, all at this mine, 4 years and 3 weeks at his current occupation. He had received training in accordance with 30 CFR, Part 48.

MSHA was notified at 11:45 a.m., on the day of the accident by a telephone call from a safety representative for the mining company. An investigation was started the same day.

The Phelps Dodge Morenci mine, an open pit copper operation, owned and operated by Phelps Dodge Morenci, Incorporated, was located in Morenci, Greenlee County, Arizona. The principal operating official was Harry M. Conger, IV, president. The mine was normally operated three, 8-hour shifts a day, seven days a week. A total of 2,700 persons was employed.

Copper ore was drilled and blasted from multiple benches in the pit and then transported by truck to various stockpiles throughout the mine. The ore was crushed, screened and conveyed through the milling process. Tailings, a waste material resulting from the milling process, was carried in slurry form through pipelines to several large impoundments located at the outer edge of the mine property.

The last regular inspection of this operation was completed on July 16, 1998.


The accident occurred at the northwest area of the No. 4 west tailings impoundment on the 4186 level. Pipelines used to distribute tailings to various impoundment locations were made of high density plastic. The main pipelines were 36 inches in diameter and were equipped with manifolds having 12-inch gate valves and connected to branch pipelines servicing different levels.

A 12-inch branch pipeline had broken at the 4204 level about 120 feet up the hillside above the gate valve connecting it to the main line.

The victim's truck engine was running when he was found. A portable 2-way radio was attached to his belt and appeared not to have been used.


On the day of the accident, Pete Sierra (victim) reported for work at 6:00 a.m., his regular starting time. He attended a safety meeting with his supervisor and co-workers. After the meeting Sierra traveled in his service truck to the No. 4 west impoundment and started inspecting the pipelines and impoundments. At about 8:30 a.m., Clark Hardcastle, assistant shift supervisor, was driving around the impoundment and saw Sierra.

At about 9:35 a.m., Raymundo Gonzales, general shift supervisor, received a call from a tailings operator reporting that tailings were running down the hillside at the No. 4 west impoundment. Gonzales reported the condition to Hardcastle.

Hardcastle and Dennis Bonn, tailings dam operator, went to the No. 4 west impoundment and found Sierra on the ground under the 36-inch pipeline, about four feet to the side of a 12-inch gate valve. Tailings from the broken pipeline were running down the hill and over the victim. Hardcastle and Bonn pulled Sierra out of the area, but were unable to detect any signs of life.

Hardcastle called the guard house for assistance. At about 10:10 a.m., the company first response team arrived and started CPR. Sierra was pronounced dead at the scene by a local physician a short time later.


The immediate cause of death listed in the coroner's death certificate was asphyxia due to aspiration of mine tailings slurry. The event which predisposed the victim to asphyxia could not be determined, but may have been due to a sudden cardiac event in the setting of hypertensive heart disease leading to altered consciousness.

All indications are that the victim saw the broken pipeline during his inspections and collapsed while attempting to close the valve to shut off the flow of tailings. He collapsed directly in line with the flow of tailings and was asphyxiated by aspirating the tailings.


Order No. 7931072 was issued on August 9, 1998, under provisions of Section 103(k) of the Mine Act:

The No. 4 West tailing impoundment was the scene of an accident where an employee was attempting to close the valve on a broken slurry line and when found, was in a sitting position and nonrespondent. This order is issued to ensure the safety of persons until the affected area can be returned to normal operations as determined by an authorized representative of the Secretary.

This order was terminated on the same day after it was determined that the mine could return to normal operations.

Persons Participating in the Investigation:

Phelps Dodge Morenci, Incorporated

Edward H. Valentine..........manager, health and safety
Ross E. and safety professional
Vesta L. and safety professional

Mine Safety and Health Administration

Larry L. Weberg..............supervisory mine safety and health inspector
James E. Eubanks............mine safety and health inspector

Related Fatal Alert Bulletin:
Fatal Alert Bulletin Icon FAB98M51