DUSTIN BLEIZEFFER Star-Tribune energy reporter | Posted: Thursday, August 9, 2007 12:00 am
Underground mines in Wyoming are designed to withstand some seismic activity, according to one industry official.
Most often, it is a mine collapse that triggers a seismic reading, and not a seismic event that causes a collapse. That has been the case in each of Wyoming's two most recent mine collapses.
The most recent major mine collapse in Wyoming occurred at Solvay Minerals' trona mine west of Green River in 2000. The collapse triggered a seismic reading of 4.4, according to the Wyoming State Geological Survey.
No one was killed or injured in that event.
One miner was killed in a 1995 collapse at the same mine, however. Investigators determined that the quarter-mile section that collapsed in 1995 was an experimental area being mined more quickly than other parts of the mine. The collapse triggered a seismic reading of 5.4, according to the Wyoming State Geological Survey.
Soon after the 1995 event, public and private donations were collected to help build a seismic station near Afton in western Wyoming.
There are several underground trona mines in southwest Wyoming, but only one underground coal mine. During the past few years, the Jim Bridger coal mine in Sweetwater County has transitioned from a surface operation to an underground operation.
The mine is owned and operated by PacifiCorp and supplies about 5.5 million tons of coal annually to PacifiCorp's adjacent Jim Bridger Power Plant. Unlike the Genwal mine in Utah - site of this week's collapse that has trapped six miners - PacifiCorp uses a longwall mining method.
Longwall mining uses an automated shearing machine to cut the coal under self-advancing hydraulic roof supports. The coal is dropped onto a conveyor for transport out of the mine. The roof of the mine is allowed to fall in as the machine advances.
"It's important for people to understand the technical difficulty of mining coal," said PacifiCorp spokesman Dave Eskelsen.
Mine portal support designs are based on rules set forth by the Mine Safety and Health Administration, and those do take into account some measure of seismic activity.
Seismic activity is a concern for underground miners. Surface mines in Wyoming constantly monitor the weather for tornados and other conditions that could put workers at risk. But monitoring the underground gyrations of the earth is more difficult.
Eskelsen said the mine does not have a seismograph on site to track seismic activity, nor does it attempt to predict seismic activity. However, mine engineers do work with the Wyoming State Geological Survey, especially when developing a mine plan.
About 180 employees work in the surface operation of the mine, and 150 work underground. Eskelsen said underground workers at the mine are equipped with self-contained fresh air equipment, which were mandated soon after the 2006 Sago mine disaster.
Energy reporter Dustin Bleizeffer can be reached at (307) 577-6069 or dustin.bleizeffer@casperstartribune.net.