Drilling for clues in Wyo town
By JEFF GEARINO - Southwest Wyoming bureau | Posted: Thursday, March 11, 2010 12:00 am
ROCK SPRINGS -- There's still a lot of movement in the old underground mine voids beneath the "Tree Street" neighborhood that was beset by damage from an underground mine subsidence project in 2007.
What's causing the movement, however, remains a mystery.
Wyoming Abandoned Mine Lands officials said Wednesday that state-contracted engineers from Colorado-based Tetra Tech Inc. will resume their investigative drilling program in the neighborhood this week. Their mission is to help determine the exact cause of the underground movement and what mitigation measures will be required to stop it.
AML administrator Rick Chancellor said the new investigative drilling at various locations in the neighborhood is being conducted to supplement data previously collected from the area.
Tetra Tech engineers have been working for several years to determine what's going on in the old mine workings, tunnels and voids scattered beneath the neighborhood.
"It's a complicated (picture) underground, and the more information we have, the better we can plan on how to move forward," Chancellor said.
The information gathered from the current investigative drilling will be used to plot the course and method of future mitigation and reclamation efforts in the neighborhood.
"Probably the main thing will be to look at our options and then choose from those options the best way to move forward," Chancellor said. "(Mitigation) may involve a combination of things, but we'll wait for the final (drilling) report and then decide what we can do and where we can do it."
Drilling rigs will be employed along with geophysical evaluations of underground mine workings as part of the new investigative drilling process. Chancellor said the work is expected to continue throughout the summer and fall.
Coal history
Rock Springs was built around coal mines, which were first developed in the 1880s to supply coal to the Union Pacific Railroad.
More than 100 million tons of coal was mined over the next century. One result is that many miles of underground mine tunnels traverse underneath most homes in the city.
More than a dozen homes were damaged by subsidence in the neighborhood during a controversial reclamation project conducted in 2007, which aimed to prepare vacant lands in Rock Springs for much-needed housing development.
The subsidence project involved the use of a pilot technique known as dynamic compaction.
For three weeks beginning in mid-July 2007, cranes pounded the ground with 25- and 35-ton weights in order to collapse the underground mine voids on a tract of land adjacent to the Tree Street neighborhood.
Several weeks into the project, residents complained that the shock waves from the ground pounding shook houses, cracked driveways and foundations, accelerated ongoing subsidence problems and severely damaged many homes.
Wyoming officials have been negotiating with homeowners for more than two years over the state's settlement offers to repair damage from the project.
In the meantime, the AML Division has been conducting subsurface investigative drilling -- which began in the fall of 2007 -- in the neighborhood to determine if any mitigation measures are required
Initial findings
In October, AML officials and engineers reported the initial findings from the first two years of investigative drilling in the neighborhood at a meeting with residents and city officials.
Engineers based their initial findings on samples taken from numerous drill holes, views from downhole cameras, examinations of old mine maps, data from sonar devices, 3-D seismic mapping, visual inspections and old subsidence reports.
The report said the subsidence risk remains high in the neighborhood.
Geological engineers with Tetra Tech told residents they still didn't have an accurate picture of the exact cause of continuing movement in the underground mine voids.
But engineers said they believe there a lot of factors contributing to the neighborhood's subsidence problems, including irregular roof pillars that are prone to collapse, fluctuations in groundwater levels in the abandoned mines, a fault line dissecting underground coal seams, and perhaps even previous mitigation and grouting efforts.
Chancellor said the investigative drilling should be completed by fall, and a final report from Tetra Tech may be available before the end of the year. The information will be the basis for any future mitigation work in the area.
"We're trying to be very careful that we don't jump in there and do something that will cause more problems," Chancellor said. "So we are being very methodical and careful in our investigation to make sure we have good information that will allow us to make a good decision" on what mitigation measures might be required to stabilize the area, he said.
Contact southwest Wyoming bureau reporter Jeff Gearino at (307) 875-5359 or gearino@tribcsp.com
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