Case Study: mine subsidence, CSM

2005-04-26 | CGS Admin

For decades, the west side of the Colorado School of Mines (CSM) main campus had subsidence issues related to historical mining activities. At one point, in the 1990s, one of the married student housing units in that area was so badly damaged that it was condemned.  In the early 2000s, after the school converted the subsidence-prone area into intramural-athletic (IM) fields, ongoing subsidence-related issues were still being reported.

Clay mining in Colorado dates back to the mid-1800s and Golden was a particularly good location for clay found in the Laramie Formation. This clay has been used for a variety of industrial purposes over the years including construction (bricks, structural tiles, sewer pipes), terracotta, refractory clays, and earthenware. The mining of kaolinitic claystones in what was later to become the western area of the Mines campus left backfilled/collapsed mine workings and the possible presence of underground void spaces. To complicate matters, that same area was also the site of coal mining in the 1880s and 1890s. In particular, the Pittsburg Coal Mine entry shaft may have been located in the vicinity of one of the observed subsidence features. This mine reportedly operated between 1876 and 1880, but is un-recorded by the State. The mining operations were thought to be on three levels at depths of 100, 150, and 225 feet running parallel to the mountains.

During the initial construction of the IM fields in 2004, depressions in the ground surface began to occur. In May of 2004 immediately to the west of the IM field near the campus sorority houses, a 6 by 10 ft (2 by 3 m) area of pavement subsidence was also observed. It grew to 15 by 20 ft (5 by 6 m) within a month with the pavement settling approximately 2 ft (0.7 m). In the spring of 2005 the subsidence process reactivated, likely from snow-melt runoff. Several open holes appeared in the field and the street was further damaged. The sorority housing structures themselves were not, as they had been constructed with deep piers on sandstone bedrock that had not been mined.

A grouting program was implemented to try to stabilize the area in 2005, however in 2006 additional street damage occurred and several new depressions were found in the field.  Grouting is an engineering solution for stabilizing voids in the subsurface where cementitious fill material is injected into the void through holes drilled from the surface.

In 2008, the Colorado Division of Reclamation, Mining and Safety (DRMS) commissioned a geophysical (DC resistivity) survey conducted by Zapata/Blackhawk Engineering. This report provided more constraints on the geology as well as the actual situation with historical backfilling of the claypits and whether there were voids in the subsurface. It also aided the development of a mitigation strategy to help avoid future damage: one essential point being the careful control of water runoff from snow melt and precipitation.