Study shows Black Hawk mine larger than first thought

A sinkhole on East Daisy Drive led to the discovery of an abandoned gypsum mine under the Hideaway Hills subdivision. Courtesy photo

BLACK HAWK — Results of a geophysical study of the Hideaway Hills subdivision show that it is very likely that the mine extends underground well beyond the known mapped areas on the northeast, east, and southeast side of the subdivision.

And, according to an attorney who is part of the team representing Hideaway Hills homeowners, the findings show that there is also the possibility that the mine extends below Interstate 90.

“This interstate find is in agreement with possibilities left open by the (South Dakota) DOT study conducted over the summer of 2020,” said Patrick Ealy, director of Research and Communications for  Fitzgerald Law Firm of Rapid City.

Mohammad Sadeghi, a professor of geological engineering at Montana Tech who has conducted multiple studies on gypsum, said in his summary of the study that data gathered in the Hideaway Hills subdivision in mid-May will help show the long-term stability of the gypsum mine located under homes there.

At least 30 people lost their homes when the ground collapsed and exposed an abandoned mine beneath the homes on April 27, 2020. There are at least two lawsuits involving homeowners seeking compensation for the loss of their homes.

Sadeghi said the problems homeowners are seeing in Hideaway Hills are because of surface water seeping into the ground.

Sadeghi said gypsum is highly soluble, and if exposed to water, the water is going to dissolve the gypsum. He said any pillar or any gypsum on the roof of the mine is going to get thinner and thinner and lose its strength that is why they are seeing sink hole.

Sadeghi was hired by the Fitzgerald Law Firm to do the study. He said his team used the Frequency Domain Electromagnetic, Self Potential and Electrical Resistivity Tomography tests to identify potential voids beyond the known extent of the abandoned gypsum mine.

“The results of the three methods were in good agreement and helped identify several locations beyond the known extent of the mine where there is a high possibility of the existence of voids,” Sadeghi said.

He said although the research showed the mine extends to the northeast, east, and southeast of Daisy Drive, it is unlikely that the mine extends much farther to the west beyond the limits of the ERT-line 7. That line is about a half a block from where the first sink holes appeared.

“This conclusion is consistent with the information obtained from the residents and old areal maps that place the western entries of the mine a few feet west of the ERT-line 7. This is also in agreement with the ERT model of line 9 (conducted on the west boundary of the investigation area) where no indication of voids or any other significant features were observed,” he said.

The study also confirmed there are no voids or “other significant features” under Daisy Drive.

Sadeghi suggests conducting a more detailed (3-D) resistivity study on the northeast and east side of the area along with the Very Low-Frequency Electromagnetic (VLF) Method to be able to develop a detailed 3D model (map) of the mined areas.

The Montana Tech team does recommend further study in the area. The research team is hosting a public news conference on June 28 to share their findings.

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