CEA Foundation Research
Document name: Are Sinkholes a Threat?
Date: Nov 14, 2020
Question: “Are sinkholes a threat?”
In general, the entire region around Grass Valley has hundreds of mine features. Most of them are undocumented and few of them are formally “closed” in a way which assures long term stability. Thus occasionally there is a sinkhole or settlement, with or without the mine reopening.
That said, a relevant the question is:
Q: Is there an added risk of sinkholes or settling from the proposed reopening of the mine?
A: Yes, there is a small additional risk.
The draw down of the water level due to dewatering the mine can increase the risk of settlement or collapse, especially when the water level is near the surface or above bedrock. The Rise application document “Geotechnical Review of Near-Surface Features, IMM Project” ( Geotechnical-Review-of-Near-Surface-Features—ADDED-6262020) evaluates existing mine features with this in mind. It identifies 15 known near-surface mine features and assesses their potential for settling or collapse. There are even cases of mine shafts or other mine features directly under commercial buildings. A recommended action is provided for each feature. Five of them are recommended for investigation and physical closure. Rise has entered into an agreement with the property owners for doing the closure work. In two other cases, close monitoring is recommended.
Therefore, strictly speaking, though the plan is to mitigate these potentially worst settlement features, the answer to the question regarding the risk of sinkholes is “yes”, due to dewatering. Note that these features of concern are mostly located in the vicinity of the intersections of Idaho-Maryland Road with Spring Hill Drive and with Centennial Drive.
What is the risk beyond those identified in the ‘Near-Surface Features’ document?
Most of the risk of settling or collapse due to de-watering is probably concentrated in the vicinity of the Centennial site where the water level is within 100’ of the surface. At the Brunswick site the surface elevation is over 200’ above the mine water level, and so the probability of direct effects of settlement due to dewatering is much smaller. However, there is an unknown increased risk of settling or collapse due to blasting, vibrations, faulting, and/or disturbances.
Idaho-Maryland Mine grew out of a series of acquisitions over time, including the Eureka, Idaho, Maryland, Brunswick, and Union Hill mines. These cover a huge area of potentially hazardous unstudied mine features. These pose risks of settling, not just as a result of dewatering, but also due to other mining activities. Per the referenced document, there are recent examples: An area of settling took place about 5 years ago at a location 100’ south of the Idaho Pump Shaft that had not been previously identified. In another example, in 1998 a sinkhole formed below a residence at 12305 Bet Road, at the site of an inclined shaft of the Old Brunswick mine area.
Statement: “Two years ago we had a sinkhole in GV near Les Schwab on Freeman Lane.”
That massive sinkhole was caused by a series of events, starting years ago with Little Wolf Creek being placed in a culvert and buried beneath over 70’ of fill during the construction of the Pine Creek Shopping Center. That culvert had drainage culverts that joined it at depths. Apparently, water erosion
from seepage at a bend and junction of the culverts eroded away the surrounding fill and the overburden collapsed, creating a massive wash out and sinkhole. Heavy rains and perhaps vibrations from nearby grading may have triggered the event. Other than being an example of poor engineering design and construction, it was not related to a mining feature.
Statement: “We also had one in Nevada City at the Stone House parking lot”
Probably another example of a sinkhole caused by an old mine feature.