2024-12-16_REVISION - M1977410 (17)

Home Browse Search

Cross Gold Mine Permit No. M-1977-410 Technical Revision No. 7 Preliminary Adequacy Review Grand Island Resources P.O. Box 3396 Nederland, CO 80466 Ms. Amy Eschberger June 12, 2020 Page 3 necessary data to provide a sufficient stability analysis for the portal slope, this revised analysis must be submitted to the Division through TR-07. A critical aspect of the stability analysis is the distribution and character of the geologic materials comprising the slope and rock mass surrounding the tunnel opening. GIR will evaluate the subsurface conditions during installation of soil anchors for the cellular concrete retaining structure, spilings installed in advance of tunnel rehabilitation after TR-05 is approved, and the ground conditions which are exposed in the tunnel itself as the work progresses. Field and laboratory testing of the various material encountered during this interim construction phase will be conducted to verify or update the material properties adopted in the analyses presented with TR-05. These data will be used in conjunction with as-built information on the final slope geometry to provide analyses which demonstrate that the slope meets the long-term slope stability requirements of MLRB policy. 5) The enclosed Memorandum from Peter Hays, DRMS recommends a monitoring program be prepared for monitoring and measuring the collapse void to prevent and/or prepare for damage to Caribou Road. Given the significant potential for off-site damage and impacts to Caribou Road due to current slope conditions (including the collapse void), the Division recommends this proposed monitoring program be submitted as soon as possible in a separate Technical Revision that can be reviewed and approved outside of the review process for TR-07 (which may be extended to allow for additional data collection and analysis). As indicated, the sides of the void have sloughed in with the Spring thaw such that the void completely daylighted to form a crater in the slope. Thus, there is no longer a void to monitor. There was no sign of settling or cracking further upslope as the area was monitored for several days through regular observation. At no time did it appear that there was a reasonable potential for failure of the slope to present imminent danger to human health, property, or the environment. Since that time, the slope has now drained considerably which increases its stability. In order to prevent additional sloughing that could destabilize the slope and endanger the County Road, GIR backfilled the pit with pervious cellular concrete on May 19. This will serve to further increase stability of the slope. Unless the level of saturation increases significantly, the slope should now be sufficiently stable for a temporary interim construction slope. The slope will continue to be monitored using an observational approach until a more permanent final configuration has been constructed. To enhance safety, GIR also proposes to place a series of concrete Jersey barriers along the edge of the road above the portal slope along this stretch of roadway.