SUMMARY OF WORK

• The Yankee Canyon County Road A-25 Subsidence Mitigation Project is located approximately 9 miles northeast of Raton in Colfax County, New Mexico. There are several small subsidence features found in the travelled way of County Road A-25 (A-25) and three known abandoned mine adits near A-25 (Sheet 3 and 4). The project area is undermined by the Turner-Urtado Mine and the Denton Mine. The Turner-Urtado Mine shows a portion of the mine workings below the subsidence features along A-25, and the Denton Mine workings explored through a known abandoned mine adit undermine A-25 (Sheet 3). A-25 is an unpaved road with steep grades and switchbacks.  The road surface is exposed bedrock in some locations, and dirt with a gravel overlay in others. The site is steeply sloped and rocky with dense stands of scrub-oak, making access to areas adjacent to A-25 difficult. The project area (Sheet 2) is partially on private land and partially on State of New Mexico land.

  This project involves drilling and grouting in underground voids to prevent further subsidence of A-25; and surface restoration and re-seeding of disturbed areas. This project will include the following work: 

  • There are four subsidence features within the travelled way of A-25 suggesting that the road alignment has mine related voids underneath. Closure of the subsidence features and providing additional support to A-25 will include:
    • Construct Support Pillars in Mine Workings: Establish support pillars in the encountered mine workings using forty-two (42) drilled boreholes, with a spacing of 10 to 20 feet, on center, near the subsidence features and approximate known mine workings, and 30 feet, on center, along the remaining road segments. Drilling will occur along approximately 810 feet of the A-25 road alignment to support the partially collapsed underground mine workings, at approximate depths ranging from 20 to 100 feet below the ground surface. Proposed drill hole locations are shown on Sheets 4 and 5. The goal of the drilling and grouting work is to address the A-25 subsidence features, map voids under the road alignment, and to support those voids with grout pillars to stop additional subsidence and stabilize the road.
    • The amount of drilled boreholes that are expected to encounter rubble or void space due to the room and pillar design of the mine is unknown at this time due to uncertainty about the exact position and orientation of the mine workings. Boreholes that do not encounter rubble or void space will not require casing or support grout. Approximately 3,000 linear feet of drilling is expected, and approximately 1,400 linear feet of casing will be required for this task. It is estimated that the volume of grout to construct the pillars will be approximately 460 cubic yards (CY) (355 CY + 30% contingency).
    • Boreholes shall be drilled through the mine workings horizon (as determined by the Engineer at each bore hole location) and five feet into the floor below the workings. Casing shall initially be set three feet above the mine floor of each hole (as determined by the Engineer) for the initial grout stage. Each grouting stage is considered complete after reaching the pressure cut off, volume cut off, other listed cut off condition, or if determined complete by the Engineer.  After grouting the initial stage, the casing will be pulled up three feet for each successive stage; grout stages shall continue to a point when the casing is at least five feet above the mine workings or rubble.  When a hole has met the criteria for the completion of grouting, the casing shall be removed, gravity fed grout will be used to backfill the hole to the appropriate depth from surface, and imported base course material shall be used to fill the remaining hole space to restore the hole to appropriate surface conditions.  Surface restorations cover the surface expression of the drill hole and are not meant to address A-25 as a whole.  The Contractor shall note that timbers were used to support the mine workings and that pieces of wood may be recovered during drilling. The Contractor shall expect to encounter timbers in various states of degradation.  Grouting shall be within the limits indicated on the project plans to meet the acceptance criteria presented in Section 03020 of these Specifications.
    • The bore holes and support pillars will be constructed as shown on Sheet 6.
  • The Contractor will hand seed and mulch disturbed areas and staging areas. The A-25 road surface, including staging areas on the road surface, will not be seeded and mulched.  In total 0.25 acres are expected to be seeded and mulched.  A mix of native seeds will be used for reseeding, and a local woody mulch that is weed-free certified will be used to stabilize the ground surface.

 Demobilization shall be conducted in such a manner to ensure that the Contractor leaves all project areas in as good or better condition than before disturbance.