- A total of 168 boreholes were spread out over 4 acres to intercept the underground mine workings.
- 26,400 cubic yards of high mobility grout was injected into the boreholes. That’s enough grout to fill 8 Olympic Swimming Pools!
- 56,500 linear feet were drilled in 136 working days for the project with an approximate average termination depth of 170 feet.Development of and implementation of the refusal criteria is an important component of Brierley’s approach to subsidence mitigation
- See the Story Map for more photos and detailed information about this project.
As a follow-on assignment to our work at Hanna Elementary, Brierley Associates’ AML team was retained to mitigate subsidence features originating from abandoned underground mine workings beneath the football field and track. The depth to the voids vary from 132 to 226 feet below ground surface. Subsidences would routinely appear in the football field as well as the surrounding areas of the school. The project also involved removal of the existing athletic field facilities, and construction of new replacement facilities. Close work coordination with the investigation team and the contractor responsible for removal of the existing athletic facilities and reconstruction was key to project success and required both teams to work side by side.
Since the exact location of mine workings that could be backfilled with Brierley’s high mobility grout mixture was unknown, our initial plan was to layout the exploration boring plan consisting of 168 locations spread out over 4 acres using a grid approach that took into account the estimated dimensions of the room and pillar mining. During the investigation, grouting and confirmation program, over 300 borings, both vertical and angled were completed. Information obtained indicated historic mine roofs ranged from about 60 feet to 220 feet below ground surface.