Geophysical Microgravity Survey
The Bull Frog Mine, underlying the Avalon Wharton site, was worked circa 1880 for magnetite (iron) ore. Based on Oweis Engineering Inc.’s geotechnical engineering effort, mine remediation work was undertaken to render the site safe for development of a proposed four-story apartment building, parking garage and site appurtenances and to eliminate possible hazards associated with the abandoned mining activity.
The mine within the property is approximately 400 feet in length extending to a depth of 525 ft. at the property line then extends to the neighboring property. The mine transects the site through the footprint of the proposed parking garage, residential building, associated utilities and retaining walls. In broad terms, the geotechnical remedial design called for filling the subsurface mine void underlying the site where less than 30 feet of bedrock was present above the mine void. This was accomplished by drilling and compaction grouting the mine stopes (mined out areas 3 to 15ft. thick) for a minimum depth of 30 feet into bedrock. A geophysical microgravity survey was conducted under the proposed parking garage and building footprint to identify any unmapped mine shafts or subsurface mining features not defined by geotechnical borings and mining analysis. The remediation procedures described above eliminated the subsurface void in the upper portion of the bedrock from the abandoned mining activities and formed a sound structural section for construction of the proposed residential development. The mine closure documents were submitted and subsequently approved by the New Jersey Department of Labor, Mine Safety Section. The utilized method of upstage grouting of the mine was an effective method of establishing a sound structural section to support the proposed structures and site elements and eliminated any predictable hazards associated with the existence of near surface mining features.