New Mexico Geological Society Annual Spring Meeting — Abstracts

In early August, 2015 a large sinkhole opened suddenly at ground surface above the abandoned Allison Mine on the west side of Gallup, NM. The mine is located in a tributary valley of the Rio Puerco. A paleovalley, filled with alluvium and not visible at ground surface, underlies the present valley. The sinkhole is within the alluvial fill of the paleovalley and lies along the channel of the present valley.

This underground room-and-pillar coal mine operated from the late 1800s to 1939. Two coal seams of the Gibson Coal Member of the Mesaverde Formation were mined, one at 80-100 feet depth and the other at 140-165 feet depth. When it closed, most entries to the mine were sealed and surface facilities were removed, but no backfilling or other stabilization of underground workings was performed.

When notified of the sinkhole, the Abandoned Mine Land Program of the New Mexico EMNRD immediately initiated an emergency response. A team of geologists, geophysicists, engineers and surveyors was assembled and began a fast-track investigation to determine the shape and extent of the sinkhole, its cause(s), and possible remedial measures to arrest sinkhole expansion and to protect public safety and impacted property. Historical records were collected and reviewed at the same time that field investigations were initiated. Mine records were fragmentary but useful in locating mine workings in the vicinity of the sinkhole. Anecdotal information from local residents was especially helpful in locating adits and shafts and enabled the team to more precisely position the mine workings map in relation to present ground features. Field investigations included land surveying, subsidence monitoring, mapping of surface features, aerial photography, geophysical surveys, and exploratory drilling.

These investigations revealed that the upper coal seam had been eroded from the paleovalley, leaving the eroded edge of the upper seam buried in alluvium approximately 60 feet west of the center of the sinkhole. During mining, an exploratory drift was advanced along the upper seam and into the alluvium. Over time, the timber supports of the drift deteriorated, and overburden progressively caved into the drift. Caving was accompanied by high-angle cracking that propagated to ground surface, and some of these cracks intercepted the present-day arroyo channel. Runoff entered these cracks and followed them into the collapsing drift, eroding alluvium along the way and forming an underground erosional chamber. Once it daylighted at ground surface, the sinkhole expanded rapidly in runoff-induced increments to more than twice the original size in less than four months.

Left unchecked, subsidence would continue as long as runoff could enter the sinkhole and eroded alluvium could be flushed into the vast underground workings. To arrest this process, the sinkhole was backfilled with crushed recycled concrete to provide structural support for the sinkhole, to be followed by grouting to minimize infiltration of surface water. The arroyo channel will be lined across the subsidence area to separate runoff from the sinkhole.