Logan Cave, located in Benton County, Arkansas, is inhabited by several
endangered and threatened species. The cave and surrounding area was designated
a National Wildlife Refuge under the control of the U.S. Fish and Wildlife
Service (USFWS) in 1989. Cave researchers access the cave through a steep-sided
sinkhole entrance, which also is one of the two access points used by endangered
bats. There is evidence of instability of one of the entrance slopes that
has raised concerns that the entrance could close if slope failure was
to occur. At the request of USFWS, we performed an engineering geologic
investigation of the sinkhole to evaluate stability of this slope, which
is comprised of soil, and other mechanisms of sediment transport into the
cave entrance. The investigation included engineering geologic mapping,
sampling and laboratory testing of subsurface geologic materials, and slope-stability
analysis. We found that the sinkhole slope that extends into the entrance
of the cave is comprised of sandy and gravelly soil to the depths explored
(6.4 meters). This soil likely was deposited as alluvium within a previous,
larger sinkhole. Based on properties of the alluvium, geometry of the slope,
and results of finite-element slope-stability analyses, we conclude that
the slope is marginally stable. Future failures of the slope probably would
be relatively thin and small, thus several would be required to completely
close the cave entrance. However, sediment is accumulating within the cave
entrance due to foot traffic of those accessing the cave, surface-water
erosion and transport, and shallow slope failures from the other sinkhole
slopes. We conclude that the entrance will be closed by sediment in the
future, similar to another entrance that we identified that completely
closed in the past. Several measures could be taken to reduce the potential
for closure of the cave entrance, including periodic sediment removal,
installation of materials that reduce erosion by foot traffic and surface
water, construction of a sediment-retention wall, and excavation of the
soil slope. Any measures taken must be carefully planned and executed so
that they have no impact on organisms within the cave. |