Engineering Geologic Conditions at the Sinkhole 

Entrance to Logan Cave, Benton County, Arkansas

By William H. Schulz and Jonathan P. McKenna

Any use of trade, firm, or product names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

U.S. Geological Survey Open-File Report 2004-1357

U.S. Department of the Interior

U.S. Geological Survey

Open-File Reports are used for the dissemination of information that must be released immediately to fill a public need or for information that is 

not sufficiently refined to warrant formal publication. As such, Open-File Reports should not be viewed as final reports of major scientific investigations. Because of their preliminary nature and possibility of being superseded, Open-File Reports do not introduce new or revised geologic, 

stratigraphic, hydrologic, or taxonomic nomenclature. In particular, Article 4(a) of the North American Stratigraphic Code excludes open-file 

releases for the naming or revising of formal geologic units.
2 

ABSTRACT

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. 

INTRODUCTION

Logan Cave National Wildlife Refuge (LCNWR) covers 

0.5 km2 (123 acres) in Benton County, Arkansas, and includes 

a limestone solution cave with approximately 2.4 km (1.5 mi) 

of mapped passageways (http://southeast.fws.gov/LoganCave/). 

The refuge was established in 1989 to protect cave inhabitants, 

including the endangered gray bat (Myotis grisescens) and 

cave crayfish (Cambarus aculabrum), and the threatened Ozark 

cavefish (Amblyopsis rosae). Logan Cave is one of only four 

known habitats for the cave crayfish, and its Ozark cavefish 

population is the second largest known. The cave also has 

historically provided habitat for the Indiana bat (Myotis sodalis). 

The U.S. Fish and Wildlife Service (USFWS) scientists and 

others are concerned that instability of a slope area above the 

main cave entrance could result in destruction of the habitat for 

the threatened and endangered species within the cave.

The main entrance to Logan Cave through which bats, USFWS 

scientists, and other researchers gain access is a small opening 

at the base of a steep-sided sinkhole. Observations made within 

the cave entrance by these scientists indicate that soil (defined as 

unconsolidated mineral and organic material) mantling bedrock 

within the sinkhole is potentially unstable and could conceivably 

close the entrance. Such a closure would most likely have adverse 

impacts on the gray bat, cavefish, and crayfish populations. 

Furthermore, scientists could be trapped within the cave, should 

a slope failure occur while they are inside. To address these 

concerns, the USFWS contacted the U.S. Geological Survey 

(USGS) and requested that we perform an investigation of the 

stability of the soil mantling the sinkhole above the entrance 

to Logan Cave. This investigation was performed during the 

summer of 2004 and is described herein. David Kampwerth and 

Carla Mitchell of the USFWS Arkansas Field Office and Holla 

Bend Refuge, respectively, provided access to the refuge, a site 

orientation, logistic support, and valuable information regarding 

the cave and slope-stability concerns. Our investigation included 

obtaining and examining aerial photographs of the area and 

the available literature regarding regional geologic conditions, 

discussions with USFWS personnel, engineering geologic 

mapping at the refuge, sampling and logging of potentially 

unstable soils, laboratory testing of these soils, slope-stability 

analyses, and analysis of our findings.

Setting

The LCNWR is located near the northwest corner of Arkansas 

(fig. 1) in an area of moderate relief dissected by mature 

drainages. The refuge is covered by oak-hickory forest and 

undergrowth (U.S. Department of Agriculture, 1977). According 

to the USGS Gallatin 7.5-minute topographic quadrangle 

(1982), elevations decrease within the refuge from about 380 m 

(1200 ft) in its northwestern corner to 310 m (1000 ft) along its 

southern boundary. A series of southeast-trending ridges and 

valleys extend through the refuge with local relief of up to about 

30 m (98 ft), and slopes inclined as steeply as about 30°. The 

sinkhole entrance to Logan Cave is located along a flank of one 

of these ridges. Springs within Logan Cave form a stream with 

a measured flow of about 0.22 m3/s (5.0 x 106 gal/d) (http://

southeast.fws.gov/LoganCave/). This stream forms Logan Spring, 

which drains to Osage Creek just south of the refuge. Osage 

Creek is a major tributary of the Illinois River, which is the main 

drainage in southwestern Benton County, and their confluence is 

about 2 km (1.2 mi) south of the refuge. 

Geologic Setting

The LCNWR is located within the Springfield Plateau geologic 

province on the southwest flank of the Ozark Dome, which is a 

broad uplift centered in southeast Missouri (Manger and others, 

1988b). The Springfield Plateau is comprised of bedrock units 

formed from sediments deposited by Paleozoic seas (Manger 

and others, 1988a). These bedrock units dip gently to the south 

(Guccione and Rieper, 1988), and many are cut by normal faults 

with downward motion on their south sides (Manger and others, 
3

1988b). The upper part of the Springfield Plateau is composed of 

cherty limestone of the Lower Mississippian Boone Formation, 

which is underlain by chert-free limestone of the St. Joe 

Formation (Manger and others, 1988b). 

The Boone and St. Joe Formations form the dominant ground 

water aquifer in northwestern Arkansas (Martin, 1994). Chert 

content within the Boone Formation ranges from a few to 

70 percent (Martin and Brahana, 1995), and laterally extensive 

chert layers have been observed to perch ground water at 

different zones within the formation (Goodman and Mack, 1985). 

Overall, ground water within the Boone-St. Joe aquifer is perched 

upon the underlying Chattanooga Shale (Martin, 1994; Stanton, 

1994). Both the Boone and St. Joe Formations are dissected by 

numerous caves and solution channels. Ground water flow within 

the Boone-St. Joe aquifer occurs preferentially through solution 

channels (including caves), fault zones, and fractures (Smith, 

1994; Bostian and Brahana, 1996). Single well pump tests within 

the Boone-St. Joe aquifer have yielded specific capacity values 

that range from 2.5 x 10-4 to 0.37 m3/d/m (0.02 to 30 gal/min/

ft) (Rezaie and others, 1979; Ogden, 1980). This significant 

variability is caused by the lack or presence of fractures, faults, 

and solution channels. The widespread occurrence of solution 

channels also affects surface water, resulting in disappearing 

streams and springs throughout the area (for example, Rezaie and 

others, 1979; Ogden, 1980; Manger and others, 1988b; Martin 

and Brahana, 1995). More than 90 percent of springs in northwest 

Arkansas with discharge in excess of 0.15 m3/s (3.4 x 106 gal/

d) are within 500 m (1600 ft) of faults, and many fault zones 

and fault-related fractures have associated parallel caves and 

solution channels that formed by preferential ground water flow 

(Bartholmey and others, 1995).

The LCNWR is underlain by generally flat-lying bedrock of 

the Boone Formation (Haley, 1993), or possibly by the St. Joe 

Formation (Manger and others, 1988a). Haley (1993) did not 

map the St. Joe Formation, although it is described by others 

(for example, Rezaie and others, 1979; Ogden, 1980; Goodman 

and Mack, 1985; Manger and others, 1988a, 1988b; Martin and 

Brahana, 1995). Chattanooga Shale is mapped just beyond the 

southern boundary of the refuge (Haley, 1993) so it probably 

underlies the Boone or St. Joe Formation at shallow depth below 

the refuge. The shale is exposed to the south because elevations 

in the area decrease to the south. A normal fault is mapped along 

the east boundary of the refuge (Haley, 1993). The regional 

water table, which occurs in the St. Joe-Boone aquifer, has been 

mapped at a depth of 23 m (75 ft) in the vicinity of the sinkhole 

entrance to Logan Cave (Ogden, 1979). It seems that Logan Cave 

may have formed because ground water perched on chert beds or 

low-permeability limestone beds flowed preferentially through 

fractures formed by tectonic activity on the nearby fault, resulting 

in dissolution of surrounding limestone.

According to the Soil Survey of Benton County, Arkansas 

(U.S. Department of Agriculture, 1977), Clarksville series soils 

cover the majority of the hillslopes near Logan Cave, whereas 

Nixa series soils cover the ridgetops. Clarksville soils form 

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4 

from very cherty limestone and generally consist of a cherty 

silt loam with high permeability and moderate strength. These 

soils typically are classified in accordance with the Unified Soil 

Classification System (USCS; Naval Facilities Engineering 

Command, 1986) as silty, clayey, or poorly graded gravel 

(GM, GC, and GP, respectively). The fine-grained fraction of 

Clarksville soils is non-plastic to slightly plastic (USCS group 

symbols of CL and ML) and has low potential for swelling and 

shrinking with variations in moisture content. Nixa series soils 

are very similar to Clarksville soils, except that the Nixa soils 

have very low permeability. Both soil series generally are brown 

to yellowish brown, although the Nixa soils grade to reddish 

brown near bedrock. Both soil series grade into weathered 

bedrock and fill fractures within the upper parts of the bedrock.

INVESTIGATION TECHNIQUES

Our investigation included office, field, and laboratory work. 

We obtained and evaluated pertinent documents prior to our 

field investigation to provide us with background information on 

the topographic and geologic conditions of the refuge. We then 

performed a field investigation to characterize the conditions 

of the sinkhole entrance to Logan Cave and supplemented that 

characterization with laboratory tests. Finally, we evaluated the 

results of our field and laboratory work and performed analyses 

of possible future conditions of the sinkhole entrance. 

Field Investigation

Our field investigation was performed July 26–29, 2004, and 

involved engineering geologic mapping and subsurface sampling. 

We created a base map of the area around the sinkhole entrance 

by surveying selected points using a total-station theodolite. 

We established a local coordinate system that we registered to 

compass directions by using a pocket transit. Surveyed points 

were flagged in the field for identification during engineering 

geologic mapping. The base map was completed by plotting 

all surveyed points at 1:120 scale using graphing software. The 

base map was supplemented by measurements made with a 

pocket transit, tape measure, and a laser rangefinder/hypsometer. 

Engineering geologic conditions then were mapped onto the base 

map while at the refuge. The final map is provided on Plate 1.

Subsurface materials were sampled at eight locations within 

and around the sinkhole entrance to Logan Cave. At seven 

locations (pl. 1), direct-push subsurface sampling equipment 

was used to obtain samples at depths up to 6.4 m (21 ft). This 

equipment consists of a 0.6-m (2-ft) -long, 5.1-cm (2-in) 

-diameter, cylindrical steel sampler into which clear, disposable, 

cylindrical plastic liners are inserted prior to sampling. The 

sampler was typically advanced in 0.6-m (2-ft) increments by 

using a 22-kg (48-lb), electric breaker hammer, then withdrawn 

by using a hand- or foot-operated jack. Upon retrieval from 

the direct-push hole, the plastic liner and sample within the 

liner were removed from the steel sampler, and the soil then 

was removed from the plastic liner. The composition and 

characteristics of each sample were evaluated and recorded on 

logs (Appendix A), after which the samples were placed in plastic 

bags, sealed, and labeled. Subsequent samples were obtained 

from progressively greater depths by installing a new liner and 

advancing the sampler down the same hole. Steel rods were 

threaded onto the sampler to collect soil from depths greater 

than 0.6 m (2 ft). At each direct-push hole location, the sampler 

was advanced to sufficient depth to permit characterization of 

near-surface geologic materials, or until refusal to advance was 

encountered. Refusal to advance the direct-push equipment may 

be caused by bedrock or by cobble-sized and larger rocks. Upon 

completion of each direct-push hole and prior to leaving the 

refuge, we used an electronic water-level sensor to determine the 

presence of water in each of the holes; however, all holes were 

dry.

At one location within the entrance to Logan Cave (pl. 1; 

labeled TS on the Southwest-Slope Geologic Map), two thinwalled, 7.2-cm (2.8-in) -diameter, cylindrical steel samplers were 

advanced by hand using a 1-kg (2.2 lb) hammer and drive block. 

These tube samples were collected for laboratory strength tests, 

which require a larger sample diameter and less disturbance than 

is possible by using the direct-push equipment. The samplers 

were driven from the ground surface to depths of approximately 

30 cm (12 in) and withdrawn. The sampler ends were capped and 

were taped to prevent loss of soil and moisture. 

Laboratory Investigation

All samples collected during the field investigation were 

returned to our laboratory in Golden, Colo. We performed 

moisture-content tests in accordance with ASTM D 2216 

(American Society for Testing and Materials, 2002) on all 

samples obtained from the direct-push holes. We determined 

the particle-size distributions (ASTM D 422; American Society 

for Testing and Materials, 2002), plastic and liquid limits, 

and plasticity indices (ASTM D 4318; American Society for 

Testing and Materials, 2002) of four samples in accordance 

with American Society for Testing and Materials standards. The 

two tube samples were packaged and were shipped to Cooper 

Testing Laboratories (CTL) of Mountain View, Calif., for tests to 

determine strength parameters, plastic limits, liquid limits, and 

unit weight. CTL performed a consolidated, undrained, triaxialcompressive-strength test with pore-pressure measurements on 

the samples in general accordance with ASTM D 4767 (American 

Society for Testing and Materials, 2002), and the authors’ 

loading and drainage specifications. The loading and drainage 

specifications were selected to represent the conditions to which 

the slope above the sinkhole entrance may be subjected. The test 

was performed in multiple stages (multistage test) on a remolded 

(reconstructed) sample. All laboratory test results are provided in 

Appendix B. 

FINDINGS

Site Conditions

Topographic and geologic conditions present at the sinkhole 

entrance to Logan Cave as determined during this study are 
5

depicted on the geologic maps and cross section (pl. 1). Two 

maps are included on the plate; a standard geologic map that 

shows surficial geology and topography and a geologic map 

with a “window” that shows a view of the part of the southwest 

slope of the sinkhole that extends into the cave. The map with 

the window was created to more clearly depict the conditions 

that we addressed during our study; stability conditions of the 

soil (mapped on pl. 1 as Qal) slope above the sinkhole entrance. 

As shown on the Southwest-Slope Geologic Map, the soil (Qal) 

slope extends from the sinkhole rim to the stream within Logan 

Cave. Note that topographic contour lines along the cave roof 

and northeast side have been omitted from both geologic maps to 

improve clarity.

The sinkhole entrance to Logan Cave is located along an 

intermittent, south-trending drainage that is deeply incised 

(about 8-m (26-ft) deep with 22° side slopes just north of the 

cave entrance) and displayed evidence of recent surface-water 

flow at the time of our field investigation. The catchment area of 

this drainage is about 12,100 m2 (130,000 ft2). As indicated on 

the geologic maps (pl. 1), most of the flow within this drainage 

probably is captured by the sinkhole. The drainage continues 

south of the sinkhole where it broadens before merging with 

another intermittent drainage a few tens of meters away. Thin 

sandy, subangular limestone and chert gravel (Qal on pl. 1) was 

observed along the drainage axis north of the sinkhole. The 

drainage enters the sinkhole at its northernmost point and exposes 

limestone bedrock along a series of small waterfalls, the last of 

which is about 2-m (7-ft) high. From the base of the falls, the 

drainage changes course as it approaches and enters Logan Cave, 

eventually draining to the northeast and into the stream within 

Logan Cave (pl. 1). 

The sinkhole within which the entrance to Logan Cave is 

located is roughly rectangular in map view with width and 

length of about 10 and 24 m (33 and 79 ft), respectively, and a 

maximum depth of about 15 m (49 ft) (pl. 1). The long axis of 

the sinkhole is oriented nearly northwest to southeast and the 

short axis about northeast to southwest. The perimeter of the 

sinkhole is marked by an abrupt slope break, with slope angles 

increasing from 0° to 23° outside of the sinkhole to between 45°

and 55° within the sinkhole. Much of the rim of the sinkhole 

displays evidence of episodic, small-scale landsliding and erosion 

by surface water, such as fresh exposures of geologic materials, 

disturbed and overhanging vegetation, and rills. We also observed 

evidence of ground water seepage from shallow soil (Qr on pl. 1) 

mantling bedrock along parts of the sinkhole rim. Limestone 

bedrock is nearly continuously exposed along the northeast slope 

of the sinkhole and is sporadically exposed along the southeast 

and northwest slopes. Only the southwest slope of the sinkhole is 

underlain by soil to significant depth, thus likely has the greatest 

potential for landsliding. Therefore, the southwest slope was the 

primary subject of our investigation.

Southwest Sinkhole Slope

The southwest slope of the sinkhole has a generally consistent 

inclination of about 47°, although it flattens to about 28° in its 

lower part just upslope of the cave. The slope is sporadically 

vegetated with immature bushes and trees. A nearly 3-m 

(10-ft) -deep, northeast-trending gully has been eroded through 

the sinkhole rim near the southern end of the southwest slope 

(pl. 1; geologic maps). This gully has side slopes inclined as 

steeply as 71° and provides the main access to the cave entrance.

The surface of the southwest slope, the gully through the 

sinkhole rim, and the west part of the southeast slope are covered 

by sandy, gravelly soil (Qal on pl. 1). Inside the cave, intermittent 

drainages have eroded as much as 30 cm (12 in) of this gravel 

down to the limestone bedrock. Other bedrock exposures were 

observed protruding through the gravel, indicating that only thin 

soil mantles bedrock within the cave. We advanced two directpush holes immediately outside of the cave entrance (GS-2 and 

GS-3, Appendix A and pl. 1). At GS-2, we encountered sand and 

gravel soil to a depth of 1.4 m (4.6 ft), where refusal occurred. 

We advanced GS-3 about 2 m (7 ft) north of GS-2 in an attempt 

to determine if bedrock caused refusal. At GS-3, we encountered 

refusal at 3.4 m (11 ft) and determined that gravel and sand 

extend to at least this depth (Appendix A and pl. 1; Geologic 

Cross Section). As shown on the Geologic Cross Section, GS-3 

indicates that soil extends to greater depth than the cave floor just 

outside the cave, whereas bedrock can be observed at the ground 

surface a meter away inside the cave. The soil obtained from 

GS-2 and 3 was sandy gravel to a depth of 0.9 m (3 ft), below 

which it was gravelly sand (Appendix A). GS-1 was advanced 

southwest of the southwest rim of the sinkhole (pl. 1; geologic 

maps and cross section) and revealed sandy gravel to a depth of 

6.4 m (21 ft) (Appendix A). Refusal was encountered at 6.2 m 

(20 ft) following removal of the 5.8–6.4-m (19–21-ft) sample, 

indicating that collapse of the hole occurred and that material 

that collapsed into the hole caused refusal, rather than bedrock. 

Finally, GS-7 was advanced west of the sinkhole (pl. 1; geologic 

maps) and revealed that sandy gravel extends to a depth of at 

least 3.6 m (12 ft) in this location (Appendix A). 

Several large blocks of limestone were observed at the base 

of the falls of the north drainage and within the cave, but these 

isolated blocks are not shown on the geologic maps. Similarly, 

isolated exposures of bedrock through the thin soil on the slope 

within the cave are not shown. In addition, much of the soil 

mapped on the lower parts of the southwest slope (Qal on pl. 1) 

has experienced additional transport by shallow landsliding, 

foot traffic, and surface water since initial deposition. Therefore, 

it would be more proper to map this material as colluvium. 

However, the deposits of transported soil are thin and not 

differentiable from the in-place soil so they were not mapped as a 

separate unit. 

Remaining Sinkhole Slopes

Continuous outcrops of undulating but generally flat-lying, 

moderately hard, weak to moderately strong, little weathered, 

nearly unfractured limestone were observed within the sinkhole 

along the axis of the drainage that enters the sinkhole from the 

north and to the southeast along the sinkhole’s northeast side. 

Small parts of the exposed limestone were weathered, weak, 
6 

and loose. We indicate on the geologic maps and cross section 

(pl. 1) that the limestone is part of the Boone Formation (pl. 1); 

however, as discussed above, it may be part of the St. Joe 

Formation. The limestone that comprises the northeast side 

of the sinkhole overhangs the cave entrance and the cave area 

immediately downslope from the entrance (pl. 1; Geologic Cross 

Section). The part of this limestone outcrop that protrudes farthest 

over the cave entrance is indicated by a heavy red line on the 

geologic maps (pl. 1). Limestone exposures along the northeast 

sinkhole slope have surface inclinations of 55° to overhanging, 

and most of them are covered with mature-appearing mosses 

or very thin soil. Soil that averages about one-meter thick and 

is comprised of gravelly silt is exposed along the top of the 

northeast slope and was identified in a direct-push hole advanced 

just northeast of the sinkhole (GS-4, Appendix A and pl. 1; 

geologic maps). 

Two joints cut the limestone exposed in the sinkhole near the 

north part of the cave entrance. These joints were planar and 

vertical with an average strike (orientation) of 17° west of north 

and generally were tightly closed and unfilled. One of these joints 

defines the east side of a block failure of the sinkhole wall. This 

failure created a cavity within the wall up to about 2-m (7 ft) high 

and 6-m (20 ft) deep (measured from the sinkhole slope face). 

The area of this failure is delineated on the Geologic Map (pl. 1) 

by the dashed 296-m topographic contour and the western of the 

two joints. Broad, low-volume seepage was observed emanating 

from the limestone wall above the failure. The block that fell now 

comprises the floor of the void that its displacement created, and 

the base of the block can be partly observed within Logan Cave 

where it rests on gravelly soil. 

The southeast and northwest sinkhole slopes generally are 

inclined between 50–55°, although sporadic limestone outcrops 

were observed along these slopes with vertical surfaces. These 

bedrock exposures protrude through a very thin (less than 

0.3-m (1-ft) thick) mantle of sandy, gravelly soil that covers 

most of these two slopes. Sparse, immature bushes and trees also 

occupy the slopes. As identified at the rim of the sinkhole and in 

direct-push holes advanced to the east (GS-5 and 6, Appendix A 

and pl. 1), thicker soil mantles bedrock outside of the sinkhole 

and consists of gravelly silt. The southeast and northwest slopes 

display scarps to about 0.3-m (1-ft) high that appear to have 

formed from shallow landsliding (pl. 1; geologic maps). A 2-m 

(7-ft) -high scarp is located along the base of the northwest slope 

and likely was created by surface-water erosion. A small amount 

of rilling was observed extending along the junction of the 

northeast and southeast slopes and is indicated by the intermittent 

drainage symbol on the geologic maps (pl. 1). Gravel and organic 

material has collected along the lower part of this intermittent 

drainage and within the cave where the drainage changes course 

toward the northeast (pl. 1; Southwest-Slope Geologic Map).

Logan Cave

Near the sinkhole entrance, Logan Cave consists of two 

parts; the lower part of the southwest slope and overhanging 

limestone bedrock and the tubular limestone passageway (pl. 1; 

Geologic Cross Section). Figure 2 shows both of these parts of 

the cave in a view to the northwest, similar to the view shown 

in the Geologic Cross Section (pl. 1). Note the soil mantling 

bedrock in the figure, the green area outside the cave near the 

upper-left corner of the figure, and the large mound of soil on the 

floor of the passageway in the lower right part of the figure. Soil 

locally mantles bedrock on the northeast side of the passageway, 

as shown in the left center of figure 3, which is a view to the 

southeast from within the passageway. A stream runs along the 

floor of the passageway and beneath the bedrock that comprises 

the cave’s northeast wall as shown on the Geologic Cross Section 

(pl. 1). The stream appears to flow primarily beneath the bedrock 

downslope of the cave entrance and primarily along the floor of 

the passageway both upstream and downstream (southeast and 

northwest, respectively) from the entrance. A thin (less than 

0.5-m (1.6-ft) thick) deposit of sandy gravel generally covers the 

floor of the passageway, but this deposit is much thicker (over 

1 m [3 ft]) in some areas downslope of the cave entrance (fig. 4). 

It is our understanding that a 50-year rainfall event occurred 

during the summer of 2003, after which soil within the cave 

entrance and passageway was first observed (Carla Mitchell, oral 

commun., 2004).

Immediately upstream from the entrance, the cave becomes 

tubular with limestone walls, ceiling and floor (with the exception 

of soil covering the floor). The cave becomes tubular downstream 

from the entrance, as well. However, a layer of sandy gravel is 

exposed along the southwest wall of the cave (fig. 5) between 

layers of limestone for at least about 10 m (33 ft) downstream 

from the entrance (the length of passageway explored during our 

study). This layer of sandy gravel can be followed through the 

northern part of the cave entrance where it merges with the sandy, 

gravelly soil mapped on the southwest slope of the sinkhole.

Laboratory Test Results

All samples obtained from the direct-push holes were tested 

for water content. Test results (Appendix B) revealed that water 

content varied little with depth at each direct-push hole location, 

and that moisture levels were typical for moist to wet soils. No 

indication of a nearby water table was observed. Grain-size 

distribution tests performed on samples obtained from GS-1, 

GS-2, GS-3, and GS-7 indicated that the soil generally is wellgraded, sandy gravel with little fine-grained material. Atterberg 

limit tests indicated that the fine-grained parts of these samples 

consist primarily of low-plasticity silt. 

The two tube samples of soil were obtained from just within 

the cave on the southwest slope (pl. 1; Southwest-Slope Geologic 

Map). The samples were obtained from this location because the 

soil appeared to be somewhat fine grained, which is required for 

performing most strength tests. CTL classified these samples as 

silt with sand and gravel, which differs from all other samples 

obtained during our subsurface exploration; therefore, the 

material in the tube samples is not necessarily representative of 

the southwest slope as a whole. These samples had a wet unit 

weight of 14.81 kN/m3 (94.3 lb/ft3) at 22.0 percent moisture 

content, and a dry unit weight of 12.14 kN/m3 (77.3 lb/ft3). These 
7

Figure 2. Photograph of the 

passageway and sinkhole 

entrance to Logan Cave. 

View is to the northwest. 

Passageway is at right, 

entrance is visible near 

the upper left (light green 

vegetation outside of the 

cave is visible), and sandy 

gravel and other debris is 

visible draping most of the 

slope from the entrance to 

the passageway and on the 

floor of the passageway. The 

width of the passageway at 

the bottom of the photograph 

is about one meter.

Figure 3. Photograph of 

the passageway (center 

of view) and part of the 

sinkhole entrance (right side 

of view) to Logan Cave. 

View is to the southeast. 

Sediment on the northeast 

wall of the cave is visible in 

the left center, as well as on 

the southwest slope (upper 

right) and passageway floor. 

Height of the passageway in 

the photograph is about two 

meters.
8 

Figure 4. Photograph 

of the passageway from 

southeast of the sinkhole 

entrance showing a thick 

accumulation of sediment on 

the floor of the passageway. 

View is to the northwest. 

The entrance is to the 

left and upslope of this 

accumulation. Passageway 

is about 2-meters wide 

at its widest point in this 

photograph.

Figure 5. Photograph of the 

passageway northwest of the 

sinkhole entrance. View is to 

the northwest. Sandy gravel 

deposits between layers of 

limestone are visible on the 

left extending down from 

the roof of the passageway. 

Height of the passageway is 

about 1.5 m.
9

unit weight values are typical for sandy gravel, as well (Das, 

1994, p. 44). Triaxial-compressive-strength tests performed on 

these samples indicated total and effective friction angles of 24.9°

and 36.3°, respectively, and no cohesion. These friction angle 

values should be considered lower bounds since the triaxial test 

was performed in multiple stages, and the sample was remolded, 

both of which tend to reduce strength test results (for example, 

Lambe and Whitman, 1969; Terzaghi and others, 1996).

Slope-Stability Analysis

No evidence of large-scale instability was identified along 

the southwest slope of the sinkhole; however, evidence of 

shallow (less than 0.3-m (1-ft) deep) landsliding and raveling 

was observed. The average inclination of this slope is 47°, 

which is quite steep for a slope comprised of gravel. Therefore, 

we performed stability analysis of the southwest slope using 

Version 7 of the finite-element program PLAXIS©. This program 

was developed specifically for geotechnical analysis, such as 

slope-stability analysis, and has been demonstrated to provide 

similar results to those of conventional slip-circle slope-stability 

analyses, yet it does not require predetermination of failuresurface geometry or special assumptions about interslice forces 

as do the slip-circle techniques (Griffiths and Lane, 1999; Savage 

and others, 2000). 

We analyzed stability of the southwest slope as it is shown 

on the Geologic Cross Section (pl. 1). PLAXIS© requires input 

of the soil unit weight, Young’s modulus, Poisson’s ratio, 

angle of internal friction (φ), and cohesion (c). Of these, slopestability analyses generally are most sensitive to variations of the 

strength properties φ and c. We used soil parameter values that 

were determined in the laboratory and selected from published 

literature.

The triaxial-compressive-strength test provided soil strength 

parameters of φ=24.9° and c=0 for the soil on the lower part of 

the southwest slope. As stated previously, the material that was 

tested to obtain these parameters is not representative of the 

majority of the material that comprises the southwest slope. We 

assumed that φ=24.9° represents a lower bound for the soil that 

comprises the slope, and that the soil must have some cohesion 

because the slope is currently stable. As an upper bound for the 

strength parameters, we chose a friction angle of 48°, which is a 

published value for angular gravel with some sand (Das, 1994, 

p. 320). The soil that comprises the southwest slope is well 

characterized as angular gravel with some sand, so this friction 

angle probably is more representative than the lower value from 

laboratory tests. Young’s modulus and Poisson’s ratio were held 

at constant values of 60,000 kN/m2 (1.2 x 106 lbf/ft2) and 0.4, 

respectively, which fall in the range of published values obtained 

from several sources (Lambe and Whitman, 1969, p. 159; Das, 

1994, p. 301). Unsaturated unit weight was determined in the 

laboratory to be 14.81 kN/m3 (94.3 lb/ft3). We increased the 

unsaturated unit weight to 17 kN/m3 (108 lb/ft3) during analyses, 

which is near the unit weight of the soil at saturation 

(17.52 kN/m3 [111.6 lb/ft3]) based on calculations made using 

laboratory test results.

The southwest sinkhole slope appeared to be stable at the time 

of our field investigation. Based on the geometry of the slope 

and the potential strength of the soil of which it is comprised, 

we assumed that the slope is marginally stable. Our stability 

analyses was designed to determine likely failure geometries 

and the degree of soil-parameter variability required to result 

in slope failure. Failure of the slope could occur due to changes 

in strength or loading conditions, both of which could vary 

due to changes in moisture content in the slope following 

prolonged rainfall. It is unlikely that ground water exists in the 

slope at depths that could affect its stability because none was 

observed during our field investigation, soil likely extends to 

depths significantly below the slope face due to the origin of 

the soil deposit (discussed in the Conclusions section), and the 

regional water table has been mapped at 23m (75 ft) below the 

sinkhole entrance (Ogden, 1980). Therefore, increasing porewater pressures within the soil due to increasing ground-water 

levels are not expected to occur and thus slope failure by this 

mechanism is unlikely. 

We analyzed the slope with two strength scenarios and two 

unit weight scenarios. Strength of the soil that comprises the 

slope primarily is derived from friction, since granular soils 

generally have very little cohesive strength. However, apparent 

cohesion due to capillary forces exerted by water held within 

the soil matrix may contribute significantly to the strength of the 

slope (Wu, 1996), and is not readily measured during strength 

testing (Lu and Likos, 2004). This capillary cohesion varies 

nonlinearly with water content and is reduced to zero as the soil 

dries completely or reaches saturation. We used the upper and 

lower bound friction angles obtained from the laboratory and 

literature to calculate the minimum cohesion required for the 

slope to be stable and then evaluated whether capillary cohesion 

could exceed this minimum. We also analyzed variable unit 

weight scenarios to evaluate the effects of rainfall increasing the 

soil unit weight.

Trial 1—Upper-Bound Angle of Internal Friction—

In-Situ Unit Weight

We analyzed the southwest slope with constant φ=48° and 

unit weight of 14.81 kN/m3 (94.3 lb/ft3), and varied c until 

we obtained results of marginal stability. Cohesion of at least 

0.7 kN/m2 (14.6 lbf/ft2) was required to provide stability to the 

slope. This value of c is quite low and is within the likely range 

of capillary cohesion for this soil type (Lu and Likos, 2004). 

Figure 6 shows the modeled slope geometry, which matches 

that of the Geologic Cross Section (pl. 1), and the distribution 

of stresses predicted by PLAXIS©. The warm colors indicate 

high shear stress, while the cool colors indicate low shear stress. 

Figure 6 indicates that the highest shear stresses, and thus the 

base of failure predicted if soil strength falls below the modeled 

values, are concentrated at shallow depth in the upper one-half 

of the slope. The predicted slope failure has a thickness of about 

1.5 m (4.9 ft) (measured normal to the slope) and is classified 

as a translational failure, meaning that landslide displacement is 

parallel to the slope. 
10 

Trial 2—Upper-Bound Angle of Internal Friction—

Saturated Unit Weight

To model the slope of Trial 1 under nearly saturated 

conditions, may occur due to heavy rainfall, we increased the unit 

weight to the nearly saturated unit weight estimated to be 

17 kN/m3 (108 lb/ft3) and held φ and c constant at 48° and 

0.7 kN/m2 (14.6 lbf/ft2), respectively. The analysis indicated that 

the slope is slightly less stable than for the scenario using the insitu unit weight (Trial 1), but still indicated a marginally stable 

slope. This result was expected because unit weight has little 

effect on the analysis as cohesion approaches zero. 

Trial 3—Lower-Bound Angle of Internal Friction—

In-Situ Unit Weight

We modeled the southwest slope with constant φ=24.9° and 

unit weight of 14.81 kN/m3 (94.3 lb/ft3), and varied cohesion to 

determine the minimum required for stability. We found that c 

of at least 6.0 kN/m2 (125.3 lbf/ft2) is required for the slope to 

be stable. As with Trial 1, capillary cohesion of this magnitude 

is reasonable for the type of soil that underlies the southwest 

slope. However, the failure predicted to occur if soil strengths 

drop below the values used during Trial 3 (fig. 7) is much deeper 

than that predicted during Trial 1. This failure is as deep as 3.5 m 

(11.5 ft) (measured normal to the slope), extends to the toe of 

the slope, and extends back about 4 m (13 ft) from the sinkhole 

edge. It is classified as a rotational failure, meaning landslide 

displacement would involve rotation along a concave-upward 

failure surface. 

Trial 4—Lower-Bound Angle of Internal Friction—

Saturated Unit Weight

The slope of Trial 3 was modeled with the soil nearly 

saturated, such as may occur due to heavy rainfall, by increasing 

the unit weight to the estimated near-saturation value of 17 kN/m3

(108 lb/ft3). The strength parameters φ and c were held constant 

at 24.9° and 6.0 kN/m2 (125.3 lbf/ft2), respectively. The analysis 

indicated that the slope would fail under these conditions. The 

failure geometry is nearly identical to that shown in figure 7.

Summary of Slope-Stability Analyses 

The conditions modeled as Trials 1 and 2 are likely to more 

accurately represent potential conditions of the southwest slope 

than the conditions modeled in Trials 3 and 4; thus, the results 

of Trials 1 and 2 are more likely representative of potential 

instability of the southwest slope. This is mainly because the 

strength parameters used for Trials 1 and 2 are representative of 

the soil identified nearly everywhere on and within the southwest 

slope. The soil on which triaxial-compressive-strength tests were 

� � � ��

�

��

�

�

Figure 6. Graphical representation of the Trial 1 slope-stability analysis results showing the modeled southwest slope 

cross section and calculated stresses. The shear stress field is represented by colors with cool colors indicating low 

shear stress and warm colors indicating high shear stress. The base of a landslide predicted by this trial would be 

located where shear stresses are highest (warmest colors).
11

performed was generally not representative of the soil observed 

on and within the southwest slope. Slope failures predicted by 

Trials 1 and 2 were thin (about 1.5-m (4.9-ft) thick) and occurred 

on about the upper one-half of the southwest slope. Failure of 

the slope would most likely occur during or following significant 

rainfall events due to a decrease in capillary cohesion. 

It is our opinion that the southwest slope is marginally stable, 

given realistic strength parameters and the slope geometry. 

Therefore, eventual failure of the slope seems likely. The 

likelihood of slope failure is greatest during intense rainfall on 

already wet soil. It is worth noting that the 50-year storm of 2003 

did not trigger large-scale landsliding, but did trigger small-scale 

landsliding and transport of soil into the cave entrance. 

CONCLUDING DISCUSSION

The sinkhole entrance to Logan Cave is a geologically active 

environment and should continue to be so in the future. Our 

investigation indicated that several geologic processes have been 

changing the sinkhole entrance for many years. An important 

aspect in predicting future evolution of the sinkhole entrance is 

understanding what has happened in the past.

Geologic History

The sinkhole entrance to Logan Cave is located along and to 

the east of the axis of a drainage that probably was created during 

the Pleistocene epoch (1,800,000–10,000 years ago; Manger and 

others, 1988b). Our investigation revealed that a sinkhole larger 

than that which currently provides the entrance to Logan Cave 

extended to the southwest and has been filled with sandy, gravelly 

soil that we conclude is alluvium (pl. 1). This sinkhole must 

have formed prior to or during the Pleistocene to have filled with 

alluvium. As a result of the previous sinkhole being filled with 

alluvium, the southwest slope of the current sinkhole is underlain 

by alluvium. No surface expression was observed of the larger, 

older sinkhole, other than, perhaps, the general broadening of 

the drainage valley in this area. The large block failure located 

adjacent to the north end of the present cave entrance rests on 

alluvium, indicating that the failure occurred while the original 

sinkhole was filling. 

While the previous sinkhole filled (or after it filled), sediment 

was transported by surface water and shallow slope failures 

into the present entrance of Logan Cave and also into a similar 

adjacent entrance located to the northwest. This entrance 

completely filled with sediment as evidenced by the sandy gravel 

exposures in the southwest wall of the passageway northwest of 

the present entrance (fig. 5 and pl. 1; Southwest-Slope Geologic 

� � � ��

�

��

�

�

Figure 7. Graphical representation of the Trial 3 slope-stability analysis results showing the modeled southwest slope 

cross section and calculated stresses. The shear stress field is represented by colors with cool colors indicating low 

shear stress and warm colors indicating high shear stress. The base of a landslide predicted by this trial would be 

located where shear stresses are highest (warmest colors).
12 

Map). The former entrance probably was a solution channel that 

was exposed in the walls of the previous sinkhole. Sediment fills 

the former entrance completely and must have partly filled the 

passageway because the deposit displays a vertical, erosional 

surface within the cave. This indicates that the stream was 

capable of transporting sediment from the passageway in the 

past, but not from the former entrance. There also is evidence 

of stream transport of sediment immediately downslope of the 

present entrance. Figure 2 provides a view to the southeast of 

the present entrance area and passageway and shows a large 

mound of sediment blocking the floor of the passageway as well 

as a small amount of sediment high on the northeast wall of the 

passageway (left side of figure). This sediment on the northeast 

wall seems to indicate that a thick deposit previously existed 

at this location but was subsequently removed by the stream. 

The ages of this deposit and subsequent removal, however, are 

unknown. At present, the stream in the entrance area flows almost 

entirely beneath the northeast wall of the passageway, thus it 

appears to no longer be capable of removing sediment deposited 

downslope of the entrance. In fact, the stream is not visible 

at all near the mound of sediment shown in figure 2; it flows 

completely beneath the northeast bedrock wall. Many episodes 

of sediment deposition in the passageway likely have resulted 

in accelerated dissolution of the northeast wall, thus the stream 

has migrated to the northeast away from the entrance. Even if 

the stream is capable of flushing sediment from the passageway 

in the future, that may not prevent the entrance from closing, as 

occurred at the former entrance located to the northwest.

Mechanisms of Sedimentation Within the 

Sinkhole Entrance 

Observations made by cave researchers during the past several 

decades have indicated that the sinkhole entrance to Logan Cave 

is progressively filling with sediment. As discussed above, the 

stream within Logan Cave probably has removed significant 

volumes of sediment in the past, but did not prohibit a former 

entrance from filling. In addition, the stream has migrated away 

from the passageway in the cave entrance area so probably is 

no longer capable of removing sediment and keeping even the 

passageway open. There are several mechanisms responsible 

for transporting sediment into the cave entrance that could be 

mitigated to keep the entrance open. These mechanisms include 

foot traffic, surface-water flow, and landsliding.

Foot Traffic

Scientists and others have used the sinkhole entrance to gain 

access to Logan Cave for at least the past 50 years and continue 

to do so (David Kampwerth, oral commun., 2004). A trail within 

the sinkhole has been used to access the cave entrance. Foot 

traffic on this trail has moved alluvium downslope and has 

created a large, deep gully through the sinkhole rim near the 

south end of the southwest slope (pl. 1; geologic maps). Much of 

the transported sand and gravel has accumulated near the cave 

entrance as indicated by the decreased slope in this area, and a 

significant amount has accumulated within the cave entrance and 

passageway. By reconstructing the pre-gully topography of the 

sinkhole, we calculate that just over 10 m3 (350 ft3) of sediment 

has been transported by foot traffic. This volume is sufficient to 

fill about 80 percent of the passageway located downslope of the 

sinkhole entrance. Transport and deposition of this sediment has 

been very gradual and probably permitted much of the material 

that traveled as far as the passageway to be removed by the 

stream. However, with the reduced sediment transport capacity 

of the stream, gravel pushed downslope into the cave likely will 

no longer be removed. The loose nature of the alluvium and 

steep inclination of the trail will result in continuing transport 

of sediment into the cave by foot traffic, based on our own 

experience and observations.

Surface-Water Flow

Alluvium and the soil that mantles bedrock, which we 

conclude is regolith (pl. 1), are prone to erosion and transport 

to the cave by surface-water flow due to their low density 

and lack of cohesive strength. The 50-year rainfall event of 

2003 demonstrated the capability of surface water to transport 

sediment to the cave. Surface-water flow occurs as sheet flow and 

concentrated flow. Sheet flow occurs on all slopes and primarily 

transports and deposits fine-grained soil. Concentrated flow 

occurs in some areas, especially within the drainage that enters 

the sinkhole from the north, near the junction of the northeast 

and southeast slopes, and down the access trail. Concentrated 

flow in these areas has resulted in more pronounced erosion than 

that due to sheet flow as well as transport and deposition of up 

to cobble-sized rocks. Surface-water flow within the sinkhole 

is due to precipitation falling in the sinkhole and overland flow 

down the north drainage and the slopes above the northeast side 

of the sinkhole. Overland flow from the slope above the northeast 

side of the sinkhole appears to occur both on the ground surface 

and within the regolith with water perched at its base on top of 

limestone bedrock. This transient ground water seeps out of the 

regolith where it encounters the sinkhole slopes and then flows as 

surface water within the sinkhole. 

Landsliding

Shallow (less than 0.3-m (1-ft) thick) landsliding seems to 

be an ongoing process within the sinkhole, especially along the 

upper parts of the slopes and in the shallow regolith mantling 

parts of the bedrock slopes. Much of this landsliding seems to 

have been associated with surface-water erosion as the landslide 

scars are adjacent to erosional gullies. Scarps shown on the 

geologic maps on the southeast slope probably were formed by 

small, shallow landslides. No evidence was observed for largescale landsliding within the sinkhole. As discussed in the SlopeStability Analysis Section, reasonable strength parameters for 

the alluvium indicate that the slope currently is in a condition 

of marginal stability. Future landslides would most likely be 

triggered by prolonged, heavy rainfall. These landslides would be 

shallow (up to about 1.5-m (4.9-ft) thick), translational failures 

of the upper part of the southwest slope, as well as much smaller 

landslides from the very shallow regolith mantling bedrock 
13

along the other sinkhole slopes. Slope failures smaller than those 

predicted by our slope-stability analysis probably occurred during 

the 50-year rainfall event of 2003 and resulted in deposition of a 

large volume of alluvium in the cave entrance and passageway. 

However, larger landslides did not occur during that rainfall 

event. It is possible that future heavy rainfall may trigger larger 

failures that could result in complete closure of the cave entrance. 

We believe that landslides of this magnitude are unlikely to occur, 

and that small, shallow landslides are very likely in the future, 

several of which could result in closure of the cave entrance.

Evaluation of the stability of the sinkhole slopes that are 

comprised of bedrock was beyond the scope of our study. There 

is a large amount of rock cantilevered over the cave entrance 

as illustrated on the Geologic Cross Section (pl. 1). However, 

discontinuities (other than bedding planes) along which failure 

of this rock mass could occur were not observed, although they 

may exist farther into the slope. As described earlier, a large 

wedge of rock failed just northwest of the cave entrance while the 

previous sinkhole filled with alluvium, and scattered deposits of 

other rock falls are visible low on the southwest slope. Failure of 

a bedrock slope within the sinkhole would likely be triggered by 

exceptionally heavy precipitation or additional undercutting by 

the stream located within the cave.

Summary of Sedimentation Mechanisms

Other than foot traffic, the mechanisms responsible for 

transporting sediment into the sinkhole entrance of Logan 

Cave cannot feasibly be eliminated. Sediment will continue to 

be transported by surface-water flow and landslides, as well 

as by foot traffic when it occurs. The stream within the cave 

passageway downslope of the entrance is no longer capable of 

flushing sediment from this area. Thus, it is our opinion that the 

sinkhole entrance eventually will fill with sediment, eliminating 

this point of access to the cave for scientists and other animals, 

including the endangered gray bat. The potential for sudden, 

complete closure of the entrance due to a large landslide is 

probably remote, but it does exist.

Mitigation Alternatives

Many alternatives exist to reduce the amount of sedimentation 

within the sinkhole entrance to Logan Cave. However, the 

potential impact of any mitigation measures on species that reside 

within the cave must be considered along with cost, disturbance 

of the natural environment outside of the cave, safety of cave 

researchers, and other issues. Also, it should be considered 

that construction-related activities could result in significant 

disruption of the cave ecosystem. Provided below are some 

alternatives to reduce the amount of sedimentation in the cave 

and the potential for slope failure to close the sinkhole entrance. 

These alternatives are provided in order of increasing cost 

associated with their construction and the degree of disturbance 

of the natural environment that they involve. Different 

combinations of the mitigation measures than those provided 

below for each alternative could be used as well.

Alternative 1—Periodic Sediment Removal 

Approximately 10 m3 (350 ft3) of sediment mantles bedrock 

within the cave entrance and the passageway downslope. This 

sediment reduces the size of the entrance and passageway, thus 

increasing the potential for the entrance or passageway to be 

blocked. Loose sediment could be manually excavated in the 

cave and transported from the sinkhole. Manual excavation 

and transport could be performed relatively quickly with the 

assistance of a cable-and-pulley system from the cave entrance 

to south of the sinkhole that would allow containers filled with 

sediment to be rapidly transported out of the sinkhole, emptied, 

and returned to the cave entrance without requiring significant 

foot traffic into and out of the sinkhole. This alternative does not 

address mechanisms responsible for transporting sediment to 

the cave so would need to be performed periodically. It does not 

directly address the potential for the cave entrance to be closed 

by sedimentation (other than by providing space into which 

sediment may be deposited). 

Alternative 2—Foot-Traffic Erosion Control and 

Periodic Sediment Removal 

Foot traffic along the trail that traverses the southwest slope 

of the sinkhole and leads to the cave entrance has resulted in 

transport of about 10 m3 (350 ft3) of alluvium from along the trail 

to the lower parts of the slope and within the cave. Placement of 

a cellular paver, geogrid, or geotextile fabric along the trail that 

is designed to reduce erosion of granular soils by foot traffic and 

surface water, such as those described at http://www.ndspro.com/, 

would greatly reduce the amount of sediment transported by 

foot traffic. Improvement of the trail would not address future 

sedimentation within the cave by landsliding or surface-water 

transport. To address sedimentation from these processes, 

sediment could be periodically excavated and removed from the 

cave passageway and entrance areas by hand as described under 

Alternative 1. 

Alternative 3—Sediment-Retention Wall, Foot-Traffic 

Erosion Control, and Periodic Sediment Removal 

This alternative addresses some of the limitations of 

Alternative 2 by nearly eliminating the potential for sediment 

to enter the sinkhole entrance by any mechanism. Sediment 

transported downslope could be retained by a wall constructed 

just southwest of the cave entrance as close to the overhanging 

cliff as possible, yet far enough away to allow access to the 

entrance. The wall should be about 1-m (3-ft) high and abut 

the cliff just beyond the ends of the entrance. The wall could 

be constructed of pressure-treated lumber secured to posts set 

in concrete in holes drilled or excavated to sufficient depth to 

support the wall and a full load of alluvium. Alternatively, the 

wall could be constructed of formed, steel-reinforced concrete 

upon a footing cantilevered into the slope. A concrete wall 

would simplify maintenance (described below) and would not 

have the potential adverse effects on organisms within the cave 

that pressure-treated lumber may have. Any wall constructed 
14 

should be designed by a registered civil engineer using the soil 

parameters of c=0, φ=47°, unit weight of 17.5 kN/m3 (111 lb/ft3), 

and a frost-penetration depth of 0.51 m (20 in.) (Naval Facilities 

Engineering Command, 1986). Drainage holes should not be 

necessary through the wall as the alluvium drains effectively 

and is unsaturated. Note that special precautions must be taken 

to construct a wall without adversely affecting the slope during 

construction. These precautions are described in Construction 

Notes below.

Erosion by foot traffic should be addressed as described under 

Alternative 2. Although a wall would retain material transported 

by foot traffic, the cost of foot-traffic erosion control measures is 

relatively small, and their benefits are high. Foot-traffic erosion 

control measures would greatly reduce the need for periodic 

maintenance at the sediment-retention wall and would enhance 

the stability of the southwest slope near the trail.

The sediment-retention wall and foot-traffic erosion-control 

measures would greatly reduce the volume of sediment 

transported into the cave entrance. However, the height of the 

wall must be maintained by periodic removal of sediment from 

against the wall to reduce the likelihood of sediment overtopping 

the wall. Removal of sediment from within the cave should 

only be necessary following wall construction if the wall is 

overtopped, which is extremely unlikely if maximum wall 

height is maintained. If a lumber wall is constructed, it must be 

periodically inspected for rotted, damaged, loose, or otherwise 

flawed lumber in accordance with manufacturer’s specifications. 

Flawed lumber must be replaced in accordance with wall designs.

Alternative 4—Slope Excavation, Foot-Traffic Erosion 

Control, and Periodic Sediment Removal 

This alternative provides measures to significantly reduce the 

amount of alluvium entering the cave from the southwest slope 

by reducing the potential for mobilization of the alluvium. The 

alternative does not address sediment input to the cave from other 

slopes, however. The likelihood of landsliding to occur on the 

southwest slope and mobilization of alluvium by surface water 

could be greatly reduced by excavating the southwest slope such 

that a consistent slope angle of no greater than 38° is attained. 

Excavation of the slope should be planned and executed such that 

smooth transitions are made between the southwest and adjacent 

slopes, which would require partial excavation of those slopes. 

This alternative likely would result in temporarily increased 

surface-water transport of sediment to the cave from the 

transition areas between the southwest slope and adjacent slopes. 

Following excavation of the southwest slope, foot-traffic erosioncontrol measures described in Alternative 2 should be constructed 

to reduce the potential for future transport of sediment by foot 

traffic. Because Alternative 4 does not address sedimentation 

from sources other than the southwest slope, and because some 

alluvium would continue to be transported down the southwest 

slope to the cave, periodic removal of sediment from the cave 

entrance would be necessary, as described under Alternative 1.

Construction Notes

Construction of any measures within the sinkhole must be 

carefully designed and implemented to reduce potential impacts 

to the cave and its inhabitants. Temporary debris catchment/

retention structures should be constructed outside of the cave 

entrance prior to construction activities. These could entail sturdy 

lumber or sheet metal sections braced across the cave entrance 

if larger failures are anticipated (such as during excavation for 

a wall or of the entire southwest slope), or silt fencing along the 

slope just above the cave entrance if only transport due to foot 

traffic is anticipated (such as during placement of foot-traffic 

erosion-control measures or manual excavation of sediment 

from within the cave entrance). If a concrete wall is constructed, 

excavation of the southwest slope to permit construction of the 

footing and wall must be performed in steps so that no more 

than 3 m of slope (measured along topographic contour) is 

unsupported at any one time to reduce the total size of potential 

slope failures during construction. We expect that any significant 

depth of excavation (such as to frost depth) will result in small 

failures of the slope near the excavation. These failures must 

be anticipated, and preparations must be made to accommodate 

them. 

We observed that gravel-sized and smaller rock fragments 

primarily comprise the southwest slope. However, large blocks of 

rock (greater than1-m (3-ft) diameter) were also observed on this 

slope, mainly along the lower part. Isolated, large blocks of rock 

probably would be encountered during excavation of the lower 

part of the southwest slope and should be accounted for on any 

construction drawings or specifications. 

Any material removed from the sinkhole (rock, soil, 

and organic debris) could be placed within the refuge in an 

environmentally-sound location approved by USFWS personnel. 

However, at no time should excavated material be placed closer 

than 10 m (33 ft) to the sinkhole rim. In addition, no material 

should be placed north or east of the sinkhole or within the southtrending intermittent drainage.

Summary of Mitigation Alternatives

In our opinion, the most cost-effective and least-disruptive 

alternative to reduce the potential for closure of the sinkhole 

entrance to Logan Cave is Alternative 3, because this alternative 

requires little disruption within the sinkhole (compared to 

Alternative 4) and will allow the cave entrance to remain at 

least partly open following all but the largest (and least likely) 

landslides on the southwest slope. Alternative 3 also would 

significantly reduce sediment input to the cave from other areas 

of the sinkhole. Alternative 4 reduces the potential for landslide 

events, but does not address transport of soil from slopes other 

than the southwest and requires significant disturbance within 

the sinkhole. Alternatives 1 and 2 do not address the potential 

for large landslides to close the sinkhole entrance, other than 

by providing void space into which landslide debris may be 

deposited. 
15

We believe the potential for sudden closure of the entrance 

due to landsliding of the southwest slope is extremely small if 

Alternative 3 or 4 is implemented. However, the results could be 

catastrophic if such a closure occurred while scientists (or others) 

occupied the cave. It is our opinion that the cave should not be 

occupied during or within several days following abnormally 

heavy, prolonged rainfall events. In addition, the alternatives 

provided herein do not address failure of the rock slopes that 

comprise the other sides of the sinkhole, although the sedimentretention wall of Alternative 3 should provide some protection to 

the cave entrance should failure of these other slopes occur. 

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Manger, W.L., Zachry, D.L., and Garrigan, M.L., 1988b, An 

introduction to the geology of northwestern Arkansas: Field 

trips: The Compass, v. 65, no. 4, p. 242–256. 

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Council, p. 319–336.
APPENDIX A

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APPENDIX B

Laboratory Test Results
USGS LABORATORY TEST RESULTS

Logan Cave National Wildlife Refuge

Cu Cc

PL LL PI Uniformity Coefficient

Boring Depth (m) Moisture Content Plastic Limit Liquid Limit Plasticity Index Coefficient of Curvature Comments

GS-1 0-0.6 18.2%GS-1 0.6-1.2 15.2%GS-1 1.2-1.8 13.2%GS-1 1.8-2.4 14.6%GS-1 2.4-3.4 12.1% 21 24 4 77 7 Tested with 3.4-4.0m sampleGS-1 3.4-4.0 13.8% 21 24 4 77 7 Tested with 2.4-3.4m sampleGS-1 4.0-4.6 16.4%GS-1 4.6-5.2 16.0%GS-1 5.2-5.8 13.9%GS-1 5.8-6.4 15.8%GS-2 0-1.2 19.8% 23 31 8 200 20GS-3 0.9-1.5 20.4%GS-3 1.5-2.1 22.0%GS-3 2.1-2.7 26.1% 22 41 20 20 2GS-3 2.7-3.4 24.4%GS-4 0-0.9 22.2%GS-5 0-0.9 40.1%GS-6 0-0.9 18.3%GS-7 0-0.6 13.5%GS-7 0.6-1.2 15.2%GS-7 1.2-1.8 12.9%GS-7 2.4-3.0 15.7% 20 26 6 143 5 Tested with 3.0-3.7m sampleGS-7 3.0-3.7 16.2% 20 26 6 143 5 Tested with 2.4-3.0m sample
Logan Cave Grain Size Distribution

Sample GS-1 (2.4 - 4.0 meters)

0

102030405060708090

100

0.01 0.1 1 10 100

Particle Diameter (mm)

Percent Finer (%)

Coarse Medium

Silt

Fine Medium Coarse

Sand

Fine Coars

G

e

ravel
Logan Cave Grain Size Distribution

Sample GS-2 (0 - 1.2 meters)

0

102030405060708090

100

0.01 0.1 1 10 100

Particle Diameter (mm)

Percent Finer (%)

Coarse Medium

Silt

Fine Medium Coarse

Sand

Fine Coars

G

e

ravel
Logan Cave Grain Size Distribution

Sample GS-3 (2.1 - 2.7 meters)

0

102030405060708090

100

0.01 0.1 1 10 100

Particle Diameter (mm)

Percent Finer (%)

Coarse Medium

Silt

Fine Medium Coarse

Sand

Fine Coars

G

e

ravel
Logan Cave Grain Size Distribution

Sample GS-7 (2.4 - 3.7 meters)

0

102030405060708090

100

0.01 0.1 1 10 100

Particle Diameter (mm)

Percent Finer (%)

Coarse Medium

Silt

Fine Medium Coarse

Sand

Fine Coars

G

e

ravel
Project:

Project No. Client: Remarks:

MATERIAL DESCRIPTION LL PL PI %<#40 %<#200

LIQUID AND PLASTIC LIMITS TEST REPORT

Source: B-1 Sample No.: 1 Elev./Depth: 1'

Figure

LIQUID AND PLASTIC LIMITS TEST REPORT

COOPER TESTING LABORATORY

USCS

411-004 USGS

Brown SILT with Sand and Gravel 42 31 11

Logan Cave - 1

5 10 20 25 30 40 31

35

39

43

47

51

NUMBER OF BLOWS

WATER CONTENT

10 30 50 70 90 110

LIQUID LIMIT

10

20

30

40

50

60

PLASTICITY INDEX

4

7 CL-ML

CL or OL

CH or OH

ML or OL MH or OH

Dashed line indicates the approximate

upper limit boundary for natural soils