# Characteristics of Slumgullion Landslide inferred from subsurface exploration, in-situ and laboratory testing, and monitorinG

# 

## William H. Schulz^1^, Jonathan P. McKenna^2^, Giulia Biavati^3^ & John D. Kibler^4^

### 

### ^1^ U. S. Geological Survey (e-mail: wschulz@usgs.gov)

### ^2^ U. S. Geological Survey (e-mail: jmckenna@usgs.gov)

### ^3^ Università di Bologna, Italy (e-mail: <biavati@geomin.unibo.it>)

### ^4^ U. S. Geological Survey (e-mail: kibler@usgs.gov)

Cite as: Schulz, W.H., McKenna, J.P., Biavati, G., and Kibler, J.D.,
2007, Characteristics of Slumgullion Landslide inferred from subsurface
exploration, in-situ and laboratory testing, and monitoring, *in*
Proceedings of the First North American Landslide Conference, Landslides
and Society: Integrated Science, Engineering, Management, and
Mitigation, Vail, Colo., June, 2007, American Society of Civil
Engineers.

\* The use of trade, product, industry, or firm names is for descriptive
purposes only and does not imply endorsement by the US Government.

**Abstract:** Continuous monitoring for 22 months of piezometer and
tensiometer nests and displacement and meteorological sensors in the
active part of the Slumgullion landslide, southwest Colorado, identified
complex relationships between pore pressure and movement. Piezometers
were located 3.4 m from a landslide margin while tensiometers were
located 21.3 m from the margin. Landslide displacement was continuous
and generally steady, although velocity spiked following spring
snowmelt. Pore-water pressures along the margin dropped 2 m. Pore-water
pressures measured away from the margin increased abruptly following
rainfall and snowmelt events but mean values were steady. Landslide
velocity correlated positively with pore-water pressures measured away
from the margin. Pore-water pressures along the margin increased
gradually following snowmelt. Cyclic pressure change along the margin
occurred with abrupt decrease during landslide acceleration and increase
during deceleration, probably due to changes in dilation of shearing
landslide debris. Repeated cycles of this type occurred and probably
involved dilatant strengthening of shearing landslide debris during
acceleration resulting in deceleration and subsequent consolidation of
landslide debris and pore-water pressure increase that aided generation
of a subsequent acceleration-deceleration cycle.

# Introduction

The Slumgullion landslide is a translational debris slide (Cruden and
Varnes 1996) located in the San Juan Mountains in Hinsdale County,
southwestern Colorado, U.S.A. (Figure 1). Part of the landslide has been
continuously active for about 300 years (Varnes and Savage 1996). The
active part of the landslide is about 3.9 km long, averages about 300 m
wide, and has an estimated volume of 20x10^6^ m^3^ and average depth of
about 14 m (Parise and Guzzi 1992). Because of its continuous activity,
the landslide presents an exceptional opportunity to study landslide
processes and has been the subject of several such efforts. For example,
Fleming *et al.* (1999) concluded that annual displacement of the
landslide had been about constant during the preceding 100 years and
found that the landslide consists of several independent kinematic units
that can have differing velocities. They found that velocity varies
seasonally, presumably due to changes in pore-water pressures. Savage
and Fleming (1996) measured landslide displacement in near-real time
during part of 1993 and also found seasonally varying velocity. Coe *et
al.* (2003) performed hourly monitoring of landslide displacement, air
and soil temperature, snow depth, rainfall, soil water content, and
apparently perched-groundwater pressures and found that the landslide
moved fastest during the spring and summer and slowest during the
winter. They identified generally direct, positive correlation between
pore-water pressure measured at a depth of 2.2 m and landslide velocity,
but also identified periods of landslide acceleration in response to
snowmelt events in the absence of increased pore-water pressure. They
inferred that greater surface-water availability and increased
pore-water pressures resulted in greater landslide velocity. Baum and
Reid (2000) proposed that low-permeability clay layers formed along
basal and marginal shear zones effectively isolates the landslide
hydrologically from adjacent areas and aids perennial landslide
displacement. The landslide has greater spatial density of springs,
sinks, ponds, and surface-water drainages than adjacent areas (Fleming
*et al*. 1999) and the locations and activity levels of these features
vary with time. Evaluation of deeper groundwater conditions within the
landslide and their relations with precipitation, snowmelt, and
landslide displacement has been lacking from previous studies. Deeper
groundwater conditions have remained unexplored because of difficult
access onto the landslide.

![](media/image1.png){width="3.5930555555555554in"
height="4.1194444444444445in"}

**Figure 1.** Map showing the location of the Slumgullion landslide.
Modified from Coe *et al.* (2003).

To better understand deep groundwater conditions at the landslide, we
advanced a borehole during July 2004 to a depth of 9.3 m at a monitoring
site (IS-1) described by Coe *et al.* (2003), which is located near the
left margin (when viewed in the downhill direction) of the active part
of the landslide (Figure 2). The 2.2-m-deep piezometer described by Coe
*et al.* (2003) is located about 1 km downslope from where we performed
our investigation. We installed equipment to monitor groundwater
pressures in saturated and unsaturated landslide debris. This equipment,
along with that described by Coe *et al.* (2003), has been monitored on
an hourly basis since installation. We performed laboratory and field
tests of material properties such as porosity, grain-size distribution,
and saturated hydraulic conductivity and installed a
subsurface-displacement-measuring device that has been monitored
episodically. This paper summarizes the results of this recent work.

![](media/image2.png){width="6.497222222222222in"
height="4.988194444444445in"}

**Figure 2.** Virtual oblique view of the active part of the Slumgullion
landslide developed from digital topographic data (Messerich and Coe
2003). View is from the elevation of the monitoring site, which is
represented by the red sphere. The width of the landslide at the
monitoring site is about 200 m and the length of the landslide is about
3,900 m.

# SETTING

The Slumgullion landslide occurs within Tertiary volcanic rocks
including basalt, rhyolite, and andesite, much of which has been highly
altered by hydrothermal activity (Lipman 1976; Sharp *et al.* 1983;
Diehl and Schuster 1996). The active part of the landslide occurs almost
entirely within inactive landslide deposits (Fleming *et al.* 1999,
Figure 3). The average inclination of the ground surface along the
landslide is 8°. Our monitoring equipment and a monitoring site (IS-1)
described by Coe *et al.* (2003) are located at about the middle
elevation of the landslide and along its left margin (Figure 2). Nearly
all displacement along this margin occurs along the bounding strike-slip
fault (Fleming *et al*. 1999). We observed en echelon fractures
extending at most about 3 m into the landslide at the monitoring site,
measured normal to the strike-slip fault. The monitoring site is about
100 m downslope from the narrowest, fastest part of the landslide
(Fleming *et al*. 1999). Average ground-surface inclination is 4° within
100 m upslope and downslope from the monitoring site and 10° beyond this
distance for a few hundred meters in both directions.

# 

# METHODS

## Subsurface exploration and sampling

Topography of the ground surface at the location of our monitoring site
is shown on Figure 3, which was developed using measuring tape, pocket
transit, and hand level techniques and referenced to the North American
Datum 1983 using monuments surveyed with GPS techniques (Coe *et al*.
2003). We constructed our borehole 3.4 m from the strike-slip fault
marking the left margin of the landslide using hand-portable,
direct-push subsurface exploration equipment manufactured by Geoprobe
and driven using an electric breaker hammer powered by a gasoline
generator. The borehole was continuously sampled using a 0.6-m-long,
5.1-cm-diameter, cylindrical steel sampler to a depth of 9.3 m where
refusal was encountered, presumably on a cobble or boulder. Soil samples
were visually classified in accordance with ASTM standards (American
Society for Testing and Materials 2002).

![](media/image3.png){width="6.497222222222222in" height="3.5375in"}

**Figure 3.** Cross section oriented normal to the landslide margin and
extending through the monitoring site. View is oriented parallel to the
landslide margin and directed upslope. Tensiometers are indicated by "T"
and their depths below the ground surface. Piezometers are indicated by
"P" and their elevations. The borehole was backfilled with grout, sand,
and bentonite as shown.

We hand excavated one soil pit within the landslide to a depth of 1.4 m.
The pit was located 21.3 m from the left margin of the landslide (Figure
3). Relatively undisturbed soil samples were obtained from depths of 30,
60, 95, and 120 cm by hand tapping 6.35-cm-diameter, 15.24-cm-long
cylindrical brass samplers horizontally into the pit walls then hand
excavating soil around the samplers to aid their removal.

## Material property testing

Grain-size distribution and porosity tests were performed on soil
samples in our laboratory in accordance with ASTM standards (American
Society for Testing and Materials 2002). Porosity tests utilized results
of specific gravity tests performed on parts of the samples.

Saturated hydraulic conductivity of landslide debris was measured in
situ at 10 locations near the monitoring site using a Guelph permeameter
(five locations) and an amoozemeter manufactured by Ksat, Inc. (5
locations). Measurements were made at depths of 23-89 cm.

## Monitoring

We installed 345 kPa, non-vented, vibrating-wire piezometers
manufactured by Slope Indicator, Inc. (+/- 0.345 kPa accuracy) into the
borehole at depths of 4.9 m, 7.3 m, and 9.1 m (Figure 3) to monitor
pore-water pressures. We also installed one length of coaxial cable that
extends the full depth of the borehole to detect shear displacment. The
coaxial cable was placed inside 2.5-cm-diameter polyvinyl chloride (PVC)
pipe, which was subsequently filled with cement grout to provide
rigidity. The borehole was backfilled so that each piezometer was
surrounded by sand and was isolated from groundwater conditions at
neighboring piezometers by at least 0.9 m of bentonite. The borehole was
backfilled with cement grout from a depth of 1 m to the ground surface.

Soil Moisture Corp. 100 kPa tensiometers (+/- 0.25 kPa accuracy) were
installed at depths of 30, 60, 95, and 120 cm into undisturbed soil that
formed one of the walls of the soil pit. An insulated box containing the
tensiometer bodies was buried in the pit during backfilling to protect
the water-filled tensiometers from freezing winter temperatures.

The length of the coaxial cable installed in the borehole was measured
periodically using time-domain reflectometry (Kane and Beck 1994, 1996).
The piezometers and tensiometers were connected to a Campbell Scientific
CR10x datalogger that was already in operation at the site. Power to
operate the datalogger and instruments was provided by a 12-volt battery
that was charged by a voltage-regulated solar panel. Instrument readings
were taken hourly and downloaded to a portable computer on a periodic
basis. Monitoring data were reduced according to manufacturer
specifications following retrieval from the datalogger. Piezometer data
were corrected for elevation and temperature. Other monitoring equipment
in operation at the site was described in detail by Coe *et al.* (2003).
The cable displacement transducer (+/- 0.46 cm accuracy) that provided
landslide displacement data used during our study was located 4 m
downslope from the borehole, while the soil temperature, rainfall, snow
depth, and air temperature sensors were located 8 m downslope from our
soil pit.

# RESULTS

## Soil properties

Soils encountered in the soil pit and borehole were generally classified
as silty gravel with sand. Soil color was typically dark yellowish
orange or moderate yellowish brown with yellowish gray and greenish gray
mottling, and consistency was generally moderately dense to very dense.
No apparent shear surfaces or zones were identified during either
subsurface exploration. Porosity of the four undisturbed soil samples
obtained from the soil pit ranged from 0.39 to 0.54; the average
porosity was 0.47. In situ hydraulic conductivity test results ranged
from 1.34 x 10^-6^ m/s -- 5.71 x 10^-5^ m/s; the average hydraulic
conductivity was 2.31 x 10^-5^ m/s. These conductivity values are
typical for silty sand to well-sorted sand (Fetter 1994, p. 99). Soil
moisture content ranged from moist to saturated; wetter conditions were
present below depths of 4.8 m in the borehole and 95 cm in the pit. The
apparent water table was encountered at a depth of about a meter in the
pit (the pit was backfilled before the water level stabilized).

## Landslide displacement and weather

Episodic monitoring of the coaxial cable detected no sign of shear
displacement, which indicated that the landslide is deeper than our
borehole (9.3 m). Continuous displacement monitoring indicated that the
landslide moved throughout the period of record (August 12, 2004-June 4,
2006; beginning date follows post-drilling recovery of water levels at
the piezometers), with total displacement of 7.6 m at the monitoring
site (Figures 4, 5). Landslide velocity at the monitoring site ranged
between 0.7-1.7 cm/d with an average velocity of 1.0 cm/d. Therefore,
the average daily displacement was about twice the maximum error of the
cable displacement transducer (+/- 0.46 cm accuracy). Displacement data
were considered in a relative sense so this level of accuracy was
assumed to be inconsequential. Landslide velocity data shown on Figures
4-6 were smoothed by taking the weekly average of the daily average
velocity calculated at each measurement time (hourly) with the daily and
weekly time intervals centered on the measurement time. Landslide
velocity was greatest following snowmelt during spring 2005 (Figure 6)
and spring 2006 and relatively constant at other times. Relatively heavy
rainfall events generally resulted in short periods of increased
landslide velocity.

## Shallow pore-water pressures and weather

Tensiometer monitoring indicated that the groundwater table was within
about a meter of the ground surface at the soil pit location throughout
the monitoring period (Figure 4). Many pore-water pressure increases
measured by the tensiometers correlate temporally with rainfall and
snowmelt events (Figures 4, 5), but the sequence of sensor response
generally differs for these types of events. For example, 2.8 cm of rain
fell on September 29, 2005 and pore-water pressure increased within a
day at all tensiometers beginning with the shallowest sensor and
progressing downward. Similarly, 2.0 cm of rain fell on October 27-28,
2004 and pore-water pressure increased nearly instantaneously at the 30
cm tensiometer and then at successively deeper tensiometers during the
next two days. In contrast, snowmelt events tended to temporally
correlate with simultaneous pore-water pressure increase at all sensors
(e.g., beginning April 2, 2006) or measured first at greatest depth then
at progressively shallower depths (e.g., April 1-6, 2005).

## Deep pore-water pressures and weather

The three piezometers measured very similar pore-water pressure
fluctuations throughout the monitoring period, although pore-water
pressures dropped and the vertical component of groundwater flow changed
during this time (Figures 4, 5). The total head in the deep (3160.77 m
elevation) and middle (3162.60 m elevation) piezometers dropped 2 m
during the period of record while the water table dropped below the
shallow (3165.04 m elevation) piezometer on April 4, 2005. The head drop
occurred at a generally consistent rate during fall and winter seasons,
although head increased about 20 cm rather abruptly during late
October-early November of both years. Pore-water pressure gradually
increased, stabilized, then dropped

![](media/image4.png){width="6.495138888888889in"
height="5.288194444444445in"}

**Figure 4.** Results of climate, groundwater, and displacement
monitoring. Tensiometer data are represented by "T" and sensor depth
(d.), and piezometer data are represented by "P" and sensor elevation
(el.). Negative tensiometer values indicate soil suction due to
unsaturated conditions, while positive values indicate saturated
conditions. "Cum. pptn. +2m" represents cumulative precipitation
beginning July 3, 2004 with a constant value of 2 m added to improve
figure clarity. "Slide cum. disp." represents landslide cumulative
displacement. The 3165.04 m elevation piezometer became dry on April 11,
2005 as the groundwater level dropped; hence, no data exist after this
date for this piezometer. Dates mentioned in the text are indicated.

![](media/image5.png){width="6.504861111111111in"
height="5.120833333333334in"}

**Figure 5.** Results of climate, groundwater, and displacement
monitoring. Cumulative precipitation was measured beginning July 3,
2004. Hydraulic gradient data are shown between given piezometers with
negative values indicating upward groundwater flow and positive values
indicating downward groundwater flow. "Shallow" represents the 3165.04 m
elevation piezometer, "middle" represents the 3162.60 m elevation
piezometer, and "deep" represents the 3160.77 m elevation piezometer.
The 3165.04 m elevation piezometer became dry on April 11, 2005 as the
groundwater level dropped, hence no "shallow-middle" and "shallow-deep"
data exist after this date. Dates mentioned in the text are indicated.

![](media/image6.png){width="6.496527777777778in"
height="5.329166666666667in"}

**Figure 6.** Results of climate, groundwater, and displacement
monitoring. Piezometer data are represented by "P" and sensor elevation
(el.). "Cum. pptn. +2m" represents cumulative precipitation beginning
July 3, 2004 with a constant value of 2 m added to improve figure
clarity. "Slide cum. disp." represents landslide cumulative
displacement. The 3165.04 m elevation piezometer became dry on April 11,
2005 as the groundwater level dropped; hence, no data exist after this
date for this piezometer.

during spring and early summer 2005, and appears to have been increasing
during spring 2006. Total head was greatest at the middle piezometer
throughout the monitoring period, indicating that the landslide debris
is heterogeneous. The vertical hydraulic gradient was upward toward the
shallow piezometer from both middle and deep zones, but was downward
from the middle to deep piezometer during the period of record (Figure
5). This downward gradient increased until about October 2005 then
generally stabilized.

Deep pore-water pressures generally lacked direct correlation with
precipitation and snowmelt events at the monitoring site. One instance
of apparent direct correlation was the 2.0 cm rainfall event of October
27-28, 2004, which was followed four days later by simultaneous
pore-water pressure increases measured by the piezometers. The gradual
pore-water pressure increase measured by the piezometers during spring
and early summer 2005 probably resulted from snowmelt that occurred
during the spring.

Deep pore-water pressures commonly changed in the absence of snowmelt or
rainfall events at the monitoring station. For example, pore-water
pressure increased at the piezometers on December 7, 2004 and May 11,
2005 in the absence of these events. Repeated cycles of pore-water
pressure decrease, stabilization, and increase occurred during periods
of minor rainfall or snowmelt on numerous occasions, including March
11-May 15, 2005, November 23, 2005-January 6, 2006, and March 6-April 3,
2006.

## Landslide velocity, pore-water pressures, and weather

Landslide velocity changes had variable temporal correlation with
rainfall and snowmelt events and shallow and deep pore-water pressure
fluctuations (Figures 4, 5). Although deep pore-water pressures near the
margin dropped by two meters during the monitoring period, shallow
pore-water pressures and landslide velocity were generally consistent,
overall.

The landslide generally accelerated as pore-water pressures detected by
the tensiometers increased. The snowmelt events of 2005 and 2006 were
followed by simultaneous pore-water pressure increase at the
tensiometers and landslide acceleration. Similar conditions were
observed following the October 27-28, 2004 rainfall event.

Landslide velocity changes generally did not follow deep pore-water
pressure changes detected by the piezometers. For instance, pore-water
pressure increases measured by the piezometers during spring-early
summer 2005 and October 19-30, 2005, and the overall pore-water pressure
decrease of 2 m during the monitoring period were not followed by
changes in velocity. On the contrary, deep pore-water pressure changes
frequently followed landslide velocity changes. Cycles of landslide
acceleration and deceleration accompanied, respectively, by deep
pore-water pressure decrease and increase were common during the
monitoring period (e.g., Figure 6). Acceleration following 2005 and 2006
snowmelt was followed by abrupt deep pore-water pressure decrease,
stabilized pore-water pressure as the landslide reached constant
velocity, and pore-water pressure increase as the landslide decelerated.
A significant increase-decrease cycle in the downward vertical hydraulic
gradient between the middle and deep piezometers also occurred during
this period. Three similar cycles of landslide acceleration-deceleration
and respective pore-water pressure decrease-increase occurred November
23, 2005-January 6, 2006, although these cycles were of much shorter
duration, lasting 13-16 days each (Figure 4). About a dozen cycles
occurred March 11-May 15, 2005 (Figure 6) as the landslide generally
accelerated following snowmelt.

# DISCUSSION 

Monitoring at the landslide has provided insight into the
characteristics of this landslide and into general relationships between
weather, groundwater conditions, and landslide displacement.

## Soil characteristics and groundwater conditions

Classification of soils around the monitoring site based on field
hydraulic conductivity measurements, visual examination, and laboratory
testing provided consistent results; the soil is classified as a silty
gravel with sand and has hydraulic properties typical for silty sand.
Groundwater pressures within the landslide at our tensiometer site
temporally responded to rainfall and snowmelt in general agreement with
field and laboratory measurements of soil properties, hence infiltration
at the monitoring site appears to occur through soil pores, rather than
through fractures or local zones of higher hydraulic conductivity that
we did not identify.

The groundwater table is located near the ground surface at the
monitoring site, which appears to be characteristic within most of the
landslide. Shallow groundwater is suggested by landslide acceleration
that occurs contemporaneously with precipitation and snowmelt events,
assuming that elevated groundwater pressures accompany these events and
trigger landslide acceleration. The common occurrence of ponds, springs,
and groundwater seepage zones on the landslide also suggests that
groundwater is located near the ground surface.

## Regional hydrologic effects on groundwater within the landslide

The regional groundwater system (groundwater around but outside of the
landslide) appears to affect groundwater conditions near the landslide
margin at our monitoring site. The pore-water pressure response to
rainfall and snowmelt events was gradual and relatively sustained at the
piezometers compared to at the tensiometers, if it was identifiable at
all, as illustrated by pore-water pressure changes that followed spring
2005 snowmelt (Figure 4). In contrast, the pore-water pressure response
to rainfall and snowmelt events was abrupt and generally direct at the
tensiometers. Some damping of infiltration events in the piezometer
record likely occurred due to the greater depth of the piezometers than
the tensiometers. This damping of the piezometer record also suggests
that groundwater along the margin was affected by other sources, such as
lateral groundwater flow from outside of the landslide. In addition,
groundwater may have been leaking from the landslide through its margin
at the monitoring site, possibly due to decreasing groundwater levels
outside of the landslide or due to the landslide displacing past soil
outside of the landslide that has relatively higher hydraulic
conductivity. This potential leakage is suggested by the decrease in
landslide saturated thickness at the piezometers from 71% to 60% of the
assumed landslide thickness (14 m) during the monitoring period while
the saturated thickness remained at a generally constant 93% at the
location of the tensiometers. The apparent isolation of groundwater
conditions within the landslide indicated by tensiometer data agrees
with the hydrological independence of the landslide proposed by Baum and
Reid (2000), but changing groundwater conditions along the landslide
margin measured by the piezometers suggests that isolation is not
complete. The lack of groundwater level decrease at the tensiometers
while it steadily decreased at the piezometers suggests that the
conditions resulting in decrease along the landslide margin are recent
and dynamic. We expect to see the groundwater table drop at the location
of the tensiometers in the near future if the groundwater table
continues to drop along the margin, which would reinforce the idea that
groundwater is leaking through the landslide margin.

## Groundwater effects on landslide displacement

Landslide displacement at the monitoring site appears to have generally
been directly affected by groundwater conditions within the landslide,
but not by groundwater conditions along the landslide margin. Landslide
velocity changes generally positively correlated with pore-water
pressure changes detected by the tensiometers but did not correlate with
pore-water pressure changes detected by the piezometers. This lack of
correlation is illustrated by the generally constant landslide velocity
during spring-summer 2005 and October 19-30, 2005 periods of elevated
pore-water pressure measured by the piezometers. The lack of correlation
is also illustrated by generally constant overall landslide velocity
during the monitoring period while pore-water pressure dropped by 2 m at
the piezometers. This lack of dependence of landslide velocity on
pore-water pressures measured along the landslide margin also suggests
that pore-water pressure decrease along the margin is a local condition.
Decreased pore-water pressures along the landslide margin should have
resulted in increased effective strength of the landslide material being
sheared, yet the landslide did not slow. The apparent dependence of
landslide velocity on pore-water pressures within the landslide as
measured by the tensiometers suggests that long-term ground-water level
decrease within the landslide due to drought or other conditions could
result in slowing of the landslide or cessation of movement altogether.
Similarly, long-term ground-water level increase due to prolonged wet
conditions could result in overall landslide acceleration.

## Landslide displacement characteristics and effects on groundwater conditions

Soil sheared at the landslide margin and base near our monitoring site
was apparently dilative, as suggested by the pore-water pressure
response measured at the piezometers during periods of landslide
acceleration and deceleration. These periods include snowmelt events of
2005 and 2006, and repeated acceleration-deceleration cycles of March
11- May 15, 2005, November 23, 2005-January 6, 2006, and March 6-April
3, 2006. Dilation of landslide debris within basal and marginal shear
zones appears to have occurred at an accelerating rate during landslide
acceleration and resulted in decreasing pore-water pressures within and
near shearing debris as groundwater flowed into larger soil pores and
fractures that opened during dilation. Downward vertical hydraulic
gradients occasionally increased during dilation as water was
essentially sucked into dilating debris along the base of the landslide,
as illustrated by monitoring results of March 24-April 10, 2005.
Decreasing pore-water pressures during dilation appear to have caused
the shearing landslide debris to strengthen and the landslide to
subsequently decelerate. Deceleration may have caused consolidation of
previously dilated landslide debris and consequent pore-water pressure
increase. Increasing pore-water pressures during this deceleration and
consolidation period may have resulted in subsequent, similar cycles of
acceleration and deceleration. Experimental studies performed using a
constant-stress ring-shear device (Moore and Iverson 2002) and
controlled landslides (Iverson *et al.* 2000) have documented behavior
very similar to that described here. Iverson (2005) developed a
mathematical model to explain this type of behavior. Dilation of
landslide debris during displacement can result in debris flow (e.g.,
Johnson and Rodine 1984; Fleming *et al*. 1989; Gabet and Mudd 2006) if
sufficient water is present to maintain relatively high pore-water
pressures and prohibit strengthening of dilating landslide debris. There
appears to be insufficient groundwater and surface water at the
landslide to permit this type of rapid failure, as evidenced by apparent
dilatant strengthening during landslide acceleration.

# CONCLUSION

We found that active landslide debris near the monitoring site is deeper
than 9.3 m. Groundwater conditions were observed to vary considerably
with distance from the landslide margin. Subdued, gradual, seasonal
pore-water pressure increases in response to spring snowmelt were
observed near the margin, and rapid, short-lived pore-water pressure
fluctuations in response to precipitation and snowmelt events were
observed just 17.9 m away toward the axis of the slide. Groundwater
level dropped by about 2 m at the landslide margin during the 22 months
of record (August 13, 2004-June 4, 2006), but remained nearly steady, on
average, 17.9 m farther into the slide. The landslide moved during the
entire monitoring period at our monitoring site with an average velocity
of 1.0 cm/d and total displacement of 7.6 m. Velocity was greatest
following spring snowmelt and generally positively correlated with
increased pore-water pressure measured within the landslide 21.3 m from
the landslide margin. Pore-water pressure measured along the landslide
margin showed an inverse relation with landslide acceleration, probably
due to dilation of landslide debris during accelerated shear. Several
cycles of acceleration and deceleration accompanied, respectively, by
pore-pressure decrease and increase along the landslide margin were also
identified. These cycles appear to have involved landslide acceleration
accompanied by dilation of shearing landslide debris and subsequent
decreasing pore-water pressures, strengthening of shearing debris and
landslide deceleration. Deceleration may have been accompanied by
contraction of sheared landslide debris and increasing pore-water
pressures, which frequently resulted in a subsequent episode of
acceleration.

# REFERENCES

AMERICAN SOCIETY FOR TESTING AND MATERIALS 2002. *Annual Book of ASTM
Standards, 4.08*. American Society for Testing and Materials,
Philadelphia, Pa.

BAUM, R. & REID, M. 2000. Ground water isolation by low-permeability
clays in landslide shear zones. *In:* BROMHEAD, E., DIXON, N. & IBSEN,
M-L. (eds) *Landslides in Research, Theory and Practice.* Thomas
Telford, London, 139-144.

COE, J., ELLIS, W., GODT, J., SAVAGE, W., SAVAGE, J., MICHAEL, J.,
KIBLER, J., POWERS, P., LIDKE, D. & DEBRAY, S. 2003. Seasonal movement
of the Slumgullion landslide determined from Global Positioning System
surveys and field instrumentation, July 1998-March 2002. *Engineering
Geology*, **68**, 67-101.

CRUDEN, D. & VARNES, D. 1996. Landslide types and processes. *In:*
TURNER, A. & SCHUSTER, R. (eds) *Landslides: Investigation and
Mitigation.* Transportation Research Board National Research Council,
Washington, D.C., 36-75.

DIEHL, S. & SCHUSTER, R. 1996. Preliminary geologic map and alteration
mineralogy of the main scarp of the Slumgullion landslide. *In:* VARNES,
D. & SAVAGE, W. (eds) *The Slumgullion Earth Flow: A Large-Scale Natural
Laboratory*. U.S. Geological Survey Bulletin 2130, 13-19.

FETTER, C. 1994. *Applied Hydrogeology.* Macmillan College Publishing
Company, Inc., New York.

FLEMING, R., BAUM, R. & GIARDINO, M. 1999. *Map and Description of the
Active Part of the Slumgullion Landslide, Hinsdale County, Colorado*.
U.S. Geological Survey Geologic Investigations Series Map I-2672.

FLEMING, R., ELLEN, S. & ALGUS, M. 1989. Transformation of dilative and
contractive landslide debris into debris flows -- an example from Marin
County, California. *Engineering Geology*, **27**, 201-223.

GABET, E. & MUDD, S. 2006. The mobilization of debris flows from shallow
landslides. *Geomorphology*, **74**, 207-218.

IVERSON, R. 2005. Regulation of landslide motion by dilatancy and pore
pressure feedback, *Journal of Geophysical Research*, **110**, 16 p.

IVERSON, R., REID, M., IVERSON, N., LAHUSEN, R., LOGAN, M., MANN, J. &
BRIEN, D. 2000. Acute sensitivity of landslide rates to initial soil
porosity. *Science*, **290**, 513-516.

JOHNSON, A. & RODINE, J. 1984. Debris flow. *In:* BRUNSDEN, D. & PRIOR,
D. (eds) *Slope Instability*. Wiley, London, 257-361.

KANE, W. & BECK, T. 1994. Development of a time domain reflectometry
system to monitor landslide activity. *In: Proceedings of the 45th
Highway Geology Symposium*, Portland, Ore., 163-173.

KANE, W. & BECK, T. 1996. Rapid slope monitoring. *Civil Engineering*,
**66**(6), 56-58.

LIPMAN, P. 1976. *Geologic Map of the Lake City Caldera Area, Western
San Juan Mountains, Southwestern Colorado*. U.S. Geological Survey
Miscellaneous Investigation Series Map I-962.

MESSERICH, J. & COE, J. 2003. *Topographic Map of the Active Part of the
Slumgullion Landslide on July 31, 2000, Hinsdale County, Colorado.* U.S.
Geological Survey Open-File Report 03-144.

MOORE, P. & IVERSON, N. 2002. Slow episodic shear of granular materials
regulated by dilatant strengthening. *Geology*, **30**(9), 843-846.

PARISE, M. & GUZZI, R. 1992. *Volume and Shape of the Active and
Inactive Parts of the Slumgullion Landslide, Hinsdale County, Colorado*.
U.S. Geological Survey Open-File Report 92-216.

SAVAGE, W. & FLEMING, R. 1996. Slumgullion landslide fault creep
studies. *In:* VARNES, D. & SAVAGE, W. (eds) *The Slumgullion Earth
Flow: A Large-Scale Natural Laboratory*. U.S. Geological Survey Bulletin
2130, 73-76.

SHARP, W., MARTIN, R. & LANE, M. 1983. *Mineral Resource Potential of
the Powderhorn Wilderness Study Area and Cannibal Plateau Roadless Area,
Gunnison and Hinsdale Counties, Colorado.* U.S. Geological Survey
Miscellaneous Field Studies Map MF-1483-A.

VARNES, D. & SAVAGE, W. (eds) 1996. *The Slumgullion Earth Flow: A
Large-Scale Natural Laboratory*. U.S. Geological Survey Bulletin 2130.