Monitoring Rock Slope Deformation Following an
Alpine Rock Slide in the Southern Japanese Alps
Ryoko Nishii · Norikazu Matsuoka • Atsushi Ikeda (University of Tsukuba)
Abstract. The head area of the Aresawa rock slide, located Based on the geodetic survey and continuous monitoring of
at about 3000 m a.s.l. in the southern Japanese Alps, has meteorological parameters, including precipitation, air and
experienced significant rock slope deformation associated ground temperatures and snowmelt conditions, discussion is
with opening of a number of tension cracks that were focused on the dynamics of rock slope and controls on
produced by a partial collapse in the spring of 2004. The variations in the surface velocity.
deformation process of the rock slope was monitored with a
total station between October 2006 and July 2008. 2. The Aresawa rock slide
Meteorological parameters were concurrently monitored. The The Aresawa rock slide is located on the eastern slope of
total station network consisting of 38 points revealed spatial Mt. Ainodake (3189 m a.s.l.), southern Japanese Alps, which
and seasonal variations in slope movement. The rate of is mainly composed of Cretaceous shale and sandstone (Fig.
movement differed significantly between the upper slope 1).
(more than ca. 40 m upslope from the head scarp) and the
lower slope (within 40 m from the head scarp). The two areas
are separated by a downhill-facing scarp 3 to 5 m high, 60 m
long and parallel to the head scarp. The lower slope moved
downslope at about 60 cm yr-1, whereas the upper slope
moved at less than 10 cm yr-1. Mapping of displacement
indicates the presence of a slip plane dipping downslope at
about 40° to 50 ﾟ below the downhill-facing scarp.
Downslope movement was very slow (<1 mm day-1) in the
snow-accumulated period during which ground surface
temperature (GST) remained below 0°C (November to May).
In contrast, the movement accelerated in the snow-melting
and snow-free periods during which GST showed just 0°C or
rose above 0°C (June to October). Thus, the snow cover and
underlying seasonally frozen ground that prevent infiltration
of water contribute to the slope stability, whereas snow
melting in spring and subsequent rainfalls in the
snow-melting and snow-free periods promote water Fig. 1 Location of the study area. Contour interval 10 m
infiltration in the bedrock and accelerate rock slip. The snow
regime controlling water infiltration condition plays an The rock strata run in the NE-SW direction and dip
important role in seasonal variations of the rock slope steeply at 50°-90°. The mean annual air temperature is about
deformation. -2°C at 3070 m a.s.l. (Matsuoka and Sakai, 1999) and
permafrost is possibly present only on the north-facing steep
Keywords. Rock slide, tension crack, monitoring, seasonal slopes (Ishikawa et al. 2003). The Norogawa observatory
variation, snow regime (1130 m a.s.l.), located 6 km east of Mt. Ainodake, has mean
annual precipitation of about 2200 mm (1990 to 1995). The
1. Introduction study area is covered with snow from late November to
Many rock slides and avalanches occur episodically, middle June. Landforms resulting from local rock mass
which have generally prevented monitoring and deformation, including tension cracks, uphill-facing scarps
understanding of pre-failure rock conditions, with recent and downhill-facing scarps (sackung features), widely
notable exceptions of detailed geophysical observations develop on the main ridge (Matsuoka, 1985). A number of
(Willenberg et al. 2008a, b; Ganerød et al. 2008). Some tension cracks were produced on the head area of the rock
studies reported that patterns of pre-failure surface slide in the spring of 2004. The rock slide was 400 m high,
movements depend on the advance of fractures and changes 250 m wide and 13 m in the mean depth (ca. 40 m in the
in stress condition in the rock mass (Saito, 1965; Petley et al. maximum depth). Post-failure movements were monitored on
2005). Therefore, the prediction of the progressive rock slope
this head area 100 m ×100 m wide, including a distinct
deformation requires detailed geodetic survey of spatial and
downhill-facing scarp 3 to 5 m high and 60 m long, and
temporal variations in surface movements on rock slopes.
small tension cracks having originated during the rock slide.
This paper describes rock slope deformation in the head area
of a rock slide, which was activated by a recent rock slope
failure. The rock slope is located in an alpine zone which is
The rock mass deformation in the head area was
characterized by a seasonal snow cover and frozen ground.
measured using a total station, the prism-type (PR) Leica
TC405 or non-prism-type (NPR) TCR 405ultra. The geodetic
surveys were performed 7 times between 14 October 2006
and 14 June 2008 with PR and twice between 14 June 2008
and 1 July 2008 with NPR. The instrument was changed to
avoid a possible risk by progressive slope instability. Two
benchmarks were placed on the bedrock by anchoring a steel
bolt. The geodetic network consisted of 38 points. The error
of the most survey points was less than 2 cm, which was
confirmed by the comparison between the directly measured
distances and triangulation-derived distances. In the
snow-melting period, snow cover prevented the measurement
of several points.
Precipitation and ground surface temperature (GST) were
recorded with a data logger every 6 h from 14 October 2006 Fig. 3 Air temperature and snow depth in snow-melting
to 1 July 2008, although the precipitation data were period (2008). Arrows indicate the date of geodetic
unavailable during the snow-accumulated period. The survey.
snow-melting regime of the central part of the head area was
visually monitored every 2 h from 6:00 to 18:00 from 8 May
2008 (0:00) to 27 June 2008 (12:00) with an automatic digital
camera (KADEC-EYE2, Kona, Japan). A survey pole
included in the images provided a scale of the snow depth. A
manual snow depth survey and the measurement of the snow
density in a snow pit 150 cm in depth was performed at the
monitoring site on 7 May 2008.
The passages of the polar front and typhoons induced
large precipitation events during July, September and October
(Fig. 2). GST continued at 0°C or slightly lower values
lacking diurnal fluctuation from late November to late June,
indicating the presence of the snow cover. GST remained at
just 0°C between early May and late June (often called “zero
curtain”), resulting from wetting and melting of the snow
cover. Freeze-thaw alternations occurred on the ground
surface several times from late October to early November,
whereas they were absent in spring under the late-lying snow
cover. Thus, based on the snow regime and GST, the
monitored period was classified into the snow-accumulated
(late November to early May), snow-melting (early May to
late June) and snow-free periods (late June to late November),
though the periods partly overlap. Fig. 3 shows the mean
daily air temperature and cumulative snowmelt in the
snow-melting period (2008). The mean daily air temperature
rose above 0°C in middle May, and concurrently snowmelt
progressed. In total, 255 cm-thick snow melted from 8 May to
27 June. The snow density was about 0.5 g cm-3.
Head scarp of the
Aresawa rock slide
× Point showing displacement less than 1 cm.
Fig. 4 The cumulative horizontal and vertical
displacements from 14 October 2006 to 14 June 2008.
Contour interval 2 m.
Fig. 2 Precipitation and ground surface temperature
Spatial variation in displacement
from 14 October 2006 to 1 July 2008. Arrows indicate
Contrasting surface displacement patterns were observed
the date of geodetic survey. Dashed square indicates the
between the upper and lower slopes separated by the
period shown in Figure 3.
downhill-facing scarp (Fig. 4). The lower slope (within 40 m
from the head scarp) moved much faster than the upper slope the rock slope would become wet by infiltration of the
(more than 40 m upslope from the head scarp). The lower snowmelt water in the snow-melting period, as indicated by
slope moved downslope by about 60 cm in both horizontal the ablation of snow by 255 cm from 8 May to 27 June. Thus
and vertical components, over the whole survey period. In a large amount of meltwater, equivalent to precipitation of
contrast, the upper slope moved horizontally by 10 cm and 1275 mm, which was estimated from the melted snow depth
vertically less than 10 cm. The rock mass moved, on the (255 cm) and the snow density (0.5 g cm-3), was supplied for
whole, toward the maximum gradient of the failure slope. 51 days in the ground around the slip plane. This water
condition favored the acceleration of the rock mass slip.
Temporal variation in displacement Moreover, water infiltration from rainfalls in the snow-free
The surface velocity, determined from the horizontal and period (2007) also contributed rock slope deformation. The
the vertical components, indicated a significant seasonal and acceleration of the movement coincided with GST reaching
inter-annual variations (Fig. 5). The surface velocity was and rising above 0°C. Thus, the seasonal variation in the
small (<1 mm day-1) and similar between points on the upper surface velocity is considered to depend primarily on the
and lower slopes in the snow-accumulated period. Then, the snow regime which controls water infiltration in the rock
velocity suddenly accelerated with a large spatial variation on slope.
the lower slope in the snow-melting period. The velocities in
the snow-melting and snow-free periods were several times as 6. Conclusions
large as the values in the snow-accumulated period. Much The head area of the Aresawa rock slide indicates distinct
faster movements (>10 times of velocities in the spatial and temporal variations in surface movements. The
snow-accumulated period) were recorded during the major movement occurs along a slip plane dipping downslope
snow-melting period in 2008. Thus, the geodetic survey at 40° to 50°, which accentuates a downhill-facing scarp. The
demonstrates the periodic motion of the rock mass, especially seasonal variation in rock slope deformation is caused by
below the downhill-facing scarp. water infiltration, mainly associated with the snow regime.
The thick snow cover and underlying frozen ground that
prevent infiltration of water maintain the rock slope relatively
stable throughout winter, whereas snow melting in spring and
subsequent rainfalls in summer promote water infiltration in
the bedrock and accelerate rock slip.
This study was supported by Grant-in-Aid of FGI from
Fukada Geological Institute. We acknowledge T. Fukasawa, A.
Nawamaki and K. Teuchi for field assistance, and K. Fukui
and T. Sone for logistic support.
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Fig. 5 Seasonal changes in surface velocities at Blikra, L. H. and Braathen, A. (2008) Geological Model
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