10th Asian Regional Conference of IAEG (2015)

A possible mechanism to explain reactivated landslides
which begin to move in the early cold season
Tatsuya SHIBASAKI (1), Sumio MATSUURA (1) and Takashi OKAMOTO (2)(1) Disaster Prevention Research Institute, Kyoto University, Japan
E-mail:shibasaki@scs.dpri.kyoto-u.ac.jp(2) Forestry and Forest Products Research Institute, Japan
Abstract
On gentle clayey slopes in weathered argillaceous rock areas, many landslides
reactivate repeatedly with slow movement. From field data monitored at landslide sites inJapan, it has become clear that some landslides with a relatively shallow slip surface begin
to move and become active in late autumn or early winter each year. In these cases, thetriggering mechanisms of landslides have not yet been determined, because landslide
initiation and movement do not always clearly link to rise in pore-water pressures (groundwater levels). In this study, we focused on the influence of seasonal variation in groundtemperature on slope stability, and investigated the effect of temperature on the shear
strength of slip-surface soils. We performed ring-shear and box-shear experiments on
slip-surface soils from two landslides in low temperature ranges. Experimental results
showed that shear strength decreased with decreasing temperature. These data indicate that
if slip surface soils contain a high fraction of smectite, decrease in ground temperature canlead to lowered shear resistance at the slip surface and to the triggering of slow landslide
movement.
Keywords: reactivated landslide, residual strength, temperature, smectite
1. Introduction
There are many reports describing slow-moving
landslides that occur and reactivate on overconsolidated clayey slopes all over the world. In
Niigata Prefecture, a well-known heavy snow area inJapan, landslides frequently occur on gentle slopes inareas of soft sedimentary rocks deposited in the
Neogene period. Landslides in this area have been thesubject of many research studies focused on theirmechanisms, including the effects of snow cover andsnowmelt on landslides. Slip-surface soils of theselandslides are sometimes composed predominantly of
the swelling (water absorbing) clay mineral (smectite)which is well known to exhibit an extremely low
residual friction angle.
From field data monitored at landslide sites in thisarea, it has become clear that some landslides withrelatively shallow slip surface, begin to move andbecome active in late autumn or early winter eachyear (e.g., Sato et al., 2004). In these cases, thetriggering mechanisms of landslides remain unclear,because landslide initiation and movement do notalways clearly link to rise in pore water pressures(ground water levels). In this study, we focus
attention on the influence of seasonal variation in
ground temperature on slope stability, andinvestigated the effect of temperature on the shear

strength properties of slip-surface soils.
2. Outline of investigated sites
2.1 Localities and geological backgrounds
Two landslides, Sumikawa in Akita Prefecture, andBusuno in Niigata Prefecture, were investigated inthis study (Fig.1). Sumikawa Landslide is located in ageothermal area near the active volcano, Mt.
Akita-Yakeyama; therefore, the landslide area iscomposed of highly altered volcanic rocks andtuffaceous rocks influenced by intense hydrothermal
activities. The size of the landslide is defined by its
total slope length of 450 m, width of 20–40 m, andmaximum depth of 8 m. Busuno Landslide is locatedin the Higashi-kubiki mountainous district, where
Neogene mudstone and tuffaceous rocks are widelydistributed. These near-surface rocks are exposed tostrong weathering. This landslide has a total slope
length of 400 m, width of 30–40 m, and average
depth of 4–6 m.
Both landslides are located in heavy snow areas andhave many common characteristics (i.e., slope angles,
sizes, elongated planar shapes of the landslide body
divided into some sub-blocks, smectite-richslip-surface
soils,
landslide
behavior,
andgroundwater conditions). The slope angles of bothlandslides are 10 º for the middle and lower blocks of1

10th Asian Regional Conference of IAEG (2015)

Fig.1 Map of investigation sites at the Sumikawa and Busuno Landslides

Fig.2 Behavior of the Sumikawa and Busuno Landslides2

10th Asian Regional Conference of IAEG (2015)

Sumikawa Landslide, and for the middle block ofBusuno Landslide. The basement rocks are highlyaltered and weathered. The ground materials
constituting the landslide bodies, are composed ofstrongly disturbed, weathered rocks and soils.
Throughout the year, groundwater levels areconstantly high and fluctuate slightly within thenear-surface ground. In these landslides, smectite is
the predominant clay mineral. Therefore, the slope
stability and behavior of the two landslides, werestrongly affected by the residual strength propertiesof smectite-rich slip surface soils.

3. Methods
3.1 Test samples
We collected slip-surface soil from two landslides.
At the Sumikawa Landslide, a ‘slickenside’ slip
surface was observed on the sidewall of the lower
block (Photo.1). We collected a disturbed soil sample
for a ring shear experiment. At the Busuno Landslide,
we collected undisturbed slip-surface soils by boringat two spots (BV-22, BV-26) in the middle block(Photo.2). The drill-core samples used for thebox-shear experiments were carefully obtained using
a rotary double-tube sampler with sleeve, following
the method of JGS Standard 1224. Analysis of theslip-surface soils provided data on their basic soil
properties (i.e., natural water content, consistency
index, grain size distribution, mineralogicalcomposition, cation exchange capacity (CEC), andexchangeable cations).

2.2 Landslide behavior and hypothesized landslide
mechanism
Fig.2a shows the behavior of Sumikawa Landslide
over two years (Sep. 2006 to Sep. 2008). This
landslide became active during late autumn and early
winter, and also reactivated during the snowmelt
period. In this landslide, groundwater levels areconstantly high throughout the year. Annual
fluctuation in groundwater levels is small. It is notclear whether landslide movement is regulated by arise in pore-water pressures. Landslide displacement
was not confirmed during extremely-heavy rainfall ina summer typhoon.
The behavior of the Busuno Landslide and themeteorological properties around the landslide site
have been investigated in detail since the late 1980’s(Matsuura et al., 2003, 2009). Field data revealed that
this landslide became particularly active during lateautumn and early winter each year, as did the
Sumikawa Landslide. Landslide movement became
negligible during the heavy snow season and did notreactivate during the snowmelt period (Fig.2b).
Matsuura et al. (2009) reported that landslide
velocities are slow and well correlated withpore-water pressures during early winter. On theother hand, long-term monitoring over four years(from Dec. 1991 to Aug. 1995) suggests that
remarkable increases in pore-water pressures andcumulative precipitation were not observed during theperiods of active landsliding.
The behavior in these two cases raises the questionof why these landslides show vigorous movement
specifically in the early cold season. Similar
behavioral patterns were reported for many landslides
in Niigata Prefecture by Sato et al. (2004). Manyslow-moving landslides that activate in the early coldseason are quite shallow, sometimes less than 10 m indepth. The near-surface ground is susceptible toseasonal variations in temperature. Monitor data forthe two landslides shown in Fig.2 also suggest thatlandslides became active during the period when thetemperatures of near surface ground drasticallydecreased. We then hypothesized that changes inground temperature could affect the slope stability.

Photo.1 Slip-surface observed at the side-wall of
Sumikawa Landslide

Photo.2 Slickensided slip-surface of a drill-corecollected from the middle block of Busuno Landslide
3.2 Ring shear test
In this study, we conducted temperature-change
experiments (cooling-event tests) on a reconstituted
soil sample from Sumikawa Landslide under the
residual strength condition. We used the conventionalring-shear apparatus. The machinery of this apparatus
is fundamentally similar to that developed by Bishop

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10th Asian Regional Conference of IAEG (2015)

et al. (1971). The size of the test specimens used in
the apparatus was 15 cm and 10 cm, in outer andinner diameter, respectively. A normal load is applied
through a pneumatic system, and normal stress acting
on the test samples is evaluated by deducting side
friction between the upper confining ring, and the test
specimen. During the shear tests, the gap between theupper and the lower confining rings was kept slightly
open to avoid ring-to-ring friction.

two landslides are shown in Table.1. In general,
clayey soils in natural conditions generally containsome amount of water. The natural water content of
the slip-surface soils at the two landslides are in therange of 35-60 %. Both soils showed high-plasticity
properties (75 < WL < 136, 46 < IP < 85) and containhigh fractions of smectite (Fig.3). Exchangeable
cations indicate that Ca-type and Mg-type smectite
are predominant at the Sumikawa Landslide, whereas
Ca-type and Na-type smectite are predominant at the
Busuno Landslide.

3.3 Reversal box shear test
We investigated the effect of temperature on shear
strength of an undisturbed slip-surface soil. Box shear
experiments were conducted on a soil sample
collected from the Busuno Landslide. The shear
apparatus used in this study was designed for reversal(cyclic) shear test for applying a large displacement
to the test specimen, and for measuring the residual
strength of soils. Both the shear displacementcontrolled test and shear stress-controlled test (creeptest) are available with this apparatus. Normal stressacting on the test sample is measured directly by aload cell located above the shear box. Normal load on
the tested sample is maintained by a computercontrolled feedback system using a servomotor. The
servomotor-controlled shear system also enables
shear stress-controlled experiments. In this study, aslickensided slip-surface within the test specimen wascoincided with the shear plane of the shear box, andshear displacement was applied precisely along the
localized slip surface, following the method of
Mayumi et al. (2003).

Crist

1000
CPS

Sm

Sumikawa clay
Side wall

Qtz
Kaol

1000

Qtz

Sm

Kaol
Chl

Qtz

Busuno clay
BV-22

Fs

Ill
Qtz

1000

Qtz

Sm
Kaol
Ill Chl00

10Busuno clay
BV-26

Fs

20

30
2θ(CuKα)

40

50

60

Fig.3. XRD patterns of slip-surface soils
Fig.4 shows the result of ring-shear experiment
performed on slip-surface soil from Sumikawa
Landslide. Cooling-event tests were conducted at theshear rates of 0.005 mm/min and 0.02 mm/min. In an
experiment at 0.005 mm/min, shear strength slightly
decreased with decreasing temperature (Fig.4a). On
the other hand, temperature-dependent shear behavior
was not recognized at 0.02 mm/min (Fig.4b),
suggesting that shear weakening with decreasing
temperature is typical under a slow shearing-rate
condition.
In addition, shear behavior also changed inaccordance with temperature. The plotted line of thefrictional coefficient, τ/σN, at the shearing rate of
0.005 mm/min, notably showed that small stressfluctuations (stick-slip behaviors) occur at room
temperature. In contrast, no stress fluctuation was

3.4 Viscosity test
To understand better the detailed temperaturedependent soil properties, we investigated viscosities
of clay suspensions with varying water contents.
Using Marsh Funnel viscometer, we conductedviscosity tests on clay suspensions of smectite-richsoils (bentonites) and kaolin clay. This apparatus is a
simple device for measuring viscosity by observing
the time that a 500ml volume of liquid flow from acone through a short tube.
4. Results and discussions
The soil properties of the slip-surface soils from theTable.1 Soil properties of slip-surface soils
SampleSumikawa Side wall
BusunoWater
Depth
content(G.L.-m)(%)

Grain Size distribution

Consistency index

Clay

WL

WP

(%)

(%)

Silt

Sand

(%)

IP

CEC

Exchangeable cations
Na+(cmol/kg)

+

2+

K
Ca
(cmol/kg)

2+

Mg

0

60.3

61

34

5

135.7

63.4

72.3

19.6

0.6

0.3

2.4

2.7

BV-22

4.61

35.8

60

32

7

76.0

29.2

46.8

34.7

9.3

0.7

19.9

2.1

BV-26

4.72

56.6

100

-

-

125.8

41.1

84.7

28.0

7.3

0.5

16.9

1.8

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10th Asian Regional Conference of IAEG (2015)displacement gradually decelerated with increasing
temperature and finally stopped at room temperature.
We did cooling-event tests three times to confirm thereproducibility of this phenomenon. If this behavior
can be applied to the field, it indicates that slow-

Fig.5 Temperature-change box shear experiments
performed on the undisturbed slip-surface soil fromBusuno Landslide (Shear rate: 0.005 mm/min).

Fig.4 Temperature-change ring shear experiments on
Sumikawa Landslide soil (σN=200kPa)
observed and stable-sliding behavior prevailed duringcooling-event tests.
Box shear experiments were performed on
undisturbed slip-surface soil collected from Site
BV-22 of the Busuno Landslide. To confirm
steady-state (residual) shear-strength condition before
the cooling-event tests, two cycles of reversal shear
tests, with the maximum displacement of 7 mm forboth forward and backward shearing directions, wereconducted. A total displacement of 56 mm wasapplied to the test specimen at the normal stress of 25
kPa and shearing rate of 0.05 mm/min. Cooling-event
tests were subsequently performed under four
different normal stresses at the shearing rate of 0.005
mm/min. Fig.5 shows the results of a box shear
experiment at the normal stress of 100 kPa. Shearstrength drastically decreased with decreasing
temperature. Shear strength envelopes for two
temperature conditions (Fig.6) show that thefrictional angle decreased with decreasing
temperature from 8.9º at 25 ºC to 7.7º at 9 ºC. The
impact of cooling on cohesion was small.
We further conducted a shear stress-controlled
experiment (creep test) on the same test specimen(Fig. 7). After the shear displacement-controlled test,
shear stress was held constant at a slightly lower
level than the steady-state shear strength. The
cooling-event test was then started. Several minutes
after cooling, shear displacement suddenly initiatedand slow-speed displacement, up to 0.015 mm/min,continued while conditions were cool. Shear

Fig.6 Shear strength envelopes of undisturbed
slip-surface soil from Busuno Landslide under two
temperature conditions

Fig.7 Shear stress-controlled box-shear experiment
performed on an undisturbed slip-surface soil fromBusuno Landslide510th Asian Regional Conference of IAEG (2015)

moving landslides can be triggered repeatedly during
any season when the ground temperature decreases.
Ring-shear and box-shear experiments revealed thatthe residual strength of smectite-rich slip-surface
soils strongly depends on temperature. To understandbetter the soil properties of smectite-rich soils, weinvestigated the effect of temperature on viscosity ofclay suspensions. Fig.8 shows the results of funnel
viscosity tests on two bentonites and kaolin clay withvarying water content. Test results showed that theviscosity of the Na-type bentonite suspension
strongly depends on temperature. A Ca-type
bentonite suspension with 5-times as much water as
bentonite (Wn=500%), showed slightly stronger
temperature-dependent viscosity than did a kaolinsuspension with the same water content. This
indicates that the rheological properties of smectite
suspensions are very responsive to temperature.
Although further research is needed, theserheological properties appear promising forunderstanding the mechanism of temperaturedependent residual strength characteristics ofsmectite-rich soils.

angle; therefore, landslides having such slip-surface
soils can easily become unstable, and are likely toreactivate even on very gentle slopes. In Japan,
reactivated-type landslides frequently occur in theareas of soft sedimentary rocks of Neogene andPaleogene Periods. As for the Sumikawa Landslide
reported in this paper, many other landslides occur ingeothermal areas around active volcanoes. In suchareas, smectite is widely distributed due tohydrothermal alteration of host rocks, and the groundtemperature condition strongly relates withgeothermal activities. In consideration of thelong-term stability of such landslides, it is also
important to pay attention to ground temperature.
This paper mainly focused on soil behaviors in a
relatively low temperature range (5–25 °C). Shear
behavior under higher temperature conditions, andthe effect of drained conditions during heating andcooling, have not yet been determined. In order toelucidate fully the mechanism of such landslides,
further research, involving both laboratory
experiments and field monitoring, is required.
Acknowledgements
This work was supported by a JSPS Grant-in-Aid
for Challenging Exploratory Research (26560190) to
Sumio Matsuura.
References
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and Brown J.D. (1971): A new ring shear apparatus
and its application to the measurement of residual
strength. Geotechnique, Vol.21, No.4, pp.273–328
Matsuura S.，Asano S.，Okamoto T. and Takeuchi Y.
(2003): Characteristics of the displacement of alandslide with shallow sliding surface in a heavy
snow district of Japan. Engineering Geology,
Vol.69, pp.15–35
Matsuura S., Asano S., and Okamoto T. (2008):
Relationship between rain and/or meltwater,
pore-water pressure and displacement of areactivated landslide. Engineering Geology, Vol.
101, pp. 49-59
Sato, H., Shiraishi, S. and Ito, T. (2004): Thebehavior of the Tertiary landslides through thesnowy period in Niigata Prefecture, central Japan.
Journal of the Japan Landslide Society, Vol.41,
No.1, pp.37–42 (In Japanese with English Abstract)
Mayumi, T., Shibasaki T. and Yamasaki T. (2003):
Evaluation of shear strength on undisturbed slip
surface by newly developed Slip Surface Direct
Shear (SSDS) apparatus. Journal of the Japan
Landslide Society, Vol.40, No.4, pp.273-282 (InJapanese with English Abstract)

Fig.8 Funnel viscosities of clay suspensions
5. Conclusions
Results of temperature-change ring-shear andbox-shear experiments showed that the shear strengthof smectite-rich slip-surface soils decreased withdecreasing temperature. Our results indicate that if
slip surface soils contain a high fraction of smectite,
decrease in ground temperature can lead to lowered
shear resistance of slip surface, and to triggering ofslow landslide movement. Except for earthquakeinduced landslides, most landslides are induced byrise in pore-water pressure during rainfall andsnowmelt seasons. Our findings newly raise thepossibility that ground temperature influences slope
stability. This means that monitoring groundtemperatures as well as pore-water pressure isimportant for predicting landslides, especially forshallow landslides having smectite-rich slip-surface
soils.
Smectite shows an extremely low residual frictional

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