10th Asian Regional Conference of IAEG (2015)

Characteristics of shallow landslides, soil layer structure and soil properties on hillslopes
underlain by granite and hornfels
cases from the disaster on 20 August 2014 at Hiroshima, Japan
Takuma WATAKABE(1), Yuki MATSUSHI(2), Masahiro CHIGIRA(2), Ching-Ying TSOU(2)and Yasuto HIRATA(1)(1) Graduate school of science, Kyoto University, Japan
E-mail:watakabe@slope.dpri.kyoto-u.ac.jp(2) Disaster Prevention Research Institute, Kyoto University, Japan
Abstract
In 20 August 2014, a heavy rainstorm triggered more than 450 shallow landslides
and debris flows in hillslopes of granite and hornfels in Hiroshima, southwest Japan. The
characteristics of landslides differ with geology even they are located under similar rainfall
condition (~150 mm/3h). Two types of shallow landsliding emerged in this area; planartranslational sliding of soil layer and gush-out failure of gravelly colluvium. Planar sliding
occurred both of granite and hornfels and it removed thin soil layer up to 1 m. The
mechanism determining the slip depth seems to be different with lithology. Planar sliding in
granite form a slip plane at a boundary of mechanical strength in soil profile, while thesliding occurred at hydraulic discontinuity for hillslopes in hornfels area. Gush-out failures
were triggered mainly in hornfels area. In many gush-out scars, open-worked gravel layer
exposed at the bottom of scar head. This layer may works as conduit for groundwaterdrainage, however, excess water supply might trigger pore pressure raise, leading toshallow landslide initiations.

Keywords: shallow landslides, granite, hornfels, soil layer structure, soil property

1.

2006) and soil layer structure (Wakatsuki et al., 2009,
Koyama et al., 2005) which should be related to theoccurrence of the different type of shallow landslides,
at varying locations and slip depths. Proper slope
hazard mitigation requires prediction of location andslip depth for the different type of shallow landslidein hillslopes. Especially, understanding of subsurface
vertical profiles of mechanical strength andhydrological properties of soil is essential to discuss
initiation processes for each type of landslide.
Different type of shallow landslides occur evenunder a similar intensity and amount of rainfall when
lithological condition set different subsurface
structures in hillslopes, as demonstrated in a case of
the disaster in August 2014 at Hiroshima, southwest
Japan. Landslides occurred with two main types: 1)
simple planer soil-layer sliding, and 2) collapse ofgravelly colluvium with gush-out of subsurface water
forming an outlet of preferential flow paths. The
difference in landslide types may reflect varyinghydrological processes and mechanical properties
owing to different soil layer structures. Surveying
these cases in detail, we may be able to reveal thecontrolling factors of the locations and slip depths ofthe different types of shallow landslides.
In this study, we categorized types of shallow

Introduction

Rainfall-induced shallow landslides initiate as aresult of various subsurface hydrological processes.
Major processes triggering shallow landslide aredecreasing of effective stress with increasing
pore-water pressure. Existence of less-permeable
layer in shallow subsurface causes perched watertable, leading to buildup of positive pore-waterpressure (e.g., Matsushi et al., 2006; Terajima et al.,2014). In some cases, preferential subsurface waterflow seems to cause slope instability (e.g. McDonnell,
1990; Dietrich and Dunne, 1993). Pore pressure insoils surrounding preferential flow paths capable toraise when water supply from upslope exceeds
drainage capacity of the conduit. Suction loss due towetting front migration also can trigger soil slip, if
weak material composes hillslopes (Matsushi et al.,2006).
Many problems remain unsolved for what factors
control location and slip depth of these different typesof shallow landslide. Although several previous
studies referred to the location prone to form a scarhead (e.g. Iida and Okunishi, 1979, Koyama et al.,2005), few studies took into account subsurfacehydrological processes (Onda, 1989, Matsushi et al.,1

10th Asian Regional Conference of IAEG (2015)

landslides for the Hiroshima case on GIS (Geographic
Information Systems), and surveyed location of eachtype of landslide and subsurface hydro-geologicalstructure of slid hillslopes. We selected typical slid
hillslopes and carried out geomorphological andgeological survey, to find out characteristics insubsurface vertical profiles of mechanical strengthand hydrological properties in each type of landslide.
2.

Study area

2.1 Geomorphological and geological setting
The study area is hills around Mt. Abu, at
northern part of the Hiroshima city (Fig. 1). The
major rivers in this area run from northeast to
southwest and northwest to southeast forming alluvial
low land. The ridgelines of mountains extend
northeast–southwest.
These
geomorphologicalcharacteristics reflect the dominant direction of joints
originating from past faulting in this area (Takahashi,1991).
The geology in this area consists of three types ofrocks: Jurassic sedimentary rocks, Cretaceous granitic
rocks and rhyolite. They are locally intruded by
granodiorite porphyry, granite porphyry, and felsite.
The sedimentary rocks located around Mt. Abu
underwent thermal metamorphism by contact with thesurrounding granitic intrusion and changed tohornfels.
The topography in the study area reflects theselithological settings. Granitic areas exhibit a dissected
landscape with relatively high valley density and low
elevation of mountain peaks, while the area underlain
by hornfels forms hill terrain with low valley density
and high-relief ridge crest.
The climate in this area is characterized by atemplate humid condition with mean temperature of
14.8 °C, and average annual rainfall of 1700 mm.
Half of the precipitation amount is supplied by frontal
activities and typhoons from June to September.
The Hiroshima region has experienced many
disasters by landslides induced by heavily rainstorm.
Recent severe disaster records are in 1945, 1951,
1967 and 1999 by typhoon and rain fronts (Ushiyama,2001). In the case of 20 August 2014, a concentrated
rain band caused a lot of shallow landslides in the
study area, close to the 1999 disaster area.

Fig. 1 Topographic and geologic settings around thestudy area with mapping of rainfall and shallow landslides
at the disaster on 20 Aug 2014. Geological boundaries weredrawn based on 1/200,000 and 1/50,000 geological maps
and topographic analysis by a 1m-mesh digital terrain
model (DTM). Red circles indicate source area of shallow
landslides. Contours (interval: 25 mm) show rainfall
amount from 1 a.m. to 4 a.m on 20 Aug based on records atrain gauges.

Rhyolite and Sedimentary rock areas show much
lower density than former two. Therefore, this study
focuses hillslopes consists of granite and hornfels(Fig. 2).
Two types of shallow landslides with different
shapes emerged in the study area; one is planar
sliding and the other is gush-out sliding (Fig. 3). We
can distinguish these landslides easily inhigh-resolution slope-shade map and by field surveys
3.

2.2 Distribution of rainfall amount and shallow
landslides in the disaster in 2014

Methods

Detailed geological survey and geomorphologicalinvestigation of landslide scars were conducted.
Shape of scars, slip depths and lithological conditions
are documented in the field. At some landslide scars,
we measured longitudinal and cross-sectional profile
of landslides with Tru Pulse 360 (Laser TechnologyInc.). We categorized the types of shallow landslides
based on the field observation linked to topographic

More than 450 shallow landslides occurred in the
study area where hillslopes received more than 150
mm of rainfall from 1 to 4 a.m. in 20 August 2014
(Fig. 1).
The landslide density is different by geology. The
landslide density is the highest in granite and hornfels.

2

10th Asian Regional Conference of IAEG (2015)

Fig. 3

Shapes of each type of shallow landsliding. (A)Planar sliding in granite. A left image shows the shape oflandslide over LiDAR image. A middle figure indicateslongitudinal and cross-sectional profile of hillslope. Brokenlines indicate paleo-surface. A right picture shows view of
sliding. (B) Planar sliding in hornfels. (C) Gush-out sliding
in hornfels.
Fig. 2cone penetration test was carried out in-situ on the
exposed pit wall by Yamanaka cone penetration tester(Daiki Rika Kogyo Co., Ltd.). Undisturbed soil
samples were collected by a 100 cm3 stainlesscylinder at different depths of 10-20 cm intervals.
Saturated hydraulic conductivity was measured in lab
for the extracted core by the test apparatus(DIK–4012: Daiki Rika Kogyo Co., Ltd.).

Slope shade map of the southwestern hills of

the landsliding area with mapping of different types oflandslide scars. The locations pointed in the map are fieldobservation sites shown in Figs. 3, 5–7.

analysis on GIS, using a LiDAR–derived topographic
data and high-resolution ortho photo (Fig. 2, 3).
Two types of shallow landslide are distinguishedbased on the scar shape and characteristics of material
movement; those were 1) planer translational sliding
of soil layer, and 2) gush-out failure of gravelly
colluvium. Several landslides cannot be categorized
to these types since they were induced in relation togeological structure (e.g., wedge failure along
geological discontinuity). Each type of landslides
were mapped on GIS and analyzed for statistic andtopographic aspects of spatial density anddistribution.
We selected typical hillslopes with different typeof landslide scars to investigate hydro-mechanicalsoil layer structures. The hillslopes were two planar
type slidings for each area of granite and hornfels,
and one gush-out failure in the hornfels area. In theseslopes, longitudinal profiles were surveyed with 1.5
m–span slope survey tools. Soil soundings wereconducted on the profile, using a simplified dynamic
cone penetrometer (revised Tsukuba–Maruto Type,with 1.5 cm diameter of 60° cone, knocking by 3 kg
weight). The dynamic cone penetration resistance, Nc
is defined as knocking numbers needed for a 10 cm
penetration of the cone.
To evaluate vertical changes of the soil hydraulicand mechanical properties, we excavated a pit at thescar head to investigate soil layer structure. Potential
slip plane were identified on the soil profile. Static

4.

Results

4.1 Shallow landslide types and density in eachgeology
Each type of shallow landslides occurred withdifferent spatial frequency with respect to lithologicalcondition. Almost all shallow landslides in the
granite area seemed to be categorized to thin (< 1 m
thick) planer sliding except for some wedge failure at
geological boundary (Fig. 2). In the hornfels area,less than 50% of landslides were planer type (~1 mthick), and more than 50% were gush-out failure of~3 m thickness of slid material. The total landslidedensity was almost the same in the granite area (24scars/km2) and in the hornfels (25 scars/km2).
The scars for these different types of landslides
emerged at different location. Planer sliding tended tooccur at upper hillslopes, while gush-out failure often
appeared at lower hillslopes in valley fill. Fig. 4
shows the plot of local gradient against contributing
area at the source for each type of landslides. We
could not find out distinct separation in gradient but
could find a tendency of larger contributing area forthe source of gush-out failure in hornfels.
For the gush-out type, open-worked gravel layer
was often observed at the bottom of the colluvium
exposed at scar head. Subsurface water flowed out

3

10th Asian Regional Conference of IAEG (2015)

5E).
In the hillslope with planar sliding in the hornfels
area (Fig. 6A), hillslope angle down from the flatridge was uniform around 30° and the bedrocksurface was almost parallel to the slope surface with
2 m thickness of soil layer. The landslide slipped
almost parallel to the slope remaining ~1 m ofweathered material. At the soil-bedrock interface Ncvalues increased sharply to reach the values of hard
unweathered rock.
In the pit at the scar head of planar sliding in
hornfels, hard bedrock emerged at the 1.5 m depth,and soil could be classified into colluvial and residualsoils according to their color (Fig. 6B). The boundary
between reddish brown colluvial soil andlight-colored yellowish brown residual soil existed
around 90–100 cm deep. This depth accords with thepotential slip surface. Yamanaka soil hardness value
decreased toward slip surface and increased again to
the depth of bedrock (Fig. 6D). Sharp decrease(~1/1000) of hydraulic conductivity observed for the
sample extracted at the 90 cm depth (Fig. 6E).
In the hillslope of gush-out failure, colluvial
materials of 2–3 m thickness cover the hard bedrock(Fig. 7A). The failure plane corresponded to the hard
bedrock surface, which seemed not to parallel to thehillslope. During the sounding survey, Nc value
frequently jumped and dropped, and in some cases, it
became impossible to penetrate at a shallow depth.
This probably due to large gravels trapped in the soil
layer supplied by soil creep or past landslides fromsurrounding hillslopes. A hole with ~50 cm diameter
filled with angular cobbles appeared just above thehard bedrock at the scar head of gush-out failure (Fig.
7B). The hydro-mechanical properties of the matrixof the colluvial soil were almost uniform in the 1.5 m
profile with weak (Nc < 6 and ~ 19 of Yamanaka
hardness) and permeable (K >10-5 cm/s) materials(Figs. 7C~E). Below the depth of 150 cm,
open-worked gravel layer existed at the bottom of thescar. This layer obviously has a much higher bulk
hydraulic conductivity due to the large pore size.

from this conduit just after the disaster occurrence,
and we often observed seepage from the layer after it
rains during field surveys. This subsurface structure
may function for converging subsurface water toslide thick colluvium.
During the field survey, we usually observed athicker weathering profile in the upper area near mainridge in the hornfels area. The spatial distribution ofhighly weathered soil layer may control the
distribution of the planar sliding in the hornfels area.
4.2 Soil layer structure and vertical change of soil
properties
The granite hillslope selected for the detailed
survey was characterized by a thick weathering zone
in ridge (Fig. 5A). A planar sliding occurred athillslope with angle about 38° that roughly
corresponded to a transition zone from gentle ridge
crest to steep slopes. At the sounding point of
HGr-1.3, cone penetrometer encountered hardsubstrates (Nc > 40) at shallow depth, which may
correspond to a corestone emerged in the weatheringprofile. In contrast to the thick weathering material inridge area, only thin soils remain below the landslide
scar.
The potential slip surface for this granite planar
sliding was recognized at the depth of 65 cm, attransition between weak colluvial soil and relatively
stiff residual dominant soil (Fig. 5B). The changes inmechanical strength were observed in profiles of Nc
values and Yamanaka soil hardness (Fig. 5C, D).
Below that depth, mechanical strength clearlychanged at boundary between soil and saprolite at
depth of 80–90 cm. Hydraulic conductivity decreased
gradually to 10-5 cm/s with increasing depth, but did
not show any transition across the slip surface (Fig.

5.

Discussion

5.1 Correlation between soil structure andlandslide type in granite and hornfels

Fig. 4

Slip surface of the planar sliding in the granite
hillslope was formed at 65 cm depth, corresponding
to the depth of slight increase in mechanical strength(Fig. 5C, D). Dynamic cone penetration resistanceshows an uniformly weak values (Nc < 2) at shallower
part (0-60 cm), but increases to a slightly larger value(Nc ~5) below 60 cm (Fig. 5C). Static cone
penetration hardness also increases from 12 to 17 mm
across the depth of potential slip surface. This
strength change accords with transition from upper

Gradient vs contributing area. Gradient indicatesaverage value in a part of source area. Contributing area
show upward watershed around scar heads.

410th Asian Regional Conference of IAEG (2015)

Fig. 5

Soil layer structure and changes of vertical direction of mechanical/hydrological properties in planar sliding in

granite. (A) Internal structure of hillslope. Nc 10 and 40 represent weakness layer and bedrock, respectively. Inset graphs
show results of dynamic cone penetration sounding. (B) Picture of scar head. White band shows slip surface depth. (C)Results of dynamic cone penetration test at HHr-1.6. Vertical change of soil layer was recognized that 0–65 cm depth iscolluvial soil, 65–90 cm is residual soil and 90 cm– saprolite. (D) Results of Yamanaka cone penetration test. Large black
circles indicate average value of test. Small black circles show original data. (E) Results of hydraulic conductivity test. Gray
broken lines (C,D,E) indicate slip surface depth.

colluvial soil to lower residual material (Fig. 5B).
Formation of slip surface at the boundary between
soft colluvial and stiff residual layers may control the
initiation of planar sliding. Saturated hydraulicconductivity was comparable in these layers in theorders of 10−4–10−3 cm/s (Fig. 5E), indicating thatboth of them are permeable. Koyama et al. (2005)
also found fragile layer (Nc < 1) at shallow depths (<1 m) of hillslopes prone to landsliding in Tottori Pref.

near Hiroshima. These findings indicate thatexistence of discontinuity in mechanical strength playa significant role in the initiation of planer shallow
landsliding.
Slip surface of the planar sliding in the hornfels
hillslope was formed at a depth of 90–100 cm,
corresponding to the hydraulic discontinuity. The
upper soil has a higher permeability (saturated
hydraulic conductivity K ~ 10−6–10−4 cm/s at 15–75

510th Asian Regional Conference of IAEG (2015)

Fig. 6 Soil layer structure and changes of vertical direction of mechanical/hydrological properties in planar sliding in
hornfels. Legends are the same in Fig. 5. (A) Internal structure of hillslope. (B) Picture of scar head. (C) Results of dynamic
cone penetration test at HHo-1.3. (D) Results of Yamanaka cone penetration test. (E) Results of hydraulic conductivity test.

cm), whereas lower material is less-permeable (K ~10−8 cm/s) below 90 cm (Fig. 6E). Saturated
hydraulic conductivity decreases significantly acrossthe slip surface. This change in permeability accords
with transition from reddish-brown upper colluvial
soil to lower residual material with color of yellowish
light-brown (Fig. 6B). Static cone penetration
hardness gradually decreases with increasing depth in
the colluvial soil (10–80 cm: from 17 to 13 mm) andgradually increases with depth in the residual material(90–140 cm: from 15 to 22 mm) (Fig. 6D). Dynamic
cone penetration resistance (Nc) shows values lessthan 2 at shallower part (0–67 cm) but increases

slightly (Nc ~5) below 70 cm at 10.5 m upslope fromthe scar head (Fig. 6A, C). Existence of residualmaterial including heavily weathered gravels may
contribute to this strength increase. These results
imply that hydrological and mechanical discontinuity
at the colluvial-residual boundary in the shallow soil
layer significantly affects the formation of slip
surface. Less-permeable layer may restrict the
migration of wetting front to deeper layer andperched water arises as a result of a saturated
subsurface water flow in shallow soil layer during
heavy rainfall (Matsushi et al. 2006; Wakatsuki et al.
2009; Terajima et al. 2014). Therefore planar shallow

6

10th Asian Regional Conference of IAEG (2015)

Fig. 7

Soil layer structure and changes of vertical directionof mechanical/hydrological properties in gush-out sliding in
hornfels. Legends are the same in Fig. 5. (A) Internal structureof hillslope. (B) Picture of scar heads. (C) Results of dynamic
cone penetration test at HHo-2.5. (D) Results of Yamanaka
cone penetration test. (E) Results of hydraulic conductivity
test.

landsliding in hornfels removed only thin soil layer(~1 m).
Slip surface of the gush-out sliding in the hornfels
hillslope was formed at depth of 200 cm along thebottom of open-worked gravel layer (Fig. 7B).
Dynamic cone penetration resistance, static cone
penetration hardness and saturated hydraulicconductivity show almost uniform value in soil above
the gravel layer (Fig. 7C–E). No data was available indepths of the gravel layer (150–200 cm) because ofmany hard angular gravels and large pores. These
characteristic soil structures may originate fromcolluvial deposits by past landsliding. We can find an
old landslide scar at the upper hillslope of thisgush-out sliding. This open-worked gravel layer may
function as a bypass to transport soil water todownslope. When water infiltration exceeds thedrainage ability of the gravel layer, excess
pore-pressure should generate, leading to a relatively
deep landsliding by slippage of whole soil layer.

break near ridgelines (Figs. 4, 5A, 6A). The locationof scar head in granite corresponds to transition zoneof hillslope segments covered by thick to thin
weathered layers (Nc<40) (Fig. 5A). The difference inthickness of weathered zone indicates upper limit ofshallow landslide, and the thin weathered zone resultsfrom removal of material by repeated shallow
landslides (Iida and Okunishi, 1979).
The spatial distribution of heavily weathered zone
and its mechanical and hydraulic properties should
affect location of landslide in areas of hornfels.
Similar control of landslide location was reported by(Matsuzawa et al. 2014). Shallow landslide could
occur in the steep edge of heavily weathered mantle,
even the bedrock had been recognized as
insusceptible to shallow landsliding. In addition to the
topography and thickness of the weathered material,
the location of scar head in the hornfels hillslope inthe Hiroshima case may be controlled by distancefrom ridge needed for occurrence of perched water on
the less-permeable layer.
Upslope contributing area at most of gush-out
sliding (102-105 m2) tends to be larger than that ofplanar slidings up to 103 m2 (Fig. 4). This indicatesthat gush-out sliding needs a certain amount of water
accumulation for failure initiation. The drainage
capacity of the open-worked gravel layer may control
the location of scar head formation. This hypothesis
should be verified by hydrological observations in thevalley filling colluvial material in the hornfels area.
6.

Conclusions

In this paper, we addressed the question why doshallow landslides due to heavy rainfall differ in theirshapes, depths, locations and density on hillslopes
underlain by granite and hornfels in northern
Hiroshima city. The conclusions are summarized asfollows.
(1) In granite hillslopes, most of landslides wereplanarslidingalongmechanicalstrength
discontinuity in soil layer, which corresponds toboundary between colluvial–residual materials.
(2) In hornfels area, planar sliding occupies 50%
in number of landslides. Less-permeable residual
layer exists beneath colluvial soil layer. Thishydraulic discontinuity may form perched water tablewithin the soil layer during the heavy rainfall, leading
to planar shallow landsliding.
(3) The other type of landslide in hornfels
hillslopes was gush-out sliding. Open-worked gravel
layer often outcropped at the bottom of the scar heads.
The gush-out landslides may occur when water
infiltration flux exceeds drainage ability of the gravel
layer.
(4) Gush-out sliding in valley fill tends to have alarger contributing area than the planar slidings nearridgeline. Subsurface water accumulation by lateral

5.2 Location of shallow landslide
Most of planar slidings in granite and hornfels
occurred at linear part of hillslope, just below a slope

7

10th Asian Regional Conference of IAEG (2015)

flow may be critical for initiation of the gush-out
slidings.
To understand the mechanisms of different typesof shallow landslide, we need to evaluate shear
strength parameters and soil thickness on hillslopes
for slope stability analysis. Measurement ofsubsurface hydrological processes is also required toreconstruct slope destabilization by rainwater
infiltration.

Matsuzawa, M., Chigira, M., Doshida, S. andNakamura, T. (2014) Landslide sites controlled by
the denudation front and weathering intensity:
sallow landslides induced by rainstorms in the area
underlain by the Cretaceous Izumi Group, Ehime
Prefecture, Jour. Japan Soc. Eng. Geol., Vol. 55, No.
2, pp. 64-76. (in Japanese with English abstract).
McDonnell, J. J. (1990) The influence of microporeson debris flow initiation, Q. J. Eng. Geol. Vol. 23,
pp. 325-331.
Onda, Y. (1989): Influence of water storage capacity
in regolith zone on runoff characteristics and slope
failure on granitic hill in Aichi, Japan, Transactions,Japanese Geomorphological Union. Vol. 10, No. 1,
pp. 13-26. (in Japanese with English abstract).
Takahashi, Y. (1991): Geology of the Hiroshima
District. With Geological Sheet Map at 1 : 50,000,
Geol. Surv. Japan, pp. 1-41. (in Japanese withEnglish abstract)
Terajima, T., Matsushi, Y. and Hattanji, T (2014)
Hydrogical aspects to shallow landslide occurrencein Izu-Oshima Island, Triggered by a heaby rainfallof typhoon 20th on October 15-16, 2013, Annuals ofDisas. Prev. Res. Inst., Kyoto Univ., Vol. 57A, pp.
17-24. (in Japanese with English abstract)
Ushiyama, Y. (2001): Characteristics of heavy
rainfall in Hiroshima city on June 29, 1999, basedon precipitation data since 1901, J. JSNDS. Vol. 20,
No. 1, pp. 59-74. (in Japanese with English
abstract).
Wakatsuki, T., Iida, T., Matsushi, Y., Kogure, T.,
Sasaki, Y. and Matsukura, Y. (2009): The effect ofrock weathering on soil formation and form of slope
failure on two mudrock slopes: a case study on
slope failures caused by typhoon 10, 2003 in Hidaka
District, Hokkaido, Transactions, Japanese
Geomorphological Union. Vol. 30, No. 4, pp.
267-288. (in Japanese with English abstract).

Acknowledgements
The authors are grateful to Aero Asahi
Corporation, provided the 1 m–mesh DTM data. This
study was supported by the JSPS KAKENHI(26900001, 26702010, 23221009), funding fromNatural Disaster Research Council, Disaster
Prevention Research Institute, Kyoto University, andfrom Institute of Sustainability Science, Kyoto
University.

References
Dietrich, W. E. and Dunne, T. (1993) Channel head,
In channel network hydrology. Beven, K. J. andKirkby, M. J. (eds.). John Wiley and sons, New
York, pp. 175-220.
Iida, T. and Okunishi, K. (1979): On the slope
development caused by the surface landslides,
Geographical Review of Japan. Vol. 52, No. 8, pp.
426-437. (in Japanese with English abstract).
Koyama, K., Sammori, T., Ochiai, H., Okumura, T.
and Honda, N. (2005): Influences of fragile layer on
surface failure in weathered granite slope, J. Jpn.
For. Soc. Vol. 87, pp. 304-312. (in Japanese withEnglish abstract).
Matsushi, Y., Hattanji, T. and Matsukura, Y. (2006)
Mechanisms of shallow landslides on soil-mantled
hillslopes with permeable and impermeable
bedrocks in Boso Peninsula, Japan, Geomorphology,
Vol. 76, pp. 92-108.

8