京 都 大 学 防 災 研 究 所 年 報 第48号 B 平成17年4月
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 48 B, 2005

The Shear Strength of Clayey Soils from Reactivated Landslides

Ivan GRATCHEV*, Kyoji SASSA, and Hiroshi FUKUOKA* Graduate School of Science, Kyoto University

Synopsis
This article presents the results of two case studies of reactivated landslides which occurredin Japan. Field and laboratory investigations, including a series of drained ring-shear tests, wereconducted in order to design a set of countermeasures and prevent further landslide movement. The
obtained results indicated that residual shear strength of soil from the sliding surface contributed
greatly to the behavior of landslide; and for this reason, a special effort was made to study the effectof clay content and clay mineralogy on the residual shear strength of soils. It was found that anincrease in clay content as well as the presence of montmorillonite significantly reduced residual
shear strength.
Keywords: reactivated landslides, residual shear strength, montmorillonite, plasticity index, clay
content

1.

Introduction

Reactivated landslides, which are usually found inUpper Tertiary mudstone-tuff formations, are common
phenomena in Japan. According to the geotechnicalclassification of landslides proposed by Prof. Sassa(1985), the reactivated type of landslides is a residual
slide that occurs on gentle slopes and moves slowly
mainly due to an increase in ground water level. Resent
studies have shown that the sliding surface is likely to beformed in clayey soils and exists in the residual state dueto the previous movements (Shuzui, 2001; Gibo et al.,2002). Decidedly, knowledge of residual shear strengthof these soils as well as factors affecting it is necessary
for the hazard assessment and the design of remedialmeasures.
This article briefly presents the results of two casestudies of reactivated landslides which occurred in
Hyogo Prefecture (“Kuchi-Otani” landslide) and in thevicinity of Biwa Lake (“Ogoto” landslide). SpecialJapan“Kuchi-Otani”
landslide

TokyoBiwa Lake

“Ogoto” landslide

Fig. 1 Location of the studied landslidesa)Movement
direction

620
610

Q-5

P-2

Q-7

Q-9

K-3

A’
Q-2
K-5

Q-4

Q-3

610

Q-658 0J-1AQ-1

A-1

N

Q-8

LegendQ-6600- Borehole

G-20- The boundary of landslide

40- Road- Rice fieldb)
AA’m
650
630

Q-6J-1

Q-4

610
Q-2590Sliding surface
584.05582.30Brownish clayey sandVery soft amber clay
Black stiff mudstone

578.13Black clayey sandwith gravel

575.98578.40Black stiff mudstone

Sliding surface
Core sample collected for laboratory examination

570.23Fig. 2 Plan of the “Kuchi-Otani” landslide (a); and a cross-section (through A-A’ on Fig. 2a) (b).
attention is given to factors affecting the residual shear
strength of clayey soils such as clay content and clay
mineralogy. This work shows that an increase in clay
content decreases shear strength of clayey soils, and thepresence of montmorillonite significantly reduces shear
strength of clayey soils.

2.

“Kuchi-Otani” landslide

The “Kuchi-Otani” landslide located in the Hyogo
Prefecture, Japan (Fig. 1), was reactivated in 2003. The
approximate 1.5 million m3 of landmass has been
creeping down progressively at an estimated rate ofabout 1 mm per month. The landslide area has been

Table 1. The properties of soils from the “Kuchi-Otani” landslide
B ro w n ish c la y e y s a n dA m b e r s ilty c la y

B la c k c la y e y s a n d

LL, %8 8 .2

9 6 .3

3 8 .6

P I, %

5 4 .5

6 2 .5

1 9 .0C la y c o n te n t% <2µm1166

8M in e ra ls

Q u a rtz , H a llo y s ite ,M ic a ,M o n tm o rillo n ite

Q u a rtz , H a llo y s ite , M ic a

100

2.1 Field investigation
To study the geology of the landslide site, a numberof boreholes were drilled (Fig. 2a). The borehole cores
were thoroughly collected and visually examined.
Then, borehole inclinometers were set to determine thelocation of the sliding surface and to monitor direction
and rate of movements of the landslide. During the one
year monitoring with measurement intervals of 10 days,
the following landslide characteristics have been
revealed: 1) the dip angle of the sliding surface wasgentle and varied from 5 to 7 degrees; 2) the maximum
depth of the sliding surface was estimated to be
more than 30 m; and 3) the sliding velocity wasconstant and its maximum value of about 1 mm/month
was measured in the central part of the landslide(boreholes Q-4, J-1 and K-5). A cross-section A-A’
(Fig. 2b) will be used to briefly introduce the landslidecharacteristics and the lithological composition of thearea. According to the data obtained by borehole’s
inclinometers (borehole Q-4), the sliding surface islocated near the boundary of stiff black mudstone andsoft silty clay, which is overlain by clayey sandcontaining rock fragments that are several millimeters
to several tens of centimeters in size. In Q-2 the zoneof deformation based on the inclinometer record wasdetected in the layer of clayey sand containing rockfragments of different sizes.

80
Finer by weight (%)

extensively used by the local people for farming andfurther movement will likely result in a significant
economic loss. In order to design a set ofcountermeasures and prevent a potential disaster, acomprehensive investigation of this area wasconducted by the Hyogo Prefectural Government incooperation with the Research Centre on Landslides,
Kyoto University.

Amber silty
clay

60

Brownish clayey sand40

20Black clayey sand00.001

0.01

0.1

1

10

Grain size (mm)Fig. 3 Particle size distribution curves for the tested
soils from the “Kuchi-Otani” landslide
To study the properties of the soils from the shear
zone, two core samples were collected from Q-2 andQ-4 at the depths marked in Fig. 2b. An initial
examination showed that the black clayey soil fromQ-2 is harder than that from Q-4. In addition, the coresample from Q-4 consisted of two parts: the upper brownish clayey sand, and the lower - very soft andplastic amber clay. The slickenside was observed in theamber clay.
2.2 Laboratory investigation
Particle size distribution and X-ray diffraction
analyses as well as a series of drained monotonic
speed-controlled ring-shear tests were performed todetermine the properties of the clayey soils.
According to the results from X-ray diffraction
analysis carried out by the Geo-Research Institute,
Osaka, quartz, halloysite and mica are the dominant
minerals in all the soils; however, the mineralcomposition of the brownish clayey sand also includes
montmorillonite (Table 1). Figure 3 presents theparticle size distribution for the soils used forring-shear tests. As can be seen, the amber silty clay
has a significant amount (≈67%) of clay fraction(<2µm), while the brownish and black clayey sands
contain only about 11 and 8 %, respectively. The index
properties of the soils were also studied (Table 1). The
relatively higher plasticity indices of the amber clay
(PI=62.5) and the brownish clayey sand (PI=54.5),

Residual strength (kPa)Black clayey sanda)

600

400

φr = 25.8°
20000

Residual strength (kPa)

600

200400Normal stress (kPa)

600

Amber silty clayb)

400

200

φr = 5.8°00200

400

600

Normal stress (kPa)

Residual strength (kPa)

600

Brownishsandc)

clayey400

φr = 17.5°

20000200

400

Normal stress (kPa)

600

Fig. 4 Results from drained speed-controlled ring-shear
tests on the soils from the “Kuchi-Otani” landslide
compared to that of the black clayey sand (PI=19.0)
can be explained by the difference in clay content (inthe case of the amber silty clay) and the presence ofmontmorillonite (in the case of the brownish clayeysand).
2.3 Ring-shear tests
Ring-shear apparatus. The ring-shear apparatus
used in this work is DPRI-4, one of a series ofintelligent ring-shear apparatuses developed andimproved at the Disaster Prevention Research Institute,
Kyoto University (Vankov & Sassa 1998). It is an
important tool for hazard assessment of rapid andlong-travel landslides (Sassa et al., 2004) as well as
earthquake-induced landslides (Fukuoka et al., 2004).
The principal structure of a ring-shear apparatus aswell as the method of sample preparation have beenreported in the literature (Sassa et al. 2004) and for thisreason only brief introduction of DPRI-4 will be given
below.
One of the advantages is a big shear box having
210 mm and 290 mm in the inner and outer diameters,
respectively. Two personal computers are set for test
control and data recording. The test can be carried out
using either shear torque control, shear speed control orshear displacement control. Pore pressure, appliedstress and displacement are measured by thetransducers and recorded automatically.
Sample preparation and test procedure. The
specimens were prepared from a slurry, and then set
into the shear box. The degree of saturation wasexamined by measuring BD value, which was definedas the ratio between the increments of generated porepressure (∆u) and normal stress (∆σ) (BD=∆u/∆σ)(Sassa 1988). The ratio for each test was ensured to be
more than 0.95, a value that approximates fullsaturation. The specimens were normally consolidated
under confining stresses of 200, 400 and 600 kPa. To
prevent generation of pore water pressure during
testing the lowest speed of 0.1 mm/min was used.
Figure 4 shows the results obtained for the black
clayey sand, the amber silty clay and the brownish
clayey sand, respectively. As can be seen, the value ofthe residual friction angle of the black clayey sand isrelatively high and approximates 25.8°. In contrast, theresults from ring-shear tests on the amber silty clay
give a very low value of residual friction angle of 5.8°

(Fig. 4b). The residual friction angle of the brownisha)124

clayey sand was found to be of about 17.5° (Fig. 4c).

A

122
118
114

B-2110

B-1

108
106
104
102
100
98

96

020 m

B-1 - Borehole
A’Ab)
B-2

Block B
B-1A’

Soil 2Block A
01020 mSoil 1
Fig. 5 Plan of the “Ogoto” landslide (a); and a cross-section (through A-A’ on Fig. 5a) (b).
3.

“Ogoto” landslide
3.1 Filed investigation

The “Ogoto” landslide was reactivated in 2004 in
Otsu-city in the vicinity of Biwa Lake (Fig.1). Due toits location in a residential area and its constant
movement, the “Ogoto” landslide can be seen as athreat to the local people and their property. For thisreason, immediate contrameasures based on the field
and laboratory investigations need to be put into place
to prevent a potential disaster.

The methods of filed investigation used to study thegeology of the “Ogoto” landslide are similar with thoseused for the “Kuchi-Otani” landslide (see section 2.1)
including borehole core examination and monitoring ofmovement by a series of borehole inclinometers. The
results are schematically presented in Fig. 5 in the formof a schematic map (Fig. 5a) and a cross-section AA’(Fig. 5b). The monitoring revealed two landslide
blocks (A and B) with a dip angle of the sliding surface

Description

Soil 1

soils from the “Ogoto” landslide
3.3 Ring-shear tests
The ring-shear apparatus as well as the test
procedure were the same as those described in section
2.3. The only difference is that the specimens from the“Ogoto” landslide were consolidated under confining
stresses of 50, 100 and 200 kPa.
250a)

Soil 2

LL, %

96.3

76.1

PI, %

57.2

49.5

Clay content %< 2µm
Minerals

≈ 67

≈ 59

Soil 1

200
Resigual strength (kPa)

to be about 11°. The estimated velocity of the A-block
is about 0.5 mm/month, which is much higher than thatof the B-block (about 0.06 mm/month).
To obtain the geotechnical characteristics of soils
from the sliding surfaces of the both blocks, coresamples were collected from the boreholes B-1 (Soil 1)
and B-2 (Soil 2) (Fig. 5b). An initial examination
showed that the Soil 1 is softer and more plastic thanthe Soil 2. The sliding surface was observed in the bothsoils.
Table 2. The properties of soils from the “Ogotoi”
landslide150

φr = 11.0°

10050

Montmorillonite, Mica, Kaolinite,Quartz, Plagioclase

0050

100

150200250200250

Normal stress (kPa)

3.2 Laboratory investigation
The results of laboratory examination that included
particle size distribution, X-ray diffraction andplasticity analyses are presented in Fig. 6 and Table 2.
The mineral composition appeared to be the same
comprising montmorillonite, Mica, Kaolinite, Quartz
and Plagioclase. The particle size distribution analysis
revealed different proportion of clay fraction (less than0.002 mm), which was about 67% in the Soil 1, andabout 59% in the Soil 2 (Fig. 6). This difference islikely to account for the higher plasticity index of theSoil 1 (PI≈57.2) than that of the Soil 2 (PI≈49.5).

250

Soil 2b)Residual strength (kPa)

200

150

φr = 17.4°

100

5000

Finer by weight (%)

100

80100150
Normal stress (kPa)

Fig. 7 Results from drained speed-controlled ring-shear
tests on the soils from the “Ogotoi” landslide

Soil 1
Soil 2Fig. 7 shows the results obtained for the Soil 1 andSoil 2. As can be seen, the value of the residual friction
angle of the Soil 1 was measured to be about 11°,which is smaller than that of the Soil 2 (about 17.4°).

60

400.001

500.010.1

14.

Shear strength of soil

Grain size (mm)Fig. 6 Particle size distribution curves for the tested

Analysis of the laboratory results indicated asignificant influence of clay content and clay
mineralogy on the residual shear strength of the tested
soils collected from the “Kuchi-Otani” landslide. For
example, the mineralogical compositions of the amber
silty clay and black clayey sand were found identical(Table 1); however, the former had a significantly
higher amount of clay, a property that presumably
accounted for its lower value of residual shear strength(Fig. 4b) compared to the latter (Fig. 4a). Comparisons
made between the samples of black clayey sand andbrownish clayey sand revealed very similar fineparticle distribution (Fig.3) but different mineralcomposition; that is, the presence of montmorillonite in
the brownish clayey sand (Table 1). This difference inclay mineralogy seemed to be responsible for the lower
residual shear strength of the brownish clayey sand(Fig. 4c) compared to that of the black clayey sand (Fig.
4a).

Laboratory examination of the soils from the “Ogoto”
landslide pointed out a great effect of clay content on
the residual shear strength. It was demonstrated that thehigher clay content of the Soil 1 (Fig. 6 and Table 2)
was the main reason for its lower residual shear
strength (Fig. 7a) compared to that of the Soil 2 (Fig.
7b).
It is known that the plasticity index of soil, being afunction of clay content and clay mineralogy, can beused as a parameter for the comparison of different
clayey soils. As can be seen in Fig. 8a, which wasplotted in terms of residual shear strength against
plasticity index, an increase in plasticity decreases theresidual shear strength of soil. The obtained results arein agreement with those obtained by Lupini et al.
(1981) and Tika et al. (1999). Fig. 8b presents therelation between the water content of soil measured
after each test and its residual shear strength. It is clearthat soil with higher water content has lower residualshear strength.
The obtained relations presented in Fig. 8 havepractical application and can be used for an
approximate estimation of residual shear strength ofsoil, for example in the process of field investigation.
However, in order to obtain a precise value of residual
shear strength it is still necessary to conduct a series ofdrained monotonic ring-shear tests.

30(a)Residual friction angle (degrees

2520151055.

0102030405060

Plasticity Index (%)
30

Residual friction angle, degrees(b)2010010203040

Water content from the sliding surface, %Fig. 8 Relations between residual shear strength and (a)plasticity index, and (b) water content

Conclusions

7050

Residual shear strength of five different clayeysoils collected from two reactivated landslides in Japan
as well as the soil’s properties such as clay content,
clay mineralogy and plasticity were studied and thefollowing conclusions were drawn:
Increase in clay content decreases residual
shear strength of soil;
The presence of montmorillonite decreases
residual shear strength of soil;
Increase in plasticity decreases residual shearstrength of soil;
Soil with higher water content tends to havelower residual shear strength.
The obtained relations between residual shear
strength and plasticity and between residual
shear strength and water content (Fig.8) canbe used for an approximate estimation ofresidual shear strength of soil provided thatwater content of soil or its plasticity is known.

Tokyo, 31-40.

Sassa, K. (1988): Geotechnical model for the motion oflandslides. Special Lecture of 5th International
Symposium on Landslides, “Landslides”, 10-15 July,
1: 37-55.
Sassa, K., Fukuoka, H., Wang, G., and Ishikawa, N.
(2004): Undrained dynamic-loading ring-shear
apparatus and its application to landslide dynamics.
Landslides, 1 (1): 7-19.
Sassa, K., Wang, G., Fukuoka, H., Wang, F., Ochiai,T., Sugiyama, M., and Sekiguchi, T. (2004):
Landslide risk evaluation and hazard zoning for rapid
and long-travel landslides in urban development areas.
Landslides, 1 (3): 221-237.
Shuzui, H. (2001): Process of slip-surface development
and formation of slip-surface clay in landslides in
Tertiary volcanic rocks, Japan. Engineering Geology,
61: 199-219
Tika, T., and Hutchinson, J. (1999): Ring shear tests on
soil from the Vaiont landslide slip surface.
Geotechnique, 49 (1): 59-74.
Vankov, D. and Sassa, K. 1998. Dynamic testing ofsoils by ring-shear apparatus. In: Proceedings of the
8th Congress of the International Associate forEngineering Geology and the Environment.
Vancouver, 1: 485-492.

Acknowledgements
The authors would like to acknowledge the Hyogo
Prefectural Government and the Kawasaki GeologicalEngineering Co. for cooperation during the field
investigation.
References
Fukuoka, H., Wang, G., Sassa, K., and Matsumoto, T.
(2004): Earthquake-induced rapid long-traveling flow
phenomenon: May 2003 Tsukidate landslide in Japan.
Landslides, 1 (2): 151-157.
Gibo, S. , Egashira, K., Ohtsubo, M., and Nakamura, S.
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Gratchev, I., Sassa, K. and Fukuoka, H. (2005): Hazard
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再活動地すべり地粘性土のせん断強度
Ivan GRATCHEV*・佐々恭二・福岡浩*京都大学大学院
要旨
ここでは日本で起きた 2 種類の再活動地すべりの研究結果を示す。対応策の設計および地すべり活動の阻止のため、現地および実験室にて調査を行った。得られた結果より、すべり面の残留強度が地すべり活動に大きく寄与することが示され、土の残留強度時における粘土含有率や粘土鉱物について研究した。これにより、粘土含有率の増加はモンモリロ
ナイト含有時と同様に、残留強度を十分に減少させることが示された。
キーワード:再活動地すべり, 残留強度, モンモリロナイト, 塑性指数, 粘土含有率