10th Asian Regional Conference of IAEG (2015)

Geological determination of a landslide body based on drilled cores
Yasuhiko WAKIZAKA(1)(1) A former member of the Public Works Research Institute, Japan
E-mail: dora1026@peach.ocn.ne.jp
Abstract
In the case of a landslide along a dam reservoir, an old landslide which has alreadystopped moving sometimes starts to move again as the reservoir water level changes. Whenassessing a landslide body, information about movements obtained by borehole
inclinometers is not useful for such old landslides. Instead, geological observation of drilledcores is the only way to determine the range of a landslide body. However, it is very
difficult to distinguish crushed rocks of landslide origin and those of tectonic fault origin
based on such observation of drilled cores. Recently, Wakizaka et al. (2012) and Wakizaka(2013) reported that crushed rocks of landslide origin can be distinguished from those of
tectonic fault origin: the former show a random texture, whereas the latter have somecomposite planar texture. Wakizaka et al. (2012) and Wakizaka (2013) examined a total of
only four landslide bodies, all of which were in accretionary complexes. Therefore, thepresent study observed drilled cores of 60 other landslide bodies distributed in various
accretionary complexes, normal sediments, schist, granite, and Miocene volcanic rocks. As
a result, the same conclusion that crushed rocks of landslide origin show a random texture,
whereas those of tectonic fault have some composite texture was obtained. However, there
are important points to note when distinguishing crushed rocks of landslide origin and thoseof tectonic fault origin: 1) crushed rocks with a random texture are distributed under the slip
surface, 2) some crushed rocks of tectonic fault origin do not have a clear texture, 3) clay
which composes the slip surface shows some textures such as a banded structure whichmakes it difficult to distinguish a fault gouge, 4) crushed rocks of tectonic fault origin are
sometimes distributed in a landslide body, and 5) it is difficult to distinguish crushed rocks
with a random texture of landslide origin and weathered and loosened originally brecciatedand gravelly rocks such as auto-brecciated lava, tuff breccia and conglomerate.

Keywords: landslide body, crushed rock, tectonic fault, composite planar fabric1.

Introduction
A landslide body is analyzed by topographicaland geological features and the results of movement
measurements such as by a borehole inclinometer.
The depth of the slip surface is determined by
movement measurements and/or geological features,
although the depth must be determined from only
geological features in the case of an old landslide
which has already stopped moving. The presence ofcrushed rocks which were formed by landslide
movement is an example of a geological feature of alandslide body. Slip surface clay is a part crushed at
the boundary between the landslide body andbedrock.
Fault breccia, fault gouge hydrothermally crushedrocks, and crushed rocks in a mud volcano, areexamples of crushed rocks distributed near the
ground surface other than crushed rocks of landslide
origin. Among these rocks, fault breccia and gouge
are widespread, and so geological determination of alandslide body is difficult. When usual drilling
methods are used for core sampling, it is impossible
to distinguish between crushed rocks of landslide
origin and fault breccia, because it is not possible tosample undisturbed cores in a crushed part.
Recently, drilling methods using a surfactant or
thickener have been developed that enable samplingof undisturbed cores (high-quality cores) in crushed
parts. Wakizaka et al. (2009) and Wakizaka et al.
(2012) found distinguishing characteristics between
crushed rocks of landslide origin and crushed rocks oftectonic fault origin based on observations of drilled110th Asian Regional Conference of IAEG (2015)

cores obtained from 19 drill holes in a landslide body
located in the Shimanto belt, southern Kyushu. This
report describes the results of geological observation
on high-quality cores obtained in 65 landslide bodies
in 26 regions.
2.

(1

Crushed rocks of landslide origin(2
2.1 Review on crushed rocks of landslide origin
There have been various previous studies on
crushed rocks of landslide origin as follows. Oyagi(1994) reported a “coagulated layer” observed in the
Nishitani landslide located in the Shimanto belt. He
stated that: “the coagulated layer is somewhat
compacted and consists of angular sandstone breccia
and powdery fine grained mudstone matrix”. Tajika(1995) divided three landslide deposits in an area of
sedimentary rocks in Hokkaido into four facies:
fractured rock, debris soil, clayey soil and surface soil.
Fractured rock is crushed and broken rocks of theinitial structures of bedrocks by cracks; debris soil iscomposed of clasts of boulder to fine gravel size andsand, silt and clay matrixes. Morita et al. (2002)
divided the crushing degree of the body of the Utsugi
landslide into seven categories based on observationsof drilled cores.

Fig. 1 Distinctive cracks of cores drilled in a landslide
body in region A (Wakizaka et al. 2012).
variable (Fig. 2). Figure 2 shows, in order, the size offragments and the amount of matrix; (1) shows
breccia which has the largest fragments and thesmallest amounts of matrix. Figure 2 (5) indicates thedrilled core where cumulative displacement wasmeasured, so this part makes clayey rock of the slip
surface.
A distinctive feature of this breccia is that thesize between fragments and matrix varies; larger
fragments are distributed in fine-grained matrix. No
planar fabrics such as a shear plane are observed in
breccias and clayey rocks, indicating a random fabric.
The characteristics described above are foundthroughout the landslide body such as the top and toe
parts, as well as surface and deep parts.
Wakizaka et al. (2012) divided breccias into thefollowing four categories based on the median
diameter of fragments and the amounts of fragments
and matrix.
1) Cr2: Breccias with random fabric. The amount of
matrix is less than 30%, the median diameter offragments is larger than 15 mm, and the breccia isfragment-supported (Fig. 2 (1) and (2)).
2) Cr3: Breccias with random fabric. The amount of
matrix is 30 to 60%, the median diameter offragments is 15 to 5 mm, and the breccia changes tomatrix-supported when fragments become small andthe matrix increases (Fig. 2 (3)).
3) Cr4: Breccias with random fabric. The amount of
matrix is more than 60%, the median diameter offragments is 5 to 2 mm, and the breccia ismatrix-supported (Fig. 2 (4)). Sometimes Cr4 forms aslip surface.
4) Cl: Sand to clay without fragments. Sometimes Cl
forms a slip surface (Fig. 2 (5)).

2.1 Cracky rocks
This section describes the characteristics of
crushed rocks (called crushed rocks of landslide
origin) in a landslide body (a landslide body in regionA) located in the Shimanto belt, according to
Wakizaka et al. (2012). Slip surfaces were determinedby cumulative displacement measured by borehole
inclinometers, and a landslide body was identified asa part which was shallower than the slip surface.
One of the distinctive characteristics of crushedrocks of landslide origin is cracky rocks. Wakizaka etal. (2012) classified these cracky rocks into thefollowing two categories.
1) Cr1a: Irregular cracks such as jagged cracks withnearly vertical dip are observed (Fig. 1(1)). These
cracks are oblique to the bedding planes. These
irregular cracks are considered to be tension cracks
based on their vertical dips and shape.
2) Cr1b: In some cases, fine-grained breccia fill theabove cracks (Fig. 1(2)). Two types of origin of thesefine-grained breccia are considered: breccia wasformed when such cracks formed, or breccia wasfilled from the upper parts.
2.2 Breccia and clayey rocks
Another distinctive characteristic of crushedrocks of landslide origin is the presence of various
breccia which consist of relatively coarse-grained
fragments and fine-grained matrix. This breccia isvery similar to fault breccia. The grain size offragments and the ratio of fragments to matrix are

2.3 Characteristics of crushed rocks except forregion A
Wakizaka (2013) found that the samecharacteristics of crushed rocks in a landslide body in2

10th Asian Regional Conference of IAEG (2015)

Table 1 Geology and number of observed landslide
bodies.
RegionGeologyA

Shimanto belt

B-1

Chichibu belt

B-2
C-1

Chichibu belt

C-2
D-1
D-2
E

Accretionary
Prism

Shimanto belt

Number oflandslide
body
1
1
4
2
6
1
3

Chichibu belt

1F

Chichibu belt

1G

Chichibu belt

1

H

Ultra-Tanba belt

1

I

Shimanto belt

3J

Ashio belt

7

Yezo supergroup2

Yezo supergroup2

Yezo supergroup

1

Izumi group

1

K

N

Normalsediments
except for
accretionary
prism

O

Schist, Sanbagawa belt

1

P

Schist, Suo belt

1Q

M iocene pyroclastics, sediments

9R

M iocene sediments, pyroclastics

8S

M iocene sediments, rhyolite

3T

M iocene volcanics, Cretaceous
rhyolite

2

U

M iocene pyroclastics, sediments

1

V

Cretaceous granite

1

W

Gneiss, granite, Hida belt

1

L
M

Total number of landslide bodies

65

region A were observed in other landslide bodies(B-1, C-1 and D-1 regions in Table 1). Excluding theabove four regions, the same characteristics of
crushed rocks in landslide bodies were observed in 60
landslide bodies in 22 regions (Table 1). Figure 3
shows the features of crushed rocks in six typicalexamples; the constituent geology of each region is asfollows: region B-1 is sedimentary rocks consistingof accretionary prisms in the Chichibu belt, region L
is sedimentary rocks of normal sediments except for
accretionary prisms in the Yezo supergroup, region P
is schist in the Suo belt, region Q is Miocene
pyroclastics and sediments, and region V is
Cretaceous granite. As shown in Fig. 3, all breccias
are random fabric with various sizes of fragments andvarious amounts of matrix; these features are theFig. 2 Distinctive breccia and clayey rock of cores
drilled in a landslide body in region A (Wakizaka et
al. 2012).

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10th Asian Regional Conference of IAEG (2015)

Fig. 3 (1) Features of crushed rocks of landslide origin and tectonic fault origin observed in high-quality drilledcores in six landslide bodies. A yellow bar in each photograph shows 2 cm length.
same as those of the landslide body in region A.
All landslide bodies shown in Table 1 areclassified into landslides in rocks and landslides inweathered rocks based on the classification proposedby Watari and Sakai (1975). The characteristics of
crushed rocks of landslide origin observed in thelandslide body in region A are considered to beuniversal characteristics in a landslide body classified
as landslides in rocks and weathered rocks.
No fabrics were observed in clayey rocks which
consisted of a slip surface in the landslide body inregion A. However, sometimes some fabrics wereobserved in other landslide bodies, and slickensideand striation were found along such fabrics.

these fabrics were also found in deeper parts than theslip surface. In landslide bodies in other regions,crushed rocks with some fabrics were observed indeeper parts than the slip surface.
Intervals of R1 shear or P foliation were several
centimeters in accretionary prisms such as the
Shimanto belt and the Chichibu belt, whereas theintervals were more than several dozen centimeters in
tertiary normal sediments. The development of
fabrics is poor in tertiary normal sediments comparedwith accretionary prisms.
Grain size was continuously distributed in fault
breccia compared with breccia of landslide origin.
4.

3.

Crushed rocks of tectonic fault origin

Geological observations of fault breccia andgouges which indicated a certain displacement in
outcrops and adits near the landslide body in region A
were performed. As a result, R1 shear (Barlett, 1980)
and/or P foliation were recognized in fault breccia,
whereas sometimes Y plane (Barlett, 1980) was
recognized, and sometimes Y plane was notrecognized in the fault gouge.
In the cores of the landslide body in region A,

Distinguishing crushed rocks of landslide
origin from those of tectonic fault origin

4.1 Differences between crushed rocks of landslide
origin and those of tectonic fault origin
A distinctive difference between crushed rocksof landslide origin and those of tectonic fault origin is
the presence or absence of fabrics (Wakizaka et al.
2009; Wakizaka et al. 2012). Crushed rocks, which
are considered to be of landslide origin because they
are distributed in shallower parts than the slip surface,
do not have fabrics. However, sometimes crushed

4

10th Asian Regional Conference of IAEG (2015)

Fig. 3 (2) Features of crushed rocks of landslide origin and tectonic fault origin observed in high-quality drilledcores in six landslide bodies. A yellow bar in each photograph shows 2 cm length.
rocks such as clayey rocks which consist of slip
surfaces indicate some fabrics. In contrast, crushedrocks of certain tectonic fault origin located in
outcrops and crushed rocks located in deeper parts
than the slip surface show some fabrics.
The distribution of grain size between fragments
and matrix is discontinuous in breccia of landslide
origin, whereas it is continuous in breccia of tectonic
fault origin (Wakizaka et al. 2012).

in crushed rocks of landslide and tectonic fault origin
was attributed to the difference of confining pressure
when rocks were crushed, based on relationships
between confining pressure and deformation featuresin triaxial tests. According to Hoshino et al. (1972)
and Paterson (1978), relationships between confiningpressure and deformation features in triaxial tests are
as follows.
1) Cracks of split shape along the σ1 direction areformed when no confining pressure acts, that is, theuniaxial state.
2) Shear planes of fixed direction oblique to σ1 areformed when confining pressure acts.
3) Conjugate shear planes are formed as the

4.2 Controlled factor of fabrics of crushed rocksof landslide and fault origin
Wakizaka et al. (2012) considered that thecontrolled factor of the presence or absence of fabrics

5

10th Asian Regional Conference of IAEG (2015)

confining pressure increases.
According to data indicated in Hoshino et al.
(2001), confining pressure when the deformation
mode changes from a split-shape crack to a shear
plane, shear plane to conjugate shear plane arecorrelated with the initial strength of the rocks. At
low initial strength, low confining pressure isrequired to change the deformation mode. However,the confining pressure needed to change thedeformation mode was not obtained by Hoshino et al.
(2001), because there were few cases of low
confining pressure. Nevertheless, based on theuniaxial strength of pre-Miocene sedimentary rocks,
volcanic rocks and granite, it is considered that highconfining pressure is required when composite planar
fabrics such as R1 shear and P foliation are formed.
The formation depth of mélange, which does notshow plastic deformation of quartz grain, is estimatedto be around 5-6 km to15 km (Kimura and Kinoshita,2009), so the formation depth of a fault in mélange isconsidered to be the same depth, whereas the depthof the slip surface is usually less than 100 m. The
difference of formation depth between crushed rocksof landslide origin and tectonic fault origin creates adifference of confining pressure when both crushedrocks were formed, and consequently a difference ofconfining pressure controls the presence or absenceof fabrics.

deformed slope.
2) Sometimes crushed rocks of tectonic fault origin
may not always have clear fabrics. Especially, faults
in post-Oligocene sediment do not have fabrics, or ifsome fabrics are present, the intervals of fabrics are
wide. The concept of fabric on fault breccia andgouges was absent in Higgins (1971) and Sibson(1977), who studied the initial classification of fault
rocks. Chester et al. (1985) first described that fault
breccia had fabrics. They reported a foliated gouge in
Punchbowl fault, USA based on observation ofoutcrops. McClay (1987) and other researchers added
foliated fault breccia and foliated fault gouge to fault
rocks.
Based on the relationships between confiningpressure and deformation mode, faults formed under
low confining pressure do not have fabrics. A fault
gouge without fabrics was observed in tectonic faults
in region A (Wakizaka et al., 2012).
3) The Y plane, which is observed as a banded
structure, is recognized in slip surface clay
determined by movement measurement. So, such aslip surface clay is difficult to distinguish from fault
gouges. Morgenstern and Tchalenko (1967) reported
that two systems of shear plane were present in
London clay. Inoue et al. (2001) described that
D-shear (corresponds to Y plane) and P shear
developed in slip surface clay. Many studies
indicated that slickenside was observed in slip
surface clay.
Some fabrics such as the banded structure areobserved in clayey parts excluding the slip surface. In
some landslide bodies, the slip surface originated in
tectonic faults. Therefore, slip surface clay is notdetermined by only the presence or absence offabrics.
Yamane et al. (2013) and Yamane et al. (2015)
investigated the microstructures of resin-fixing
specimens of half-cut high-quality cores in andaround slip surfaces located in region B-1 as shownin Table 1. They reported that reverse and normal
fault senses determined by composite planar fabricand asymmetric structures were found. The parts
which show normal fault sense are considered to be
the slip surface, while the parts which show reverse
fault sense are estimated to be tectonic faults.
4) A certain landslide body sometimes includes
crushed rocks of tectonic fault origin (Wakizaka,2014). Usually, crushed rocks of tectonic fault origin
are not present in a landslide body. The absence ofcrushed rocks of fault origin is attributed to thestrength of the fault, which is very low comparedwith the surrounding bedrocks, so the fault isselectively re-deformed by landslide movement. The
presence of crushed rocks of tectonic fault origin
indicates bedrock.
However, crushed rocks of tectonic fault origin
were found in the landslide bodies located in regions

4.3 Distinguishing crushed rocks of landslide
origin from those of tectonic fault origin
Basically, crushed rocks without fabrics(random fabric) are considered to be of landslide
origin, whereas crushed rocks with fabrics are
considered to be of tectonic fault origin; therefore, it
is possible to distinguish crushed rocks of landslide
origin and those of tectonic fault origin. However,there are some important matters when distinguishing
the origin of crushed rocks using the presence orabsence of of fabrics:
1) Crushed rocks with random fabrics are sometimes
observed below certain slip surfaces. Therefore,
when the bottom of the landslide body is determinedby the presence of crushed rocks with random fabric,
the determined bottom is deeper than the true bottom.
It is considered that the origin of these crushed rocks
distributed in deeper parts than the slip surface iscrushed rocks formed by gravitational deformation
before landslide movement, or crushed rocks affectedby upper landslide movement (Wakizaka et al.2012).
Chigira et al. (2013) classified plastically
deformed rock masses into cracked, jigsaw-1,
jigsaw-2, disintegrated, or pulverized, based on
observation of high-quality cores obtained in a
gravitationally deformed slope. This classification
corresponds to Cr1a, Cr1b, Cr2, Cr3, Cr4 and Cl of
Wakizaka et al. (2012). Crushed rocks with randomfabric are also distributed in a gravitationally610th Asian Regional Conference of IAEG (2015)

B-2 and L. In the landslide bodies in regions B-2 andL, crushed rocks of tectonic fault origin were foundin weakly crushed blocks by landslide movements.
Therefore, the presence of crushed rocks of tectonic
fault origin is not an indicator of bedrock.
5) It is difficult to distinguish crushed rocks oflandslide origin from loosened initially brecciated andgravelly rocks such as auto-brecciated lava, tuff
breccia and conglomerate. When initially brecciatedrocks and conglomerate distributed near the groundsurface were affected by weathering and release of insitu stress, these rocks became loose. Loosened rocks
are composed of fine grained matrix along cracks andfragments, and the distribution of matrix andfragments is random. Fragments consist of theoriginal rock, so the appearance of these loosened
rocks is very similar to that of crushed rocks oflandslide origin. Thus, observation of matrix andfragments of these loosened initially brecciated rocks
and conglomerate is very important.
5.

confining pressures, Geological Survey Japan
Report, No.244, 200p.
Hoshino, K., Kato, H., Tanaka, S., Omata, A.,
Moriguchi, Y., Hattori, M. and Imamura, T. (2001):
Handbook of mechanical properties of the Japaneserocks under high confining pressure, Geological
Survey of Japan, AIST, 479p. (JwE)
Inoue, M., Yamada, T., Tanaka, H. and Kitagawa, R.
(2001): Shear structure of sliding surface andlandslide process of Noda landslide developed in the
Triassic stratum area, Okayama Prefecture, Japan,
Journal of the Japan Society of Engineering
Geology, 42, pp.88-100. (JwE)
Kimura, G. and Kinoshita, M. eds. (2009):
Accretionary prisms and megathrust seismogenic
zones, Toward a better understanding of the Nankai
Earthquakes, University of Tokyo Press, 281p. (J)McClay, K. (1987): The Mapping of Geological
Structures, Geological Society of London,
Handbook Series, 161p.
Morgenstern, N. R. and Tchalenko, J.S. (1967):
Microstructural observations on shear zones fromslips in natural clays, Proceedings GeotechnicalConference, Oslo, pp.147-152.
Morita, T., Yoshimura, N. and Yokoyama, S. (2002):
Internal Structure of moving Body of Utsugi
Landslide in Kochi Prefecture, Proceedings ofAnnual Meeting, Japan Society of Engineering.
Geology, pp.115-118. (J)Paterson, M. S. (1978): Experimental RockDeformation – The Brittle Field. Springer- Verlag,
Berlin, 254p.
Sibson, R. H. (1977): Fault rocks and fault
mechanisms, Journal of the Geological Society
London. Vol.133, pp191-213.
Tajika, J. (1995): Internal structure and depositionalfacies of landslide material originated fromsedimentary rocks, Report of the Geological Surveyof Hokkaido, No.67, pp.59-145. (JwE)
Wakizaka, Y. (2013): Characteristics of crushedrocks observed in drilled cores in landslide bodies inaccretionary complexes, Tectonophysics, 605,
pp.114-132.
Wakizaka, Y. (2014): Remaining tectonic faults
hinder distinguishing a landslide body and basement
rock, Proceedings. 12th IAEG.
Wakizaka, Y., Kozuma, M. and Watatani, H. (2009):
Breccia which characterizes landslide mass,
Proceedings of Annual Meeting, Japan Society ofEngineering. Geology, pp.41-42. (J)Wakizaka, Y., Kozuma, M. Watatani, H. andToyoguchi, Y. (2012): Characteristics of crushedrocks in a landslide body –An example of alandslide located in the Shimanto belt-, Journal of
the Japan Society of Engineering Geology, 52,
pp.231-247. (JwE)
Watari, M. and Sakai, A. (1975): A view of roughinvestigation and investigation of landslides,

Conclusions

It is difficult to distinguish crushed rocks oflandslide origin and those of tectonic fault origin,
because both crushed rocks are very similar. Detailed
structures can be observed in undisturbed cores
drilled by high-quality drilling methods. As a resultof observing such high-quality cores obtained fromvarious geologies, crushed rocks of landslide origin
show a random fabric, whereas those of tectonic fault
origin indicate some fabrics such as R1 shear and /or
P foliation and/or Y plane. Therefore, crushed rocksof landslide origin and tectonic fault origin can be
distinguished by the presence or absence of fabrics.
However, there are some important issues when using
the presence or absence of fabrics as such anindicator.
References
Bartlett, W. L. (1980): Experimental wrench faulting
at confining pressure, M. S. thesis, Texas A&M
Univ., College Station, Tex.
Chester, M., Friedman, M. and Logan, J. M. (1985):
Foliated cataclasites, Tectonophysics, 111, pp.139146.
Chigira, M., Hariyama, T. and Yamasaki, S. (2013):
Development of deep-seated gravitational slope
deformation on a shale dip-slope: Observations
from high-quality drill cores, Tectonophysics, 605,
pp.104-113.
Higgins, M. W. (1971): Cataclastic rocks, UnitedStates Geological Survey Professional Paper.
No.687, 97p.
Hoshino, K., Koide, H., Inami, K., Iwamura, S. and
Mitsui, S. (1972): Mechanical properties ofJapanese Tertiary sedimentary rocks under high7

10th Asian Regional Conference of IAEG (2015)

Technical Memorandum of Public Works ResearchInstitute, No.1003, 70p. (J)Yamane, M., Yamada, M., Sengoku, A., Wakizaka, Y.
and Akamatsu, K. (2013): Composite planar fabrics
which distinguish slip surface clay from fault gouge,
Proceedings of Annual Meeting, Japan Society ofEngineering. Geology, pp.193-194. (J)Yamane, M., Yamada, M., Sengoku, A., Wakizaka, Y.
and Akamatsu, K. (2015): Composite planar fabrics

capable of distinguishing between slip surface clay
and fault gauge -An example of landslide ofaccretionary complex in the Chichibu belt-, Journalof the Japan Society of Engineering Geology, 56.
(accepted). (JwE)(JwE): written in Japanese with English abstract(J): written in Japanese8