10th Asian Regional Conference of IAEG (2015)

Crushed Rock Mass in Landslide Body of the Cretaceous Sedimentary Rocks

Toshio MIZUTANI, Takeshi NAKAMURA and Keii BAN(1)(1) Shikoku Branch, NIPPON KOEI CO., LTD.
Abstract
We have got an opportunity to observe the cutting slope of a massive landslide bodyof the Cretaceous sedimentary rocks. Various rock masses, which have been deformed,
crushed or rotated by the past landslide movement, appear inside of the landslide body. As aresult of the observations, distribution and characteristics of crushed rocks possibly resultsfrom the geologic structures such as bedding or joint planes and the extent or progress oflandslide movement. Therefore, it is effective to investigate how the geologic structures
have remained, continued or changed in order to analyze landslide mechanisms. It is
important to survey and examine carefully about distribution and characteristics of crushedrocks along bedding planes especially if stratified rocks had been continuously distributed
like in this area.
Keywords: landslide body, observation at slope, characteristics of crushed rocks
1.

Introduction
It has been reported that landslide bodies
consist of not only randomly crushed rocks but also
fault rocks in some case (Wakizaka et al.,2012).
When to discriminate between landslide bodies andbasement rocks geologically, it is important toexamine carefully about quality and continuity ofgeological fracture based on rock types andgeological structure of it.
We have got an opportunity to observe thecutting slope of a massive landslide. Various rockmasses deformed, crushed and rotated by the pastlandslide movement were found inside of thelandslide body. Then we report the correspondenceof the crushed rock mass with geological structureand notes of observation at outcrops and borehole
core.

shale layers along bedding planes are accompanied
by several centimeters thick clay.
High-quality cores
Normal cores753

62

Cut slope
41

This area

50m

2.

General information of the landslide
The landslide of this report is facing southwest
along the Koutou River in Kagawa prefecture,
Japan. Basement rocks of the landslide consist ofmainly sandstone and shale in the Cretaceous Izumi
Group. Layers are tilting towards slightly upstreamside to the bedding plane dipping 30 ～ 40° at
south or southeast. The size of the landslide isgeographically estimated about 300m long and200m wide (Fig. 1). About twenty cores were
drilled into the landslide body (Seven of them werehigh-quality cores). Rocks inside the landslide bodytend to be broken into some blocks caused by many
cracks along bedding and joint planes. And someFig. 1 Location map of the slope and borings
3.

Observation at the cut slope
At the slope cutting through the landslide bodyabout 100m long and 50m high, we observed
geology and geological structure and to classify
grade of crushing (Fig. 2, Photo 1～4, Table. 1).
Shale, sandstone, tuffaceous shale and tuff aremainly distributed on the slope. There are some slip
surfaces ① ～ ③ which are formed along
bedding planes inside the landslide body. Rockmasses between each slip surfaces appear different
grade of crushing, which tend to more crushed1

<Legend>

Gravel and Soil

Clay layer

TuffShale

Tuffaceous shaleSandstone

Slip surface ①

Slip surface ②

32･some open cracks･slightly deformed

2

5

Slip surface ③

2･strongly crushed･mainly gravel

Fig. 2 Outcrop of the slope (showing the landslide bodies)

1･kink and shear deformation･gravel along the slip surface

4

3

6･gravel and soil
･including clay

10th Asian Regional Conference of IAEG (2015)

10th Asian Regional Conference of IAEG (2015)

Photo. 1 Slip surface ①
Almost 15 centimeters thick clay layer is distributed. There are some linear marks caused through shearing
processes, called slickenside.
Bedding: N55°E42°S, Linear marks: S11°W32°

Photo. 2 Slip surface ②Almost 5 centimeters thick black clay is underlain
by crushed rock (Cr3 grade). The slip surface isharmonic with bedding planes. Tuffaceous shalerocks have been crushed into block and gravel.
Bedding: N57°E34°S, Linear marks: S47°W7°<Legend>3

Cr1aCr3

Cr1bCr4

Cr2Cl

10th Asian Regional Conference of IAEG (2015)<Legend>
Cr1aCr3

Cr1bCr4

Cr2Cl
Photo. 3 Slip surface ③

Strongly crushed shale rocks are distributed at upstream area. The slip surface is partially harmonic withbedding planes.

Photo. 4 Boundary of the shale layer
Some clay layers are intermittently distributed along the layer.

410th Asian Regional Conference of IAEG (2015)

toward upstream side corresponding to upper layer.
Moreover, rotated rock lumps, kink and shear
deformation by landslide moving along bedding
plane have been found as grade of crushingincrease.
Characteristics of slip surface ① ～ ③ areshown below (Table. 1).
Table. 1
Slip
surface

tuffaceous shale are rarely deformed and rotated,including thin crushed zone.
4.2 Common characters of slip surfaces
We considered about common features oflithofacies, grade of crushed rocks and slip surfaceof ① and ② existing in borehole cores,focussing on the depth of boring No. 5 and 2 by thestrike and dip of bedding planes on the slope.
<Slip surface ①>・Boring No.5 : the depth of around 25 meters
・Boring No.2 : the depth of around 33 meters
・The slip surface is distributed around the upper
part of sandstone.
・It occurs as several centimeters thick crushed zone
with some planer structures.
・Rocks of just upper and lower layer of the slip
surface remain the grade of Cr1a or nevercrushed.
<Slip surface ②>・Boring No.5 : the depth of around 15 meters
・Boring No.2 : the depth of around 20 meters
・The slip surface is distributed around the lower
part of tuffaceous shale.
・Rocks around the slip surface are crushed to thegrade of Cr2 or Cr3.
・The contrast between upper and lower side ofgrade of crushed rocks is viewed quite clearly.
Compared with the features of slip surfaces on
outcrops of the slope, the features of slip surfaces inborehole cores seems to be less crushed in general.
4.3 Evaluation of the probability of slip surfaces
It has been quite difficult to discriminate
landslide bodies especially like as slip surface ①
from fault rocks only by the previous method of
observation at borehole cores. Then it will be
helpful to evaluate of the probability of slip
surfaces by careful examination about quality andcontinuity of geological fracture.
(1) As geological features in this area, relatively
continuous planes are distributed. And it iseffective to examine about distribution of them.
Layers of sandstone and tuffaceous shale arethought to be a factor of parting top anddownstream side of the landslide. Because shale
is softer than another rocks in this area, a crushedzone has been easily formed around boundaries
between shale and another rocks. Surveys about
factors of parting bottom and upstream side of thelandslide are in progress.
(2) It is important to examine about quality andcontinuity of geological fracture especially along
bedding planes. A slip surface is thought not to bewholly crushed in early stages that totaldisplacement of the landslide has been very small.
Slip surfaces in this case are easy to be evaluatedfault rocks by mistake because there is somepossibility that some planer structures areCharacteristics of slip sufaces
Characteristics(on outcrops of the slope)①・clay layer distributed within shale
just upper layer of sandstone・almost 15centimeters thick・slightly deformed shale rocks
・harmonic with bedding planes②

・clay layer distributed between shale
and tuffaceous shale・ including black clay, several
centimeters thick・crushed tuffaceous shale rocks into
block and gravel・harmonic with bedding planes③・gravel and soil layer within shale・ strongly crushed shale rocks at
upstream area
・ partially harmonic with beddingplanes

4.

Notes of observation at borehole core
We compared high-quality cores No.5 and 2(Table. 2, Photo. 5～10) with outcrops of the slope.
Then we considered about notes of observation at
borehole cores.
4.1 Characteristics of borehole cores
Shale, tuffaceous shale and sandstone aremainly distributed under boring No. 5. The
tuffaceous shale ranges in depth from around 14 to
16 meters. And the range of sandstone extends fromthe depth around 28 to 37 meters. In terms of gradeof crushed rocks, the upper layer of tuffaceous
shale is slightly deformed to the grade of Cr1a or
Cr1b with some open cracks, partially crushed to
the grade of Cr2 as brecciated rock having themaximum particle diameter of around 20
millimeters. On the other hand, rocks in the lower
layer of tuffaceous shale are rarely deformed androtated with few oxidized cracks. Almost all therocks of this layer remain the grade of Cr1a ornever crushed, including several centimeters thickcrushed zone with some planer structures.
As the same situation as No. 5, grade of crushedrocks of boring No. 2 seems to be changed at theborder of tuffaceous shale that extends in the depth
around 20 meters. Rocks in the lower layer of5

10th Asian Regional Conference of IAEG (2015)

Table. 2 Grade of crushed rocks of the outcrop of the slope and borehole cores
Grade

Characteristic

Cr1a・some open cracks
・very little oxidized
・very small amount ofsand and soil alongcracks
・slightly deformed

Cr1bOutcrop of the slope

Borehole Cores

No.5 26.10-26.25m

・some open cracks
・slightly oxidized
・filled with sand andsoil along cracks
・slightly deformed
No.2 30.75-30.90m

Cr2・mainly gravel
・5 ～ 30mm or larger
in diameter・clearly rotated andshear deformation
No.2 20.70-20.85m

Cr3・gravel and soil
・about 5 ～ 10mm indiameter・strongly crushed
・wet and soft soil
No.1 21.80-22.00m

Cr4・mainly soil
・about 1 ～ 5mm indiameter・strongly crushed
・wet and soft soil
No.1 35.50m

Cl・mainly clay
・including sand andsilt
・spring water comesout sometimes in therain

No.2 5.40m

610th Asian Regional Conference of IAEG (2015)
lithofacies

grade of crushed rocks

oxidation・slightly deformed to the grade of Cr1a or
Cr1b with some open cracks
・partially crushed to the grade of Cr2 as
brecciated rock having the maximum
particle diameter of around 20
millimeters

Photo. 6 Borehole core No.5 (GL-15m)
・the slip surface is distributed around the lower
part of tuffaceous shale・rocks around the slip surface are crushed tothe grade of Cr2 or Cr3・rarely deformed and rotated with few
oxidized cracks
・almost all the rocks of this layer remain
the grade of Cr1a or never crushed<Legend>Gravel and Soilnot crushed

Sandstone

Cr1a, Cr1b

Tuffaceous shale

Cr2Shale

Cr3

TuffCr4

Photo. 7 Borehole core No.5 (GL-25m)・several centimeters thick crushed zone withsome planer structures

Photo. 5 Borehole core No. 57

10th Asian Regional Conference of IAEG (2015)

lithofacies

grade of crushed rocks

・the grade of Cr1b with some opencracks

oxidation

Photo. 9 Borehole core No.2 (GL-21m)
・crushed to the grade of Cr2-3 as brecciatedrock having the maximum particle diameterof around 10 millimeters・almost all the rocks of this layer remain
the grade of Cr1a or never crushed

Photo. 10

Borehole core No.2 (GL-33m)

・crushed zone with some planer structures
5.

Conclusions
As a result of the observations, there is apossibility that influence of crushing such as
bedding or joint planes and the progress and extent
of landslide movement appears on distribution andcharacteristics of crushed rocks. Therefore, it iseffective to considerate about continuity of thepreviously mentioned geological factor foranalyzing mechanism of landslides. It is importantto survey and examine carefully about distribution
and characteristics of crushed rocks along bedding
planes especially if relatively continuous planes had
been distributed like in this area.

<Legend>Gravel and Soilnot crushed

Sandstone

Cr1a, Cr1b

Tuffaceous shale

Cr2Shale
Tuff

Cr3
Cr4

Photo. 8 Borehole core No. 2

Acknowledgements

included in slip surfaces and landslide bodies.
Therefore it is necessary to examine about quality
and continuity of geological fracture by observing
more than one borehole core. In addition, it isquite important to analyze about consistency
between topographic features and grade ofcrushed rocks, considering about generation anddevelopment of the landslides.
Some slight displacements have been recordedat the depth of slip surface ① or ②. Surveys
about the mechanism of the landslides are inprogress, keeping the previous mentioned notes inmind.

The authors are grateful to the members of
Kagawa Prefecture and the member of Public
Works Research Institute, who gave the technicalassistance of this Landslide.
ReferencesWakizaka, Y., Kozuma, M.,Watatani, H.,Toyoguchi, Y. (2012): Characteristics of Crushed
Rocks in a Landslide Body : An Example of aLandslide Located in the Shimanto Belt，Journalof Japan Society of Engineering Geology ，Vol.52(2011-2012), No.6, pp.231-247

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