京 都 大 学 防 災 研 究 所 年 報 第47号 C 平成16年3月
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 47 C, 2004

Geo-disaster Prediction and Geo-hazard Mapping in Urban and Surrounding Areas

Susumu IAI, Shinya INAZUMI*, Masahiro CHIGIRA, Toshitaka KAMAI,
Roy, C. SIDLE, Mamoru MIMURA, Hiroshi SUWA, Takashi SAITO,
and Tetsuo TOBITA* Graduate School of Engineering, Department of Urban Management, Kyoto University(formerly, COE researcher, DPRI, Kyoto University)

Synopsis
Urban development rapidly expanding from lowland to surrounding hills andmountains poses increasing risks in geo-hazards, including liquefaction during
earthquakes, and failure of artificial and natural slopes. Objective of this study is todevelop methodologies for assessing vulnerability to these hazards, and technologies for
improving the performance of geotechnical works in urban areas. This paper
summarizes the results of the study with respect to (1) performance of retaining walls at
waste fill in waterfront areas, (2) hazard mapping of natural slope failures throughmonitoring using laser scanners, and (3) hazard assessment of residential areas in valley
fills and hills.
Keywords: environment, geo-disasters, hazard mapping, hills, mountains

1. Introduction
Urban areas were highly developed in lowland
and rapidly expanding toward surrounding hills andmountains. The rapid expansion causes increasing
hazards associated with geological and geotechnicalprocess. In particular, liquefaction during earthquakes
causes potential risks to infrastructures in urban areas.
Failure of artificial and natural slopes poses potential
threats to residents and communities. Objective of
this study is to develop methodologies for assessing
vulnerability to these hazards, and to propose
techniques for improving the performance ofgeotechnical works in urban areas.
This paper summarizes the results focusing on
the following priority issues.
(1) Waste fills in waterfront areas have been rapidly
expanding and pose new threats due to failure or
mal-function of retaining walls. Vulnerability ofthese retaining walls against waves, tsunamis, andearthquakes should be appropriately evaluated andnew techniques for improving the performance of
these walls should be proposed.
(2) Prediction of shallow landslides was difficult
because of site-by-site variations in geotechnicalconditions. However, occurrence of shallow
landslides is widespread, and poses a serious threat
to residents and communities in the area. A new
methodology should be developed for assessing
susceptibility of shallow landslides to mitigate
landslide hazard.
(3) Valley fills and artificial slopes have been
increasing for developing new residential areas
conveniently located to large cities. Slope failuresin these valley fills and hills pose immediate
threats to residents. Study should be performed onthese aspects of geo-hazards.

2. Performance of steel pipe sheet piles in coastal
landfill site
2.1 Introduction
Due to strict environmental requirements anddifficulties in land expropriation in Japan for inland
landfill construction, coastal waste disposal landfills
are increasingly being adopted and require large scale
reclamations in ports and harbors near urban areas. In
design of vertical cutoff barriers, conservation of port
and harbor area as well as quality of waste material isconsidered. The cutoff must have both mechanicalstrength and low hydraulic conductivity in order tocurb leakage of leachate from contained wastematerial to the open sea. Tokyo metropolitan city, forinstance, disposes 84.7% of its waste into coastal
waste disposal facilities in the sea and man-madeislands. Coastal waste disposal facilities are in threedesigns: (a) rubble mound seawall, (b) caisson wall,and (c) double wall cutoff steel pipe sheet piles(SPSP), as shown in Fig. 1. The rubble mound ismainly for stable (non-toxic) waste material but cansuffer loss of riprap which leads to erosion of the toe
by wave action and requires a large area since it must
slope gently for stability purposes. Caisson type is for
toxic waste materials; however, caisson has a shallow
penetration depth. SPSP has good penetration depth,
cheap and easy to construct as a cutoff barrier incoastal waste disposal landfills, but at the current
state, there is a high risk of leakage of the leachatefrom contained waste through traditional SPSP joints.
Traditional joints in SPSP include L-T, P-P, and P-T
joints as shown in Fig. 2; among their shortcomings
are as follows:(1) The joints must be grouted with mortar for
sealing and increasing strength of the joint. The
grouting process causes pollution leading toenvironmental degradation.
(2) In the current practice, mortar is pumped into thejoints after installation of SPSP is complete; in thisconstruction procedure, it is difficult to control thequality of grouting, hence, existence of uncertainty
Geotextile membrane
L.W.L
(a)

RiprapTie rod

H.W.L

Armor Blocks
Improved ground
1：2

L.W.L

W.L
Waste Material

RubbleSeabed

L.W.L

(b)

H.W.L

CaissonRubble bedding

H.W.L
Improved groundTie rod
Backfillsoilwith regard to effectiveness of such treatment.
(3) Further, since the joints are submerged in a water
body, injected mortar is packed to prevent
segregation and the eventual dilution by thesurrounding water body; this procedure increases
the hydraulic conductivity of the grouted joints.
(4) Excessive displacements and inclinations cause
crushing of the traditional joints, especially,
towards the tip of the piles making it near
impossible to seal.
(5) SPSP with traditional joints have low bending
rigidity owing to vertical shear movements that
occur at the joints under lateral load.
(6) Only one pile is installed at a time leading to
unnecessary long construction periods and highoperation costs.
With the backdrop of these numerous problems
in SPSP with traditional joints, Kamon et al. (2001)
assessed leachate migration paths in double wall
SPSP applied as cutoff material in coastal waste
disposal landfills. They proposed that a hydraulicconductivity of 10-8 cm/s is required of SPSP toeffectively prevent leakage of leachate. To maintainthis hydraulic conductivity over the whole SPSP wall,
waterproof treatment of SPSP joints must be done toachieve hydraulic conductivity of the order 10-7 cm/s
(Kamon et al., 2001). It is difficult to meet therequired low hydraulic conductivity value using the
traditional joints filled with mortar, Kimura et al.
(2003) have therefore developed H-joint SPSP andproposed H-H joints to alternate in series with the Hjoints. In this chapter, H-joint and H-H joint aredescribed, evaluation of the hydraulic conductivity ofH-jointed SPSP with H-H joints is done byexperiments. Water-swelling sheets are used to seal
the H-H joints.
2.2 Developed H-joint
Kimura et al. (2003) in an innovative contribution
developed a new H-joint from a simple idea in which
two steel pipe piles are connected by H-steel section
welded on them as shown in Fig. 3. The H-steel
section is what is referred to as the H-joint and will
still have to alternate in series with the traditional
joints pending development of another suitable joint.
The connected piles are easily driven simultaneously
by any of the existing pile driving methods. Among

Sandproof sheet
Gravel CoverSteel sheet pile

Waterproof
material
BackfillgravelWaste Material
1
Gravel Mat ：2

Gravel Cover
Waste Material
Steel pipe sheet piles(c)

P-P joint

Fig. 1 Types of cutoff walls for coastal waste
landfills commonly used in Japan

P-T joint

L-T joint

Fig. 2 Types of traditional joints with water-paths

advantages of H-joint piles are as follows:(1) H-joint SPSP have high bending rigidity because
H-steel section is welded rigidly and continuously
against two steel pipes. Kimura et al. (2003) haveshown by centrifugal modeling on rectangular Hjoint SPSP foundations that H-joint SPSP exhibits
higher lateral capacity than the SPSP withtraditional joint connections.
(2) Precision of construction is high, with anaccuracy of 3 mm in the horizontal direction; thisis because one side of H-joint pile has flat-likeshape that minimizes inclination of the pile during
driving (Kimura et al., 2004).
(3) H-joint is completely water-proof because of thecontinuous welding, and can prevent backfill sandbehind a seashore protection retaining wall frombeing washed away by water undercurrents.
(4) The use of the H-joint is environmentallyfriendly since it reduces the number of grouted
joints.
(5) Two connected piles are driven simultaneously
thus reducing the number of driven piles resulting
in shortened construction period and reduction in
operation costs (Kimura et al., 2004).
(6) The H-joint increases strength per unit length
which means smaller foundation dimensions canbe designed hence less amount of steel material isrequired.
Clearly, the H-joint satisfies permeability and bearing
capacity requirements. The only point ofcontention is the fact that it will still alternate withthe traditional joints. The authors are proposing HH joint, described below, to alleviate this condition.

a set of H-jointed SPSP

Steel pipe

welded

P-P joint(a) H-joint alternating with P-P joint

H-joint

2.3 Proposed H-H joint
Traditionally, when H-joint SPSP is used as avertical cutoff barrier, two adjacent H-joint SPSP is
still continuously connected by traditional joint
connection, such as P-P joint. Although the H-joint
SPSP itself can exhibit complete impermeability, therisk of water leakage exists in the traditional P-P joint
part between two H-joint SPSPs. Therefore, theauthors have proposed “H-H joint”, which isdesigned to use H-steel sections with different sizes
to replace the traditional joint part between two Hjoint SPSPs, as shown in Fig. 4. The H-H joint isformed from two H-steel sections of different sizes;
their web heights and flange width sizes are such thatthe two H-steel sections just fit into one another. One
H-steel section is welded on one side of a H-joint
SPSP. Two such H-joint SPSPs are connected toform the H-H joint by driving a second SPSP
adjacent to the first one so that their H-steel sections
interlock. The H-H joint SPSP does not only maintainthe advantages of H-joint SPSP, but also possesses
enough waterproof property by using a waterswelling paint coated onto the H-H joint connection
instead of filling mortar. That is, the H-H joint SPSP
improves the joint part of two adjacent H-joint SPSPwith a H-H steel joint connection, as a result, much
more waterproof property than the traditional joint
has been achieved. Furthermore, the steel ribs are
installed in order to improve the tension resistance ofthe H-H joint connection, as shown in Fig. 4.
2.4 Permeability test on sealed H-H joint
The object is to evaluate the performance of H-H
joint sealed by water-swelling paint sheet when
exposed to 0.02 - 0.5 MPa water pressure in a coastal
waste disposal facility; it is done by carrying out
hydraulic conductivity tests. The thickness of thepaint coat affects its swelling volume and theresulting swelling force; this determines waterproof
strength of the paint at the H-H connection andtherefore a suitable thickness of the paint coat thatwill produce required pressure resistance will be
estimated.
The water-swelling paint swells when it comesin contact with water; it is a solid with fluidity
blending of water-absorbing polymer, filling-agent,and fluxing material into synthetic-resin elastomer as
B

H jointed SPSP
H-H jointwater-swelling paint sheet/coat(b) Photograph H-jointed SPSP
Fig. 3 H-joint SPSP

steel ribInterlocking
surfaceFig. 4 The proposed H-H joint and slip restraining ribs

a base material. Water extracted from dried coating
film of the water-swelling paint has been tested andfound to satisfy the standard of water-purity based on
the food hygiene law, and to suit the reduction ofenvironmental impacts. The water-swelling paintused in the experiment begins to swell after 1 to 2
hours after it comes in contact with water, and thevolume will be increased to about 20 times of theoriginal volume after 24 hours achieving a hydraulicconductivity of 1.42 x 10-9 cm/s.
The H-H joint specimen was welded at thebottom and the sides of a 450 x 350 x 300 mm steel
chamber shown in Fig. 5 so that water paths were
only at the H interlocking surfaces. Rubber packing
placed over the top of the chamber is for ensuring
watertight conditions in the chamber. The steel plateis bolted over the rubber covered chamber. The
influent flow and collected effluent flow wasmeasured by the calibrated water reservoir and by
weight using a weighing machine shown in Fig. 5,
respectively.
In Japan, the specified standard hydraulicconductivity of waterproofing structures in coastal
waste disposal landfills is 10-6 cm/s; it is based on
hydraulic conductivity of a 50 cm thick soil layer.
Since the current standard do not expressly specify
standards for SPSP as cutoff walls in waste disposal
landfills, the measured hydraulic conductivity of the
SPSP are thus converted to equivalent hydraulicconductivity for an homogenous soil layer whose
dimensions are equal to those used to develop theJapanese hydraulic conductivity (k ≤ 10-6 cm/s )standard as shown in Fig. 6.

2.5 Permeability of H-jointed SPSP with H-H joint
Figure 7 shows measured hydraulic conductivity
for H-jointed SPSP with H-H joint sealed with waterswelling paint sheets of thickness 1, 2, and 3 mmagainst water pressure; also shown are equivalenthydraulic conductivity for P-T joints sealed by mortar
and rubber (Oki et al., 2003). It is observed that ke for
H-H joints depends on water pressure; it increases
with increasing water pressure, this may be becauseof the elastic nature of the water-swelling paint sheet.
Paint sheets with thickness of 2 mm and above meet
specified hydraulic conductivity of ke ≤ 10-6 cm/s
(Kamon and Inui, 2002) for water exclusion
structures up to water pressures of 0.5 MPa. On theother hand a 1 mm paint coat meets the standard ke of≤ 10-6 cm/s up to a water pressure of 0.4 MPa.
In coastal waste disposal landfills, maximum
water level difference between the contained waterand the outer sea level must not exceed 2 m (Kamonand Inui, 2002); this level is controlled during heavy
rains, water tide variations, and high water waves.
Reports, however, indicate that some landfills havebeen filled to 5 m way beyond the specified limit of 2
m water level difference. It is important therefore toevaluate ke of H-jointed SPSP with H-H joints forapplication as a cutoff material in waste disposal
landfills based on the 5 m (equivalent to 0.05 MPa∆h
L = 45 cm

φ = 100 cm
B = 275 cm
air pressure
regulator

effluent line

H 400 x 400

bolt

Q [cm3/s]
Conversion

steel plate(a) H-jointed SPSP with H-H joint

rubber packing

reservoir

300
pressure gageinfluent line

450

∆h

350

(a) Schematic diagram of hydraulic conductivity test set-up (Unit: mm)
50 cm = T
welded

50

L

55.5
20
11

8
20

water-swelling sheet: Effluent port

B

: Air valve

Q

ke =

: Bolt50( i ) Aerial viewke

: Influent port
10

Q ⋅T
[cm/s]B ⋅ L ⋅ ∆h(b) Equivalent homogeneous soil layer( ii ) Interlocking details(b) Schematic model aerial view and H-H joint interlocking details

Fig. 5 Hydraulic conductivity test layout, model
aerial view and details of H-H joint interlock

Fig. 6 Conversion of the size of H-jointed SPSP with
H-H
jointsintoanequivalenthomogeneous soil layer

water pressure ) rather than the specified 2 m(equivalent to 0.02 MPa) water level difference.
From Fig. 7, ke for H-H joints coated with 1 and
2 mm paint sheets was very low in the order of 10-8
cm/s at a water pressure of 0.05 MPa; ke for the 3 mm
paint coated joint was too small and could not bemeasured, since there was not effluent flow collectedin the 6 hour test duration, ke can be said to be below
1 x 10-9 cm/s. The authors propose use of ke = 10-8
cm/s in design of H-H joints sealed with this waterswelling paint.
When compared with findings of Oki et al.
(2003), at 0.05 MPa the H-H joint has ke of 2-orderdifference compared with the P-T joint sealed withmortar. The P-T joint sealed with mortar and rubber
membrane is found to have ke = 10-8 cm/s like wasthe case with H-H joint, however, this value is basedon laboratory findings in which application of mortar
and the rubber membrane can easily be controlled;
the case may not necessarily be true in actual
construction practice because P-T joint treatment isdone after construction and SPSP with this joint have
low installation accuracy. Based on the advantages ofthe H-H joint SPSP mentioned in section 2.3 andbacked by the measured low hydraulic conductivity
values, the use of H-jointed SPSP with H-H joints
sealed by water-swelling paint is highly
recommended as a cutoff material in coastal waste
disposal landfills, shoreline retaining walls, and in theconstruction of bridge pier SPSP foundation among
other applications.
In this study the H-H interlock surfaces weresealed using water-swelling sheets, however, othermethods of applying the water-swelling paint exist
namely applying it in the form of paste. Unlike usinga paint sheet which can easily peel off during
construction, the pasted water-swelling materialbonds more strongly with the steel material than the
sheet type hence recommended.
● P-T joint with mortar filling◆ P-T joint with rubber plate and(Oki et al., 2003)

mortar filling (Oki et al., 2003)

mortar filling

Equivalent hydraulic conductivity [cm/s]

rubber platemortar filling

10-5
Technical standard for vertical cutoff barrier
Coating thickness(ke ≤ 1 x 10-6 cm/s)
1 mm

10-6
2 mm

10-7

3 mm

10-8

10-9
0

0.1

0.2

0.3

0.4

0.5

Water pressure [MPa]Fig. 7 Equivalent hydraulic conductivity versus

resisted water pressure

0.6

3. Proposal of landslide susceptibility index inmountainous areas by using new technique,airborne laser altimetry
Shallow landslides have occurred in many
locationscontemporaneouslybyrainstorms,
indicating that there is a kind of susceptibility that ischaracteristics to geology and geomorphology. We
have successfully applied airborne laser scanning(ALS) technique to detect shallow landslides in
granite and ignimbrite areas, of which results gives us
a basis for the landslide-susceptibility evaluation in awide area.
The areas investigated, Nishimikawa, Fukushima,and Hiroshima, experienced severe rainstorminduced landslide disasters in 1972, 1998, and 1999
respectively, and ALS performed after the disasters
extracted many old landslide scars before thesedisasters as well as new landslide scars that areclearly observed on aerial photographs after the
events, indicating that these areas could have beenrecognized as susceptible areas to landslides before
these disaster events. The landslide densities per
square kilometers of slopes steeper than 20 ゜ in
Nishimikawa granite area were 114 before the 1972
disaster and 293 by the 1972 event. The landslidedensity per square kilometers of slopes steeper than
20゜in Fukushima disaster area was 117 per square
kilometers before the 1999 disaster and 115 by thedisaster. The landslide densities, which could be
defined as above by using ALS, thus provide
landslide susceptibility.
3.1 Case study for the 1998 Fukushima rainstorm
hazard – Identification of shallow landslides andhazard assessment in an area of ignimbrite
overlain by permeable pyroclastics–
We applied the airborne laser scanner techniqueto the Nishigo area of Fukushima rainstorm disasterthat occurred from 26 to 31 August 1998 mainly by
shallow landslide. More than a thousand landslides
occurred on slopes of vapor-phase crystallized
ignimbrite overlain by scoria, pumice, and ash.
During the time of this disaster, five people were
killed before dawn in a welfare house, which became
a turning point for the Japanese government toestablish a new law for the disaster prevention
against slope movement hazard with the following
disaster occurred in Hiroshima, western Japan in
1999.
We identified landslides during the 1998 disasteron aerial photographs with a scale of 1:8,000 and also
by ground-truth geological investigation (Chigira,2002; Chigira et al, 2002). Airborne laser scanningwas performed for an area of 2.5 km2 within the 1998
disaster area of Fukushima in late April, 2002. The
system was ALTM1225. The ALTM sensor aboard ahelicopter AS350B operated at 25000 pulses/s at awavelength of 1064 nm with scanning angles 10 20 ゜ . This technique provides highly precise

horizontal (approximately 0.5 m) and vertical heightlocations (approximately 0.15 m). From the above
data, 1-m grid DTM was made and a topographic
map with 1-m contour interval was made.
(1) Geology and geomorphology
Small plateaus, about 60 to 100 m high above thenearby fluvial plains, occupy a wide area of Nishigo
Village. The tops of the plateaus are essentiallydepositional surfaces of pyroclastic flow covered bythin tephra layers. The bedrock of the plateaus is theearly Quaternary Shirakawa pyroclastic flow,
overlain by thin beds of pyroclastics, which arethought to have erupted from Nasu volcano 200 to
350 thousand years ago. The Shirakawa pyroclastic
flow has been divided into several flow units, and theflow in and around the investigated area has been
fission track dated to 780 ka, and consists of dacitic
ignimbrite (tuff), which is a weakly consolidated,massive, and intact tuff which had been subjected tovapor-phase crystallization. This vapor-phasecrystallized tuff has characteristic weathering profile,which consists of hydrated, exfoliated, anddisintegrated zones toward the ground surface(Chigira et al., 2002). The tuff of hydrated zone issignificantly deteriorated but is still massive. The tuff
of exfoliated zone is exfoliated to slope-parallel
plates or lenses a few to 5 cm thick. The disintegrated
tuff has lost its original rock structure and is soil-like.
The tephra layers overlying the Shirakawa
pyroclastic flow are exposed in landslide scars andartificial cuttings; elsewhere they are covered byvegetation. These beds, which consist of mudflow
deposits (diamicton made up of tuffaceous fines andandesite blocks) and air-fall deposits of scoria,
pumice, and ash, are nearly horizontal, hence theiroutcrop traces fall along contour lines. The
thicknesses of the mudflow deposits vary from 2 m to
20 m. The total thickness of the scoria and pumice
beds varies from 0.5 m to 3.5 m, and the thickness of
the ash occupying the top of the plateau is notprecisely known but is estimated to be up to 10 m.
The permeabilities of the underlying tuff is very low
in comparison with the overlying tephra layers,which was one of the major causes of the landslides.
(2) Geomorphological and geological features of thelandslides generated by the 1998 rainstorm andprevious
landslides
Aerial photographs taken from 10 to 11
September 1998 revealed that 203 landslides
occurred within the laser-scanned area at the time ofthis disaster, but it is not easy to recognize whether
previous landslide scars were present or not on theaerial photographs (Fig. 8A). Three dimensional
images and maps made by laser scanner, however,clearly show previous landslide scars as well as thoseduring this event (Fig. 8B, C). One hundred and two
previous landslide scars are identified on the basis ofFig. 8 Aerial photograph (A, Kokusai Kogyo,co.), three dimensional image (B Asia Air
Survey co.), and maps (C and D) made byusing airborne laser scanner, near the welfare
facility, Taiyo no Kuni in Fukushima.
Airborne laser scanner can identify previous
landslides hidden by trees. Blue areas
indicate landslide scars of 1998 event andgreen areas indicate landslide scars before
this event. (Chigira et al., in preparation)

morphological characteristics, which will be
described in the following.
As has been reported by Chigira (2002) andChigira et al. (2002), three types of landslide
occurred during this disaster. They are landslides ofpyroclastic deposits overlying weakly consolidated
ignimbrite, landslides of weathered tuff andcolluvium, and landslides of depression fill. Thesetypes of landslide strongly reflected thehydrogeological structure. Typical morphology ofthese landslides is shown as laser-scanner maps andprofiles in Fig. 9, in which profiles made by the laser
scanner and by ground survey are both shown.
scanner and by ground survey are both shown.
1) Landslides of pyroclastic deposits overlying
weakly consolidated ignimbrite
These landslides involved pyroclastic air-falldeposits overlying diamicton of mudflow deposits on
weakly consolidated ignimbrite as well as debris on
top. They were aligned along the trace of the air-falldeposits on gentle convex breaks along the peripheryof the small plateaus. The hydrogeological structure,that is permeable beds overlies impermeable rock,effected the infiltrating behavior of rainwater:
rainwater on small plateaus firstly
infiltrates vertically and then flows
laterally within permeable air-falldeposits and finally gushing out at the
periphery of the plateaus to cause
landslide.
The geometry of this type oflandslide is clearly shown on a map
made by laser scanner (Fig. 9A). The
landslide scar of this type of landslide
is an amphitheater or bowl-like
depression and the ground surface
along the path of debris transportation
was essentially not eroded. The size ofthe bowl varies from 5 to 15 m indiameter and a few to 5 m in depth.
Mudflow deposits usually are exposedat the bottom of the bowl, and air-falldeposits crop out in the upper part ofthe scar. The lower part of the bowl isvery gentle and inclines from 5˚ to 15˚and the upper part 30˚ to 45˚. The
height of the head scarps varied from a
few to 5 m.
2) Landslides of weathered tuff andcolluviumThis type of landslide occurred on
weakly consolidated ignimbrite withthe sliding surface within the exfoliatedzone, which forms the bottom of theintensively weathered zone (Chigira etal., 2002). This type of landslide
stripped away most of the slidematerial, leaving bright-color rocksurface of exfoliated zone at the landslide scar. The
scars dip 25-45˚ with an average of 32˚, a few to 5
degrees steeper than the juxtaposing slope. The
landslide scar itself is planar along the exfoliatedzone (Fig. 9B), in contrast to the bowl-likemorphology of the landslide scars of air-fallpyroclastics on the mudflow and the ignimbrite as isshown in contour maps and cross sections made by
the air-born laser scanner. In comparison with thecross section made by ground survey, the crosssection made from laser scanner gives somehowrounded edge of the top of landslide scar, but theshape of landslide scar is generally clearly detected.
The size of the scar varies from 5 to 30 m in widthand length and less than a few meters in the depth.
3) Landslides of depression fill
The intense rainfall also triggered failure ofcolluviums or ash that filled depressions, although
this type of failure seemed less common than theother two types. A typical example of this was alandslide that occurred near Daishin Junior HighSchool, 15 km northeast of Taiyo-no Kuni, where thelandslide, leaving a scar 5 m deep and 80 m long,exposed a former buried stream again. The hollow

Fig. 9 Laser scanner maps and profiles of three types oflandslides that occurred in Nishigo village by the 1998 disaster(Chigira et al., in preparation). Contour interval is 1 m. Top:
Landslides of pyroclastic deposits overlying weakly
consolidated ignimbrite; middle: Landslides of weathered tuff
and colluvium; bottom: Landslides of depression fill. Upper
left profiles are made by ground survey.

upslope of this landslide had old depressions a few
meters in diameter and less than 1 m deep, indicatingthat underground erosion and ground settlement
preceded the landslide. The gradient of this hollow
before the slide is assumed to be 15 ゜ from thefollowing survey. In addition, this old depression
fill was cut artificially for construction at the lower
part of the slope and this landslide seems to havestarted at this cutting.
Fig. 9C shows the laser scanner map of this typeof landslide. Two landslides of depression filloccurred on slopes above knick points of hollows
located at the heads of ravines, retrogressing the
knick points upslope (cross section of Fig. 9C). The
gradients of the slopes with underground erosion
awere 12゜ and 15゜.
3.2 Granitic rock areas
Granitic rocks have been experiencing manylandslides by rainstorms in Japan (Fig. 10, Chigira,2001). We applied airborne laser scanner to Obara
village, central Japan, where devastating disasteroccurred by large number of landslides during therainstorm with the precipitation of 238 mm per 6
hours in July 1972. By examining the map incomparison with the aerial photographs taken after
the disasters, we identified many landslide scars that
occurred at the time of the disaster (Fig. 11, Tobe andChigira, 2004). We also
recognized many old
landslide scars that had
already existed at the timeof the disasters but had
been difficult to identify
in aerial photographs.
This ALS landslide map
and geologic map that we
made by intensive fieldsurvey clearly showed thestrongcontroloflandslide density by rocktypes as has been reported
by Yairi et al. (1973).
The landslide densities
per square kilometers ofslopes steeper than 20 ゜
were 114 before the 1972
event, 293 by the 1972
event, and 408 altogether,respectively.
Corresponding landslidedensities in granodiorite
area were 28, 13, and 41.
These numbers clearly
indicates that we could
have known that thisgranite area is very
susceptible to landslide.

3.3 Areal susceptibility evaluation from theairborne laser scanner
As has been described, airborne laser scannerperformed under the good condition, say less tree
leaves, can identify old landslides while aerial
photographs can only detect new landslides if they
are not large. The occurrence of an old landslide ishard to be dated, but landslide distribution for long
periods records the susceptibility of a given area
against landslide. For example, Fukushima disasterarea had a landslide density of at least 117 within one
km2 of slopes steeper than 20 degrees before the1998 event and had a new landslide density of
233/km2 at the event. These numbers are quite large
and on about the same order of the landslide densityof devastating disaster that occurred in weathered
granite area of Obara village, central Japan, which
had a landslide density of 294/km2 at the event and at
least 114/km2 before the event (Tobe and Chigira
2004; Duan et al., in preparation). The repetition ofshallow landslides must be closely related to the rateof production of slide materials, that is mainly byweathering, but constantly high landslide density
suggests high susceptibility.

Fig. 10 Landslide disasters in granitic rock areas (Chigira, 2001)

4. Landslide hazard prediction in residential areasof hillside
4.1 Neural-network for hazard mapping of valley
fills

GraniteFig. 11 Laser scanner map in Obara village (upper)
and aerial photograph of the same area (lower, Aero
Asahi Co.). Landslides in blue color are by the 1972
disaster event and those in green color are previous
ones. (Tobe and Chigira, 2004)(1) Background
Recent destructive earthquakes in urban regions,such as the 1978 Miyagiken-oki earthquake, the 1993
Kushiro-oki earthquake and the 1995 Hyougokennanbu earthquake, have triggered landslides in many
gentle slopes of residential areas around Sendai,
Kushiro, Nishinomiya and Kobe. The earthquakeinduced slope instability that has occurred is closely
related to these artificial landforms, especially valley
fills (embankments). More than 60% of the unstableslopes in the Kobe-Nishinomiya urban region are in
artificial valley fills. This instability was caused bystrong ground movements during the 1995
Hyougoken-nanbu earthquake.
Investigation of past artificial landform changes
and multi-variate analysis of case studies of pastearthquake disasters show that differences in the
shape of fills, such as depth, width, inclination angleof the base, and cross-sectional form, may be the key
discriminating factors of slope instability.
Triggering mechanisms (e.g. earthquakes) need
to be considered in the analysis for accurate
estimation, however, it is difficult to include
earthquake parameters in such linear multi-variate
analysis (quantification theory II). Neural network
analysis is applied to assess large fill slope instability
in urban residential areas.
(2) Neural network model
The developed neural network model includes
both causative factors (shape of fills, groundwatercondition, age of construction) and the triggeringfactors (distance from the fault, moment magnitude,
direction to fault). The model consists of nine input
factors, twelve intermediate neurons, and two output
items (stable or unstable) as shown in Figure 12.
Back propagation method (BP method) was used toinvestigate most appropriate set of the weight ofconnections between the three layers of the model.
The control parameters of BP method, learning ratio,
number of intermediate neurons, and number oflearning times, were determined to minimize the total
error between the input and the out put.
Summation of square of weight between input
layer and intermediate layer represents the power of
influence of input factors. Each sum of square ofweight divided by total sum of square of weightshould be used as percentages contributed of the
input factors. The thickness of fills and direction tofault are the first (29.9%) and second (16.1%)
contributing factors as shown Table 1.

Fig. 12 Neural network structure of the prediction model

Table 1 Weight of bonds between input layer and intermediate layer (M-1〜12:neurons of intermediate layer)

Table 2 Ratio of correct interpretation by the neural network analysis(3) Results of the analysis (learning process)
Table 2 shows results of neural network analysis,that are number of samples using analysis, number ofsamples given correct answers, and ratio of correctinterpretation. The ratio of correct interpretation
higher than 90 % means high accuracy for detectionof instability of fills by strong
earthquake motion.
(4) Sensitivity analysis
The developed neural networkmodel was independently checkedagainst another data set (not used in thelearning process) and sensitivity analysis
was conducted. The results of analysis
on the independent data set show thehigh accuracy of the model remains thehigh ratio of correct interpretation evenless comparing to the results of learning
process.
The dependence between each input
factors
andoutputvalue
wasinvestigated by sensitivity analysis.

Value of a selected input factor changes graduallyduring that value of the other input factors were holdin average value. As shown Figure 13, instability ofvalley fill increases in the shape of less than 5 m inthickness, wider than 60 m, larger than 20 of ratio of
thickness vs. width, steeper than 15 degrees of baseangle at central part of valley, and the axis of valley

Fig. 13 Results of sensitivity analysis

perpendicular to fault. Earthquake greater than 7.2 ofmagnitude, and earthquake more than 80 km far fromvalley fill increase instability of the fill.
(5) Performance evaluation of the model
The three major earthquakes in 2003, theMiyagiken-oki earthquake in 26 May, the Miyagikenhokubu earthquake in 26 July, and the Tokachi-oki
earthquake in 26 September, should evaluate theperformance of the developed model. Eight fills innorthern part of Miyagi prefecture including fivestable cases were investigated by using high precision
surface wave exploration and dynamic penetration
tests (mini SPT tests). The N-value of the mini SPT
calibrated the results of the surface wave exploration(S-wave velocity).
The shape of valley fills determined by the crosssection of S-wave velocity of each fills was made as
input data to the model. The seven fills including four
stable cases were classified correctly; one stable casewas classified to be unstable one (incorrect answer)by using the developed model. Thus, the fineperformance and high reliability of the model was
shown in disasters by the two northern Miyagi
earthquakes.

case of the Tokachi-oki earthquake will be resulted
from the unexpected conditions of input factors thatare the both type of movement and earthquake. These
performance evaluations of the model suggest that
instability mechanism should be studied to be beyond
analysis of experiences including the neural network
analysis.
(6) Application to hazard mapping
The neural network analysis appears to haveadvantages over the multi-variate analysis because
the neural network model can represent a kind of
non-linear regression model to be fine performancewith limited number of samples (experiments) evenin complicated cases. The newly proposed neuralnetwork model can consider the triggering factors(earthquake parameters) for each expected rupture offaults. It means that the hazard mapping of valley
fills in urban residential areas should be possible toconduct by using the neural network model.
Figure 14 shows a result of hazard mapping ofvalley fill type landslides in Tokyo-Yokohama
district.
More than half of the fill-slopes should be classified
into a high-risk group. The regional difference of risk

Fig. 14 Maps showing the results of the stability assessment of valley-fill type landslides in TokyoYokohama district by using both the multivariate analysis and the neural network analysis
Several fills developed in Sapporo and Kushirourban regions became unstable by the Tokachi-oki
earthquake. The developed model checked the fourcases of these, however, all of them were interpreted
incorrectly. Cases in Sapporo were strongly related toliquefaction of limited shallow part of large valley fill,
and the landslide in Kushiro occurred in the part oflarge fill slope (not to be valley-fill type). The
distance from the rupture zone of the Tokachi-oki
earthquake was over 100 km at the both cities. It
means that the poor performance of the model in theon the fill-slope landslides is obviously depends on
geology. Vertical elevation drop between alluvial
plain and terrace surface controlled the depth ofvalleys in this district. Shallow dissected valleys
developed widely in the northern part from the TamaRiver (named the Musashino terrace) that is lower(younger) than the terraces in southern part (named
the Shimo-sueyoshi terrace, the Tama hills). Valley
fills in the Musashino terrace are shown in high-risk
compared to the fills in southern part because of its
shape of valley fills.

The landslide potential by heavy rainfall inTokyo-Yokohama district such as shallow collapse ofsoft material on seep slopes of the Kanto Loam will
decrease by the effects of protection works from1970’s that have still continued by the Government.
However, the risk at the earthquake, especially by theartificial valley fill type landslide, still remains, andincreases by the continuous human activity in this
district. It is thought that the loose and uncompacted
material of filling slopes with poor drainage systems
is responsible to the occurrence of the landslides.
Landslides associated with artificial valley fills will
be common in Tokyo-Yokohama district at the next
large earthquake disaster (2xxx Tokyo earthquake).
Many cities in Japan have almost the same conditions
at risk of urban landslides such as the Hanshin district
and Tokyo-Yokohama district; where, there is somesignificance to conduct hazard mapping on thepotential of urban landslide disaster.
4.2 Effects of duration of rainfall on landslides
Several damaging landslides and debris flows
on July 20, 2003, in southern Kyushu, Japan,
attracted international attention and resulted in one of
the major natural disasters of recent years. Largeamounts of rain fell on July 19-20 as a Baiu front
passing over the Sea of Japan met a high-pressurezone moving up from the southeast over the Pacific
Ocean. Altogether, 21 lives were lost related to thesediment disasters and more than 240 homes were
either damaged or destroyed by landslides, debris
flows and flooding. Nevertheless, such natural
disasters related to landslides and debris flows occur
frequently in Japan and local residents generally
acknowledge their potential exposure to thesehazards, but risk and vulnerability may becomplicated by other social and economic factors.
Initial news reports following the July 20th storm
suggested that only a few slope failures occurred.
While all fatalities occurred in two debris flows in
Minamata (Hogawachi and Fukagawa) and alandslide in Hishikari, damage by mass movements
was widespread in southern Kyushu. In the mostimpacted areas of Minamata (4 km2) and Hishikari (2km2), 37 and 14 landslides were identified,
respectively. Almost all landslides in the MinamataHishikara region were underlain by weathered
andesite, tuff-breccia, and tuff, and the slope
gradients of 20 inspected landslides were generally
only moderately steep (initiation zone 20 – 38°).
None of the debris flows that were examined in thechannels appeared to initiate as debris flows, nor did
they rapidly convert from landslides to debris flows
as has been suggested in other studies [Fannin andRollerson, 1993; Iverson et al., 1997]. Deeper
landslides typically traveled further downslope if
gradients were steep and often developed into debris
flows. Rainfall intensities during the 2- and 6-hperiods up to and including the time of mostdamaging landslides at Hishikari and Minamata werevery intense – 53 and 89 mm h-1, respectively. Total
storm precipitation prior to failure was very high at
Hishikari (< 337 mm; ≈ 3.5 km west of the fatallandslide), but not extraordinary in Minamata (< 265mm; ≈ 2 km from the two major debris flows). Only
moderate rainfall occurred at Hishikari and Minamata
on the day prior to the disasters and antecedent 10day rainfall was only 75 and 70 mm, respectively.
For the largest and most damaging landslide/debris
flow at Minamata (Figure 15), it appears that porewater pressure likely developed at the base of theweathered andesite within the limited space in asystem of interconnected fractures and interstices.
This probable scenario of rapid pore pressure
accretion and subsequent slope failure of therelatively deep regolith is in contrast to theoreticallyderived responses in homogenous porous media,
which suggest that thin soil mantles become unstable
during short-term, high-intensity storms, while
deeper regoliths fail during prolonged storms ofmoderate intensity [e.g., Haneberg, 1991]

Fig. 15

Landslide and debris flow at Hogawachi in

Minamata, Kyushu, on 20 July 2003; 15 people killed and13 homes destroyed.

5. Conclusions
Methodologies for assessing vulnerability to thegeo-hazards, and techniques for improving theperformance of geotechnical works in urban areas areinvestigated. Major results obtained from this study
may be summarized as follows.
(1) Steel sheet pile walls for retaining waste fill inwaterfront areas should meet the performancerequirements that should be structurally stable aswell as prevent the pore water migration throughthe wall. A new technique using the steel joint,called H-H joints, is proposed.
(2) Airborne laser scanning (ALS) techniqueachieves high precision in detecting shallow
landslides in granite and ignimbrite areas. Since
these shallow landslides occur repeatedly within aperiod of 100 years, density of landslides per unit
area obtained by the ALS technique can be a goodsusceptibility index of landslides.
(3) Neural-network technique is applicable to theprediction of failures in valley fills during
earthquakes.
(4) Some of the large damaging landslide/debris flow,such as those at Minamata in 2003, may be causedby the pore water pressure developed at the base of
the weathered andesite within the limited space ina system of interconnected fractures and interstices.
Acknowledgments
This paper summarizes the results of the COE
research performed by Geo-disaster Division, DPRI.
Primary part of this paper, i.e. Chapters 2 through 4,
is based on the contributions by the 2nd through 5th
authors. The first author compiled the paper as
project leader in 2003. The research presented in
Chapter 2 was performed under the supervision ofProf. Makoto Kimura, Graduate School ofEngineering, Kyoto University and Prof. Masashi
Kamon, Graduate School of Global Environmental
Studies, Kyoto University. The authors wish to
express their sincere gratitude to these professors.
This research was supported by the Japanese
Ministry of Education, Culture, Sports, Science andTechnology (MEXT) 21st Century COE Program for
DPRI, Kyoto University (No.14219301, Program
Leader: Prof. Yoshiaki Kawata) as well as the Grantin-Aid for Scientific Research (B) (2) (No. 15310129,
Principal Investigator: T. Kamai, Kyoto University).
This manuscript partly follows the instructions given
in IAHS Press (1995). The study in Chapter 3 wasalso funded by the Ministry of Education, Culture,
Sports, Science and Technology (MEXT) (Special
Coordination Fund for Promoting Science andTechnology, Aereal Prediction of Earthquake andRain Induced Flow Phenomena (APERIF Project),Principal Investigator: Kyoji SASSA) and the Japan
Landslide Society.
References
Chigira, M. (2001): Micro-sheeting of granite and its
relationship with landsliding specifically after theheavy rainstorm in June 1999, Hiroshima
Prefecture, Japan. Engineering Geology. 59，219231.
Chigira, M., (2002): Geologic factors contributing tolandslide generation in a pyroclastic area: August
1998 Nishigo Village, Japan. Geomorphology 46,
117-128.
Chigira, M., Duan, F., Yagi, H., and Furuya, T. (inpreparation) Identification of shallow landslides
and hazard assessment in an area of ignimbrite
overlain by permeable pyroclastics by using
airborne laser scanner – Case study for the 1998
Fukushima rainstorm hazard –Chigira, M., Nakamoto, M., Nakata, E., (2002):
Weathering mechanisms and their effects on thelandsliding of ignimbrite subject to vapor-phasecrystallization in the Shirakawa pyroclastic flow,
northern Japan. Engineering Geology 66, 111-126.
Duan, F. and Chigira, (in preparation) Mapping
shallow landslide and evaluating slope stability
using digital terrain analysis with high resolution
DTM data
Fannin, R.J., and T.P. Rollerson, (1993):Debris
flows: some physical characteristics and behaviour,
Can. Geotech., J. 30, 71-81.
Haneberg, W.C. (1991): Pore pressure diffusion andthe hydrologic response of nearly saturated, thinlandslide deposits to rainfall, J. Geol. 99, 886-892.
Iverson, R.M., M.E. Reid, and R.G. LaHusen, (1997):
Debris-flow mobilization from landslides, Annu.
Rev. Earth Planet. Sci. 25, 85-138.
Kamai,T., and H. Shuzui. (2002): Landslides in urban
region, Riko-tosho, 200. (in Japanese)
Kamai, T., Shuzui, H., Kasahara, R., and Y.
Kobayashi, (2004): Earthquake risk assessments oflarge residential fill-slope in urban areas,
Landsides – Journal of the Japan Landslide Society,157, 29-39 (in Japanese)
Kamon, M. and Inui, T. (2002): Geotechnicalproblems and solutions of controlled waste disposalsites, Journal of Geotechnical Engineering, JSCE,
No.701/III-58:1-15 (in Japanese).
Kamon, M., Katsumi, T., Endo, K., Ito, K, and Doi,
A. (2001): Evaluation of the performance of coastal
waste landfill with sheet pile containment system,
Proceedings of the Fourth Japan National
Symposium on Environmental Geotechnology, 279284 (in Japanese).
Kimura, M., Too, A.J.K., Inazumi, S., Isobe, K., andNishiyama, Y. (2004): Innovative development ofsteel pipe sheet pile joint, Proceedings of the Third
Civil Engineering Conference in the Asian Region,
submitted.
Kimura, M., Too, A.J.K., Isobe, K., and Nishiyama,
Y. (2003): Offshore construction of bulkhead waste
facilities by H-joint steel pipe sheet piles,
Proceedings of BGA International Conference onFoundations: ‘Innovations, Observations, Design
and Practice’, 443-452.
Oki, T., Torizaki, K., Kita, H., Yoshida, M.,
Sakaguchi, Y., and Yoshino, H. (2003): Evaluationof impermeability performance of the verticalimpermeable walls by using steel sheet piles andsteel pipe sheet piles, Proceedings of the Fifth Japan

National
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Yairi, K., Suwa, K., Masuoka, Y. (1973)

Landslides by the rainstorm of July 1972 Obara
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Prefecture. Yano, K. ed. Report of the Grant-in
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92-103. (in Japanese)

要 旨
低平地を中心として急速に周辺丘陵地へと拡大する都市域では，地震時液状化，宅地造成地盤崩壊，人工・自然斜面崩壊など，地盤災害の危険性が増している。本研究は，これらの地盤災害に対する都市域の脆弱性診断技術と危険度評価技術の高度化，地盤基礎構造物の性能向上技術の開発を目的とする。平成１５
年度のとりまとめとして，本論文では，廃棄物埋立護岸の環境脆弱性と性能向上，レーザースキャナー技術を用いた山地での表層崩壊発生危険度評価の提案，丘陵地斜面における地すべり災害の予測モデルの開発についてとりまとめた．
キーワード: 環境，危険度評価，丘陵地，山地，地盤災害

COE

都市および周辺地域における地盤災害予測とハザードマッピングに関する研究
井合進，稲積真哉，千木良雅弘，釜井俊孝，Sidle, R.C.
三村

１．はじめに
低平地を中心として急速に周辺丘陵地へと拡大する都市域では，地震時液状化，宅地造成地
盤崩壊，人工・自然斜面崩壊など，地盤災害の危険性が増している。本研究は，これらの地盤災害に対する都市域の脆弱性診断技術と危険度評価技術の高度化，地盤基礎構造物の性能向上技術の開発を目的とする。平成１５年度のとりまとめとして，本論文では，廃棄物埋立護岸の環境脆弱性と性能向上，レーザースキャナー技術を用いた山地での表層崩壊発生危険度評価の提案，丘陵地斜面における地すべり災害の予測
モデルの開発についてとりまとめた．
２．廃棄物埋立護岸の環境脆弱性評価と性能向上
海面埋立処分場における埋立護岸は、廃棄物、建設発生土、および浚渫土砂の海面埋立処分に対応して、港湾保全との整合を図りつつ、埋立処分する空間を確保するためのものである。廃
棄物埋立護岸は、波浪、高潮、津波など海上特有の諸条件に対して十分安全であるとともに、廃棄物からの浸出水が海域へ流出することのな
い構造でなければならない。近年では、施工効率や経済性を考慮して、鋼管矢板を用いた廃棄物埋立護岸が採用されている。本研究では、既往の鋼管矢板護岸における継手箇所の問題点に着目し、遮水性能に関する課題を明らかにした。
さらに、実海域における鋼管矢板の施工性や遮水性向上に貢献し得る H-H 継手を介した連結鋼
管矢板の性能を透水試験によって評価し，H-H
継手を介した連結鋼管矢板の適用が効果的であることを明確にした。
３．山地での表層崩壊発生危険度評価指標の提案−レーザースキャナー技術を用いて―表層崩壊は，個々の規模は小さくても広い範囲に多数発生することが多く，それが引き起こす災害は甚大になることが多い。表層崩壊に対して危険な斜面を広い地域から抽出することは，地盤の多様性のために難しいが，広い範囲の危険度を評価することは可能になりつつある。
我々は，地質特性とレーザースキャナー地形計衛，諏訪浩，斉藤隆志，飛田哲男

測技術を用いて地域の表層崩壊危険度を評価す
る新たな手法を開発した。レーザースキャナー
計測は，航空機から地表に向けてレーザーパル
スを発射し，樹間を通過して返ってくる反射をとらえ，地表を精密に測量する技術である。
本研究では，1972 年西三河災害箇所および1998 年福島県南部豪雨災害箇所に，レーザース
キャナー計測を適用した。その結果，花崗岩地域で崩壊が密集して発生したことと，花崗閃緑
岩地域では崩壊が極めて少なかったことが明確になり，災害前にレーザースキャナー計測が実施されていれば，花崗岩地域の崩壊危険性の高さを数値的に推定可能であったことが明らかになった。
上述のように，レーザースキャナー計測によって，100 年オーダーの期間に発生した崩壊密度を読み取ることが可能であり，この崩壊密度は，
特定地域の崩壊危険度インデックスとして用いることができると考えられる。今後，崩壊跡の持続時間と崩壊の免疫期間や植生のレーザース
キャナー計測結果への影響について検討することによって，この実用化をはかることができる。
４．丘陵地斜面における地すべり災害の予測モデルの開発
丘陵地斜面における地すべり災害の予測について、新たな予測モデルの開発とケーススタデ
ィを行った。１９７８年宮城県沖地震、１９９３年釧路沖地震、１９９５年兵庫県南部地震等、都市に被害を与えた地震では、人口密集地に形成された多数の宅地盛土（多くは谷埋め盛土）が、大規模に変動（地すべり）した。そこで、
谷埋め盛土の変動・非変動事例 314 例について，
ニューラルネットワークを用いた変動予測モデ
ルを構築した。モデルの感度解析及び未学習データを使用した精度評価試験を行った結果、いずれにおいても良好な成績を得た。また、２００３年の水俣災害の発生メカニズムについて検討し、深部における間隙水圧の全般的な上昇が、表層崩壊の急激な発生をもたらした原因であることを明らかにした。