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

Geo-disaster Prediction and Geo-hazard Mapping in Urban and Surrounding Areas
Progress Report in FY 2004

Masahiro CHIGIRA, Susumu IAI, Roy C. SIDLE, Toshitaka KAMAI,
Mamoru MIMURA, Hiroshi SUWA, Takashi SAITO,
and Tetsuo TOBITA
Synopsis
Urban development rapidly expanding from lowlands to surrounding hills andmountains poses increasing risks for geo-hazards, including liquefaction during earthquakes,
and failure of artificial and natural slopes. In order to develop methodologies for assessing
vulnerability to these hazards and for making spatial and temporal predictions, and to develop
technologies for improving the performance of geotechnical works in urban areas, we studied:(1) seismic performance of river embankments; (2) water infiltration behavior and creep
movement of decomposed granite slopes; and (3) landslides triggered in residential areas invalley fills and hills by the 2004 Chuetsu earthquake.
Keywords: environment, geo-hazards, hazard mapping, urban area, hillslopes, mountains

1.

Introduction

Urban areas developed in lowlands have been
rapidly expanding into the surrounding hills andmountains. This expansion increases hazards
associated with geological and geotechnicalprocesses as well as exposure of residents. 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; such disasters
were unfortunately realized during the 23 October
2004 Chuetsu Earthquake.
The objective of this study is to develop
methodologies for assessing vulnerability to thesehazards and making spatial and temporal prediction,
as well as proposing techniques for improving theperformance of geotechnical works in urban areas.
This paper summarizes the study results focusing
on the following priority issues:(1) Seismic performance of river embankments
Failure of river embankments during earthquakes
poses potential hazards to urban areas due to flooding
and inundation. The conventional analytical method
for evaluating the seismic vulnerability is based on
the pseudo-static limit equilibrium method and is notcapable for evaluating the settlements. In this study, amethod based on effective stress analysis for
simulating the varying degree of settlements isstudied through a series of centrifuge model tests andanalyses.

(2) Prediction of shallow landslides
Rainstorms have been inducing widespread
distributed landslides, but water infiltration behavior
within weathered materials has not been wellunderstood. In addition, even shallow landslides seemto be preceded by creep movement, but the creep
movement has not been studied in detail. We focus
on these issues to make spatial and temporal
predictions of shallow landslides. We monitored
water infiltration behavior within a decomposed
granite slope, which is one of the typical rocks
susceptible to landslides, as well as creep movementof the surface loosened layer.
(3) Valley fills and artificial slopes have been
increasing related to the development of newresidential areas conveniently located near large cities.
Landslides in these valley fills and hills pose
immediate threats to residents. We studied the sitecharacteristics of the landslides triggered by the 2004
Chuetsu Earthquake in the suburb of Nagaoka andOjiya cities, central Japan.
2. Seismic performance of river embankments
2.1 Introduction
River embankments in urban areas are often
constructed on old river channels and other loosely
deposited sandy layers. These embankments often
suffered serious damage during earthquakes. Failureof river embankments during earthquakes poses
potential hazards to urban areas due to flooding andinundation. These hazards may be caused by Tsunami
run-up through a river or those caused by heavy rains
before or immediately after the earthquakes. These
hazards are typically governed by the settlements ofthe embankments. However, the conventional
analytical method for evaluating the seismic
vulnerability is based on the pseudo-static limit
equilibrium method and is not capable for evaluating
the settlements.
Ozutsumi et al (2002) performed a series ofeffective stress analyses of liquefaction-induced
deformation in river embankments using an effective
stress model developed by one of the authors and hiscolleagues (Iai et al, 1992) and found that thisanalytical tool has the potential ability to simulate theliquefaction-induced residual deformation during
earthquakes. In this study, the capability of thisanalytical tool for simulating the varying degree of
settlements is studied through a series of centrifuge
model tests and analyses.
2.2 Case history of damage to embankments
A case history of damage to the embankments at
the mouth of the Yodo river in Osaka city, Japan,
during 1995 Hyogoken Nambu earthquake(magnitude=7.2) was used as a reference in this study.
This reference was systematically varied with respect
to the level of shaking and foundation soil conditionin the centrifuge model tests as described later.
The embankments were located within 40 km
from the epicenter. The peak ground acceleration at
the site of the embankments was about 0.2g as
recorded at a nearby strong motion site (Matsuo et al,2000). Typical damage to the embankments is shownin Fig. 1. The settlement of this embankment was 2.7
m. The other embankment located nearby settled only
0.3 m. These embankments were made of a core ofsoil materials covered with a concrete and asphalt as
erosion protection and a parapet wall 8.1 m above thesea level.
The cross sections and soil profiles of theseembankments are shown in Fig.2 (JICE, 2002). These

Fig. 1 Typical damage to river embankments (Yodoriver embankment during 1995 Hyogoken Nambu
earthquake) (after Ozutsumi et al, 2002)(a) Embankment with serious damage(b) Embankment with minor damage
Fig. 2 Cross sections and soil profiles (after Ozutsumi
et al, 2002)
embankments about 3 m high were constructed on a
Holocene deposit about 20 m thick. The deposits
consist of, from the top, the upper sandy layer (As2)
about 10 m thick, clay layer (Ac) about 10 m thickand the lower sandy layer (As1) about 1 m thick. The
SPT N-value of As2 layer is about 10. The soil
profiles at both sites are similar to each other except
for the existence of a cohesive layer Ac about 2 m
thick below the embankment at the site of theembankment having smaller settlement as shown in
Fig. 2(b). Foundation conditions significantly affectthe seismic performance of embankments.
2.3 Centrifuge model tests of embankments
Centrifuge model tests were performed in 50g
centrifugal acceleration using the geotechnicalcentrifuge at Disaster Prevention Research Institute,
Kyoto University, with an arm length of 2.5 m. The
cross section of an embankment was determined byreferring to that at the Yodo river and shown in Fig. 3.
The model embankment and foundation ground wasmade of Soma No.5 sand. The foundation groundcondition was varied using a loose deposit for one
series of tests, and a dense deposit for other. In eachseries of tests, amplitude of input motion was varied

in three levels. Total of six cases of mode tests were
performed. Relative density of model foundation
ground was about 30% for loose deposit, and 70% fordense deposit. Input motions were sinusoidal wave,
1Hz in prototype scale, with 20 cycles of loading.

loose foundation ground after shaking (Case3)

Fig. 5 Measured deformation of embankment on
dense foundation ground after shaking (Case6)

Fig. 3 Cross section of an embankment in centrifuge
model tests (1/50 scaling at 50g field)
2.4 Results of centrifuge model tests
Typical examples of residual deformation of theembankment model are shown in Figs. 4 and 5. The
one shown in Fig. 4 is the results for loose foundation
ground subject to a peak acceleration of 313 Gal(Case3). Excess pore water pressures in the loose
foundation ground gradually increased as cyclic shear
is applied due to shaking. Foundation groundgradually softened due to the excess pore waterpressure rise. The embankment pushed down thesoftened foundation ground, resulting in settlement at
the crest associated with lateral spread deformation at
the foundation ground.
The deformation shown in Fig. 5 is for dense
foundation ground subject to a peak acceleration of
574 Gal (Case6). Failure mode of embankment in thiscase is represented by slide of embankment soil along
the surface of embankment. Settlement of the crest is
induced by this slide type of failure. There was no
significant deformation in the dense foundation
ground.

2.5 Effective stress analysis through multiple
shear mechanism model
The constitutive model used for the analysis isbased on multiple inelastic shear mechanism model(Iai et al, 1992). The computer code incorporating
this model is called FLIP (Finite element analysis ofLiquefaction Program). The model takes into accountthe effect of rotation of principal stress axis directions,the effect of which is known to play an important rolein the cyclic behavior of anisotorpically consolidated
sand. In addition to the conventional assumption of
hyperbolic relationship assigned for each shear
mechanism, the model uses the extended Masing rule
to reproduce realistic hysteresis loop for cyclicloading. The excess pore water pressure generation
due to dilatancy is modeled using the concept ofliquefaction front, which is defined in the equivalentnormalized stress space shown in Fig. 6, where S
represents the mean stress ratio relative to the initialmean stress, and r represents the deviator stress ratio
relative to the initial mean stress.

Fig. 6 Liquefaction front modelFig. 4 Measured deformation of embankment on

The analysis is performed in two steps. In the firststep, static analysis is performed in drained condition
by applying gravity (i.e. centrifugal acceleration in
the model tests) over the analysis domain. In thesecond step, dynamic analysis is performed inundrained condition by applying input motion at thebase of the analysis domain. The analysis in thisstudy was performed for prototype scale with modelparameters that correspond to SPT N-value of 5 forloose foundation ground and 40 for dense foundation
ground and the embankment.
Computed deformation of embankments is shownin Figs. 7 and 8. The one shown in Fig. 7 is theresults for loose foundation ground.

Fig. 9 Settlements at crest of embankment at various
levels of acceleration
Fig. 7 Computed deformation of embankment onloose foundation ground after shaking (Case3)
mode is in good agreement with that measured andshown in Fig. 4. The agreements include the overall
pattern of settlements associated with the lateral
movement of foundation ground.
The computed deformation shown in Fig. 8 is for
dense foundation ground. The agreement with themeasured deformation shown in Fig. 5 is apparently
not very good, especially the analysis was not
successful in simulating the sliding of embankment
slopes as shown in Fig. 5. This discrepancy ispresumed to be due to the fact that, in the analysis,
the nodes at both ends of the slopes share the same
displacements as those of the foundation ground,
whereas, in model tests, the soil of the embankmentis free to slide over the foundation ground. If muchfiner meshes or other improvements in the analysis
are adopted, the agreement would be improved.
Within the scope of the present study, the results ofanalyses and model tests agreed in that deformationof embankment is confined within the embankment
itself, without significant deformation in the dense
foundation ground.

Fig. 8 Computed deformation of embankment on
dense foundation ground after shaking
2.6 Settlement vs. acceleration level
In order to evaluate the overall performance ofembankment subject to various levels of shaking, themeasured and computed settlements obtained from all
the six cases are shown in Fig. 9.

For a loose foundation ground, settlements
reached 1.4 m at the maximum, equivalent to about
50% of the original height (3 m) of the embankment.
For a dense foundation ground, the settlement was
only 0.1 m at the maximum, equivalent to about 3%of the height of the embankment. Both the computed
and measured results showed similar trends insettlements with increasing levels of shaking,although the computed results for loose foundation
ground are smaller than those measured and thecomputed results for dense foundation ground arelarger than those measured. In order to achieve better
agreement, more studies may be necessary. The
settlements for dense foundation ground are reduced
to only 0.5 to 6.5% of those for loose foundation
ground, indicating that the densification offoundation ground is effective in improving theperformance of embankment.
2.7 Mechanism of settlement
In order to identify the mechanism of settlement
in embankment, computed settlements are plotted inFig. 10, superposed by the plot of axial strain
difference in soil element in the foundation groundbelow the embankment. The results indicate that thesettlement is closely correlated with the axial strain
difference in foundation soil for loose condition. For
dense foundation ground, the correlation is not good.
Consequently, the settlement of embankment
constructed on loose foundation ground is induced bydeformation of foundation ground associated withaxial strain difference of soil whereas the settlementof embankment on dense foundation ground is
induced by due to deformation of embankment itself.

Fig. 10 Settlement and axial strain difference infoundation soil below the embankment

2.8 Summary
A series of centrifuge model tests and effective
stress analyses were performed on performance of ariver embankment during earthquakes. The following
conclusions are obtained:(1) Settlements of embankments constructed on loose
foundation ground are governed by deformationinduced in the foundation ground, that settles andspread laterally.
(2) Settlements of embankments constructed on dense
foundation ground are governed by deformation in
the embankment itself.
(3) The effective stress analysis performed in thisstudy captures the essential features of thoseseismic responses of embankment and can be auseful tool to assess the seismic performance ofriver embankment.
(4) Condition of foundation ground significantly
affects settlements of embankments. Densificationof foundation ground is an effective measure to
improve performance of embankments during
earthquakes.
3. Landslides triggered by the 2004 Chuetu
earthquake
3.1 Outline of the earthquake
A series of earthquakes (the strongest with amagnitude of 6.8 in JMA-Japan Meteorological
Agency; 6.5 on the Richter scale) and aftershocks
jolted northern Japan on October 23, 2004, killing atleast 30 people and injuring more than 2000 people
largely as the result of building collapse. The
strongest earthquakes with magnitudes greater than
6.0 on JMA scale occurred within a time frame ofless than an hour, and had the epicenters spread
across Ojiya city and Yamakoshi village, Niigata
Prefecture. The major earthquakes were characterized
by a shallow focal depth that generated strong levelsof ground motion, resulting in extensive damage
throughout the region. Hundreds of landslides were

Fig. 11 Locality map of the Chuetsu
earthquake.
triggered by the earthquakes and aftershocks withmuch associated to residential areas throughout
southern Niigata Prefecture. Although heavy rain fellin the region two days prior to the earthquake, ourfield reconnaissance a few days after the earthquake
revealed that most soils in shallower failures wererelatively dry or only slightly wet along the sliding
Fig. 12 Landslides
and ground failures in
the residential area of
Otoyoshi shuraku:(a) overview of the
area
and
the
earthflow on which
the community wasbuilt; (b) tension
cracks in the area
showing evidence ofaccelerated earthflow
activity during theearthquake; and (c)damage to home androad due to failure ofartificial fill material.

surface.
3.2 Landslides in roads and residential areas
Residential development, particularly cuts andartificial fills, is known to decrease slope stability(Sidle et al., 1985; Kamai et al., 2004). Earthquakes
exacerbate such potential instabilities by the groundmotion induced and the enhancement of pore waterpressure in wet soils. Within our reconnaissance area,
we observed earthquake triggered landslides thatwere highly affected by road cuts, road fills, andresidential fills. Road cuts remove support along theuphill side of road corridors (Sidle et al., 1985;
Megahan, 1986); thus, ground motion arising during
an earthquake often triggers numerous landslides inthese sites. Road fill materials on hillslopes naturally
destabilize road embankments by adding weight tothe hillslope, oversteepening the outside side of theembankment, and sometimes incorporating poormaterials or insufficient consolidation of materials
within the embankment. Hundreds of road cut slope
and fillslope failures we observed as the result of theOctober 23rd earthquake. Additionally, much cut andfillslope material remained tenuously unstableawaiting failure during forthcoming rainstorms or
periods of snowmelt.
Additionally, we investigated numerous failures
triggered by the Chuetsu earthquake in residential fillslopes constructed on reclaimed land in Nagaoka city.
Damages to houses and roads due toearthquake-induced landslides in Otoyoshi shuraku of
Nagaoka city were significant (Fig. 12b,c). The entire
housing development (Fig. 12a) was actually built onan old earthflow that was reactivated during the 23
October 2004 earthquakes. The tension crack in Fig.
12 b is evidence of the earthquake-induced
reactivation of the earthflow, whereas the damaged
house and road in Fig. 12 c is a consequence of theearthquake–induced failure of a valley fill. The
geological structure in this area is Middle Pleistocene
sediments, consisting of gravel, sand and mud (withweathered reddish soil) (Kobayashi et al., 1991).
Another residential area (Takamachi danchi) of
Nagaoka city incurred substantial damage to houses
and roads due to seismically-induced failures ofartificial fill slopes. The entire road surrounding thisdevelopment was constructed on fill material; theroad was partly destroyed during the earthquake.
Takamachi danchi covers about 4.1 ha, and lies on
Pliocene to Middle Pleistocene sediments composedof sand, silt and gravel. About 70 of the 522 homes inthe development were damaged due toseismically–induced deformations and failures ofartificial fill slopes. Apparently, the fill material inboth Otoyoshi and Takamachi regions was partly
saturated partly due to the rainstorms during theprevious series of typhoons that struck the region a
few days prior to the earthquake. Poor drainage
systems within the retaining structures supporting thefill contributed to the accumulation of water in the fillslopes and exacerbated the instability.

Fig. 13 Landslide types triggered by the 2004
Chuetsu earthquake in mountainous areas
near Nagaoka and Ojiya cities.

3.3
Geologicalandcharacteristics of landslides

geomorphologicalThousands of landslides of natural slopes occurredin hilly areas in the Higashiyama Hills, where
Plio-Pleistocene sedimentary rocks are distributed.
The most common landslides of natural slopes
triggered by the earthquake were shallow disrupted
landslides on steep slopes without geologic

Fig. 14 Higashi-takezawa slide(Nakanihon Air Service CO., LTD)

preference, which has been common to many
previous disastrous earthquakes in the world (KEEFER,
1984). The Chuetsu earthquake also triggered many
deep landslides, damming streams and flooding
villages. Field investigation and interpretation of
aerial photographs taken before and after theearthquake clarified that many of these slides had
occurred due to the reactivation of previous
landslides (CHIGIRA, 2004). These had planar sliding
surfaces along bedding planes or along the oxidation
front (Fig. 13). The bedding-parallel sliding surfaces
were made at the boundary between the overlying
permeable sandstone and underlying siltstone (1 inFig. 13, Fig. 14) or along the bedding planes of thealternated beds of sandstone and siltstone (2, 3, and 4
in Fig. 13). Sliding surfaces along the oxidation front
were made in the area of marine black mudstone (5 inFig. 13). New landslides (rockslide-avalanches)
occurred with the sliding surfaces in an a-few-cm
thick tuff interbedded in siltstone (6 in Fig. 13). Mostof the deep landslides occurred on slopes undercut by
erosion or artificial excavation, notwithstanding theywerereactivated
or
new
ones.
One
rockslide-avalanche occurred on a slope where
buckling deformation preceded the earthquake (7 inFig. 13). Gentle valley bottom sediments were
mobilized in many locations, probably because they
were saturated and partial liquefaction occurred by
the earthquake (8 in Fig. 13).
3.4 Summary
The 2004 Chuetsu earthquake triggered manylandslides of road cut slopes, residential fill slopes inurban area, as well as on natural slopes, isolating
villages. Precipitation of about 100 mm two days
before the earthquake seems to have affected thelandslide. The most common landslides of naturalslopes were shallow disrupting landslides, but morethan 100 deep slides also occurred and affected wide
areas and dammed streams. Most of these deep slides
were due to the reactivation of old landslides and had
planar sliding surfaces along bedding planes or theoxidation front. In addition, most of the deep slides
were undercut before the earthquake and destabilized.
Mobilization of sediments filling gentle valleys also
occurred.
4. Stability of urban natural slopes
Urban areas are growing into neighboring
mountainous areas by cutting natural slopes or by
utilizing areas at the foot of mountain slopes.
Particularly, Japan has very small flat areas, so thissituation is becoming very common.
4.1 Water filtration behavior within decomposed
graniteOne of the rock types, which are very susceptible
to landslides and has been suffering serious damage
by rainstorms, is granite (Fig. 15, (CHIGIRA, 2001).

Fig. 15 Landslide disasters occurred in the
granitic rock areas.

Fig. 16 Locality and topographic map anda cross section of the study site.
Granite occupies wide areas in urban areas and also
in candidate sites of new Japanese capital, partly
because it forms wide low-relief hills and are easily
excavated by machine without explosion. In spite of
the above situation, landslide mechanism ofweathered granite is not well understood so spatial
and time prediction methodology of those landslides
is not yet developed.
In order to clarify the mechanism of landslide ofweathered granite and water filtration behavior,
which is essential to establish the methodology to
predict landslide by rainstorms, we monitored water
filtration behavior and surface creep movement of adecomposed granite slope. Details of the results andthe conceptual model of water filtration have beenwritten in a paper submitted elsewhere (CHIGIRA etal., submitted).
(1) Study site and methods
The investigation site is located in the southernpart of Shiga Prefecture, central Japan (Fig. 16). This
site is a hilly area with elevations from 450 to 650 m
above sea level, and is underlain by the Cretaceous
Shigaraki granite (Collaborative Research Group for
the Granites around Lake Biwa, 1982). The landform
has low relief with a relative height of less than one
hundred meters from flat river bottom. The granite is
deeply weathered, and almost all outcrops above theriver bottom consist of decomposed granite except
for sporadically exposed core stones, while beneaththe stream bottom less weathered and sound graniterock is expected. This area was struck by a heavy
rainstorm in 1953; widely distributed, destructive
landslides occurred, killing forty-four people.
We monitored rainfall, discharge amount,electrical conductivity and the chemistry of waterfrom sub-horizontal drill holes just beneath thegroundwater table within a decomposed granite slope
as well as suction and creep movement at the slope
surface (Fig.16). The monitoring interval was from
April 2003 to November 2004. Water sampling forchemical analysis was done manually.
(2) Results and a conceptual model of water
filtration
Typical response of electrical conductivity anddischarge amount against rainfall events is shown in
Fig. 17. We found that when precipitation exceeds
about 30 mm, electrical conductivity decreases
spontaneously, then soil water with high electricalconductivity is pushed down and subsequently dilute
rainwater itself reaches to the groundwater table. The
conceptual model of water filtration suggested by themonitoring results is shown in Fig. 18. The pushingdown occurs at almost all rainfall events exceeding
10 mm precipitation. The immediate decrease of theelectrical conductivity is due to quick inflow ofrainwater to the groundwater table through “express
ways” like cracks, and its increase is due to theFig.
17
Monitored
dataof
precipitation, discharge, and electricalconductivity.
pushing down of soil water. The electricalconductivity reached to its maximum 20 to 40 hours
after the peak of rainfall events and then decreased,
while the discharge from the drill holes increased
further after the peak of the electrical conductivity to
become its maximum 2 to 5 days after the peaks ofprecipitation. The time delay of the maximum ofdischarge amount from the maximum of the electric
conductivity suggests that the pushing down of thesoil water is caused by the pressure propagation by
the downward movement of the wetting front as was
suggested by Bianchi and Haskell (1966).
The soil water that is pushed down seems to be acapillary water above the groundwater table. The
SiO2 concentration of this water is apparently
controlled by the solubility of cristobalite and is richin Ca, which is supplied from plagioclase with SiO2.
The cation concentrations in the unsaturated soil
water and the capillary water are affected by ion
exchange.

Fig. 18 Conceptual model of water filtration in a
decomposed granite.

4.2 Creep movement of a surface loosened layer
Surface deformation by creep movement of aloosened layer was observed on the monitored slope.
Figure 19 shows the displacements measured withextensometers along with precipitation and matricsuctions in the loosened layer in the depth of 50 cm.
Monitored suction data indicated that positive porewater pressure occurred only several times during themonitored interval. Accumulated creep displacement
during 7 months was about 3 mm. The displacement
was not directly related to the amount of precipitation
but to the fluctuation ranges of matric suction as is seenin Fig. 19. In July and August, there were not so heavy
rainstorms in comparison with the other months, but
most of the displacement occurred. Figure 19 also
shows that the fluctuation ranges of suction were thelargest in these two months. These findings indicate that
displacement occurs in response to the repetition ofwetting and drying rather than to positive pore pressures.
4.3 Summary
Water infiltration behavior observed by themonitoring the amount, chemistry, and electricalconductivity of water from just beneath the groundwatertable in decomposed granite suggests that infiltrationnear the groundwater table has three steps: Quick inflow
through short-cuts like cracks, pushing down of thecapillary water, and then inflow of rainwater throughmatrix of decomposed granite. The surface loosened
layer creeps in response to the repetition of drying andwetting before catastrophic sliding.
5. Concluding remarks
During this fiscal year, we performed the studies on(1) seismic performance of river embankments, (2)water infiltration behavior and creep movement of
decomposed granite slope, and (3) landslides triggered inresidential areas in roads, valley fills and hills by the2004 Chuetsu earthquake. In addition to the study in FY
2003, which includes (1) performance of retaining walls
at waste fill in waterfront areas, (2) hazard mapping of
natural slope failures through monitoring using laser
scanners, and (3) hazard assessment of residential areas
in valley fills, we are approaching to our goal, to develop
methodologies for assessing vulnerability to geohazards
and for making spatial and temporal prediction, and to
propose techniques for improving the performance ofFig. 19 Relationship among thedisplacement, precipitation, and matricsuction in a surface soil.
geotechnical works in urban areas.
Acknowledgements
The authors are grateful to former students, Chizuru
Imai and Hideaki Hijikata for performing
geohydrological monitoring in the field.
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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
平成16年度報告
千木良雅弘，井合 進，Sidle R.C.,釜井俊孝，
三村 衛，諏訪 浩，斉藤隆志，飛田哲男
要旨
本研究は，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,発生場の時空的予測技術の開発，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１６年度のとりまとめとして，本論文では，河川堤防の耐震性能，風化花崗岩斜面
内での地下水浸透挙動，表層土のクリープ挙動，2004年中越地震によって発生した斜面移動現象の発生場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
平成１６年度報告
千木良雅弘，井合 進，Sidle R.C., 釜井俊孝
三村 衛，諏訪 浩，斉藤隆志，飛田哲男
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,発生の時と場所の予測技術の高度化，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１６年度のとりまとめとして，本論文では，河川堤防の耐震性能向上，2004年中越地震によって発生した地すべり・斜面崩壊現象の発生場の解明，風化花崗岩斜面内の降雨浸透挙動に関する研究結果をとりまとめ
た．
２．河川堤防の耐震性能
低平地に展開する都市域の耐水土構造物の耐震性能の診断技術の高度化を目的として，河川堤防を対象として，遠心力場の模型振動実験および有効応力解析を行った。既往の耐震性診断技術としては，等価静的慣性力などを考慮した力のバランスに基づく簡易解析法（円弧すべり法）があるが，この方法では，耐震性能を支配する堤防天端の沈下量を予測することができない。対象とした断面は，1995年兵庫県南部地震
において液状化により崩壊した淀川堤防（高さ3m）である。遠心力場の実験では，基礎地盤（飽和砂地盤）の条件を緩詰めおよび密詰めの２種類の条件，それぞれの地盤条件において入力波の振幅を変化させて３ケースずつ，延べ６ケース実施した。基礎地盤が緩詰めの場合には，基礎地盤を堤防が押し下げる形で堤防の沈下が発生したのに対し，基礎地盤が密詰めの場合には，
堤防盛土部分の表層すべりによる沈下となっている。これらの堤防の変形形態は，有効応力解
析により，概ね再現できることが確認された。
また，沈下量についても，概略レベルではあるが，有効応力解析により再現可能であることが確認された。あわせて，河川堤防の耐震性能向上のためには，地盤の締固めなどによる基礎地
盤の耐震性向上が著しい効果を発揮することが明らかにされた。
３．2004年中越地震による斜面変動現象
2004年中越地震は，代表的な地すべり地帯で，しかも長岡市や小千谷市といった都市とその近郊山地の直下を震源として発生した．その結果，1000を超える膨大な数の地すべりと崩壊が発生
し，丘陵に拡大していった住宅地の地盤に変形を生じ，また，山間地の集落を孤立させた．2004
年中越地震は，いわばわが国の地方都市に発生する地震災害の典型的な事例でもあった．
長岡市の丘陵を造成した住宅地では，いたるところで盛り土がすべった．特に，不安定になった盛り土下の旧谷の方向と地震動の方向との間に関連が見られた.より山地側では，地すべりと表装崩壊が数多く発生し，川が閉塞され，一部の集落は水没した．比較的規模の大きかった
地すべりの大部分は古い地すべりの再活動であり，また，それらは脚部を侵食などによって切り取られており，不安定化していたものであった．これらの結果は地震時に発生する地すべり
危険箇所の判定に大きな参考となる．
４．風化花崗岩斜面内の降雨浸透挙動と表層ク
リープ
花崗岩はわが国の主要都市の近郊に広く分布
し，都市は，その分布域に拡大している．また，花崗岩は首都機能移転候補地にも広く分布している．一方で，風化花崗岩は従来の豪雨によって数多く崩壊してきたが，その崩壊メカニズムについては不明な点が多く残されている．そこで，風化花崗岩斜面において，表層のサクショ
ンと地下水面近傍からの水の量，化学組成，電気伝導度を連続的に観測した．その結果，強い降雨があると，降水は最初短絡経路を通って地下水面に達し，その後に風化花崗岩を浸透する
水が毛管帯の水を押し下げ，その後降水そのものが地下水面に到達する，といった現象が連続的に起こることがわかった．実際の崩壊は地下
水面が上昇，あるいは表層の緩み帯に宙水が形成して発生するものと考えられるが，その前段階の浸透挙動が明らかになったといえる．
風化花崗岩の表層での変位測定結果では，表層の変位は，特に強い降雨の時に大きく発生す
るというよりも，サクションが大きく変動する期間に特に集中することが明らかになった．これは，崩壊の発生に至るような注意すべき降雨パターンに示唆を与える．