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posilureforlocknert ets dn cfaileslop

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Engineering Geology 122 (2011) 115128

Contents lists available at ScienceDirect

Engineering

j ourna l homepage: www.e lsevaluate displacement of a rigid soil block along a xed slip surfacebased on a double integration of acceleration exceeding a yield valueacting on it. In actual slope failures, however, sliding debris may notnecessarily behave as a rigid body but deforms continuously withmovable slip surfaces. It sometimes tends to become destructive dueto a shift from slow rigid-block slide to fast debris ow because thefriction coefcient decreases drastically after failure starts.

In order to evaluate slope failures including ow failures from theirinitiation to termination, an energy approach was rst proposed byKokusho and Kabasawa (2003) and further developed by Kokusho

with a minus sign so that Ep or Ep is a positive value.Once failure starts, the amount of the dissipated energy is critical

to determine if it develops as a ow-type failure and how far it ows.In some time increments when earthquake shaking has alreadyceased (EEQ=0 orEk=(Ep)EDP), and ifEDP is smallerthan Ep, then it is clear that Ek is positive and the debrisaccelerates. It can also be inferred that a shift from slow slide to fastowmay occur not only due to an increase inEp but also due to adecrease ofEDP associated with pore-pressure buildup in liqueablesoil and strength loss in high-sensitivity clay. In fast ow failures,and Ishizawa (2007). In that method, fourpotential energy change Ep, earthquake

Corresponding author.E-mail address: kokusho@civil.chuo-u.ac.jp (T. Koku

0013-7952/$ see front matter 2011 Elsevier B.V. Aldoi:10.1016/j.enggeo.2011.03.019ris reaches down-slope.s modications by usingsi and Seed, 1978) can

Note that the potential energy change before and after the failure Epin Eq. (1) or Ep in Eq. (2) is always negative and hence expressedThe Newmark method (Newmark, 1965) or itFinite Element Model analyses (e.g. Makdi1. Introduction

Seismically induced slope failuresbased on force equilibrium on a potforce approach can evaluate a safety fapredict slide deformation, once failurperformance-based design and risk eimportant to know not only the safdeformation develops and how far faiormally been evaluatedsliding soil mass. Thisainst failure, but cannotrs. From a viewpoint ofon of slope failures, it istor but also how large

slope failure EEQ, dissipated energy in the sliding debris EDP, and itskinetic energy Ek are correlated in the following equation;

Ep + EEQ = EDP + Ek 1

or as an incremental form;

Ep + EEQ = EDP + Ek: 2energies; gravitationalenergy contributing to

debris will keeppotential energythanEDP, thenand comes to aconsumed. Thus,dissipation mechevaluate the runapproach.sho).

l rights reserved. 2011 Elsevier B.V. All rights reserved.

Back-calculation increasing volume of failedEnergy approach to seismically induced scase histories

Takaji Kokusho a,, Tomohiro Ishizawa b, Keisuke Koa Department of Civil Engineering, Chuo University, Japanb National Institute on Earthquake & Disaster Research Institute, Japan

a b s t r a c ta r t i c l e i n f o

Article history:Accepted 28 March 2011Available online 12 April 2011

Keywords:Seismic wave energyPotential energyLandslideTravel distanceFriction coefcient

An energy approach is proseismically induced slope fadissipated in large ow dconsideration on a rigid bevaluated by the proposed eslope failures during recenMobilized friction coefcienslope inclinations. The frictioslopes, indicating that thepe failure and its application to

mi a

ed here to make a simple evaluation of travel distance of debris duringes. In the evaluation, earthquake energy and gravitational potential energy aremations. Shake table tests on dry sand slopes together with theoreticalmodel are revisited to show that measured slope displacements can be

gy approach if an appropriate friction coefcient of the slope is specied. Then,arthquakes are investigated, and the energy approach is applied to them.uring failures are back-calculated, revealing their strong dependency on initialoefcients are found to be smaller than the initial slope inclinations for gentlerd debris tends to accelerate. The friction coefcients tend to decrease withes, which is consistent with previous case studies on huge landslides.

Geology

evie r.com/ locate /enggeoowing until kinetic energy plus the subsequentchange is all dissipated. Namely, ifEp is smallerEk is negative, hence the debris decreases its speedhalt when the reserved kinetic energy Ek is allprovided that the earthquake energy and the energyanism in owing debris are known, it is possible toout distance even in ow-type slides by the energy

In this paper, model shake table tests and comparative studieswith a rigid block model which have been performed in previousresearch (Kokusho and Ishizawa, 2007) are reviewed to explain theenergy balance in a model slope made from dry sand and to propose asimplied evaluation method for slope deformation based on theenergy concept. The energy-based simple evaluation method is thenapplied to a number of slopes that failed during the 2004 NiigatakenChuetsu earthquake (Kokusho et al., 2009a) and the 2008 Iwate-Miyagi Inland earthquake to back-calculate mobilized friction co-efcients and to discuss how the friction coefcients are determinedaccording to various parameters of slopes.

2. Previous works on shake table tests and rigid block modeling

A spring-supported shaking table shown in Fig. 1(a) was utilized totest a model slope made from dry sand, called Model-A. The slopeangle was parametrically changed as 29, 20, 15 and 10, consideringthe angle of repose of the model slope (35.4) determined from astatic test in which the same slope was statically inclined until theinitiation of failure. The table was initially pulled to several differenthorizontal displacements and then released to generate decayed freevibration. The frequency of the vibrationwas set to 4 discrete values of2.7, 2.5, 2.2 and 2.0 Hz.

In order to single out the energy dissipated due to slope failure, notonly in Model-A but also in Model-B, a pile of rigid concrete columns ofexactly the same weight, was tested in the same way (see Figure 1(b)).

116 T. Kokusho et al. / Engineering Geology 122 (2011) 115128The decays in the amplitude were measured in both Model-A and B.Results of a typical experiment are shown in Fig. 2. Note that thedifference in the amplitudes grows larger with the number of cycles,though the initial table displacement and the vibration period of thetable are almost the same in the two models. It seems reasonable toassume that this difference reects the greater energy dissipated inModel-A (the model slope) due to its failures than in Model-B.

The earthquake energy increment used in the model slopeEEQ inEq. (2) is evaluated from the loss energy per cycle in Model-A WA,and that in Model-B, WB as EEQ=WAWB, because theenergy in the two models can be assumed identical except thatdissipated inside the sand slope. The total energy EEQ calculated as asum of EEQ in all cycles to the end of the vibration represents the

(a) Spring support shake table

(b) 2 models compared; Model-A (left) & Model-B (right)Concrete columns

Surface marker

Vertical side marker

Pull handleLucite box Accelerometer

LVDT

Releaser

Load cell320

(Unit mm)

500

800 400

Supporting spring

Fig. 1. Shake table test apparatus for model slopes (a) and 2 models compared (b).

Kokusho and Ishizawa, 2007.amount of earthquake energy used in producing the residualdisplacement in the model slope.

In order to correlate the energy EEQwith the residual displacementof the slope, incremental and total horizontal displacementsrs andrs of slope surface markers were quantied by means of videocameras and a laser scanning technique, respectively. Details ondenition and measurement of slope displacements are available inother literature (Kokusho and Ishizawa, 2007).

The potential energy Ep was calculated from the change inslope surface geometry in the video images and a laser scanningtechnique (Kokusho and Ishizawa, 2007) as;

Ep = gBzdxdz: 3

Here, =soil density (assumed constant), g=acceleration of gravity,and B=thickness of the 2-dimensional model perpendicular to thecross-section. Coordinates x and z are in horizontal and verticaldirections of the slope and the integration in terms of x and z is carriedout over the cross-sectional area of the slope. Then, the dissipatedenergy EDP in the model test can be readily evaluated from Eq. (1) inwhich Ek=0 if the energy balance after the end of slope failure isconcerned.

In Fig. 3, the residual displacements rs are plotted versus thevibration energy EEQ contributed to slope failures for 4 different slopeangles of 29, 20, 15 and 10 under 4 different input frequencies. Foreach slope angle, all plots can be represented as a single curve,indicating that the energy can serve as a unique determinant for slopedisplacement even under different shaking frequencies. Fig. 3 alsoindicates that the gentler the slope is, the greater is the energy EEQ toattain the same residual displacement rs. It is further noted that thereseems to be a threshold energy, corresponding to each slope angle,below which no residual displacement occurs, indicating that theenergy uniquely determines not only residual displacements but alsothe initiation of slope failure. This indicates that, unlike the currentdesign practice, slope failures may be actually controlled by theenergy principle instead of acceleration or inertia force (Kokusho andIshizawa, 2007).

Next, the test data was utilized to develop simplied evaluationmethod for slope deformation based on the energy balance in a rigidblock model. Sliding displacement r of a rigid block shown in Fig. 4(a)gives thepotential energy changeEp and thedissipated energy EDPas;

EP = Mgr tan 4

EDP = Mgr1 + tan2

tan

1 + tan tan 5

where M=mass of sliding soil block, =tan (=slope angle) isslope inclination and =tan (=friction angle) is frictioncoefcient (Kokusho and Ishizawa, 2007). Then, starting fromEq. (1) and using Ek=0 in comparing the conditions before andafter slope failure, the earthquake energy is correlated with r as;

EEQ = EDP EP = Mgr tan : 6

The ratios of EEQ to Ep is;

EEQEp

=tan

tan7

If the slip plane is saturated and seismically loaded in undrainedcondition, however, it should be considered that the seismic inertiaforce normal to the plane is all carried by temporary pore-waterpressure and does not change the effective normal stress, and hencethe shear resistance. In this case, it is easy to understand that the

dissipated energy EDP can be expressed by the shear resistance along

the slip plane, n0A tan /cos , multiplied by the displacement alongthe slip plane, r/cos , such that,

EDP = rAn0n0=n0 tan= cos2 = rAn0 tan= cos2= Mgr tan

:

5

Eq. (6) is also replaced by

The energy ratio; Eq. (7), is replaced by Eq. (7) accordingly.

EEQEp

=tan tan

tan7

From Eqs. (6) and (6), the residual slope displacement for the rigidblock model can be formulated for the unsaturated case as;

r =1

tan EEQMg

8

and for the saturated undrained case as;

r =1

tan tan EEQMg

: 8

In Fig. 5, the residual displacements rs (considered here to beequivalent to r in the rigid block model) already used in Fig. 3 fordifferent slope angles and different input frequencies are plotted againversus the normalized earthquake energies EEQ/Mg. The weight of thedisplaced soil mass Mg was evaluated from Eq. (4) using the measuredpotential energy Ep and the measured displacement rs to complywith the rigid block theory. It is remarkable that if =0.857 is chosen,Eq. (8) canpredict the residual slopedisplacement almost perfectly for allslope angles andall input frequencies. This indicates that if an appropriatefriction coefcient is known in advance, the rigid block model shown inFig. 4(a), which apparently simplies the failure mechanism of the sand

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0Slope angle : 29

Dis

plac

emen

t u (cm

)

Time t (s)

Model-A Model-B

Fig. 2. Decay vibrations measured by displacement gage in Model-A and B.Kokusho and Ishizawa, 2007.

,

,

,

,

,

117T. Kokusho et al. / Engineering Geology 122 (2011) 115128EEQ = EDP EP = Mgr tan tan

: 6

In the above two equations, n0=Mg cos 2/A is the total stressnormal to the slip plane, n0=n0u0 is effective stress normal tothe plane where u0=the initial pore pressure, A is the horizontal areaof the sliding soil mass and tan *=(n0/n0)tan .

2.5

3.0

3.5Slope inc. : , 201 2.7Hz :

2 2.5Hz :

3 2.2Hz :2.0Hz :Q

(J)

,

,

,

,

290 2 40.0

0.5

1.0

1.5

2.015

Residual displ

10

4

Earth

quak

e e

nerg

y E E

Fig. 3. Residual slope displacements rs versus vibration energy EEQKokusho and Ishizawa, 2007.slope shown in Fig. 4(b), can successfully predict the realistic failure.

3. Evaluation method for runout distance

Based on the model test results and their interpretation in terms ofthe rigid block theory, an energy-based evaluation method for run-outdistance of earthquake-induced slope failure was proposed by Kokushoet al. (2009a). It is outlined here, again, together with some additionalexplanations.

6 8 10

Thresholds

acement rs

(cm)

20

29

15 10

,

,

,

,

,

for 4 different slope angles under 4 different input frequencies.

In the method, it is necessary to properly evaluate site-dependentseismic energy. In the present research, the energy is temporarilyevaluated by a simple formula assuming the spherical energy radiationof the bodywaves,whichhave been employed for engineering purposesin evaluating site-specic wave energy and liquefaction potentialevaluations (e.g. Sarma, 1971; Davis and Berrill, 1982). In order toknow its applicability, the formula was compared with the down-holeseismic records actually obtained during recent strong earthquakes in

which is postulated to radiate from the hypocenter and determinedusing the empirical equation by Gutenberg (1956) as

log E0 = 1:5Ms + 1:8 10

where Ms is Surface Wave Magnitude (Note: Japanese EarthquakeMagnitude, MJ, almost equivalent to Ms is used here).

Data points for the calculated energy from the records at depths

29

r

Mg

Friction

Slope gradient = tan

(a) Rigid block model

Shaking direction

Dyed sand marker

Stick marker

Dry sand

001 002

100

(Unit:mm)

(b) Dry sand slope

Fig. 4. Comparison of models of rigid block (a) and dry sand (b) slope.Kokusho et al., 2009a.

118 T. Kokusho et al. / Engineering Geology 122 (2011) 115128Japan (Kokushoet al., 2007;Kokusho, 2009; Kokusho and Suzuki, 2011).Fig. 6 shows the data points of input wave energy per unit horizontalarea, EIP/A, plotted versus hypocentral distances R on a loglog diagramcalculated from down-hole seismic records at the depths of around100 m below the ground surface during recent strong earthquakes inJapan (1995 Kobe EQ., 2003 Tokachi-Oki EQ., 2004 Niigataken ChuetsuEQ. and 2008 Iwate-Miyagi Inland EQ.). Straight lines in the chartrepresent the above-mentioned empirical formula:

EIP=A = E0= 4R2

9

assuming the spherical energy radiation of body waves for theindividual earthquakes. Here, E0 is the total wave energy in the unit kJ,

3.5

4.0

4.5

Slope angle:

:

:

rgy

E EQ/

Mg :

1 2.7Hz

2 2.5Hz

3 2.2Hz0 1 2 3 40.0

0.5

1.0

1.5

2.0

2.5

3.0

15

E

10

:

Nor

maliz

ed

earth

quak

e e

ne

Slope residual d

4 2.0Hz

Fig. 5. Earthquake energy versus residual slope displacement for different sKokusho et al., 2009a.tend to be generally consistent with Eqs. (10) and (11), though theagreement may not be good for some plots with longer hypocentraldistances in particular, presumably because fault mechanisms such asfault type, dimensions, directivity and asperity (e.g. Somerville, 1996)are completely ignored in this simple formula. Needless to say,improvement in predicting site-specic seismic energy consideringthe fault and path mechanism is needed as a future study. In themeantime, the input energy per unit area EIP/A at a base layer duringthe earthquake may be roughly computed from the earthquakemagnitude and the focal distance for engineering purposes.

Then, a sloping ground is idealized as an equivalent horizontal 2-layersystem consisting of an upper layer, which includes the slope, and a baselayer. By subtracting the energy Ed, that is reected downward into the

, , ,

, , ,

, , ,

, 20 15 10, ,5 6 7 8 9 10

q.(8) for =0.857

2920

, , ,

eformation rs (cm)lope angles by different input frequencies obtained by shake table tests.

displacementOQ(r) corresponds to theglobal inclination=tan of theslope from Eq. (4), hence,

Ep =Mgr

= tan 12

Corresponding to the earthquake energy, the centroid of the slidingsoilmassM canbe considered to rise fromP toP as shown in Fig. 7 by EEQ/Mg (the dimension is length). The inclination of the line P'Q, or the ratio ofthe height P'O expressed as (Ep/Mg+EEQ/Mg) to the horizontaldisplacement (r), OQ, can be expressed using Eqs. (4) and (6) as

Ep =Mg + EEQ =Mgr

= tan + tan 13

For cases of saturated slip plane, the same inclination can beexpressed from Eqs. (4) and (6) as:

Ep =Mg + EEQ =Mgr

= tan 13

100101

10

100

1000

M =8.0M =7.2

M =6.8

Inci

dent

wav

e en

ergy

EIP

(kJ

/m2 )

Hypocentral distance R (km)

M=6.7M=6.9

Fig. 6. Incident seismic wave energy versus hypocentral distance calculated from

119T. Kokusho et al. / Engineering Geology 122 (2011) 115128base layer due to the impedance contrast at the layer boundary, from theinput energy EIP, the earthquake energy EEQ, that is transmitted into theupper layer, can be computed (i.e., EEQ=EIPEd). Assuming that all theenergy EEQ transmitting into the upper layer is absorbed by the upperlayer due to the slope failure as observed in the shake table model tests,the energy ratio EEQ/EIP can be formulated as (Kokusho et al., 2007):

EEQ=EIP = 4=1 + 2 11

where =(Vs)upper/(Vs)base is the impedance ratio of the upperlayer to the base layer. A small portion out of the energy transmittedinto the upper layer (EEQ) may be dissipated by cyclic straining of soilor internal soil damping. If this portion is denoted as EEQ, the energycausing the slope failure is (EEQEEQ). In the following, EEQ is assumedto be negligibly small compared to EEQ, because soil conditions arenormally not so soft or liqueable in sloping ground.

For slopes that are not straight as illustrated in Fig. 7, Eqs. (1)(8) canstill be used if =tan is taken as a global inclination of a straight line PQ(directly connecting the centroids of a soil mass before and after failure)different from the initial inclination 0, and =tan as the averagemobilized friction coefcient over the travel distance. The sliding massMin Fig. 7maybedeterminedby conventional slip surface analyses,where apotential slip surface having the lowest factor of safety is found. However,

vertical array records during recent earthquakes in Japan, compared to a simple theoryof spherical energy radiation.Modied from Kokusho and Suzuki, 2011.in quite a few natural slopes, the potential slip surface may be reasonablyassumed to coincidewith a beddingplaneor aweak seamobserved in siteinvestigations. The drop height PO (Ep/Mg) divided by the horizontal

=

+

pEMg

EQEMg

P

Q

P

: Saturated slip plane

: Unsaturated slip plane

1tan =1

0 0tan =O

Initial slope angle

Average slope angle

rHorizontal travel distance

( )tan tan +

1tan = : Friction angle

*tan

Fig. 7.Graphical evaluationmethod for run-outdistanceof seismically inducedslope failure.Kokusho et al., 2009a.Consequently, the procedure for runout distance evaluation is:

1) Determine the dimension and weight of a potential sliding soilmass and its centroid P.

2) Determine the mobilized friction coefcient .3) Evaluate the earthquake energy EEQ by Eqs. (9)(11).4) Locate Point P, which is higher than P by the length EEQ/Mg as

shown in Fig. 7.5) Starting at Point P, draw a line having an inclination of tan +tan

() or tan * for a unsaturated or saturated condition,respectively, until it intercepts the slope surface (Point Q). Thenfrom the geometry of the slope, r can readily be obtained based onEq. (13) or Eq. (13).

This very simple procedure may be conveniently used to evaluatethe runout distance for seismically induced slope failure in developinghazard maps if the mobilized friction coefcient =tan of aparticular slope is known in advance.

0 5 km

Slope failures (red spots)Epicenter of

main shock

Koi ponds (blue spots)

Landslide dams

Fig. 8. Center part of the damaged area with countless slope failures, Koi-ponds andlandslide dams during the 2004 Niigataken Chuetsu earthquake.

Kokusho et al., 2009a.

4. Slope failures during recent earthquakes in Japan

Before applying the energy approach to actual slope failures, let ustake a look at slope failures during two recent large earthquakesoccurred in Japan: the2004 Niigata-ken Chuetsu earthquake (Kokushoet al., 2009a) and the 2008 Iwate-Miyagi Inland earthquake.

4.1. 2004 Niigata-ken Chuetsu earthquake

be a variant of Type-A, underlain by dipping mudstone, the displacedsoil mass was highly weathered because of repetitive slope failures inthepast andhencedeveloped into amudow. This typeof failure seemsto beunique to this regionbecause of themanyKoi-ponds located in thedamaged area. The failure was obviously associated with the ponds incausingow-type failure, involving colluvial soils of highwater-contentwith long travel distance.

Type-B Type-A Type-CKoi-pond

Sedimentation plane (Dip plane)

Fig. 9. 3 types of slope failures, A, B, and C, occurred during the 2004 Niigataken Chuetsu Earthquake.Kokusho et al., 2009a.

120 T. Kokusho et al. / Engineering Geology 122 (2011) 115128During the Niigata-ken Chuetsu earthquake of October 23 (MJ=6.8,thrust fault, focal depth 13 km), 2004, more than 4000 slope failuresoccurred 200 km north of Tokyo in the main island of Japan. All thefailed slopes are tabulated with geological and geotechnical propertiesin the web site (http://www.civil.chuo-u.ac.jp/lab/doshitu/eq_reports/2009/data_base/database_slope_2004niigata.htm). The damaged areashown in Fig. 8 belongs geologically to GreenTuff Region and is knownas a landslide-prone area with geological structures of active folding(JSCE (Japan Society for Civil Engineers), 2007). Not only themain shockbut a quite a few large aftershocks of MJ6 occurred for a few daysreecting the complexity of the fault rupture mechanism. The majorfault line, though it did not appear clearly at ground surface, was on theright side in Fig. 8with its strike NNESSW, indicating that the damagedarea was on the hanging wall of the thrust fault.

Slopes were composed of weak sedimented rock of Neogene age,consisting of interbedded layers of strongly weathered sandstonesand mudstones, and bedding planes had a strong effect on the slopefailures. A number of red spots in Fig. 8 indicate slope failures, some ofwhich blocked streams making landslide dams. Many blue spots alsoshown in the gure represent Koi ponds constructed on mountainslopes by farmers, who have cultivated Koi as an important localindustry in this region from ancient times.

The slope failures due to this particular earthquake may beclassied into 3 types, as illustrated in Fig. 9:

Type-A: deep slips parallel to bedding planes (dip planes), in gentleslopes of around 20. In many cases, the displaced soil/rock masseshad originally been destabilized by river erosion or road construc-

0.8

1.0

1.2

1.4

2008 Iwate-Miyagi Inland EQ.2004 Niigataken Chuetsu EQ.

sion

stre

ngth

: q (M

Pa)0 20 40 60 80 1000.0

0.2

0.4

0.6

Unco

nfin

ed c

ompr

es

Fines content Fc (%)Fig. 10.Unconned compression strength of samples from slopes failed during the 2004Niigataken Chuetsu earthquake and the 2008 Iwate-Miyagi Inland earthquake.tion, and they glided as rigid bodies along slip planes at the bottomof the sandstone. The displaced soil volumeswere very large and thesoil blocks sometimes showed little surface disturbance.

Type-B: shallow slips of 12 m deep not parallel to bedding planesin slopes of around 30 or steeper. These failures far outnumberedthe Type-A failures, but the individual soil volumes were not verylarge. Moving masses were highly disrupted internally, andsometimes left trees with deep roots in their original locations.

Type-C: slope failures inhighlyweathered colluvial soils inplaceswhereKoi-pondsandterracedpaddyeldswere located. Though this typemay

Fig. 11. Higashi-Takezawa slide (Type-A) during the 2004 earthquake seen from top ofscarp (large soil mass slid down as a block along the arrows, lled the valley, climbed upto the other side, and dammed the river).Kokusho et al., 2009a.Fig. 12. Photograph of Haguro Tunnel Entrance slide during the 2004 earthquake(Type-B) where surface shallow soil slid down and disintegrated into pieces.Kokusho et al., 2009a.

In most of the slope failures, sandstones were largely responsiblemainlybecauseof theirweaknessdue to strongweathering. Compressiontest results on samples taken from failed slope scarps are shown on thechart of q (unconned compression strength) versus Fc (nes content:particle size smaller than 0.075 mm) in Fig. 10 with open circles. The

Fig. 13. Photograph of Musikame slide during the 2004 earthquake (Type-C) where aKoi-pond triggered long runout distance failure.After http://www:ajiko.co.jp (Kokusho et al., 2009a.

E

121T. Kokusho et al. / Engineering Geology 122 (2011) 115128Epicenter

DFaul

t pla

ne

10 km

CB

A

Fig. 14. 1800 slope failures occurred mostly in the hanging wall of the fault and alongseveral valleys, which are regionally grouped from A to E, during the 2008 Iwate-MiyagiInland earthquake.strengths of intact sandstones (Fc030%) are qu=0.1 MPa or smaller,considerably weaker than those of interbedded mudstones (Fc100%)with qu0.8 MPa. Also noted is that the sandstones consisting of poorlygradedne particles had higher permeability (of the order of 103 cm/s)than that ofmudstones (of the order of 104106 cm/s) andhencemayhave served as aquifers (Kokusho et al., 2009b).

The most representative example of failure Type-A is shown inFig. 11 (Higashi-Takezawa), where a block of highly weatheredsandstone (actually weakly cemented sandy soil), of about 1 mil-lion m3 and 15 m thick, slid about 100m on a mudstone slip plane of20. The displaced soil mass blocked a river, making a naturalreservoir on the right side of the photograph.

One of the largest Type-B failures is shown in Fig. 12 (HaguroTunnel Entrance) where about 80,000 m3 of soil debris ran out morethan 100 m. A soil mass 48 m thick, disintegrated into small piecesslid down a slope steeper than 35 and reached houses below.

Fig. 13 shows a typical slope failure of Type-C (Mushikame), where3

1.2 km

0.8 km

Type-AMan-made reservoir

Main scarp

1.2 km

0.8 km

Type-A

Sliding direction

Man-made reservoir

Main scarp

Fig. 15. Aratozawa slope failure during the 2008 Iwate-Miyagi Inland earthquake a 1.2 kmby 0.8 km slide moved almost horizontally about 360 m along a deep-seated slip plane.about 160,000 m of soil with highwater content ran downmore than100 m into a river below as a mud ow. A Koi pond seems to haveplayed an important role in triggering the failure because it kept watercontent high making the slope seismically instable, and internalerosion by pond water eventually caused the large-volume failure.However, a lot still needs to be learned before the exact mechanism ofthe Type-C failure is fully understood.

4.2. 2008 Iwate-Miyagi Inland Earthquake

Iwate-Miyagi Inland earthquake (MJ=7.2, thrust fault, focal depth8 km) occurred in June 14, 2008, 400 km north of Tokyo in the mainisland of Japan. During the earthquake, very strong ground motionsweremeasured near the fault; PGA (PeakGround Acceleration) of 12 gand PGV (Peak Ground Velocity) of more than 50 cm/s horizontally.About 1800 slope failures occurred mostly in the hanging wall of thefault and along several valleys,whichare regionally grouped into 5 areasas shown in Fig. 14 (Geospatial Information Authority of Japan: http://zgate.gsi.go.jp/iwate2008/index2.htm). The fault plane is indicatedwitha blue line rectangle with a N-S strike dipping westward in the gure.The bedrockwasmostly of volcanic rocks ofMiocene and Pleiocene age;consisting predominantly of welded/non-welded tuff, sandstone, andsiltstone. All the failed slopes are tabulated with geological andgeotechnical properties in the web site (http://www.civil.chuo-u.ac.jp/lab/doshitu/eq_reports/2010/pdf/slope_data_iwate_Miyagi.xls). The

unconned compression strengths of samples from failed slopes duringthe earthquakeare shown in Fig. 10with solid squares. The strengths arevery variable (q=0.21.2 Mpa) and seems to increase with increasingnes content (Kokusho et al., 2009c). It may be judged that the rocks inthis area are stronger than those of the 2004 Chuetsu earthquake forrockswith small Fc in particular andalso that sandymaterialswith lower

4.3. Statistics of failed slopes in the two earthquakes

All slope failures during the two earthquakes (4321 and 1821slopes, each) are statistically analyzed based on air-photographstaken just after the earthquakes. Fig. 17 shows the percentage of thenumbers of failed slopes corresponding to several classes of affectedarea, 100 to 106 m2. The affected area is dened as a total area of slopefailure, encompassing scar, path and deposit. It is noted that, despitedifferent geological and topographical conditions, the two earth-quakes show very similar trends in relative numbers of failed slopesbelonging to individual area classes. There exists a clear peak at thearea of 1000 to 3200 m2, below which the number of failed slopesdecreases clearly. This may somehow reect a technical limitationthat slope failures smaller than this threshold (around 1000 m2

according to Figure 17) may be difcult to locate in air-photographs.Fig. 18(a) and (b) show variations of numbers of failed slopes and

affected areas per 1 km2 in concentric circles of stepwise epicentraldistances for the two earthquakes. Despite somewhat different trends

14 7a

0 400 800 1200 1600 2000200

300

400

500

Horizontal distance (m)El

evatio

n (m

) before after slip surface

Fig. 16. Cross-section along the center line of the large Aratozawa slide triggered during the 2008 Iwate-Miyagi Inland earthquake.

122 T. Kokusho et al. / Engineering Geology 122 (2011) 115128Fc and lower strength served as slip planes during this earthquake, too.Fig. 15 shows the largest landslide, at Aratozawa, where a mass

1.2 km long by 0.8 km wide slid almost horizontally more than 350 malong a deep-seated slip plane toward a man-made reservoir. Thetotal volume was about 35 million m3. The cross-sectional view alongthe center line is shown in Fig. 16. Though the exact depth of themajor slip plane is difcult to identify exactly, it is estimated as thedashed line, with a dip angle of around 5. The landslide, whichmovedin a direction slightly oblique from a path directly toward thereservoir, collidedwith amountain in front, while part of the landslidemoved into the reservoir, triggering a small tsunami.

Slope failures during the 2008 earthquake have also been classiedinto 3 types as follows, although the classication may not be so clearas the 2004 earthquake, presumably because the rocks are of volcanicorigin without clear bedding plane which are prevalent in this area.

Type-a: large slides moving almost as rigid bodies along deep slipplanes.

Type-b: medium size slides with characteristics in between Type-aand Type-c.

Type-c: small shallow slides with disintegrated debris.

The Aratozawa landslide may be classied as Type-a, consisting ofrigid body movement along a deep slip plane, which was probablysaturated and under high pressure, though local failures alongshallower slip planes associated with the major slip plane made thefailure more complex.

40

50

60

lope

s (%

)

2004 Niigataken-Chuetsu EQ (Total No.:4321)

2008 Iwate-Miyagi Nairiku EQ (Total No.:1821)0

10

20

30

106