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ORIGINAL ARTICLE

Surface geophysical investigations of landslide at the Wiri areain southeastern Korea

Man-Il Kim Ji-Soo Kim Nam-Won Kim

Gyo-Cheol Jeong

Received: 20 July 2009 / Accepted: 28 September 2010 Springer-Verlag 2010

Abstract A geophysical survey was undertaken at Wiriarea of the Andong in southeastern Korea to delineate sub-surface structure and to detect the fault zone, which affectedthe 1997 mountainhill subsidence and subsequent roadheaving initiated by the intense rainfall. Electrical resistivitymethods of dipoledipole array proling and Schlumbergerarray sounding and seismic methods of refraction andreection proling were used to map a clay zone, which wasregarded as the major factor for the landslide. The clay zonewas identied in electrical resistivity and seismic sections ashaving low electrical resistivity ( \ 100 X m) and low seismicvelocity ( \ 400 m/s), respectively. The clay zone detectedby using geophysical methods is well correlated with itsdistribution from the trench and drill-core data. The resultsof the electrical and seismic surveys showed that slopesubsidence was associated with the sliding of saturated clay

along a fault plane trending NNWSSE and dipping 10 20SW. However, the road heaving was caused by the slopemovement of the saturated clay along a sub-vertical NNE-trending fault.

Keywords Landslide Fault Surface geophysicalsurvey Electrical resistivity Seismic velocity

Introduction

The term landslide is applied to a wide range of massmovements ranging from soil creep to rock avalanche, andwithin a single slip the type of movement and the degree of disruption may change as the movement proceeds (McCannand Forster 1990 ). Landslides are therefore composed of materials of varied lithology and physical properties, whichmay range from unconsolidated sediments to hard rock.

There are many factors affecting the stability of slopes.Change in any of them or a combination of disturbingfactors can alter the stability of a slope and lead to failure.In some cases, increase of disturbing forces acting on aslope can be caused by a relatively sudden triggering eventwhether natural (e.g., earthquake) or human generated,such as explosion. In some cases, on the other hand, a slopefailure can take place slowly as a result of intense precip-itation during a rainy season or storage water in a reservoirof a dam construction (Murck et al. 1995 ).

The main factors that inuence stability of slopes are (1)gradient of slopes; (2) hydrological and hydrogeologicalcharacteristics of the slope area; (3) structural geology of the slope area; and (4) occurrence of a triggering event.From a geological point of view, therefore, a validassessment of a landslide requires bringing a solution forgeological and geotechnical problems of a slope resting on

M.-I. KimOfce of Environmental Geology,Korea Rural Community and Agriculture Corporation,487 Poil-Dong, Uiwang, Gyeonggi-Do 437-703, Koreae-mail: [email protected]

J.-S. KimDepartment of Earth and Environmental Sciences,

Chungbuk National University, Chongju 361-763, Koreae-mail: [email protected]

N.-W. KimWater Resources and Environmental Research Division,Korea Institute of Construction Technology,Goyang 411-712, Koreae-mail: [email protected]

G.-C. Jeong ( & )Department of Earth and Environmental Sciences,Andong National University, Andong 760-749, Koreae-mail: [email protected]

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a mountainhill. These problems are mainly: strike and dipdirections of strata and slope, relief of bedrock and facturezone, water content of clay, groundwater level and internalcomposition of a slope.

In slope stability assessment, the results of the aboveinvestigation are used. In the application of surface geo-physical exploration to landslides, electrical resistivitysounding and seismic refraction surveys have been themethods most commonly employed with the primary aimof providing information about the geological framework and the mechanism of landslide (Bogoslovsky and Ogilvy1977 ; McCann and Forster 1990 ; Caris and Van Asch1991 ; McGuffey et al. 1996 ; Hwang et al. 2000 ; Mauritschet al. 2000 ; Bichler et al. 2004 ; Otto and Sass 2006 ;Goktu rkler et al. 2008 ; Jongmans et al. 2008 ).

In this study, the subsurface electrical and seismicstructures of the Wiri area of Andong, Korea, wereinvestigated using electrical dipoledipole mapping andseismic reection methods (Fig. 1). The geophysicalresults were correlated with trench and drill-core data. TheWiri area is well known for its large mass movements androad fracture activities, which took place in a heavy rainyseason in 1997. The objectives of this study were mainly togain an improved understanding of the clay-rich fault zone,which was reported to be the major factor contributing tothe 1997 slope failure (Jang and Jang 2000 ). Geophysicalmethods were applied to delineate discontinuities of faultand fracture zones, which facilitated the slope movementwithin the clay layer, and to further investigate the rela-tionship between the land subsidence and road heaving.

Because vertical electrical sounding indicates a changeof apparent resistivity with depth at the measuring point, itis capable of detecting the water-saturated clay, which ischaracterized as a zone of lower resistivity. The fault clayin this case means a clay driven and caught in the faultzone. The 2-D resistivity section was produced by using adipoledipole conguration and it was interpreted in termsof the dip, strike and thickness of the fault clay. A pseudo-3-D resistivity volume constructed from the interpolationof sounding data was correlated with the 2-D resistivitysection to provide a better understanding of the mechanismof landslide. Complex interpretation of the study area wasachieved by incorporating the electrical resistivity struc-tures with the seismic velocity structures from refractiondata, known to be an effective tool for landslide analysis. Inaddition, a short seismic reection line was carried out toprovide information on the sub-vertical fault zone.

Location and history of landslide

The study area, located in Imdong Myun, Andong, Korea(Fig. 1), suffered heavily from landsliding, which caused

road failure and ground fracture after a heavy rainfallseason in 1997 (Fig. 2). Road 999 had been built in 1989along the mountain valley in time for the construction of the Imha dam. Since then, this area has been continuouslysuffering from being prone to landslide due to reduction of loading; mountain-hill sliding in the eastern part and roadheaving in the western part. The slide block was dormantuntil 1997, when unusually heavy precipitation raised thewater table and initiated intermittent movement of the clay,which was facilitated by the natural planes of weaknesssuch as bedding planes in the sedimentary rock. The slipsurface, dipping to SW with an angle of 11 , was exposed

Fig. 1 Geological map of study area. Electrical dipoledipole lines( D1 , D2 , D3 and D4) and seismic lines are deployed to examineNNE- and NNW-trending faults. The inset shows 16 electricalsounding sites. Trenching (TR-1, TR-2) and drill coring (WB-1,WB-2) are performed in the vicinity of the faults. The location of theslip surface is represented by asterisk

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at the northeastern margin of the survey area (Fig. 3). The

clay was struck by the sub-vertical fracture zones at theroad bend while moving southwestward, which raised italong the roadside at the rate of approximately 20 cm/day(Fig. 3b, c).

Geology and trench/drill-core data

Geologically, the area under investigation belongs to thenorthern margin of the Cretaceous Kyungsang Basin,located in southeastern Korea. The rocks involved aresedimentary rocks consisting of mainly arkose sandstone,red mudstone and shale deposited at the western margin of the small Yungyang Basin. The Wiri area is at the top of the Cretaceous Donghwachi formation, in contrast with theGasongdong formation in Kyungsang basin, and consists of alternating sandstone, ne silty sandstone and shale in theWiri area shown in Fig. 1.

Two major faults intersect in the northern part: a steeplydipping NNE-trending fault (NNE fault), which crosses theroad bend in the southwestern part, and an NNW-trendingfault (NNW fault) starting at the northeastern part with agentle southwesterly dip. In 1997, the sliding massesresting on the failure plane of NNW fault were reported tomove southwesterly and then uplifted through sub-verticalfracture zones by forming a slickened line when they struck the NNE fault (Jeong 1998 ). On a larger scale, the heavingoccurred along the roadside, which was prone to weakerloading, compared with the stronger load at the easternmountainside, which is related to the occurrence of landsubsidence. The failure is assumed to have occurred alongthin clay layers (ranging from a few millimeters to520 cm), which are observed in the hillsides shown inFig. 4.

Trenching (TR-1, TR-2) and drill coring (WB-1, WB-2)were carried out to investigate the stratigraphy and geo-logical structures of the study area. The trenches areapproximately 2-m deep and dug close to road 999. Trenchresults show that the major lithology is sandstone withsome interbeds of thin clay layers dipping toward SW(Fig. 5a) and SE (Fig. 5b). The SW- and SE-dippinginterbeds appear to be associated with the attitudes of theNNE and NNW faults, respectively. For the drill-coreanalysis, an NX-sized drill was used for the 10-m longcores at each site. Drill coring was carried out at WB-1,where upheaval of the clay layer occurred, to investigatethe clay layers along an NNE fault caused by the horizontalloads from the eastern side. The analysis revealed thatsandstone and siltstone layers exist beneath the clay zone(Fig. 4a) with their attitude affected by the NNW fault.Drilling at WB-2 was set a bit off the NNW fault to check the distribution of the clay layers, which were interlayeredwithin sandstone layers (Fig. 4b). Interlayers of sandstone,

Fig. 2 Rainfall and landslide data for the Wiri area, shown fromJanuary to July 1997. The red dotted line depicts the timing of slidingat Wiri area on 7 July 1997

Fig. 3 During the 1997 landslide, a sliding plane exposed with a 11

SW dip, b road 999 was fractured, and c clay heaved on the roadsideat the rate of approximately 20 cm/day

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siltstone and clay were identied. On the basis of obser-vations of silty clays at the upper depth from WB-1 and atthe lower depth from WB-2, the silty clay dips south-westward and straties at a depth of 10 m near the NNEfault.

The study area has been stabilized presently afterreduction of the overburden load (Fig. 6), placing therubble debris appropriately and establishing drainage underthe roadside. These remediation steps took into consider-ation the results of the in situ permeability, drawdown/slugtests and stability analysis.

Electrical resistivity mapping

The resistivity of soil is dependent on the degree of satu-ration, the resistivity of pore uid, porosity, and shape andsize of solid particles. Resistivity can vary from 105 X m indry sand to 10 X m in wet sand (Fukue et al. 1999 ). Theelectrical properties of clay are more complicated in termsof fabric, because the diffuse double layer formed on andbetween the particles may show different conductivityfrom the free pore water (Jackson et al. 1978 ). According

Fig. 4 Lithological descriptionof drill cores: a WB-1 andb WB-2. Southwest dippingtrend is indicated by theobservation of silty clays, whichare deeper (a depth of 10 m ormore) in WB-2 than in WB-1

Fig. 5 Photographs of trenched outcrops. Major lithology is sand-stone with some interbeds of silty clay toward a SW for TR-1 andb SE for TR-2

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to the statistical data from Reynolds ( 1997 ), resistivity isvery dependent on water content: typical resistivity for clay

with 3040% water content is 1100 X m, which is muchlower than dry clay of 100200 X m.

Taking into account the interpretation that the 1997landslide and deformation were primarily caused by thedecrease of shear strength of the water-saturated clay zone,this investigation is mainly concerned with electrical DC-resistivity mapping and sounding of the clay layer with anemphasis on the determination of the attitudes (dip, depthand extension, etc.) of the fault clay.

A total of 16 electrical sounding sites were selected foran investigation of the change of resistivity with depth. Inaddition, four dipoledipole lines (D1, D2, D3 sub-parallelto road, and D4 crossing the road) shown in Figs. 1 and 6were run to provide 2-D resistivity structure.

Data acquisition

The electrical resistivity survey was designed to penetrateto a depth of a few tens of meters. The data acquisitionsystem used was the ABEM SAS 300C. To increase S/N,electrical current was input at a maximum of 250 mA byemploying an SAS 2000 Booster. Soundings were closelyspaced as it is known that geo-electrical conditions inlandslide areas can change rapidly with distance (Caris andVan Asch 1991 ; Bogoslovsky and Ogilvy 1977 ).

In this study, vertical electrical soundings were carriedout using a Schlumberger array at 16 sites on the at areabetween dipoledipole lines D1 and D2. A Schlumbergerarray was employed for two reasons: (1) Using the Wennerconguration requires all four electrodes to be movedbetween successive depth soundings, whereas with theSchlumberger array the potential electrodes are maintainedat xed spacings. (2) Electrical effects for the xedpotential electrodes are integrated equally into all the

recordings and therefore electrical inhomogeneities causedby the movement of the potential electrodes are effectively

overcome. Soundings were made with a maximum AB/2 of 25 m. This provided the capability to probe the subsurfacestructures to approximately 25 m. However, interpretationwas limited down to a depth of 12.5 m based on a criterionof the depth of investigation (Barker 1989 ). Current elec-trode cables were spread in a northsouth direction on theat area to overcome topographic effects shown in Fig. 6.

The dipoledipole proles were surveyed employing a 5or 10-m dipole spacing, a 21-station array, and a maximumdipole separation n equal to 8, which resulted in a maxi-mum imaging depth of approximately 25 or 50 m,respectively.

The vertical electrical sounding and dipoledipole datawere processed for this study using Soundpro ( 1996 ) andDipro ( 2000 ), respectively. Dipro is a two-dimensionalelectrical resistivity data processing software based onWindows 95/98/NT/2000 operating system. As for theDipro package, the measured data are inverted to trueresistivity image by the 2.5-dimensional inversion that isbased on nite element modeling with topographic cor-rection and the active constraint balancing (ACB). TheACB method is employed to enhance image resolutionwhile inverted. The simple equation of the ACB method isshown in Eq. 1 as a type of variable Lagrangian multiplierbased on the automatic parameter resolution analysis formodel regularization (Yi et al. 2001 2003 ).

e2 k x; z f x; z 2ffi 0 1

where e is an error between eld observed value and the-oretical value, k is a constant of the Lagrangian multiplier,and k ( x, z) is a function of x and z automatic determinationbased on the model resolution distribution.

The modeling and inversion algorithms incorporate thetopographic correction, which is included in the modeling

Fig. 6 Site view of electricalresistivity and seismic surveylines . The solid dots betweenthe line D1 and D2 indicate thevertical electrical soundingpoints. Note that the panoramicview appears to be distorted dueto the projection angle of thecamera

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stage. Inversion employed in Soundpro adopts the least-square method with smoothness constraint (Soundpro1996 ).

Vertical sounding data

In 1997, landslide initiated in the northeastern part of thearea and the sliding mass moved southwestward toward theroadside. The estimation of the bulk attitude of the claylayer and the NNW fault can be effectively examined in thearea some distance away from the roadside. Therefore, 16vertical sounding areas were selected in the at region(15 m 9 15 m area) between dipoledipole lines D1and D2, and approximately 100 m distant from the road(Figs. 1, 6).

The sample data for the four points spread in theNESW direction (inset in Fig. 1; VES 1, 7, 11, and 13) areall characterized by H-type resistivity curves (Burger 1992 ),

as shown in Fig. 7. A southwest dipping event, which is theclay zone associated with the 1997 landslide, can be seenby connecting the depths of the lowest apparent resistivitypeak.

In an attempt to investigate the sounding curves inthree dimensions, the 16 inverted resistivity proles wereinterpolated using the cubic-spline method (IDL 1996 ) toprovide 2-D resistivity sections at dened depth levels.A pseudo-3-D resistivity volume was constructed by usingthe 2-D resistivity sections (Fig. 8a). The lower resistivityzone (\ approximately 100 X m) appears at the northeast-ern edge in the 0.5 m-depth slice, at the diagonal in

the 3 m-depth slice, and at the southeastern edge in the5 m-depth slice. Connection of these events conrms thepresence of the approximately 15 SW-dipping eventidentied in Fig. 6. Such a southwest dipping event isagain observed in another section of depth-to-low resis-tivity peak (silty clay), indicated by arrows in the bottompanel of Fig. 8b.

Dipoledipole data

The SW-dipping fault clay, previously identied from thevertical sounding data, was investigated using dipoledipole resistivity mapping. To map the resistivity distri-bution of the fault clay in two dimensions, dipoledipolearray proling was carried out in the southwestern part of the survey region (covering an area of approximately200 m 9 200 m). The survey lines D1, D2 and D3 ransubparallel to the road to examine the attitude of the faultclay associated with the landslides. Line D4 crosses theroad to conrm the presence and posture of the sub-verticalfault (Fig. 1).

Figure 9 illustrates the inversion process of the Diprofor the line D1. The processing strategy is to determine theresistivity structure that brings the theoretical data as closeas possible to eld data by minimizing the differences on aleast-square basis between the eld data and the theoreticaldata. The observed data (Fig. 9a) and the computed resis-tivity value (Fig. 9b) show a close t, which conrms thereliability of the 2-D resistivity structure (Fig. 9c) imagedthrough the inversion process shown in Table 1.

The best t resistivity structure determined from theinversion results is interpreted in terms of four individuallayers shown in Fig. 10: silty clay ( \ 100 X m), sandy silt-stone (100200 X m), sandstone (200400 X m) andweathered rock ( [ 400 X m). The silty clay is approxi-mately 5-m deep at stations 1214 on the line D1

Fig. 7 1-D electrical resistivity prole from four sounding sites:VESs 1, 7, 11 and 13 (see Fig. 1 for location). Peaks indicate the claylayer dips southwestward

Fig. 8 Electrical resistivity volume constructed by interpolation of electrical sounding data. Southwesterly dipping layer is claried indepth-to-silty clay as shown by arrows . a Pseudo-3-D resistivityvolume for the each depth. b Depth-to-low resistivity peak (silty clay)

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(Fig. 10a) and 10-m deep at stations 1014 on the line D2(Fig. 10b). With respect to attitude of the fault clay, thelow resistivity zone is estimated to be gently dipping as lowas approximately 12 SW. Such a dipping layer is mostlikely associated with the activity of the 1997 landslidefailure.

Results for line D3 (Fig. 10 c), located close to the road,illustrates the clay fault imaged down to approximately25 m, which, projected to the clay layer on line D1, indi-cates that the clay layer dips at approximately 18 . The dipof the clay tends to increase toward the road. There appearsto be a sub-vertical boundary near station 8, which isprobably a sub-vertical fault zone associated with the roadheaving in 1997. It should be noted that the low resistivityzone of the clay layer no longer extends southwesterly nearthe road.

The SW-dipping clay and the sub-vertical fault wereidentied on line D4 (Fig. 10d), with the same attitude asthat on line D3. It is concluded that the fault clay in theeastern part did not move southwesterly near the road, butmoved within the sub-vertical fracture zones developedwest of the road.

The silty clay also correlates well with the observa-tions from the drill-core data (Fig. 4a, b); the silty claydips southwestward (toward WB-2) to a depth of 10 m ormore near the NNE fault. The slope-failure terrain iseffectively identied in the fence of resistivity sections,shown in perspective viewing NW (Fig. 11 ). The claylayer appears to be gently dipping between D1 and D2,steeply dipping between D2 and D3, and sub-vertical atthe roadside. The activity of the 1997 road heaving isexplained by the movement of the saturated clay, whichrose along the weak zone of NNE sub-vertical fault zoneat the roadside.

Seismic survey

Seismic refraction and reection data were collected sep-arately in the vicinity of the road along a 46-m segment of line D3 (Figs. 1, 10c) to seismically investigate the NNEand NNW clay fault shown in Fig. 1. Seismic refraction (atotal length of 46 m) and reection (a total length of 23 m)lines were deployed and the seismic results are correlatedto the electrical resistivity structures.

Data acquisition and processing

Field acquisition parameters for the seismic survey wereselected to provide the capability of delineating refractorsand mapping reectors as close to the surface as possible.A 24-channel ABEM Terraloc Mark 6 was used as therecording system with a 4.7-kg hammer source. Signal 10and 100 Hz geophones with a spacing of 1 m were used inrefraction and reection mapping, respectively. To increasethe S/N, vertical stacking of six repetition phones (tenrepetition phones for reection mapping) was used. Thesampling interval and low-frequency cut lter were set as0.5 ms and 10 Hz, respectively.

Seismic refraction data were processed using the soft-ware GLI3D ( 1998 ) employed with the generalized linearinversion (GLI; Hampson and Russell 1984 ). For moreprecise delineation of the structures near the fault zone,

Fig. 9 Inversion process with the software Dipro using data acquiredalong line D1. a Apparent resistivity pseudosection. b Apparentresistivity theoretical pseudosection. c 2-D resistivity structures

Table 1 The error of t values tabulated for each inversion

Iteration 1st 2nd 3rd 4th 5th

RMS error 2.81 9 10- 1

1.88 9 10- 1

1.57 9 10- 1

1.47 9 10- 1

1.41 9 10- 1

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tomographic grid-based inversion (TGI) was also per-

formed. Refraction analysis was performed by incorpo-rating all of the 48 shot gatherers (24 shots 9 2experiments).

For the reection data, the software SU ( 1995 ) andVISTA ( 1999 ) were jointly used. In the processing, toreduce the artifacts produced during the S/N enhancementas much as possible, common processing steps for reec-tion imaging were taken: edit bad traces, AGC, bandpass/fk ltering, CDP sorting, NMO correction, residual statics,stretch muting and stacking.

Data interpretation

The sample refraction shot gatherers near the fault zone areshown in Fig. 12a, and their travel-time curves wereinvestigated in terms of intercept time, and slopes of directand head waves as shown in Fig. 12b. The velocities of theupper and lower layer were estimated to be 450520 and800920 m/s, respectively. The lateral continuity of thestructure was effectively achieved by incorporating all of the 48 refraction shot gathers (Fig. 13a). The position of the deep low velocity zone matches closely with the

location of the fault zone (station 9) previously mapped inthe electrical dipoledipole line D3 (Fig. 10 c).

For further investigation of the deep discontinuity nearthe fault, tomographic grid-based inversion (GLI3D 1998 )was performed. The four units previously mapped from theelectrical resistivity sections were again distinguished interms of seismic velocity: silty clay (400 m/s), sandy silt-stone (400800 m/s), sandstone (8001,000 m/s) andweathered rock ( [ 1,000 m/s) as shown in Fig. 12b. How-ever, sandy siltstone and silty clay do not have a sharpboundary as indicated from the resistivity data. This isprobably related to the increase of density and velocity of

clay material with water content (Fukue et al. 1999 ). Theattitude of the fault fracture zone matches well with theresult of the GLI inversion for the two-layer case: the sub-vertical fault is approximately 19-m deep at an offset of 10 m (Fig. 13a).

The sub-vertical fault zone mapped from the seismicrefraction survey is also conrmed in the seismic reectionline. In the brute stack (Fig. 14 a), the weak reectionevents are suppressed by the strong inclined featuresassociated with the surface waves and scattered energy

Fig. 10 Seismic velocity structures based on the dipoledipoleresistivity section for lines a D1, b D2, c D3 and d D4, respectively

Fig. 11 Resistivity fence-diagram in perspective viewing NW. Siltyclay dips southwesterly and heaves near the sub-vertical fault, asshown by the arrows

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from inhomogeneities such as the sub-vertical fault (Kim

et al. 1994 ). Thus, the apex of the inclined feature on thebrute stack provides information on the location of sub-vertical inhomogeneity. Using the basic processing stepsand parameters, the inclined features and uncorrelatednoise are greatly attenuated, resulting in well-focusedreections of the sub-vertical fault at 11.5 m in the nalstack (Fig. 14b). Intense reection events with sub-hori-zontal and a discontinuous geometry occur at depths of 0.025 s (approximately 8 m) to 0.1 s (approximately24 m). Three noticeable sub-horizontal events A, B and C

between 11.5 and 23 m are probably associated with thelayer boundaries of silty clay/sandy siltstone, sandstone/ weathered rock and weathered rock/bed rock, respectively.

Discussion

On the basis of the schematic movement model (Fig. 15 )provided from the results from the trench and the drill-coredata, and electrical resistivity and seismic refraction sur-

veys, it is suggested that the 1997 landslide failure wascaused by the decrease in shear strength of saturated faultclay due to intense rainfall, with the sliding mass movingalong the NNW fault plane. The physical properties forelectrical resistivity and seismic velocity of the layers aregiven in Table 2. The road fracture which occurred isassociated with the upward movement of the clay along theweak zone of the NNE sub-vertical fault near the roadside.

In this study, the effect of rainfall on the landslidefailure was not directly investigated because the survey sitewas already in a state of restoration and stabilization usingtreatments such as drainage works and debris lling under

the roadside. The water content of 61% of the area in therainfall season is interpreted to be sufcient to decrease theshear strength on the sliding surface (Fukue et al. 1999 ;Jang and Jang 2000 ; Zhang et al. 2001 ; Bichler et al. 2004 ).On the basis of the geophysical mapping of the clay layerand the fault zone near the road surface, we cannot elim-inate the possibility of such a slope-failure hazard. How-ever, such a landslide is less likely to occur again in theclay layer because of the completion of drainage works anddebris lling under the roadside.

Fig. 12 Results of refractionshot near the fault zone: a threeshot gathers selected fromseismic refraction data (forward,reverse and midpoint shooting)and b their travel-time curves .The arrows represent thecrossover distance

Fig. 13 Seismic refraction results using a generalized linear inver-sion (GLI) and b tomographic grid-based inversion (TGI). Verticalfault zone is shown as the valley at an offset of 10 m

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Conclusions

By employing electrical resistivity and seismic methods,the clay layer and the fault zone associated with the 1997landslide failure were successfully mapped. The results areas follows:

(1) According to the trench and drill-core analysis, thestudy area consists of sandstone and siltstone withclay interbeds, in which the landslide occurred by thedecrease in shear strength of saturated clay interbedsand the NNE and NNW faults highly disturbing thebedding. These observations correlate well with theshallow geophysical images.

(2) In the geophysical sections, the NNW fault (associ-ated with the landslide) is mapped to be gentlydipping 1020 NW, whereas the NNE fault (asso-ciated with the road heaving) is sub-vertical.

(3) Four individual layers can be distinguished in termsof electrical resistivity and seismic velocities. Thesilty clay layer was characterized by it lower electricresistivities ( \ 100 X m) and lower seismic velocities(\ 400 m/s), respectively. The fault clay was inter-preted to be the major factor in the 1997 landslidefailure.

(4) The results of the vertical electrical resistivitysounding and dipoledipole mapping showed thatthe subsidence of slope was probably associated withsliding of wet clay on the NNW fault plane, while theheaving of road was probably caused by upwardmovement of the wet clay along the sub-vertical NNEfault zone. These observations correlate well with thetrench and drill-core analysis.

(5) The multi-geophysical survey approach employed inthis study shows that it can be effectively used as atool for landslide failure problems.

Fig. 14 Seismic reectionmapping: a brute stack; theinclined feature is related to thesurface wave and scatteredenergy from verticalinhomogeneities and b nalstack. Events A, B, C areinterpreted to be the layerboundaries of silty clay/sandysiltstone, sandstone/weatheredrock and weathered rock/bedrock, respectively

Fig. 15 Schematic geometric depiction of the subsurface structuresat the investigation area: a before slope stabilization and b after slopestabilization

Table 2 Electrical resistivity and seismic velocity of the layers forthe slope-failure terrain

Physical properties Siltyclay

Sandysiltstone

Sandstone Weatheredrock

Electrical resistivity ( X m) \ 100 100200 200400 [ 400

Seismic velocity (m/s) \ 400 400600 6001,000 [ 1,000

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Acknowledgments This research was supported by grant (2-2-3)from the Sustainable Water Resources Research Center of 21stCentury Frontier Research Program.

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