Engineering Geology and Stability of the Laprak Landslide, Gorkha ...

Engineering Geology and Stability of the LaprakLandslide, Gorkha District, Western Nepal

NARAYAN GURUNG

Department of Civil Engineering, Indian Institute of Technology, Delhi, 110 016,Hauz Khas, India

WILLIAM C. HANEBERG1

Haneberg Geoscience, 3063 Portsmouth Avenue, Cincinnati, OH 45208

G. V. RAMANA

MANOJ DATTA

Department of Civil Engineering, Indian Institute of Technology, Delhi, 110 016,Hauz Khas, India

Key Terms: Landslides, Rainfall, Seepage, StabilityAnalysis, Finite Element Analysis, Nepal

ABSTRACT

The Laprak landslide is a complex consisting oftranslational and rotational landslides, debris flows,and rockslides triggered by exceptionally heavy rainfallin July 1999, with continuing movement duringsubsequent monsoonal wet seasons. The active portionof the complex is a structurally constrained wedge ofcolluvium with a triangular or trapezoidal cross section,the geometry of which is controlled by two or three setsof foliation and joint planes, derived from phyllitic andquartzose metamorphic bedrock of the High Himala-yan Crystalline Sequence. Stability analyses, con-strained by field observations, limited geotechnicaltesting, and finite element simulation of steady-stateseepage, show that the landslide most likely moves asthree sections, with the middle section being more stablethan the upper and lower sections. Non-slope-parallelseepage simulated by the finite element model appearsto locally decrease instability by allowing drainage intothe discontinuous bedrock. The stability analyses alsosuggest that the landslide will continue to move duringthe wet season when pore-water pressures are moder-ately high (although complete saturation of the slope isnot required). Moderate earthquakes would exacerbatewet season instability but do not appear sufficient totrigger movement during the dry season, when pore-water pressures are low. The remote location ofLaprak, which is a two- to three-day walk from the

nearest road-head, and lack of money limit remedialoptions to those that can be undertaken using manuallabor and native materials, for example, replacement ofdry stone masonry buildings with lightweight woodenstructures and surface drainage improvement.

INTRODUCTION

The village of Laprak, which is home to about3,500 people, is located at 28u1394.80N, 84u48912.70E,at about 2,150 m elevation along the Raizo Kholavalley in the Gorkha District of western Nepal(Figure 1). There is no road access, and a two- tothree-day trek, depending on weather conditions,through mountainous terrain is required to reach thevillage. On July 3, 1999, heavy rain in excess of340 mm in 24 hours, much of it apparently occurringduring about 3 hours, triggered a series of rockslidesand debris flows that led to the reactivation of part ofan existing landslide of unknown age and upon whichthe village is built. One woman was killed, ten homeswere destroyed, and about 12 hectares of cultivatedland were taken out of production along the banks ofthe adjacent valley during the initial episode ofmovement. The landslide has since moved duringeach summer monsoon season, with more damagingmovement in 2002, 2006, and 2007, destroying anadditional 14 homes and taking an additional11 hectares of cultivated land out of production(Figure 2). Because most buildings in Laprak areconstructed with unreinforced dry stone, and manyhave already been damaged by landslide movement,seismic shaking with or without additional landslidemovement poses an added threat to the village. Theremote location of the village, lack of road access,1Email: [email protected].

Environmental & Engineering Geoscience, Vol. XVII, No. 1, February 2011, pp. 23–38 23

reliance on manual labor, and severe budget limita-tions all place significant constraints on mitigationoptions.

Purpose and Scope

This paper summarizes the engineering geology andstability of the portions of the Laprak landslide thatbegan moving in 1999 and that continue to moveepisodically during yearly monsoons. Our analysis—undertaken to both understand the initiation of the1999 landslide and elucidate conditions that maytrigger future movement—is based upon a series ofreconnaissance visits to Laprak, topographic and

engineering mapping of features related to thelandslide, digital terrain modeling to visualize thegeomorphology of the landslide and bedrock controlson colluvium accumulation, a limited program oflaboratory soil testing, and slope stability analysis ofseveral static and seismic scenarios with and withoutcontributions from bedrock groundwater flow.

Initial Movement

The Laprak landslide began on the morning of July3, 1999. While walking at about 6:30 am, Mr.Dambar Gurung, the village peon or local adminis-trator, noticed newly deposited boulders belowquartzite cliffs at the head of Chhelong Gully, whichforms the northwestern lateral boundary of thecurrent landslide. He also witnessed small rockfallsor rockslides at that time and estimated that the maindebris mass had been deposited around 4:00 to 5:00am. After a period of increasing rainfall intensity,debris began to flow down Chhelong Gully at about7:30 am, sweeping away homes built along the gullyedge and existing check dams within the gully.Because no pre-flow topographic maps exist, it isimpossible to determine the amount of debris-flowscour and/or deposition along the gully. Movement ofthe main landslide mass is likely to have beencontemporaneous with or subsequent to the largedebris flow. As will be discussed, small-scale quarry-

Figure 2. Annual, monthly maximum, and daily maximum rainfall measured at Barpak, near the Laprak landslide. The maximum dailyand maximum monthly rainfall values are for the months during which landslide movement occurred. The yellow bars indicate years inwhich landslide movement was reported along with the damages for each year.

Figure 1. Index map of Nepal showing the location of theGorkha District and Laprak.

Gurung, Haneberg, Ramana, and Datta

24 Environmental & Engineering Geoscience, Vol. XVII, No. 1, February 2011, pp. 23–38

ing of the quartzite near the gully head may have alsocontributed to the initial movement.

The nearest rain gauge is in the village of Barpak, ahalf-day walk over a 3,000-m-high ridge fromLaprak. The 1999 movement occurred in responseto extreme short-term rainfall of 342 mm in 24 hoursand 10-day antecedent rainfall of 235 mm at Barpak.According to residents of Laprak, most of the rainfallthat triggered the 1999 landslide occurred during thethree hours before movement began. Based onanecdotal accounts of the rainfall duration, the 1999storm appears to plot near the landslide-triggeringintensity-duration threshold developed for the NepalHimalaya by Dahal and Hasegawa (2008) and wellabove the sediment-producing threshold developedfor the Annapurna region of Nepal by Gabet et al.(2004). Although damaging movement of the land-slide in both 1999 and later in 2007 was associatedwith extreme rainfall events, other damaging episodesin 2002 and 2006 were associated with average tobelow average rainfall. Less significant movementappears to occur almost every year during thesummer monsoon season.

Geologic Setting

Laprak lies above the Main Central Thrust (MCT),which is one of three major thrust faults running thelength of the Himalayas. The MCT defines the HighHimalayan Front and brings predominantly Precam-brian metamorphic rocks of the High HimalayanCrystalline Sequence to the surface (Figure 3).Bedrock in the vicinity of the village, which isexposed locally along riverbanks and ridges, consistsof garnet mica schist, calcite schist, and quartzite.Instrumental records (Pandey et al., 1999) show aband of seismicity coincident with the High Himala-yan Front defined by the MCT, and several segmentsare inferred to be capable of producing earthquakessimilar in size to the 1934 M 5 8.1 Nepal-Biharearthquake. Lave et al. (2005) described paleo-seismicevidence for an even larger Mw 5 8.8 earthquake inNepal sometime around 1100 A.D.The Laprak landslide is located on a steep

colluvium-mantled slope with strong bedrock struc-tural control (Figure 4). Foliation dips steeply towardthe northeast and is generally oblique to the average

Figure 3. Generalized geologic map showing the location of Laprak relative to the Main Central Thrust (MCT) and other major geologicfeatures. The hx map unit is the High Himalayan Crystalline Sequence. Modified from Amataya and Jnawali (1994).

Laprak Landslide, Nepal


topographic slope in and around Laprak. In someplaces near the head of the landslide, however,discontinuities in quartzite are nearly parallel to thelocal topographic slope. The quartzite at the ridgenear the headscarp of the landslide contains jointsand fissures that make rockslide and rockfall poten-tial an issue, in addition to landsliding further downthe slope. There is a 5-m- to 13-m-thick layer ofcolluvium, as estimated from stream-bank exposures,above the bedrock in most places along the slope.Topographic slope in the vicinity of Laprak

averages about 30u and in some places exceeds 45u;there is a ridgeline above its head and the RaizoKhola (river) at its toe. More than 70 percent of thearea is steeper than 25u, and more than 30 percent issteeper than 30u. Elevation ranges from about 2,500 mat the ridgeline to about 1,750 m at the toe along theRaizo Khola. The landslide is about 1,550 m long and200 m to 650 m wide, and there are 25- to 60-cm-wideand 4- to 12-m-long tension cracks in several places.Springs also appear during the monsoon season. Inparticular, soil along the banks of the adjacentChhelong Gully is chronically unstable, and thehouses along the edge of the gully are more vulnerablethan those on other parts of the landslide. New cracks

continue to develop in and around the village witheach episode of landslide movement.The landslide is a complex consisting of several

translational landslides, rotational landslides, debrisflows, and rockslides (Varnes and Cruden, 1998); thetranslational components are predominant. Fieldobservations and interpretation of mountain-scalegeomorphology using shaded relief images andcontour maps produced from a 30 m ASTER(Advanced Spaceborne Thermal Emission and Re-flection) satellite DEM (digital elevation model)obtained for this project suggest that the currentLaprak landslide may be part of a much largerancient landslide complex of unknown age and depth,which may or may not involve bedrock (Figure 5).

FIELD, LABORATORY, ANDOFFICE INVESTIGATIONS

Fieldwork

The descriptions in this paper are based uponfieldwork performed during four visits to Laprak.Preliminary observations were made in November1999, three months after the landslide. Additional

Figure 4. View to the southeast across the Raizo Khola valley toward Laprak and the landslide. Locations denoted in the photograph are:(A) area of planar rockslides at head of Chhelong Gully, (B) quartzite boulder field, (C) active headscarp, (D) area of stable shallow bedrockand the village school (white buildings), (E) typical translational slide developed in thin soil on a foliation dip surface, (F) recent rotationallandslide component associated with widening of Chhelong Gully, and (G) older stable landslide of unknown age. L1 through L4 mark theapproximate locations of soil samples collected for laboratory analysis.



visits occurred in July 2007, May 2008, June 2008,and January 2009.

A detailed 1:2,000 topographic map of the landslidewas prepared by contouring points surveyed using atotal station and global positioning system (GPS) unit.The locations of footpaths, surface drainages, andrepresentative buildings in the village were also sur-veyed; however, the density of construction and steepslopes within the village made it impossible to survey allof the building locations during the time available forfieldwork. Colluvium thickness, as exposed in gulliesadjacent to the landslide, surface drainage paths, andseepage locations, were noted during mapping.

Because the colluvium contains a considerable pro-portion of sand and rock fragments, it was not feasible toobtain undisturbed samples for testing. Disturbedsamples, however, were obtained from depths of about1 m at four different locations shown in Figure 4.

The strike and dip of foliation in the schist, joints,and the ground surface slope were measured near the

headscarp, along the western edge, and along thesouthern edge of the landslide to obtain informationabout rock fractures and constrain kinematic slopestability analyses. A modified Q-system chart (Bar-ton, 2002) was used for rock mass classification in thefield. Spacing of discontinuities, roughness of discon-tinuities, groundwater flow, discontinuity length,aperture, or separation, infilling materials, andweathering grade were also recorded.

Digital Terrain Modeling and Landslide Morphology

The topographic contours were interpolated onto a2 m grid to create a digital elevation model (DEM)using the geographic information system (GIS)software GRASS 6.3. This was accomplished byimporting the contours, rasterizing them, and theninterpolating values between the rasterized contours.The resulting DEM and geomorphic derivative maps

Figure 5. Areas of suspected landsliding along the Raizo Khola drainage in the vicinity of Laprak, as inferred from field observations andinterpretation of a 30 m ASTER DEM. Simulated illumination is from an inclination and azimuth of 30u/315u.



created from it were used to visualize the surfacemorphology and lateral extent of the landslide, as wellas to ascertain the control of bedrock structures oncolluvium accumulation, using a suite of shadedrelief, contour, slope angle, and slope aspect mapsproduced using standard GRASS functions to sup-plement the topographic contours (Figure 6A–C).The head of the active portion of the landslide

consists of an arcuate crack and attendant topo-graphic break at the southwestern end of the slide.

The northwestern (left) lateral boundary coincideswith the Chhelong Gully. The southeastern (right)lateral boundary is indistinct but likely skirts a knobof shallow to exposed bedrock upon which the villageschool, which has not been damaged by landslidemovement, is built and may follow the path of severalsmall surface drainages in that area. Near the toe, thelandslide widens downslope toward the Raizo Khola.The derivative maps created from the DEM suggest

that the Laprak landslide consists of four segments,

Figure 6. Landslide and erosional scarp features superimposed on the digital elevation model of the Laprak landslide. (A) Shaded reliefwith simulated illumination from an azimuth/inclination of 315u/30u. Surveyed locations of selected buildings are shown in yellow toillustrate the village location. Surface drainage paths (rivulets) are shown as single blue lines. (B) Slope angle. (C) Slope aspect. (D) Areas ofslope aspect in close agreement with observed and inferred bedrock structures.



three of which constitute the active portion of thelandslide. The inactive portion of the landslide, whichextends approximately 200 m upslope from thecurrently active headscarp, consists of a quartziteboulder field derived from quartzite cliffs near theridgeline. No cracks or other signs of ongoingmovement are visible in this portion of the landslide,which is covered largely by cultivated fields. The threeactive portions of the landslide consist of a steep andthick upper portion, a less steep and possiblyshallower central portion, upon which much of thevillage is located, and a steep, incised, and bulging toethat may consist of several subsidiary components.Both the upper and lower active portions are largelycovered by terraced agricultural fields, and breaks inthe slope angle map (Figure 6B) suggest that the toemay consist of at least two large landslides in additionto several smaller superficial landslides and erosionalscars. The slope aspect map (Figure 6C), in particu-lar, appears to illustrate bedrock structural controlson colluvium accumulation. The southeast- andnorthwest-facing slopes (green and yellow-orange,respectively, in Figure 6C) share orientations similarto metamorphic foliation and joints observed in thefield. This bedrock signature is faint in the upperinactive and upper active portions of the landslideabove most of the village, but it is much stronger inthe middle portion that underlies most of the village,where the colored bands are narrow and well defined.Evidence of bedrock control extends downslope fromthe village, but the width of the bands on the aspectmap increases and definition decreases, which mayindicate that the bedrock is deeper near the toe thanbeneath the village and that thick colluvium precludesthe topographic expression of all but the largestbedrock irregularities.

The relationship between surface topography andunderlying bedrock structures can be further exam-ined by exploiting the fact that if there is no soil, theslope aspect will coincide with the dip direction of thebedrock structures that dominate the local topogra-phy (e.g., Jaboyedoff et al., 2007). In situations wherebedrock structures are covered with colluvium orother surficial deposits, however, the dip angle of thebedrock may not be accurately reflected by theground surface slope. Therefore, only the slope aspector dip direction was considered during this project.Figure 6D is a recoded version of the aspect map inwhich the red patches represent areas in which theground surface dip direction agrees closely with themetamorphic foliation dip direction (D1: 330u to360u) and the green patches represent areas in whichthe ground surface dip direction agrees with joints(D3: 120u to 150u). The slope aspect map alsocontains distinct zones that occur only in close

association with the foliation-parallel slopes (D2:295u to 325u). These zones are shown in purple inFigure 6D and, because of their spatial and geometricassociation with observed foliation attitudes, mayrepresent intrafolial shears not originally observed inthe field. The yellow areas in Figure 6D representareas in which the dip direction is similar to thediscontinuities along which planar rock slope failuresoccur near the head of Chhelong Gully (D4: 025u to055u). Although the D4 discontinuities were observedonly in quartzite near the gully head, the presence oflarge yellow patches in Figure 6D suggests that theD4 discontinuities may influence the general topo-graphic slope and geometry of the colluvium masscomprising the main landslide in areas where they arenot visible at the surface. Figure 7 is an upslopelongitudinal view of the landslide created using thedigital elevation model and discontinuity locationstaken from Figure 6D, illustrating the role of bedrockstructural controls on planar rocksliding at the headof Chhelong Gully and the accumulation of acolluvial wedge dipping downslope toward the RaizoKhola.

Soil and Rock Geotechnical Properties

Soil samples collected from four locations (L1, L2,L3, and L4 in Figure 4) were returned to the IndianInstitute of Technology in Delhi for classification,index property measurement, and shear strengthtesting using standard methods. The specific gravity,density, liquid limit, plastic limit, and grain-sizedistribution for each sample were determined, anddirect shear tests were conducted using remoldedmaterial. Pertinent properties are summarized inTable 1. A large 30 cm by 30 cm Shear Trac II directshear test box was used to estimate the cohesion andstrength of the soils. The soil samples were collectedfrom an average depth of 1 m, and the applied normalloads for the direct shear tests ranged from 100 kN/m2 to 300 kN/m2. The shear strength testing wasconducted on dry samples.The grain-size distribution curves of the soils are

shown in Figure 8 and summarized in Table 2. Soilsfrom all four sites have more than 70 percent coarsematerials, and their grain-size distribution curves aresimilar except soil at sample 2 (location L2), whichcontained a considerable amount of fines (about 17percent silt). Three of the soil samples fall into theUnified Soil Classification System (USCS) GP(poorly graded gravel) category, and one falls intothe USCS GP-GM (poorly graded gravel to siltygravel) category.Field-scale rock mass strength parameters were

estimated using a modified Q-system approach. The



cohesive strength of the fractured bedrock wasestimated using (Barton, 2002)

c~RQD

Jn

1

SRF

UCS

100!1"

where c is cohesion in MPa, RQD is the rock-qualitydesignation, UCS is the uni-axial compressivestrength, Jn is the joint set number, and SRF is astress reduction factor. The friction angle wasestimating using (Barton, 2002)

w~arctan JrJwJa

! "

!2"

where f is the friction angle in degrees, Jr is ajoint roughness factor, Jw is a joint water reductionfactor, and Ja is a joint alteration factor. Using field-based estimates for all of the required input, Eq. 1and Eq. 2 yield estimates of c 5 480 kN/m2 andf 5 37u.The saturated hydraulic conductivity of soil sands

with 0.1 mm # D10 # 0.3 mm can be estimated as(Hazen, 1892; Bear, 1972)

ksoil~c f ns! "D210 !3"

where ksoil is in m/s, D10 is the effective grain size inmm, nsoil is the porosity of the soil, c is an empirical

Figure 7. Upslope visualization of the Laprak landslide based on the digital elevation model created for this project, illustrating the wedge-like nature of the structurally controlled colluvium accumulation and the discontinuities responsible for rockslides at the head of ChhelongGully. Representative discontinuity orientations, locations, and colors were taken from Figure 5D and are shown as large disks.Coordinates are in meters (UTM zone 44).

Table 1. Geotechnical parameter values used for slope stability analyses.

MaterialThickness

(m)Cohesion(kN/m2)

Dry Unit Weight(kN/m2)

Saturated UnitWeight (kN/m2)

FrictionAngle (u) Porosity

Saturated HydraulicConductivity (m/s)

Soil 10–20 0 22 24 34 0.35 5.4 3 1024

Rock NA 480 24 25 37 NA 6.1 3 1026



constant, and the porosity function is

f n! "~1z10 ns{0:26! " !4"

For the soil at Laprak, laboratory test results suggestvalues of c 5 0.01, ns 5 0.35, and D10 5 0.2 mm.Those values yield a saturated hydraulic conductivityof ksoil 5 5.4 3 1024 m/s, which is a reasonable valuefor the fine sandy soil comprising the colluviummatrix. Therefore, more complicated formulae werenot evaluated.

The hydraulic conductivity of the bedrock wasestimated as (Goodman, 1989)

krock~cw6m

! "

a3

s

! "

!5"

where krock is hydraulic conductivity of the rock inm/s, cw is the unit weight of water in N/m3, m isviscosity of water (13 1023 N?s/m2 at 20uC), a is jointaperture in mm, and s is joint spacing in mm.Although Goodman (1989) described Eq. 5 as beinguseful for back-calculation of aperture, a, from in situhydraulic conductivity test results, in this project itwas used to estimate k from field-based estimates of aand s. Field observations of discontinuities in thequartzite bedrock exposed at the head of Chhelong

Gully suggest typical values of 1 mm (which we inferto decrease by an order of magnitude to a 5 0.1 mmat depth) and s 5 300 mm, so that Eq. 5 yields afractured bedrock saturated hydraulic conductivityestimate of 6.1 3 1026 m/s and a soil:bedrockhydraulic conductivity ratio of 88:1.

Rock Slope Stability Analysis

Because the Laprak landslide is a complex made upof components with different mechanisms thatinteract in ways difficult to ascertain after the fact,it is impossible to conduct a single overall slopestability analysis. Therefore, three kinds of analyseswere performed to better understand different aspectsof the landslide. First, kinematic analysis was used toestablish the nature of rockslides in quartzite near thehead of the landslide, which may have played a keyrole in triggering the larger soil landslide in 1999.Second, the potential for rockslides along planarsurfaces in the quartzite (as suggested by thekinematic analyses) was evaluated for dry static,saturated static, and seismic conditions using limitequilibrium analysis. Third, the nature of andpotential for movement of the main colluviumlandslide mass with composite slip surfaces wereevaluated using limit equilibrium analyses for dry,saturated, and seismic conditions as described in asubsequent section of this paper. Pore-water pressuresin the main colluvium landslide were estimated usingtwo approaches: subjective specification of a phreaticsurface (water table), taking into account the loca-tions of known monsoon-season seeps, and finiteelement simulation of steady-state flow through thecolluvium and underlying fractured bedrock.Field measurements show that a prominent discon-

tinuity set in quartzite outcrops at the head ofChhelong Gully is oriented 48u/039u (dip/dip direc-tion), whereas the ground surface is oriented 52u/044u,or nearly parallel to the predominant discontinuitiesin that area. Discontinuity width is less than 1 mmwithout filling materials but increases to more than2 mm in areas where filling is present; it may be less atdepth. Discontinuity spacing in the quartzite isgenerally less than 0.7 m, and joint surfaces areslightly smooth. Kinematic analysis of the discontin-uous quartzite using Markland’s (1972) method andthe estimated rock friction angle suggests that theslope is susceptible to rocksliding along the slope-parallel discontinuities, as opposed to wedge ortoppling instabilities (Figure 9). Subsequent limitequilibrium rock-slope stability analyses using Roc-Plane 2.0 software yield factors of safety againstsliding of 1.25 for dry static conditions, 0.92 for fullysaturated static conditions, and 0.98 for dry seismic

Figure 8. Grain-size distribution curves for soil samples L1, L2,L3, and L4 from the Laprak landslide.

Table 2. Grain-size distribution (by weight) and Unified SoilClassification System categories for soil samples from theLaprak landslide.

Sample Gravel (%) Sand (%) Fines (%) USCS Category

L-1 49 47 4 GPL-2 34 48 18 GP-GML-3 71 26 2 GPL-4 61 33 6 GP



conditions with a pseudo-static horizontal accelera-tion coefficient of 0.12 based upon local codes.Figure 10 illustrates the geometry and results for thefully saturated static analysis; the dry static andseismic analyses used identical geometry but differentloading conditions.

Soil Slope Stability Analysis

The limit equilibrium slope stability analyses andsupporting finite element simulations of groundwaterflow for the colluvium comprising the main portion ofthe Laprak landslide were performed using thecommercial slope stability software Slide 5.0. Foreach case evaluated, factors of safety were calculatedusing the Morgenstern-Price, Janbu Corrected, andSpencer methods. Although Slide 5.0 can calculatepore-water pressures for variably saturated steady-state or transient flow as described by the Richards(1931; also see Fredlund and Rahardjo, 1993)

Figure 9. Markland (1972) kinematic slope stability analysis fordiscontinuous quartzite exposed in the head of Chhelong Gully,indicating the potential for rockslides along planar slip surfaces.

Figure 10. Limit equilibrium analysis of rock slope stability for discontinuous quartzite exposed at the head of Chhelong Gully for wetstatic conditions. Identical geometry was used for dry static and seismic analyses of the slope.



equation, this analysis used a high steady rechargerate equal to the saturated hydraulic conductivity ofthe soil (ksoil 5 5.4 3 1024 m/s) to ensure completesaturation under steady-state conditions as a limitingsituation. This was done to avoid the complications ofextreme uncertainty in the initial conditions, soilmoisture characteristic curves, and precipitation inputnecessary to constrain the highly non-linear problemof transient flow in variably saturated soil and rock.Instead, the steady-state simulations were supple-mented with a simpler order of magnitude estimate ofthe hydrologic response time of the Laprak landslide.Analysis of debris-flow mobilization and movement,which is an additional hazard at Laprak, is beyondthe scope of this paper.

Stability analysis of the main colluvium slope atLaprak was performed using the parameters inTable 1 and a vertical surface load of 2 kN/m2 inthe area occupied by the village in order to simulatethe weight of the stone buildings. For the dry seasonsimulations, the phreatic surface was specified alongthe soil-bedrock contact except at the locations ofsprings above the village and at the toe of the slopealong the Raizo Khola, where it was allowed to rise tothe ground surface. For the wet season simulationusing a specified phreatic surface, the phreatic surfacecoincided with the ground surface along the entirelength of the slope, but the bedrock was assumed tobe impermeable, eliminating the possibility of flowinto or out of the bedrock. The topography used inthe analyses was a representative profile taken downthe centerline of the landslide. Results for the staticand pseudo-static seismic analyses with a horizontalseismic coefficient of 0.12 and vertical seismiccoefficient of 0.08 are shown respectively in Tables 3and 4. Differences among the Morgenstern-Price,Janbu Corrected, and Spencer methods were negligi-

ble for each case. Figures 11 through 14 illustrate themodel geometry and Morgenstern-Price factor ofsafety distributions for multiple slip surfaces asgenerated by the Slide 5.0 software.As would be expected from its history of wet season

movement, the main colluvium slope is calculated tobe stable under both dry static and dry seismicconditions. Both the specified phreatic surface andfinite element seepage results show the upper portionto be the least stable and the middle (village) portionto be the most stable parts of the main colluviumslope, for both static and seismic wet seasonconditions. The finite element seepage results, how-ever, yield consistently higher factors of safety for allportions of the slope than does the specified phreaticsurface model (see Tables 3 and 4). For wet seasonstatic conditions, the finite element seepage resultssuggest that the middle (village) portion may benearly or perhaps even marginally stable, with afactor of safety ranging from 0.96 (Spencer method)to 0.98 (Morgenstern-Price method).The consistent differences in calculated factor of

safety values obtained using the specified phreaticsurface and finite element seepage models suggestthat, even though the soil:bedrock hydraulic conduc-tivity ratio of 88:1 implies that flow into and out ofthe bedrock might be minimal, it is significant enoughto produce noticeable changes in slope stability. Thefractured bedrock functions as a drain that allowsdownward seepage of water out of the colluvium anda concomitant increase in local slope stability (seeIverson and Major (1986) for a discussion of seepagevector orientation). Factor of safety differencesbetween the phreatic and finite element modelsranged from modest (about 4 percent for the villageportion of the slope under wet static conditions) topotentially significant (about 27 percent for the lower

Table 3. Composite surface factors of safety against sliding for static dry and wet season conditions.

Method

Dry Season (Specified Phreatic) Wet Season (Specified Phreatic) Wet Season (Finite Element Method)

Lower Upper Lower Village Upper Lower Village Upper

Morgenstern-Price 1.63 1.55 0.75 0.95 0.63 0.85 0.98 0.71Janbu Corrected 1.67 1.59 0.74 0.93 0.64 0.82 0.97 0.68Spencer 1.63 1.51 0.74 0.92 0.65 0.87 0.96 0.70

Table 4. Composite surface factors of safety against sliding for seismic dry and wet season conditions.

Method

Dry Season (Specified Phreatic) Wet Season (Specified Phreatic) Wet Season (Finite Element Method)

Lower Upper Lower Village Upper Lower Village Upper

Morgenstern-Price 1.24 1.21 0.59 0.73 0.51 0.78 0.83 0.65Janbu Corrected 1.22 1.24 0.57 0.74 0.52 0.76 0.84 0.66Spencer 1.22 1.23 0.59 0.74 0.50 0.76 0.85 0.67



portion of the slope under wet seismic conditions),although in no case did the finite element resultspredict stability under conditions for which thephreatic surface model did not. Had the finite elementsimulations not been performed, this situation wouldnot have been intuitively obvious given the largesoil:rock hydraulic conductivity contrast.

DISCUSSION

The Laprak landslide is a complicated feature, andunderstanding its reactivation is made more difficultby the fact that it occurred in a remote region with nobaseline data or detailed pre-slide maps for reference.Moreover, information about the events leading up tothe reactivation is limited to the anecdotal accounts ofresidents. Nonetheless, it is possible to make someimportant inferences about the sequence of eventsleading up to reactivation of the landslide and likelyfuture patterns of movement.Reactivation of the landslide apparently began with

a series of small planar rockslides at the head ofChhelong Gully, where discontinuities dip nearlyparallel to the local topographic slope. Quarrying

for building stone may have exacerbated the rockslidepotential by locally over-steepening the slope orremoving key blocks. Rock slope stability analysessuggest a factor of safety against sliding of 0.92 forcompletely saturated static conditions, which is areasonable supposition given the antecedent rainfalland intense rainstorm on July 3, 1999. The rock-slide(s) may or may not have been related to themobilization of the debris flow. One possibility is thatdebris-flow mobilization was facilitated by rapidloading and pore-pressure increases as a consequenceof the rockslide(s). Another possibility is that theheavy rainfall alone was sufficient to saturate thecolluvium and generate small landslides near the headof Chhelong Gully that subsequently mobilized into adebris flow, without the contribution of rockfallloading. The available field evidence does not allowus to favor either hypothesis over the other.Regardless of its trigger, the debris flow scoured

Chhelong Gully and led to the development ofrotational slumps along the lower reaches of thegully. The scouring and slumping may have reducedthe factor of safety of the colluvium mass andtriggered the main landslide; however, it does not

Figure 11. Results of limit equilibrium slope stability analysis of the Laprak landslide for dry static conditions obtained using theMorgenstern-Price method (Table 3).



appear to have been a necessary event because factorsof safety are well below unity for both saturatedgroundwater options evaluated as part of this studyeven without considering scour-induced modificationof the toe. According to the results of the stabilityanalyses, the colluvium landslide would have movedeven if the scouring and slumping had not occurredhad pore pressures been sufficiently high. The wetseason factors of safety % 1 in Table 3 suggest thatmovement does not require complete saturation, butinstead can occur if the soil column is only partlysaturated.

The relationship between the dominant discontinu-ity and local topographic slope orientations in thequartzite outcrops at the head of the slope suggestplanar rocksliding occurred in that area (Figure 9).Visualization of discontinuity orientations and thegeneral correspondence between those orientationsand the colluvium surface, however, suggest that themain colluvium landslide mass is a structurallycontrolled soil—not rock—wedge with a triangular

or trapezoidal cross section. In the case of a triangularcross section, the wedge shape would be controlled bythe red (D1) and green (D3) bedrock discontinuitiesshown in Figures 6 and 7. In the case of a trapezoidalcross section, the wedge shape would be controlled byred (D1), green (D3), and yellow (D4) discontinuitiesshown in those two figures. Although the yellow (D4)discontinuities were observed only in quartziteoutcrops at the head of the slope, large areas ofyellow ground in Figure 6D suggest that they maypersist beneath the colluvium and truncate thebottom of a triangular wedge bounded by the redand green discontinuities. The remote location ofLaprak makes it impossible to confirm the inferredbedrock control on colluvium wedge geometry withoriented cores or borehole televiewer surveys, but ourinferred geometry seems reasonable given the localstructural setting.Calculated factors of safety for the colluvium above

the village are significantly less than those for thecolluvium below the village, and both the upper and

Figure 12. Results of limit equilibrium slope stability analysis of the Laprak landslide for wet static conditions obtained using theMorgenstern-Price method (Table 3). Pore-water pressure was specified by assuming impermeable bedrock and a phreatic surfacecoincident with the ground surface.



Figure 13. Results of limit equilibrium slope stability analysis of the Laprak landslide for wet static conditions obtained using theMorgenstern-Price method (Table 3). Pore-water pressure was specified using a finite element model of steady-state seepage through bothsoil and bedrock (soil:rock hydraulic conductivity ratio of 88:1).

Figure 14. Results of limit equilibrium slope stability analysis of the Laprak landslide for wet seismic (pseudo-static) conditions withhorizontal and vertical accelerations of 0.12 g and 0.08 g, respectively, obtained using the Morgenstern-Price method (Table 4). Pore-waterpressure was calculated using a finite element model of steady-state seepage through both soil and bedrock (soil:rock hydraulic conductivityratio of 88:1).



lower portions both appear to be less stable than thevillage portion. The wet season static factor of safetynear unity for the village portion suggests that itwould be very nearly stable, or perhaps evenmarginally stable, on its own but is moving as aconsequence of its location between two much lessstable portions of the slope. Baum and Fleming(1991) demonstrated that the zones of maximumdisplacement in some large landslides occur in therelatively undeformed middle portions betweendownslope zones of shortening and upslope zones ofstretching. Although there are no comparable dis-placement or strain measurements available for theLaprak landslide, the existence of a relativelyundeformed (but highly displaced) middle sectionbetween more highly deformed upper and lowersections appears to be consistent with the observedand calculated conditions at Laprak.

Even though the modeled bedrock hydraulicconductivity is nearly two orders of magnitude lessthan the colluvium hydraulic conductivity, thedifference between the specified phreatic surface andfinite element seepage results suggests that thediscontinuous bedrock is permeable enough to allowdrainage from the colluvium into the bedrock andprovide for local decreases in instability that rangefrom modest to potentially significant. This suggeststhat common assumptions of perfectly impermeablebedrock and a phreatic surface coincident with theground surface may in general be an oversimplifiedapproach. Although consideration of bedrock seep-age in this case proved to be less conservative than themore common phreatic surface assumption, thisrelationship may not be universally applicable, andit should be evaluated on a case-by-case basis whenperforming slope stability analyses.

The seismic factors of safety in Table 4 suggestthat while a moderate (horizontal pseudo-staticcoefficient of 0.12) dry season earthquake shouldnot be expected to trigger future movement, a wetseason earthquake could exacerbate the effects ofhigh pore-water pressures and produce factors ofsafety against sliding as low as 0.5. Although a dryseason earthquake producing the modeled accelera-tion might not trigger landslide movement, it maystill result in collapse of or damage to unreinforcedstone buildings. Moderate seismic acceleration mayhave some effect on rock slope stability near thehead of the landslide, but it should not be assignificant as the effect on the main colluviumlandslide.

Mitigation measures currently under considerationinclude surface-water control, minor drainage proj-ects, and perhaps replacement of unreinforced stonebuildings with lightweight and flexible wooden

structures. The remote location and economic con-siderations preclude heavy construction projects orsuch remedial measures as horizontal drains or toebuttresses. To that end, relocation of the village isalso a possibility. However, there are no nearbylocations that offer both stable slopes and a reliablewater supply. Wholesale relocation of the communityto a remote region is likely to entail considerablesocial and cultural difficulties, and in that regard isnot an attractive option.

ACKNOWLEDGMENTS

This paper is based largely on the Master ofTechnology thesis work of Gurung at the IndianInstitute of Technology (Delhi) under the supervisionof Ramana, Haneberg, and Datta. Assistance fromthe residents of Laprak during fieldwork, rainfall dataprovided by the Department of Hydrology, Nepal,financial support by the Kadoorie Agricultural AidAssociation (KAAA), British Ghurkas Nepal (BGN),and the AEG Foundation are gratefully acknowl-edged. Reviews by Tom Badger, Bill Gates, and PaulSanti helped to improve the quality of the paper andare greatly appreciated. The opinions expressedherein, however, are those of the authors alone.

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Engineering Geology and Stability of the Laprak Landslide, Gorkha ...

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