Shallow soil slope instability analyses at horticultural farm, Daman, Central Nepal

Transcription

1 Bulletn of the Department of Geology Central Department of Geology Bulletn of the Department of Geology, Trbhuan Unersty, Kathmandu, Nepal, Vol, 007, pp 7 7 Krtpur Shallow sol slope nstablty analyses at hortcultural farm, Daman, Central Nepal Suman Manandhar Central Department of Geology, Trbhuan Unersty, Kathmandu, Nepal ABSTRACT Slope stablty analyses were carred out for slopes around the hortcultural farm at Daman, Central Mahabharat Regon of Nepal Daman les n the Mahabharat zone wth ntruded grante as the basement rock These grantes are hghly to completely weathered and decomposed to few metres depth from the exposed surfaces The hortcultural farm s stuated oer these decomposed rocks wth some colluum along the hll slopes The ranstorm of 9- July 993 deasted the hortcultural farm wth numerous landsldes and gully erosons Based on the lmt equlbrum analyss theory and computed ndex as well as strength propertes of the sol, analyses of three of the faled slopes were carred out The analyses reealed that sldes were unstable only durng fully saturated condtons Correspondng author: INTRODUCTION The hortcultural farm s stuated on the rdge of Daman at an alttude of about 300 m along the Trbhuan Hghway It les south of the Kathmandu Valley n the Maharabharat Zone, Central Deelopment Regon of Nepal (Fg ) The Daman rdge s formed of ntruded grante that has been hghly to completely weathered to certan depth It s subject to numerous landsldes and gully eroson (Manandhar 000) Besdes, the unprecedented ranstorm of 9- July 993 affected the entre area wth numerous landsldes and gully eroson These landsldes were caused due to smallscale shallow slope falures The slope nstablty analyses of the slopes were carred out to fnd the factor of safety of the slopes durng dry and fully saturated condtons GEOLOGICAL SETTING The Daman area comprses of the Palung grante ntruded n the Bhmphed Group of rocks (Stökln E-mal address: geosuman@gmalcom and Bhattara 9) These grantes are hghly to completely weathered and hae conerted to yellow brown sol Howeer, along the streambeds, grantes are moderately weathered and closely jonted The grante consttutes quartz, plagoclase, botte, and perthte as essental mnerals wth few tourmalnes These mnerals are small-graned (less than 3 mm) LANDSLIDES AND GULLY EROSION There are about 73 landsldes and gulles wthn a total area of 5,00 m The total rock/sol dsplaced was 3,0 m 3 of whch the landslde had dsplaced 3,00 m 3 and gully eroson had dsplaced,300 m 3 The area coered by landsldes s,50 m The aerage depth of falure s 0 m (Dhtal et al 993) The sol depth of the area ranges from < to > m The aerage depth s about 3 m Descrptons of studed landsldes n the hortcultural farm are gen n Table METHODS Geotechncal propertes were computed to fnd strength parameters of sol slopes for computatonal analyses Factor of safety of translatonal and rotatonal sol slopes were determned

2 S Manandhar/ Bulletn of the Department of Geology, Vol, 007, pp Palung 5 03 Study Area Daman Fg Locaton map of study area Stablty Analyses of Infnte Slopes Chtlang Markhu Kulekhan Reseror Translatonal slde often ntates wthout any sgnfcant change n geometry of the unstable regon An nfnte slope denotes a unform slope of an extent large enough that a typcal element can be consdered representate of the slope as a whole, and rregulartes at the toe and the crest of the slde can be gnored The sol propertes and porewater pressures at any gen dstance below the ground surface were assumed to be constant (Graham 9) When the depth to a planar surface was small compared wth length, the stablty was analyzed as an nfnte slope If a typcal slde of depth z and wdth b bounded by the ertcal lnes IJ and KL s solated for attenton, then from consderatons of contnuty t follows that the resultant forces Q L and Q r on the sdes of the slce must be equal, opposte and collnear (Fg ) The ertcal force across the base of the slce must equal weght W whch can be resoled nto normal (P) and tangental (T) components For stablty, the downslope shear stress must not exceed the shear strength t f of the clay The safety factor n the slope can be defned n terms of effecte stresses by the rato, t f /t that s: F = {(C (g g w ) tan f }/tanb () If the groundwater leel s at the slope surface (m=), and sol s cohesonless (C ) s zero then F = {(g g w ) tan f }/tanb () For dry condton, unt weght of water (g w ) becomes zero, the equaton () becomes as follows: F = tan f /tanb (3) Smplfed Janbu method for fnte slopes The fnte slope s used to desgnate a nonunform slope There are number of elements and rregulartes Such slopes may expose curlnear rather than planar to the surface (Graham 9) Fg 3 shows a potental sldng surface wth centre O and radus R (Graham 9) Water on the left and rght of the secton produces moments A l a l and A r a r The slde mass s dded nto slces by ertcal planes and a typcal slce IJKL s shown The method assumes that the resultant forces Q L and Q r on IJ and KL, respectely are equal and opposte and parallel to the base of the slce JK The smplfed Janbu method was used to determne the stablty of the slde mass The method descrbes a relately smple analyss for generalzed slde surfaces whch neglects the nterslce forces n the expresson for P It satsfes ertcal force equlbrum for each slce, as well as oerall horzontal force equlbrum for the entre slde mass The factor of safety for fully saturated sol mass was calculated usng the followng expresson (Graham 9): Fc = å = N = [ c ' b ( DW - ub) tan f '] sec a ma å = N = DW tan a () Durng dry condton, porewater (u) was zero and the equaton (5) as gen below was used Fc = å = N = Where, [ c ' b DWtanf '] sec a ma å = N = DW tan a tana tanf' ma= cosa F a (5) () The reported Janbu safety factor F was determned by multplyng calculated safety factor F c by a 7

3 Shallow sol slope nstablty analyses at hortcultural farm, Daman, Central Nepal A Fg Planar falure n nfnte slope (based on Graham 9) a l GWL Z Slde Surface I J W b T L K P Radus R GWL b sec ß Slope ß Pore Water Pressure O mz mz cos ß u = g cos w mz ß a r 3 m 0 3 m A A LEGEND Landslde Scar Escarpment Tree, Bent Tree Foot Tral Profle Hghly Weathered Grante Sol Colluum Bush, Shrub Crown A N A L Sol Strength c,f Q L I b L GWL A r Fg Sketch of the translatonal sol slde SL- at the Hortcultural Farm, Daman Slp Surface Pore Water Pressure u T Fg 3 Crcular arc sldng surface n fnte slope llustratng the elements of modfed Janbu method modfcaton factor f 0, then: F = f 0 x F c (7) Ths modfcaton factor that s a functon of the slde geometry and the strength parameters of the sol was obtaned from slope geometry and sol type J N a K W Q r The parameters, d and L were substtued for slope geometry where they belonged to depth of slde and length of slope profle For conenence, ths modfcaton factor was computed usng the equaton () below: f 0 = b d d () L L Where, b ared accordng to the sol type Values of b was 09 when f=0, that was when there was coheson only Smlarly, b was 03 and 050 when c =0 and f = 0, c ¹ 0 condtons, respectely (Abramson et al 00) 73

4 S Manandhar/ Bulletn of the Department of Geology, Vol, 007, pp m Water Tank 0 3 m B B N X X m 5 0 Stream N 5 m Fg 5 Sketch of the translatonal sol slde SL- at the Hortcultural Farm, Daman (see legend n Fg ) Fg Sketch of the translatonal sol slde SL-3 at the Hortcultural Farm, Daman (see legend n Fg ) Equatons () and (5) nclude the calculated factor of safety on both sdes of the equaton A tral alue of F a was frst assumed and the safety factor was computed untl ts alue concded wth F For the stablty analyss, crcle of radus of 5 m was constructed passng through the crown and toe of the slde assumng a slp crcle Then whole block was splt up unequal parts for the determnaton of safety factor The slp crcle was assumed wth dsplaced materal from the crown to the toe Geotechncal Propertes of the Sol Feld dentfcaton was used to dentfy the sol types and to fnd out the relate denstes of noncohese sols Natural mosture content, partcle dameter, specfc graty, unt weghts, and strength parameters of the sol were obtaned by laboratory tests on collected sol samples At frst, the samples were ar-dred and natural mosture contents were determned Then, partcle sze dstrbutons of sols were plotted n the semlog graph paper and specfc graty was computed thoroughly The strength parameters were obtaned by drect shear tests performed n a remolded sol specmen of cm x cm x cm sze wth predetermned normal stresses equalent to the oerburden pressure and shearng rate of 00 mm/mnute under draned condton The angle of nternal frcton and coheson parameters were found at the resdual state to analyze the stablty analyses of slopes 7

5 Shallow sol slope nstablty analyses at hortcultural farm, Daman, Central Nepal Table Descrpton of the landsldes n the Hortcultural Farm, Daman Locaton Daman SL- Daman SL- Daman SL-3 Types of slde Translatonal sol slde Translatonal sol slde Crcular sol slde Length (m) Wdth Depth (m) (m) Area (m ) Sedment produced (m 3 ) Natural slope (Degree) Faled slope (Degree) Slope drecton Wdth of tenson crack - m Man cause SE Porewater pressure NE Porewater pressure NE Porewater pressure Damage Hortcultural Farm and forest Hortcultural Farm and forest Hortcultural Farm and forest Table : Geotechncal propertes of sol Sample number Natural mosture content (w %) Specfc graty Relate densty (r %) Unfed sol classfcaton system Coheson (knm Angle of nternal frcton (f ) SL Well graded sand SL Poorly graded sand 00 3 SL Poorly graded sand wth few fnes ) CHARACTERISTICS OF SOIL SLOPES The sol slopes SL- (Fg ) and SL- (Fg 5) are stuated at the northeast and south of the hortcultural Farm Slope SL- s concae n nature whle slope SL- s concae from crown to 7 m downslope and remanng porton s conex n nature Lengths and wdths of both slopes are m, 9 m and 3 m, m respectely The south facng and north facng slopes hae and 0 natural slope 30 and when faled The faled slopes are almost parallel to the natural slope formng translatonal sol slde Few tenson cracks were obsered at the top part of the slde and were measured to be 0 mm to 0 mm wde and 0 mm to 50 mm wde n both slopes (Table ) The sol slope SL-3 (Fg ) s stuated at the south of the farm The sol slope s concae n the upper part and conex at the md-part A stream s flowng from west to east near the toe of the slope Length and wdth of the slope are m and m, respectely The south facng slope has natural slope and forms arable slopes rangng from -35 after slde Tenson cracks of wdth from 30 mm to 90 mm were obsered at the top part (Table ) RESULTS The frst eent of landslde was occurred due to contnuous precptaton on 9- July 993 The entre area of Daman and hortcultural farm was affected through seeral landsldes and gully erosons Numerous small-scale shallow sol sldes occurred n mostly resdual sol of grante and affected the farm and forest Snce both nfnte and fnte slopes of low depth of falure exst, the stablty condtons of them were computed To determne the factor of safety, geotechncal propertes were determned Sol slope SL- was well graded sand whle SL- and SL-3 were well graded and poorly graded sands (Fg 7) Specfc graty ared from 5 to 7 (Table ) Relate densty was % for all three samples Drect shear test results of two samples SL- and SL- Fg 7 Partcle sze dstrbuton cures of sols from landsldes at the Hortculture Farm 75

6 S Manandhar/ Bulletn of the Department of Geology, Vol, 007, pp 7 7 (a) Fg 9 Normal stress and shear stress cure to determne coheson and angle of nternal frcton SE (B ) NW (B) (b) Assumed falure surface 0 m 0 m Fg Profle of the translatonal sol slde SL- along A-A n Fg (c) Fg (a) Stress-shear dsplacement cure of SL- nfnte sol slope, (b) Stress-shear dsplacement cure of SL- nfnte sol slope and (c) Stress-shear dsplacement cure of SL-3 fnte sol slope were non-cohese wth angle of nternal frctons of 35 and 3 respectely Instead, SL-3 showed lttle coheson of 5 knm - wth angle of nternal frcton 33 (Fgs and 9) Angle of nternal frcton, coheson, unt weght, and depth of falure were the essental parameter whch were used for fndng the stablty condton The profles of SL- and SL- ndcated nfnte slopes (Fgs and ) The Infnte slope analyses of SL- and SL- showed stable durng dry season (Table 3) Factors of safety of them were computed 7

7 Shallow sol slope nstablty analyses at hortcultural farm, Daman, Central Nepal as 9 and 3 In fully saturated condton, slopes showed unstable and the factors of safety reduced to 07 and 05 (Table 3) Smlarly, profle of SL-3 ndcated crcular nature (Fg ) The fnte slope analyss usng smplfed Janbu method showed that the factor of safety exceeded unty (Table 5) The factor of safety durng dry season was 575 Assumng fully saturated condton, factor of safety declned to 0 (Table ) Snce, factor of safety of all sol slopes reealed stable durng dry perods and unstable whle saturated, the contnuous precptaton were responsble to dsturb such sol slopes around the hortcultural farm Proper water dranage s the most mportant technque for the correcton or preenton of landsldes Temporary remedal measures should be consdered to dert water runoff from falure zone or sealng cracks wth surface coatngs to reduce water nfltraton or coerng the ground surface temporarly Surface runoff should be derted away from the slde mass as a permanent soluton The channels should be placed at the head of the slope and along berms Then, egetaton (grass and shrubs) should be planted to stablze to slopes for long term bass CONCLUSION The results of stablty analyses of the slopes ndcated that slopes were only unstable durng fully saturated condtons The hgh precptaton durng the unprecedented ran n July 9-, 993 fully saturated the top sol layers n Daman area whch caused the slope falures Based on the nature of the slope nstabltes, the most sutable and economcal preente measures such as surface dranage work and egetate practces are recommended for safer slopes ACKNOWLEDGEMENTS Author s thankful to the Central Department of Geology for prodng necessary materal to prepare ths paper Author s also thankful to U B Shrestha, Department of Mnes and Geology and M S Dhar, Insttute of Engneerng for prodng comments on ths paper REFERENCES Abramson, LW, Lee, TS, Sharma, S, and Boyce, GM, NW (A ) 0 m 0 m 0 r = 50 m SW (X ) NE (X) m m Assumed falure surface Slope Stablty and Stablzaton Methods, Second Edton, A Wley-Interscence Publcaton, John Wley & Sons, Inc, 7p Dhtal, MR, Khanal, N, and Thapa, KB, 993 The Role of Extreme weather Eents, Mass Moements, and Land Use changes n Increasng Natural Hazards A report of the prelmnary Feld Assessment and workshop on causes of the Recent Damage Incurred n O SE (A) Fg Profle of the translatonal sol slde SL- along B-B n Fg 5 0 m Fg Profle of the translatonal sol slde SL-3 along X-X n Fg 30 77

8 S Manandhar/ Bulletn of the Department of Geology, Vol, 007, pp 7 7 Slce No South-Central Nepal (July 9-0, 993), ICIMOD, Kathmandu, pp7 Graham, J, 9 Methods of Stablty Analyss, Department of Cl Engneerng, Unersty of Mantoba, Slope Instablty: In D Brunsden and DB Pror (Eds), John Wley & Sons Ltd, pp 7 5 Table 3: Stablty analyses of nfnte sol slopes, SL- and SL- Parameters SL- SL- Unt weght at w % mosture content, g, kn/m 3 79 Unt weght of water, g kn/m3 9 9 w g = g g, b w kn/m Depth of slde, H R, m 3 7 Angle of landslde profle wth horzontal, b 30 o o In Dry Condton Factor of Safety, F (Equaton (3)) 9 3 In Fully Saturated Condton Factor of Safety, F (Equaton ()) b (m) h (m) Table : Representaton of Slce data for fnte sol slope c (kpa) Slce No f a c b DW u = g w bh DW sna (deg) (deg) (KN) (KN) (KN) Table 5 Modfed Janbu s method of analyss at present condton (dry condton) S3570 [(c b m a (Y) X/Y DW tan f )] cosa (X) F a =3 F a =575 F a =3 F a = S530 S90500 S For Frst Tral, Fa = 3, F c = 55, F = 5 For Assumed, Fa = 575, F c = 57, F = 575 Slce No [(c b ( u) tan f ] cosa m (Y) X/Y DW a (X) F =07 F =0 F =07 F =0 a a a a For Frst Tral, Table Modfed Janbu s method of analyss at fully saturated condton S957 S0357 S97 For Assumed, Fa = 07, F c = 0, F = 055 Fa = 0, F c = 00, F = 0 Manandhar, S, 000 Engneerng Geologc Studes of the Landsldes n the Kulekhan Catchment, Central Nepal, Lesser Hmalaya (unpublshed), 30p Stöckln, J, Bhattara, 9, Geology of Kathmandu Area and Central Mahabharat Range, Nepal Hmalaya, New York, pp 3