中国牛人外文翻译

ORIGINAL PAPERStability assessment of a slope under a transformer substation using numerical modellingH.J.Li •F.C.Dai •W.C.Li •L.Xu •H.MinReceived:2April 2010/Accepted:26July 2010/Published online:17September 2010ÓSpringer-Verlag 2010Abstract This paper presents an analysis of the defor-mation mechanism and stability assessment of a slope in Yunnan Province,China.Field investigations indicated that the deformation of the slope was caused by the com-bined effects of the unfavorable topographical,geological and hydrogeological conditions and the placement of man-made fill.The stability of the slope was assessed by 3D numerical modeling which showed that the factor of safety of the slope was 1.1in the natural state but reduced to 1.03after fill was placed.Pile reinforcement was undertaken,which raised the factor of safety to 1.27.Keywords Fill slope ÁDeformation mechanism ÁNumerical modelling ÁStrength reductionRe´sume ´L’article pre ´sente une analyse du me ´canisme de de´formation et une e ´valuation de la stabilite ´d’une pente dans la province de Yunnan,en Chine.Les travaux deterrain ont indique´que la de ´formation de la pente re ´sultait d’effets combine´s en rapport avec des conditions topo-graphiques,ge´ologiques et hydroge ´ologiques de ´favorables et la construction d’un stabilite´de la pente a e´te ´e ´value ´e a `l’aide d’une simulation nume ´rique 3D qui a montre´que le facteur de se ´curite ´de la pente valait 1,1en conditions naturelles mais passait a`1,03apre `s construction du remblai.Des renforcements par pieux ont e´te ´re ´alise ´s,ce qui a fait passer le coefficient de se´curite ´a `1,27.Mots cle´s Remblai sur pente ÁMe ´canisme de de´formation ÁSimulation nume ´rique ÁDiminution de re´sistance IntroductionIn order to develop the provincial economy and satisfy the need for power,a 220kV transformer substation was constructed in Yanjin,located in northeastern Yunnan Province,PR China.The construction of the transformer substation commenced in November 2004,and the site was filled in March 2005.The total volume of cut and fill was 152,000m 3.The compaction was accomplished using a sheeps-foot roller with a layer thickness of 200mm.During heavy rainfall between July 18and July 20,2005,a creeping slide was observed in the slope at the northwest of the construction site.It led to tensile cracks in the retaining wall with a maximum crack width of 20cm.Meanwhile,the foundation of the 110kv framework located on the top of the slope subsided by 29cm.The moving slope was 340m long and 100m wide.The unstable slope threatened the safety of the transformer substation,the end tower of the 110kv power output and the lives and prop-erties of the residents nearby.After deformation,the fill slope was reinforced immediately with 23rectangular piles in the northwest of the site and 41circular piles in two rows at the northwestern corner of the site.Each rectangular pileH.J.Li (&)Institute of Mine Safety Technology,China Coal Research Institute,Beijing 100013,People’s Republic of China e-mail:geolhj@F.C.Dai ÁW.C.Li ÁL.XuInstitute of Geology and Geophysics,Chinese Academy of Sciences,P.O.Box 9825,Beijing 100029,People’s Republic of China H.MinInstitute of Rock and Soil Mechanics,Chinese Academy of Sciences,Wuhan 430071,People’s Republic of ChinaBull Eng Geol Environ (2011)70:385–394DOI 10.1007/s10064-010-0318-7was 2.5m 91.8m while the circular piles had a diameter of 1m;in each case the spacing was 2.5m.The bases of the piles were extended into the bedrock.The paper presents a detailed analysis of the deforma-tion mechanism and slope stability assessment undertaken using numerical modeling,in the natural condition,after the placement of the fill and after reinforcement.Geological conditions GeomorphologyThe transformer substation is located in the Wumeng mountain area in the northeast of Yunnan province.The dominant geomorphic feature is a deeply incised canyon.The aspect of the slope is approximately N15°E and the gradient is in the range of 10°–15°with multi-step topog-raphy.The step height is between 1and 3m and the topographic relief between 35and 40m (see Figs.1,2).LithologyThe ground investigations for the second exploration stage (stability assessment)consisted of 13boreholes (BH12–BH24)and four exploratory wells (Fig.2).The results indicated that the slope consists mainly of Quaternary deposits and is underlain by Triassic detrital rock.The lithological components are illustrated in Fig.3.For the sake of simplicity,the Quaternary deposits are subdivided into three lithotypes:(a)Landfill:Light yellow to light grey in color,and composed of soft silty clay.(b)Top soil:Composed of upland field soil and irrigatedfield soil,distributed widely across the site.Theupland field soil is dark in color and composed of loose silty clay;the irrigated field soil is grey and consists mainly of soft,wet,plastic clay.The thickness is less than 0.40m.(c)Silty clay:Light yellow to light grey in color with 15–20%breccia or gravel from mudstone and pelitic siltstone.The thickness varies from 3.1to 16.2m,being thinnest at the toe and thicker along the line from BH4to BH20(Fig.3).The bedrock is a suite of detrital rock formations and is divided into completely to highly weathered mudstone and moderately to slightly weathered sandstone in the light of their different characteristics.The purple completely to highly weathered mudstone is weak and could be crushed by hand;the peak strength of the undisturbed samples being /p 0=28.1°,c p 0=40.5kPa,and the residual strength /r 0=21.5°,c r 0=14.6kPa (Fig.4).The bore-holes indicated that the completely to highly weathered mudstone is only found in the upper concave region and not at the slope toe (Fig.3).The grey moderately to slightly weathered sandstone is strong and dense.Geological structureThe site is located on the southern flank of an east–west syncline and the strata strike N80°E–S85°E with dips to the north of 10°–15°.The site investigation indicated two joint sets for the moderately to slightly weathered sandstone.The N30°E to N50°E joints had dip angles of [80°,rough and moderately weathered joint surfaces with a spacing of 0.2–0.7m while the N15°W–N40°W joints had dip angles [74°,smooth and moderately weathered surfaces and a spacing of 0.3–0.8m.The joints are generally closed although a few had a \1mm infilling of mud.HydrogeologyThe groundwater is mainly pore water in the Quaternary formation.The permeability of the completely to highly weathered mudstone is 10-8m/s hence a high water level developed above the slip surface—at some 1–3m depth because of the multi-step topography.Mechanical properties of soil and rockFrom the site investigation the materials can be grouped into four lithotypes:(a)Landfill:Mechanical strength parameters for the landfill adopted were the empirical parameters /0=14°,c 0=20kPa,given by the Yunnan Electric Power DesignInstitute.Fig.1Overview of the site area.Movement zone is outlined in red ;solid line observed,dashed line interpreted386H.J.Li et al.(b)Silty clay:In view of the similarity of the index properties of the thin top soil and the silty clay,their shear strength parameters were considered together based on samples of silty clay obtained from a man-made well.The physical properties are shown in Table 1.In order to determine themechanicalFig.2Map of the slopeshowing contours and boreholepositionsStability of a slope using numerical modelling 387parameters of the silty clay,both consolidated drained (CD)and undrained(CU)tests were carried out on four61.8mm diameter,125mm high samples.The axial strain velocity was0.011mm/min for the CD tests and0.073mm/min for CU tests.The consolida-tion stresses were25,50,100and200kPa,respec-tively,and samples were saturated by back-pressure in triaxial compression tests.The results were/ 0=26.4°,c0=8.5kPa;/0=15°,c0=18kPa for the CU and CD tests,respectively.Similarly,the friction angle(CU)of the saturated clays was lower than that of the CD tests for the same clay due to increase in pore pressures under undrained conditions (Budhu2000).(c)Weathered mudstone:X-ray diffraction tests wereconducted on the completely to highly weathered mudstone and indicated that the mudstone was com-posed of quartz(50%),chlorite(20%),illite(10%)and hematite(10%).The occurrence of hematite indicatedthat the process of weathering was in an oxidizing environment and that the mudstone was completely weathered.The physical properties of the soils are shown in Table1.The shear strength parameters were obtained using a shear rate of0.02mm/min and for the undisturbed samples indicated peak strength parame-ters of/p0=28.1°,c p0=40.5kPa,and residual strength parameters(four reverals)of/r0=21.5°,c r0=14.6kPa.However,using these peak and resid-ual parameters,the numerical simulation did not show creeping and deformation.As the slope has been deformed,shear tests were carried out on saturated remolded samples,which gave residual strength parameters of/r0=21.7°c r0=1.0kPa(Fig.4),i.e.,a friction angle similar to that of the undisturbedsamples but a lower cohesion.Adjusting the strength parameters of the completely to highly weathered mudstone to/r0=16.5°,c r0=1.0kPa,the slope progressively crept and deformed in a manner similar to the observed failure.The factor of safety was1.03 by the shear strength reduction method,close to the critical state.Therefore,the optimized residual strength of remolded samples was used in simulation (Table2).(d)Sandstone:To estimate the rock mass strength andstiffness of moderately to slightly weathered sand-stone,the Hoek–Brown(2002,2006)failure criterion was used together with the uniaxial compression strength and the characteristics of the structural surface.Uniaxial compression testing was undertaken following the National Standard of People’s Republic of China(1999)Code for Water Resources and Hydropower Engineering Geological Investigation, GB50287-99,and allowing for the anisotropy of the rock.The average values of uniaxialcompressionTable1Physical properties of silty clay and mudstoneSoil Soil propertiesWet density(g/cm3)Water content(%)Specific gravity Liquid limit(%)Plastic limit(%)Plasticity indexSilty clay 2.0(8)22.2(8) 2.69(8)36.9(8)22.0(8)14.9(8) Mudstone 2.1(8)16.9(8) 2.68(8)32.8(8)12.8(8)20.0(8)Values in parentheses represent numbers of samples testedTable2Geotechnical parameters of rock and soil adopted for numerical modelingMaterials Elastic modulus(GPa)Poisson’s ratio Unit weight(MN/m3)Cohesion(MPa)Friction angle(°)Landfill0.040.350.0180.0214.0Silty clay0.070.350.0200.01815.0 Weathered mudstone0.030.370.0210.00116.5 Sandstone14.000.220.026 3.052.0388H.J.Li et al.strength were183MPa(perpendicular to the bed-ding)and160MPa(parallel to the bedding).Clearly some anisotropy is present and hence,to be conser-vative,a value of150MPa(derived from the Yunnan Electric Power Design Institute)was adopted in the stability analysis.To establish the structure and surface conditions,55GSI estimates were made and an m i of18was obtained(Hoek et al.2002;Hoek and Diederichs2006).Using these parameters,values of m b=3.61,a=0.5and s=0.0067were obtained.The Hoek–Brown failure envelope was calculated assuming an undisturbed rock mass and then trans-lated to a linear Mohr–Coulomb envelope using RocLab software(RocScience2002). Deformation characteristics and mechanism Deformation characteristicsAfter site works involvingflattening andfilling,the retaining wall in the transformer substation cracked (Fig.5a)and other foundations showed evidence of dis-tress.In addition,tensile cracks up to50m long and 20mm wide appeared,coalescing at the top of the concave slope.Within the transformer substation the cracks appeared to run in an east–west direction,while near the backscar they had a north-east trend.Tensile cracks and differential subsidence were also observed in the retaining wall of the transformer substation(Fig.5b).In addition,a small-scale local collapse and rock fall occurred at theslope toe.From the geometry of the movement,it would appear that the landslide had been caused by the placement fill towards the top of the slope.Analysis of deformation mechanismWhilst thefilling was clearly a cause of the landslide,other inherent factors also contributed to the slope deformation, including:(a)Topography:The concave shape can result in easycollection of groundwater in the slope.In addition,the depth/location of the sliding surface would clearly be conducive to slope deformation.(b)Geological structure:The Quaternary gravelly depos-its are highly permeable.In addition to the difference between the peak strength and the residual strength of thefiner material,once a shear zone has been created the frictional resistance would be significantly reduced once movement has begun.The presence of low permeability completely to highly weathered mudstone results in a perched water table such thatthe overlying Quaternary deposits can slip relatively easily along the interface as the dip of the strata is in the same direction as the slope profile.(c)Ground water:In addition to rainfall,as the areabelow the transformer station was irrigated,there is an almost continuous supply of water into the zone with perched water conditions such that the depth to saturated conditions is only1–3m in the stepped topography;groundwater seepage was observed at the drainage hole in the retaining wall(Fig.6).The sketch map of the creep deformation mode is shown in Fig.7.Stability analysisModel setupIn this study thefinite differences program FLAC3D(Fast Lagrangian Analysis of Continua in3-Dimensions)was used for slope stability assessment(Itasca2002).This Fig.5Inner(a)and outer(b)retaining walls of transformer substationStability of a slope using numerical modelling389method has been widely used in analyzing slope stability and deformation mechanisms (Guadagno et al.2003;Kinakin and Stead 2005;Brideau et al.2006;Deng et al.2007).The mesh for finite differences modeling is shown in Fig.8.The model was 420m long and 250m wide with a maximum height of 121m.The model was composed of 128,581elements and 25,839grid points.According to the slope lithology,the materials are divided into four groups:(a)landfill (b)silty clay (c)completely to highly weatheredmudstone (d)moderately to slightly weathered sandstone.The retaining wall was taken as linear elastic material.The slope materials were assumed to be perfectly elasto-plastic and satisfy the Mohr–Coulomb failure criterion.Piles were simulated by pile structural elements which are suitable for modeling structural-support members (Itasca 2002).Roller boundaries were applied on both the four sides of the model and the bottom while the slope surface was free.The geotechnical properties for the materials are shown in Table 2;bulk modulus (K )and shear modulus (G )were converted from Young’s modulus (E )and Poisson’s ratio (m ).The initial stresses under gravity and hydrostatic pressure were calculated under natural conditions.Loading and reinforcement piles were added after the displacement and velocity were set to zero.Simulation processThe simulation process involves two conditions:one is filling,and the other is reinforcement.The fill simulation condition is composed of three steps.1.Initial stress:Linearly elastic material behavior and homogeneous elastic properties before filling were assumed to generate simple initial stresses due to gravity and the hydrostatic pressure of water under natural conditions;the displacements are set to zero.2.Natural condition:The material model of the soil and rock mass was switched from linearly elastic to elastic-perfectly plastic and the stresses corrected accordingly.The factor of safety was calculated by the shear strength reduction method.3.Filling:The zones are ‘filled’according to the scope of fill.The factor of safety under the filling condition was calculated through the shear strength reduction technique.The reinforcement condition consists of two steps.1.Initial stress:In the actual condition,the structure was reinforced by piles when the slope was deformed after filling.The initial stress was generated according totheFig.6Groundwater seepage at the drainage hole of retaining wall (a )and water level from well (b)390H.J.Li et al.actual deformation situation and the displacements were subsequently set to zero.2.Reinforcement:The slope was reinforced by pile structural elements according to the field construction.The factor of safety with the reinforcement state was calculated using shear strength reduction.The pore water pressure was calculated from the site investiga-tion evidence and the ground water table was assumed to be adjacent to the ground surface.Failure criteria and shear strength reduction technique To date there is no uniform failure criterion for numerical simulation.Griffiths and Lane (1999)and Dawson et al.(1999)took non-convergence as an indicator for failure in finite element methods while Matsui and San (1992)assumed the contour of shear strain to indicate failure;a slope will fail if the contour of shear strain exceeds 15%developing from the toe to top.Deng et al.(2007)took the plastic zone as the indicator of the failure;a slope will fail if the plastic zones are all connected from the toe to the crest of the slope.For FLAC 3D ,four indicators may be used to assess the system state,including (a)maximum unbalanced force,(b)grid point velocities,(c)grid point displacements and (d)plastic indicators (Itasca 2002).In this paper,the criteria listed above are adopted to assess the slope stability and the factor of safety is calculated using the shear strength reduction technique.There are two ways commonly used to calculate the factor of safety,limit equilibrium and numerical methods (Zienkiewicz et al.1975;Dawson et al.1999;Duncan 1996).The shear strength reduction method has advantages over the limit equilibrium,as the critical failure surface is found automatically,to satisfy translational and rotational equilibrium.In addition,it can provide a more realistic indication of the behavior of a slope.The shear strength reduction method was first proposed by Zienkiewicz et al.(1975).The numerical simulation is run for a series of trial factors of safety (F s ).The cohesion c and friction /are reduced for each F s simultaneously according to the fol-lowing equations until failure occurs:c 0¼c =F sð1Þ/0¼arctan ðtan /=F s Þð2ÞAs Duncan (1996)pointed out.F is the factor by which the shear strength of the soil would have to be divided to bring the slope into a state of barely stable equilibrium.Deng et al.(2007)evaluated the stability of landslides based on a three-dimensional strength reduction technique.This paper adopts this technique to assess the stability and calculatetheFig.8Mesh forcalculationFig.9Calculated shear strain increment (a )and plasticity state (b )Stability of a slope using numerical modelling 391safety using the FLAC 3D program in which the command SOLVE FOS can find the slip surface automatically.Modeling results Natural stateA shear band is first formed at the natural state,as shown in Fig.9a, b.No dramatic increase of displacement is observed at this stage.The shear failure occurs at the toe and in the middle of the slope,but the slip band does not completely reach the residual condition.The results from the numerical simulation showed that the factor of safety in the natural state was 1.1.Effect of fillFill was loaded onto the natural condition model in steps until the system reached equilibrium.The displacement contours after filling are shown in Fig.10,which indicates the displacement at the middle and near the crest of the slope is greater than that near the toe.Shear strains increased after filling (compared with Fig.9a)and the failure surface (as indicated by the accumulation of the shear strain increment)propagated almost from the toe to the crest (Fig.11a).The slope is in active shear failure,whereas tensile failure occurs at the slope crest (Fig.11b).The shear strength reduction method indicated a factor of safety after filling of 1.03,i.e.,close to the criticalstateFig.10Generation of ground displacement for filledconditionFig.11Calculated shear strain increment (a )and plasticity state (b)Fig.12Generation of ground displacement for reinforcement condition392H.J.Li et al.such that creep or deformation would occur under unfa-vorable conditions (e.g.,rainfall).Effect of reinforcementThe slope was reinforced with piles when the deformation reached the actual situation at the site (Fig.2).The cor-responding displacement increment after reinforcement is shown in Fig.12,which indicates that the deformation has been controlled and the small displacement occurs at the toe rather than the crest.After stabilization the shear strain increment was not developed at the rear but the small values were concentrated at the front (Fig.13a).With the continued time-stepping,failure (shear-n)is occasionally observed at the toe but reinforcement prevents it affecting the crest (Fig.13b).The factor of safety was 1.27after the installation of the reinforcement piles (calculated by the shear strength reduction technique).From Fig.13c,the FOS does not apply to the whole slope but only the upper part which is stabilized by the piling.Summary and conclusionsField investigation in the study area indicated that the unfavorable topographical,geological and hydrogeological conditions were the inherent factors contributing to the creep movement of the slope while man-made filling was the main external factor.The deformation mechanism of the slope was analyzed using a numerical model which indicated:(a)A shear band first formed in the natural state with shear failure occurring in the middle and toe of the slope.The factor of safety under natural conditions was 1.1,an insufficient margin to accommodate the subsequent placement of fill.(b)After filling,creep slipping or deformation would belikely to occur under unfavorable conditions (e.g.,rainfall)when the FOS would decrease to 1.03.When the fill was placed,the factor of safety would be reduced by 6.4%.Failure would be likely to occur in the form of shear (in the slope)and tension (at the crest).The modeling results were in good agreement with the actual deformation 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