Simulation analysis of construction process of high rock slope's stabilization

Journal of Harbin Instiute of Technology (New Series), Vol. 15, No.2, 2008Simulation analysis of construction process of high rock slope’s stabilizationZHU Zhan-yuan'i2, LING Xian-zhang'，WANG Xuan-qing' , ZOU Zu-yin?朱占元，凌贤长,王宣青,邹祖银(1. School of Civil Engineering, Harbin Inslitutle of Teehnology, Harbin 150090, China, E-mail :zhuzywau910@ 163. com; 2. Information and EngineeringTechnology Clle,Sichuan Agriculural University, Ya'an 625014,Chinu; 3. Zhengyuan Construct Compary of Shandong Province,Yantai 26402, China)Abstract: A sel-developed elasto-plastic finite element program was used to analyze the construction sequenceof high rock slope’s stabilization in a coal-coking plant, and the result was compared with that employing theultimate equilibrium method. Based on the results of finite element analysis, the stress contour graphs and dis-placement vector graphs at different constniction steps were obtained, and the behavior of the slope during stabi-lization construction process was analyzed quantitatively. Based on the analysis of safety factors of three diferentschemes of stabilization and two different constnuction schemes, the assessment of stability and bracing design ofthe construetion process were performed. The results show that the original reinforcement design is improper;the stability of the rock slope is cotolled by a developed structural plane , the stability factor after excavation isless than 1, and the free surface should be braced in time; for stability, the construction sequence should adoptthat bracing follows excavation step by step up to down; the local slide occurred during the construction processagrees with the dangerous slide delermined by the numerical analysis 。which proves the validity and rationalityof the adopted method.Key words: high rock slope; stability; stabilization; construction process simulation; elasto-plastic FEMmethouCLC number: U416. 3Document code: AArticle ID: 1005-9113(2008 )02-0188-06From construction mechanics point of view, influ- measures to cover. etc. resulted in cracks on structuralence of construction step of excavation and bracing onplane 1 -1 and local rockslide in zone II, as shown inside slope stability is a very popular subject that deserv-Fig.1. On the background, self-designed elasto-plasticed investigation. Compared with classical ultimate e finite element program ZZYFEM which can simulatequilibrium methods, the finite element method based on construction process of slope was used to do numericaldeformation analysis has many unique advantages ，simulation analysis on three stabilization schemes andwhich provides new method to analysis of rock slope two construction schemes, and comparative sludy wasand has been successfully applied in many engineering done with the results calculated by ultimate equilibriumprojects 1-21. At present, some rockslide took place method.during construction period, in which construction se-quence is one of the important reasons. For deeply dig 1 General Situation of the Projecthighwall slope, excavation and bracing sequence havegreat influence on stability of slope , thus simulation a- 1. 1 Geologic Conditionnalysis for construction process is very important toThe rock highwall slope of the Cokeoven Plant isslope stabilization. Now, only a few papers about simu-close to a workshop used to store coal. For building ex-lation analysis for the construction process using finite tension of the workshop, excavation of the western up-element method can be found. Zhao Shang-yi utilized land shaped a high and steep manual slope which grad-ANSYS to make numerical simulation of the construc-ually heightens from south to north. Height of the slopetion process of slope cutting“; W ang Min-qiang simu- is between5 - 56 m, length about 300 m, slope anglelated the construction process and analyzed the stability. average at about 60° and some part nearly upright. Theof a slope beside a cetain workshop adopting, visco-site mainly belongs 10 low massif physiognomy and ac-elasto-plastic model and a new rheological model4. Jn cording to height, slope gradient, occurtence mode ofexcavation process of rock highwall slope in a coke-oV- the structural plane and level of rock integrity, theen Plant, because of bad geologic condition, neglect oflope is divided into 4 zones marked I, II, I1，andstructural plane 1 - 1 in sile survey coupled with exces-I中国煤化工nsity idegee.sive excavation, late bracing, shortage of protective It wassions basin, faultYHCNMHGReceived 2006 -04 -20.Sponsored by Scienife and Technological Suppont and Guidance Plan Projots of Zhejiang Province( Grant No. 2008C23019).●188●Journal of Harbin Institute of Technology (New Series), Vol. 15，No. 2, 2008structure was not found in this area and the stratum is400 mm x 300 mm; anchors adopt completely felt steelalmost continuous. The stratum in the whole slopebar HRB335 with diameter 28 mm, spacing 2 m xrange is uniclinal structure , the occurrence is NE55° -2 m; Set antislip piles at the slope toe, spacing5 m,75°∠29° - 35°, the thiekness is about 0.8 m, silt-section size 2 mx1.5 m.stone and shale exists partly between of them, addi-This paper adopls 11 - 11 section plane of zone IItionally, the coalescent between these layers is bad-as the calculating section, and mainly compares theworst. Joint fssure is well developed, the whole rockthree different stabilization schemes and two differentis separated into disintegrated structure or crannyconstruction schemes. The three reinforcement schemescrumb structure by joint fissure，the continuity of cran-are shown in Fig. 3. Stabilization scheme 1，the initialnies is very good and the ground water level of thescheme; Scheme 2, prolong all the anchors to3 m af-nearby slopes is buried comparatively deep. There areter the structural plane; Scheme 3, set a 50 m pres- .mainly two groups of disadvantageous combination 0tressed anchor into stable rock through structural sur-side slope trend with joint cracks ( advantageousface 1 -1. The two different construction schemes is asplane) within the sector, the structural plane prone-follows:①Bracing after each step of excavation, asness are 170° and 49° and obliquity is about 80°Fig.4 shows, initial slope- + manmade slope due to ex-paper makes simulation analysis only on constructioncavation-→construction step 1 ( cutting the upper slopeprocess of zone II with height between 14-56 m, asto form pack way) - construction step 2 ( bracing theshown in Fig. I. There are two main original structuralupper slope) - + construction step 3 ( stabilizing theplanes (Fig.2) at NE55° -75°∠29° - 35° flld withlower slope) - construction step 4 ( bracing the lowersoft mud and calcareous cement.slope) - →+ construction step 5 ( set antislip piles) ; .②construction upper lattice beams ，lower latticebeams and antislip piles after excavation completed.Fig.1 Original rock highwall in zone II( a)Scheme l:initial designSlpp urdare h erdvaion美感“Eontrl n m dring, 21-1 atutund plarndmnd和entrfin54.5029.3076.20(b)Scheme 2: lengthen ruck bolt to 3 m after the structural plane160.00Fig.2 Geologic section plane of zone II (m)1.2 Stabilization and Construction SchemesFor zone II, the original scheme plans to clearand dress the cover manually.2.5 m wide pack way isset along 45 m contour line, the grade under the packway is 1:0.87 and the grade above the pack way is中国煤化工1:0. 90; lttice beams of reinforced concrete and spraycose anckoranchored bracing system are set on the slope. Therein,MHCNMHGspacing of lattice beam 2 m x 2 m，section sizeFg.J suaouzauun scnemes ana calculating modelJournal of Harbin Instiute of Technology (New Series), Vol. 15, No. 2, 2008Sthilimntion constnu.tion steplIn which, M represents number of excavated elements(Excavating uppeslopeyin j construction step; [B], element strain matrix;Sabilization conesturtion strpl ‘(Excawating lower slope){σ} j1, element stress in stepj - 1; [N], elementSuieation construrtionCaxawded rck mw during邮2(Sat 4Per moxktrhdisplacement shape function matrix; {y}, body force只另waleshep biling rqersinof excavated element in the step. There for , unloadingste 2(Set Jower mckhot))force in the first excavation step is:Slubilizntion orsnxtio.step 5 (St anisip pie)1SP|,= E[B]"(olodV- 2」,[N]"1y}dV(3)where { σ}。represents initial earth stress. If M excava-54.5029.3076.20ted elements are in the step, we can caleulate dis-' 160.00placement increment △8}1 and stress incrementFig.4 Construction scheme①{△σ}j. Without regarding to displacement from initialearth stress before construction, stress and displace-2 Simulations of Construction Processment after the first excavation step is{σl| = {σ}o + {Sσ}i(4)2.1 Construction Step and Increment Step{8}; = {O8{;(5)Since most geotechnical engineering is constructed2.2 Finite Element Modelin steps, construction step and increment step has toThe longitudinal length of the slope is large, sobe used to simulate construction process of excavationcomputational model can be simplified as plane strainincludes bracing, loading and unloading'3-4I. To eachproblem.In the model, there are 2915 nodes, 3043construction step, expression of FEM analysis in inere-elements ( including 152 Goodman interface elements,ment loading process is2681 quadrilateral elements and 148 anchor elements[K]。{Oδ|y = {OF|，.ay+ {0Giy+ IOP|and 62 beam elements) totally; the horizontal dis-(i = 1,m;j = 1,n).where m represents construction steps; n, total incre-placements of right and left side boundary are set zero,ment loading times in each construction step; L K]y,the underside boundary is fixed and the silt clay is sim-plified to distributed load instead of building elements ,stiffness matrix inj increment step, i construction step;as shown in Fig. 3.{ OF};, equivalent unloading nodal force vector on ex-2.3 Constitutive and Mechanics Parameterscavation boundary in i construction step; ay , coefficientIn the slope excavation and stabilization site, ma-of unloading on excavation boundary in j incrementterials include rock mass , anchor rod, beam and struc-step, i construction step, which can be set to crosstural plane etc. Structural plane is simulated by Good-some steps and' 2 a; = 1 when unloading processman interface elements and constitutive relation adopt-ed is nonlinear elastic model. Constitutive relation ofcompleted; { OGlg, equivalent nodal force vector ofother materials adopted is ideal elasto-plastic model;new added weight in j increment step, i constructionDrucker-Prager yield criterion is adopted for rock;step; { AP}g, equivalent nodal loading force vector injmeanwhile, Mises yield criterion adopted for anchorsincrement step, i construction step. To most geotechni-and beams- .。The physical mechanics parameterscal problem, gravitational field is a component of initialused in this paper mainly base on results of site surveystress field and unloading force due to excavation in jto the slope along with degree of destructiveness andstep can be calculated byweathering", and values used in similar project, 8aslisted in Tab. 1.{SF|,= E []'{o},.dV- E [N]Iy\dV.(2)Tab.1 Physical and mechanics parameters of the slope materialsYoung6Poison'TangNormal .Cohesive Intemal frictionGravitymodulusratiostiffnessstrengthangleSort of rock massE/GPaK/ (kBa.m~l) Rn/ (kRa.m)C/RPn中()___ y/ (kN.m-3)Intense weathered quartz sandstone 1. 250.3003022.5Moderate weathered quartz sandstone 3. 120.27中国煤化工23.51 -1 struetural plane4810MYHCNMHG19.22 - 2 structural plane12000545000552419.8●190●Journal of Harbin Institute of Technology (New Series)， Vol. 15, No. 2, 20083 Safety FactorIn most conditions, collapse of slope appears asrockslide mass sliding along interface between slidemass and base rock. Utilizing fnite element method tocalculate the stress of the interfaces can conquer thedisadvantage of ultimate equilibrium method, by whichforce under the bottom of rock piece can not delegatethe real stress condition. Further more, the safety fac-torF, in whole can be directly derived by the formermethod [7].(a)Maxinurn shear stres Tm. (kPa)2T, 2((C +σm tanp,)A; +P tanp,; +P。F, .年=年ET2TA(6)where N is number of slide surface in slide mass; T; isslide force in sliding direction; T is slide resistance ofslide surface i in reverse direction to T; A is area ex-cept regions in tension on the skid surfacei; Pi is nor-mal component of ultimate bearing capacity caused by(b) Vertor gaph of principal stres( kPa)anchor along slide surface i; P, is tangential compo-nent of ultimate bearing capacity caused by anchor a-long the slide surface i.4 Analysis of Calculated Results回4.1 Stress Field and Deformation Field of theSlopeFrom numerical calculation of the self programmedZZYFEM，we can get stress contour graph and dis-placement vector graph before and after shape of theslope at each stabilization construction step. Befor(c) Vector graph of displacemnent( m, 300 muliplies)new part of the workshop is built and the slope isFig.5 Stress field and displacement vectors after slope shapedshaped, stress distribution is comparatively uniformnwith a few yield elements at certain place on the coverDuring stabilization construction process, elasticand turning point of the structural plane. Along withback effect caused by cutting off the upper slope resultsexcessive excavation to the hill and building extensionin great stress release in the upper slope and the struc-of the workshop, stress field changes obviously. Simul-tural plane 1 - 1, stress concentration is obvious neartaneously, unloading leads to great stress reduction atthe pack way and number of yield elements is less thanthe slope toe and nearby the cover , and obvious deflec-before. But, as shown in Fig. 6, some tensile elementstion in principle stress trace; at the slope toe and turm-in the slope combined with the structural plane form aing point of the structural plane, stress concentrationsdangerous slide mass，whose stability should be caredaggravated and yield element number increased; at freespecially during construction. If dealing improperly, itface and top of the slope ，unidirectional tensile ele-would cause local slide of upper slide mass and evenments appeared;, at slope toe, elastic recovery dis-wholly collapse. Local land slide under the pack wayplacement appeared because of unloading; slide massduring construetion ( Fig. 7) agrees with simulation re-has obviously sliding trend towards free face and thesults中国煤化工calculation. Othermaximal displacement is about 18 mm which happenedconslYH_ence on stress andat the interface of the slope and the structural plane 1displCN M H Geferto Rel.[4] for-1, as shown in Fig. 5.details..191●Journal of Harbin Instiute of Technology (New Series)， Vol. 15, No.2, 2008Scheme①requires bracing after each excavation step,which may give help to distortion control and stabilityof the slope; antislip piles have obvious effect on im-DungerousL slidleemassproving stability of structural plane 2 - 2; safety factorcalculated by ultimate equilibrium method agrees withresults calculated by finite element method, while theformer is about 5% lower. It can be explained as fol-lows: in ultimate equilibrium method, in order to getdefinite solution, we must make assumptions more orless on forces between slices. These assumptions re-lease restrictions between slices to some degree andFig.6 Stress state of slope elemnent at stabilization step 1leads calculated safely factor lower7?.+ Scheme 1; itial design，年 Sthme 2:knyghen nxk Folt o 3 m dfer the atuxtumld plune古Stheme 3; add mne long pestesesed anchor复12: \、111.009Initial Slope Step 1 Step2 Step3 Step 4 Step 5formedConstruction stepFig.7 Local land slide of zone II during constructionFig.8 Safety factors of three different schemes for stabili-zation in construction process ( FEM, scheme①)4.2 Change of Stability Safety Factor DuringConstruction1.In the condition without structural plane, the safe-ty factor is 1.21 by finite element strength discountingmethod9- 10 .. Considering the infuence of structural1.6plane 1 -1 and structural plane 2 - 2, safety factors ofi 1.5slide slope during its formation and alteration is shownin Fig. 8, which is calculated according to Eq. (6) u1.3-1.2.sing calculated stress by simulation analysis in con-struction process. I can be found from the figure thatbecause of excessive excavation, the dangerous high-0.wall slope was shaped; during construction, cutting ofInitial Slope Step 1 Step 2 Step 3 Step4 Step 5the upper slope gives a certain help to stability and cut-ting the lower part has disadvantage to its safety; dueFig 9 Relations between safety factors, structural planeto structural plane 1 -1, the initial scheme is improp-and construction scheme of scheme 3er; both scheme 2 and 3 can make safety factor of theslope satisfy requirements of relative code. Because the5 Conclusionsbulk of land slide on structural plane 1 -1 is not greatand anchors project has been on, scheme 3 is more e-From the completion of the slope stabilizationconomical.(Fig.10) in Oct. 2004, displacement of the slope isTo scheme 3，according to imbalance pushingsmall and distortion tends to be converged. Accordingforce method of ultimate equilibrium method ( UEM)，to comprehensive analysis, the slope has achieved sta-we can calculate the safelty factor al each constructionbilization for ever. From the simulation analysis onstep and compare it with that calculated by finite ele-construction process and shaoing Drocess of the slope ,ment, results are shown as Fig. 9. From the figure, we中国煤化工:can find that stability of the slope is controlled by_nd safety factor dur-structural plane 1 - 1; construction scheme has greatingMYHCN M H Gby the progan, ac-influence on safety factor during construction process.cording to them we can value construction scheme ra-●192.Jounal of Harbin Insiute of Technology (Nev Serie), Vol. 15, No.2, 2008tionally and provide reference to construction displace-[4] Zhu Zhan-yuan. Reearch on High Slope Stability and Se-lection of Stabilization Scheme about Rock Mass in thement monitor.MA' ANSHAN Steel Company. 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