Underground Execvations

PartIVAnalysisofUndergroundExcavations

NUMERICAL MODELLING IN ROCK ENGINEERING

ObjectivesAnalysisanddesignofundergroundmineexcavations Priordeterminationofstabilityproblems

Notnecessarilytopreventdeformationandfailurebuttopreventlarge,uncontrolleddisplacementsandfailurefromoccurringduringexpectedexcavationlife

Stressdistributionnearminingexcavationface Designofsupportsystems


UndergroundExcavations


GeneralDescriptionInsitustresses Estimationoffield(insitu)stresses

Materialproperties Materialconstitutivemodel

Supportsystems Rockbolts,shotcrete,steelsets,etc.


RockMassPropertiesEstimationofYoungsmodulus,E Intactrockstrength GSI(Table)

Rockmassstrength e.g.,HoekBrown mi Intactrockstrength

GSI(Table)


RockMassPropertiesE =173MPa, =0.3Rockmassstrength=0.3MPa

HoekBrowncriterion

Insitustress=1.35MPa

Strengthtostressratio=0.22


ApproximateGuidelinesFewstabilityproblems,nosupport

Designbyrocksupportinteraction

Carefulsupportdesign

Seriousstabilityproblems,careful

sequencingrequiredImpossibletosupport

00.17 0.3c p

00.05 0.17c p

00.05c p

00.3 0.5c p

0

0.5c p


StressesatMiningFaceRedistributionofstressesduetoexcavation

3Dflowofstressesaroundadvancingface

Tunnel advancement

Tunnel displacement

21

Support installation


3DFaceStressDistributionExamine3D modelofdrift


ProfileofTunnelDisplacements


ConvergenceConfinementMethodConvergenceConfinementMethod(CCM) 2Dapproach Reasonablyestimatestunneldeformationspriortosupportinstallation Estimatesloadsimposedonsupportsystemsinstalledaroundtunnels


ConvergenceConfinementMethodIdealizedconditions Circulartunnel Hydrostaticinsitu stressstate

Comprisesthreebasicparts LongitudinalDeformationProfile(LDP) GroundReaction/ResponseCurve(GRC) SupportCharacteristicCurve(SCC)


Whyuse2Danalysisfor3Dproblem?Fewexisting3Dsupportmodelling tools

Absenceofmeshalgorithmforarbitrary3Dgeometries

2D relativelysimpleapproach Givesreasonablygoodresults


ConstructionofLDPVariousmethods Panet Chern Vlachopoulos &Diederichs Axisymmetric FEManalysis


ExampleLDP Chern Equation

T

u

n

n

e

l

f

a

c

e


ExampleLDP Chern Equation


GroundReactionCurve(GRC)Relatesradialdisplacementoftunnelwallstointernalpressureactingontunnelwalls


GroundReactionCurve(GRC)Generation Elastoplasticsolutionsofdeformationaroundexcavation Numericalanalysis(Phase2) twomethods Supportpressure:progressivelyreducesurfacepressure(Panet,1982) Materialsoftening(coresoftening):progressivelyreducemodulusofdeformation,E(Swobada,1979)


SupportPressureApproachAdvancing Tunnel

r r r

r = (1-)o

=0 0

SupportPressureApproachCharacteristicline(Groundreactioncurve) Crownandsidewall

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

P

r

e

s

s

u

r

e

Displacement

Crown

Side wall


MaterialSofteningApproach

Advancing Tunnel

EE

Stiffness reduction factor

Initial stage stage 1 stage 2 Final stage


SupportPressureApproachStagedmodellingofinternalpressure

Useoffieldstressvectordistributedloadplusstagedfactors

Plotofradialdisplacementsagainststages

PlotofGRCinExcel

Identificationofmaximumradialdisplacementandradiusofplasticzone


ExampleofGRC

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 0.5 1 1.5 2 2.5

I

n

t

e

r

n

a

l

P

r

e

s

s

u

r

e

(

M

P

a

)

Tunnel Convergence (%)

Ground Reaction Curve


CreationofLDP

Requires Distancefromfaceatwhichsupportisinstalled Maximumtunneldisplacement Radiusofplasticzone Tunnelradius Radialdisplacementattunnelface


CreationofLDPEssenceofVlachopoulos andDiederichs approach LDPdependsonsizeofplasticzonearoundtunnel

Equations


CreationofLDP

Step1:calculatePr

Step2:calculateratiouo/uMaxStep3:calculateuoStep4:plotratioofwalldisplacementstomaximumradialdisplacementasfunctionofdt wheredt =X/Rt , X isthedistantfromthetunnelfaceandRt isthetunnelradius

Step5:determinetunnelconvergenceattimeofsupportinstallation


SupportCharacteristicCurve(SCC)Relatespressureonsupporttoradialdisplacementsoftunnelwall

Hoek &Brown(1980)tables/equations provide YoungsmodulusKs ofsupportsystem(compositeofconcreteandsteelmember)

Maximumcapacityofsupport


SupportStiffness&Capacity


ExampleofSCC

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.5 1 1.5 2 2.5

I

n

t

e

r

n

a

l

P

r

e

s

s

u

r

e

(

M

P

a

)

Tunnel Convergence (%)

Support Characteristic Curve


AlgorithmforCCMStep1:determineLDP(Vlachopoulos &Diederichs)

Step2:fromdistanceofsupportinstallation,L,andLDPdeterminetunnelradialdisplacement,us,attimeofsupportinstallation


AlgorithmforCCM


AlgorithmforCCM

Step3:onplotofGRC,placeSSC(startingatus)

Step4:determineintersectionofGRCandSCC thisgivesestimateoffinalloadactingonsupport(equilibriumconditions)


SchematicDrawingofCCMTwocurves GRC SCC

Factorofsafety


FinalConvergence&FS

FS =pimax forsupport/pi atequilibriumconditionNUMERICAL MODELLING IN ROCK

ENGINEERING

GeneralizedSupportDesignRevisitPhase2modelforcreatinggroundreactioncurve

Modify includestagefactorthatgivesconvergenceattimewhensupportisinstalled

Compute


GeneralizedSupportDesign

PlotSupportCapacityCurvesforrequiredfactorofsafety Alsoshowswhereactualsupportloadsarerelativetosupportcapacityenvelope

Ifactualsupportloadsliewithinenvelopesupportisadequate Ifnotthenincreasesupportstrength


SupportCapacityPlotsforLinerPlotSupportCapacityCurvesforgivenliner


SupportDesign ExampleV1Tutorial Tunnelinweakrock Simulationof3D

facebehaviour Surfacesettlement Applicationofliner

support Materialsoftening


MaterialSofteningMaterialsofteningratio, =(0,1)

Surfacesettlement

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5 6 7Settlement (mm)

1-Youngs modulus of Softened Soil (kPa)

0.1180,000

0.4120,000

0.680,000

0.840,000

0.920,000

0.9510,000

0.9755,000

0.9951,000

0.9975500


SupportInstallationInstallsupportatspecifiedsettlement Softensystem(byspecified)atStage1 AddlineratStage2


ContoursofTotalDisplacement

Stage1(softenedmaterial) Stage2(installedsupport)


ForcesinLinerSupport


EndofPartIV


Documents

Underground Execvations