Evaluation of the earth pressures in inclined backfilled stopes based on the Coulomb active state coefficient

Abtin Jahanbakhshzadeh

In the proceedings of: GeoVancouver 2016: 69th Canadian Geotechnical Conference

Session: ENERGY RESOURCES - I Mining and Tailings

ABSTRACT: Backfilling is commonly used in underground mine operationsto ensure the stability of openingsand safety of theworkers.Inthis regard,it is crucial toevaluatethe stress state in the fill material and surrounding rock mass. Recentwork has shown thatthefillsettlementin the stopeproduces shear stresses along therockinterfacesthatlead to a loadtransfer to the abutments. The magnitude of thisredistributiondepends on many factors,including stope geometry andfill properties.Thisarticlepresentsa new solutionfor the stress stateininclined stopes, which uses the Coulomb activeearth pressure coefficient. This solution takes into account the variation of the stresses along the opening widthandheightfor various inclination angles.The solution andkeyresults aredescribedand validated(in part)using numericalsimulations.R•SUM•Le remblayage est couramment utilis† dans les op†rationsmini‡ressouterrainesafin d'assurer lastabilit† desouvertureset la s†curit† des ouvriers.A cet †gard,ilestcrucial d…†valuer l…†tatdes contraintes dans le remblaiet lemassif rocheux.Les travauxant†rieursont montr† que letassement duremblaitengendre descontraintesdecisaillementauxinterfacesrocheux, ce qui produit untransfertaux †pontes. L…ampleur de cette redistributiond†pend deplusieursfacteurs, y compris la g†om†trie du chantier et lespropri†t†sdu remblai.Cet articlepr†senteune nouvellesolution pour l'†tat descontraintesdansleschantiersinclin†s, — partir du coefficient de Coulomb pour la pression desterres en condition active.Cette solution prend en compte la variationselon la position, pour toute la la largeuret lahauteurde l'ouverture,etpour diff†rents angles d'inclinaison. La solution etdesr†sultatsrepr†sentatifssontmontr†setvalid†s(en partie)— l'aide de simulations num†riques.1INTRODUCTIONThere aremanyreasons for using backfill in undergroundmines,such as reducingthe volume of solid wastes onthe surface,improving ground stability, and ensuring thesafetyof workers.Differentbackfilltechnologies arecurrently used inmines(e.g. Hassani and Archibald1998; Liston 2014).The fillingmaterialsplaced in stopesare generally muchless stiff than the surrounding rock mass.The self-weightsettlementof the fillafterdepositiongeneratesshearstresses along the rough walls of the opening, whichproduces a transferofthe overburden pressuresto therock mass(e.g. Askew et al. 1978; Aubertin et al. 2003; Liet al. 2003, 2005;Potvin et al.2005).The sizesoftheopenings,theconditionsalong thefill-rockinterfacesand the backfill propertiesarethe mainparametersintroduced inanalytical solutions commonlyused to evaluate the stresses in backfilledstopes.However, the effect of wall inclination isgenerallyneglected in thesesolutions, despite results showingthatthis can havea major influenceon the stress state(Li etal. 2007; Li and Aubertin 2009a;Jahanbakhshzadeh2016).These results indicate that thearching effect,related to the stress transfer between the fill androckmass, istypically less developed in inclined stopesandthat thestressescandiffersignificantlybetweenthehanging wall and footwall.In thisarticle,an alternative plane strainsolution isproposed for the stress distribution inthebackfillplaced ininclined openingsbased onthe Coulomb active earthpressure coefficient. Thesolution isvalidated, in part, bycomparingcalculatedstresseswith thoseobtained fromnumericalsimulations.Someimplications of these resultsarealso discussed.2ALTERNATIVE ANALYTICALCALCULATIONSimulationsresults obtained with FLAC (Itasca2002)show that a decrease ofthe wallangle(from 90–to 60–)typicallyleadsto adecrease in the vertical stressesalongthe hanging wall(Jahanbakhshzadeh et al. 2015;Jahanbakhshzadeh 2016). Suchsimulations also indicatethat thesestressescanvary along the stope width (fromthe hanging wall to the footwall)and height,and with thefill properties.Most analytical solutions neglect these factors. Thecorresponding equations, which are generally based onthe Marston (1930) solution, include a unique value forthe earth pressure coefficientKto representthe ratiobetween the horizontal()and the vertical()effective

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