Estimating earth pressure exerted by the backfill on the vertical pillars in underground mine stopes

Ali Saeidi

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

Session: SOIL AND TERRAIN CHARACTERIZATION - VII Instrumentation & Monitoring

ABSTRACT: Backfilling of underground stopes and galleries in mines has become a common practice in the mining industry. Backfillincreases the stability of mine excavations, and using mine tailings as backfill material yields environmental benefits inthat it reduces the amount of waste requiring disposal at the surface. Stresses exerted by backfill on the walls ofexcavations must be accurately determined. The earth pressure coefficient (K) is the ratio of the horizontal stress to thevertical stress anddepends on the material properties.Determining an accurate value of this coefficient in backfills is keyto evaluating mine pillar stability.That determination must take into account the geomechanicalparameters, whichinevitably vary from one backfillsite to another. A predetermined standard K value is generally used to determine thestresses on the walls of a backfilled stope.This standard K valueisofteninappropriate,because important parametervariations ultimately lead to significant errors instress estimates. This study demonstrates the variability of K in varioussituations. A method is proposed, using two graphs for adjusting the Marston analytical method to evaluate proper earthpressure values,based on variousgeomechanicaland geometricalparameters. These graphs can be used as charts toestimate K values with a better accuracy based on the characteristics of the stope backfill.R†SUM†Le remblayage deschantierssouterrains dans les mines est devenu une pratique courante dans l'industrie mini‡re.Cecipermet d…augmenter lastabilit•des excavations. De plus,l…utilisation des rejets miniers comme mat•riel de remblayagepr•sente desavantages environnementaux,car cecir•duitla quantit• de rejets en surface.Par contre, ces remblaisexerceront des contraintes significatives sur les murs de sout‡nement. Les contraintes exerc•es par le remblai sur cesmurs doivent—tre d•termin•es avecexactitudepour •viter la rupture des murs de sout‡nement. Plus le poids des terresest •lev•, plus les contraintes verticales et horizontales seront •lev•es. Le coefficient de pression des terres Kpermetde donner une •quivalence entre ces deux contraintes exerc•es. Il repr•sente le ratiode la contrainte horizontale (–hz) ƒla contrainte verticale (–vz) etest d•pendant des propri•t•s des mat•riaux en jeu. D•terminer une valeur appropri•e ducoefficient Kest donc primordial pour augmenter la pr•cision des r•sultats. Cet article sugg‡re que le coefficient depression des terres doit —tre adapt• en fonction des param‡tresqui changeront in•vitablementselonla g•om•triedeschantierset la g•om•caniquedesremblais.1INTRODUCTIONMine backfilling is a technique that has been used fordecades in Canada and worldwide(Hassaniet al.1989).This method has several advantages such as stabilizingthe galleries and stopes of a mine and increasing safetyfor workers. Backfilling serves to fill excavations in a mineand allowsore to be extracted more completely becausethe underground excavations are better supported.Environmental benefits result from the use of tailings asbackfillmaterial: problems related to surface tailingsdisposal can be reduced because substantial quantities ofpotentially polluting tailings can be returned underground.This backfill material commonly consists of a mixture oftailings, water and cement.Mechanical interactions occur at the interface betweenthe backfill and the rock mass. The mechanical propertiesof these two media are very different from each other. Forexample, the backfill is of lower density and rigidity thanthe host rock; these contrasting properties must be takeninto account in a thorough analysis of the mechanicalinteractionsbetween the two media. The freshly placedbackfill tends to compress under its own weight(Belem etal. 2007)and generates shear stress at the backfill⁄rockinterface due to friction. A certain amount ofbackfillweight is transferred laterallyto the rock mass. Thistransfer is associated with arching, which occurs mostly innarrow stopes and where a less-dense material is placedbetween denser materials(Aubertin et al. 2003; Li et al.2003). The stresses exerted on the mine pillars by thebackfill must be determined to avoid failure of the pillars,which requires accurate estimates of the geomechanicalparameters of the backfill andtherock mass.Several authorshave demonstratedthe arching effect,based on field measurements or using analytical andnumerical models. Important studies onthis topicincludethose of Li andAubertin(Aubertin 1999; Aubertin et al.2003; Li and Aubertin 2009; Li et al. 2005, 2007), whichpresentthe results of large-scalenumerical investigationswhere the emphasis was on the geometry of the stopeand the properties of the backfill. These authorsconcluded that the parameters with the greatest effect on

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