Role of Pore Pressure in Upper Slope and Crest Area Stability of a Large Open Pit

S O'Neill

Dans les comptes rendus d’articles de la conférence: GeoVancouver 2016: 69th Canadian Geotechnical Conference

Session: ENERGY RESOURCES - III Mining and Tailings

ABSTRACT: Precipitation, antecedent rainfall events and recharge are important variables for upper slope stability. Infiltration causes changes in material weight, matric suction, and effective stress. The effects of a rising and falling phreatic surface, due to recharge, can result in a rapid change in pore pressure in the upper slope of a large open pit. Low intensity, long duration, rainfall events provide the time to permit infiltration into the rock matrix resulting in a smaller failure area. High intensity, short duration recharge events, where the infiltration capacity of the rock matrix might be exceeded, could result in the percolating water rapidly reaching the preferential flow domain causing a larger water pressure increase and a potential larger failure area. In this paper, conceptual upper slope failures mechanisms have been modelled in a 2D steady state, coupled, hydromechanical (HM) model, to determine: The change in fracture permeability with shear dilation after a recharge event The variation in water table causing a variation in local stress/strain effects due to the contrasting HM behaviour for the high and low permeability elements Modification in rock strength from both Mohr-Coulomb and Hoek-Brown, due to recharge, resulting in a direct effect on potential upper slope stability

RÉSUMÉ: La précipitation présente et antécédente, ainsi que la recharge des nappes phréatiques, sont des variables importantes gement du poids des matériaux, de la capacité de succion de la matrice et de la contrainte effective. Les fluctuations de la surface libre suite à un événement de recharge peuvent causer des changements rapides dans la pression interstitielle dans la pente supérieure des grandes mines à ciel faible intensité et de longue durée. Par contre, les précipitations de forte intensité et de courte durée peuvent mener à un n rapide de la Dans cet article nous avons modélisé la rupture des pentes dans une mine à ciel ouvert selon un modèle : lité des fissures en fonction de la dilatation de cisaillement après un événement de recharge La différence des propriétés hydromécaniques dans les domaines de grande et de faible perméabilité, pouvant mener à des variations locales dans le régime de contrainte et de déformation suite à un changement de la surface libre La modification de la solidité de la roche causée par Mohr-Coulomb et Hoek-Brown, et engendrée par la recharge, menant à une déstabilisation de la pente 1. INTRODUCTION The movement of water through a porous rock modifies its mechanical behavior (Detournay and Cheng 1993). Hydromechanical (HM) coupling is the physical interaction, between hydraulic and mechanical processes in fluid saturated geological porous media or fractures, caused by a change in an external load and/or a change in the internal pore fluid pressure (Olsson and Barton 2001). Pore pressure can introduce a time dependency through the coupling of diffusion with deformation. HM coupling provides a framework to understand the relationship between stress and permeability for a given rock mass (Rutqvist and Stephansson 2003). Hydrogeologists think in terms of fixed hydraulic parameters for a given calibrated model and tend not to take account of the effects of slope deformation on hydrogeological parameters. The hydrogeological parameters used for the initial slope design will not be valid during future recharge events on that same slope due to constantly changing stress. 2. ROLE OF TRANSIENT PORE PRESSURE IN THE UPPER SLOPES AND CREST AREA Between 13% and 18% of pit slope failures occur in the upper benches due to the downward percolation of recharge (transient pore pressure) (Szwedzicki 2003) and (Baecher and Christian 2005). Understanding how and

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Citer cet article:
S O'Neill (2016) Role of Pore Pressure in Upper Slope and Crest Area Stability of a Large Open Pit in GEO2016. Ottawa, Ontario: Canadian Geotechnical Society.

@article{3774_0722094816,author = S O'Neill,title = Role of Pore Pressure in Upper Slope and Crest Area Stability of a Large Open Pit,year = 2016}