Numerical investigation of the lateral earth pressure coefficient along the VCL of vertical backfilled stopes

Mohamed Amine Sobhi, Li Li, Michel Aubertin

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Mining Geotechnics

ABSTRACT: Obtaining the stress distribution in backfilled stopes is a critical concern for underground mines. Several analytical solutions have been proposed for this purpose. Many of these include the lateral earth pressure coefficient K, defined as the horizontal to vertical normal stresses ratio. For vertical backfilled stopes, some have suggested to use the at-rest earth pressure coefficient K0, arguing that the rock mass is very stiff and the walls displacement is negligible upon backfill placement. Others have applied Rankine's active earth pressure coefficient Ka, based on numerical modeling and experimental results. To clarify this confusing situation, additional numerical simulations were performed to assess the lateral earth pressure coefficient along the vertical central line (VCL) of backfilled stopes. The results indicate that K is usually close to Rankine's active coefficient Ka for cohesionless backfills. An explanation is proposed. RÉSUMÉ L'estimation des contraintes dans les chantiers remblayés est une préoccupation majeure pour les mines souterraines. Diverses solutions analytiques ont été proposées à cette fin. Plusieurs de celles-ci impliquent le coefficient de poussée des terres K, défini comme le rapport entre les contraintes normales horizontale et verticale. Pour les chantiers remblayés verticaux, certains ont suggéré d'utiliser le coefficient au repos K0, en considérant que le massif rocheux est très rigide et que le déplacement des murs causé par le dépôt du remblai est négligeable. D'autres ont proposé d'appliquer le coefficient de poussée des terres actif de Rankine Ka, sur la base de la modélisation numérique et de résultats expérimentaux. Pour clarifier cette situation ambigüe, des simulations numériques additionnelles ont été réalisées pour évaluer la valeur de K le long de la ligne centrale de chantiers verticaux. Les résultats indiquent que K est habituellement proche du coefficient actif de Rankine Ka. Une explication est proposée. 1 INTRODUCTION Stopes backfilling is widely used in Canada and elsewhere, in part because it helps reduce the environmental impact of mining operations by placing mine wastes underground (Aubertin et al. 2002; Bussière 2007; Benzaazoua et al. 2008). Nonetheless, the main purpose of mine backfill is to improve ground stability, provide a safer work space and reduce ore dilution (Hassani and Archibald 1998). A key issue for the design of backfilled stopes is the determination of the stresses in the backfill. To that effect, many analytical expressions were developed. These are mostly based on an approach proposed by Marston (1930), who made use of the Janssen (1895) arching theory to calculate the vertical stress v (kPa) in backfilled trenches with buried conduits. The vertical stress in this case can be expressed as follows: --=hBKKBvddgstan2exp1tan2 [1] where B (m) is the width of the trench, h (m) is the depth, g (kN/m3) is the unit weight of the backfill, (°) is the friction angle along the interfaces between the backfill and walls, and K (-) is the lateral earth pressure coefficient, defined as a ratio between the horizontal (h) and vertical (v) stresses (kPa). Eq. 1 leads to vertical stresses that are typically smaller than the overburden stresses, due to a load transfer from the soft backfill to the stiff walls, thus producing an arching effect. This equation was applied to mine backfilled stopes by Aubertin et al. (2003), using = , where (°) is the internal friction angle of the backfill. This assumption implies that the strength along the fill-wall interface is controlled by the backfill strength, hence considering that yielding occurs in the backfill itself rather than directly along the rough walls. This approach has also been extended to inclined backfilled stopes by Caceres (2005) and Ting et al. (2011, 2014). The effects of cohesion and of a three-dimensional geometry (Li et al. 2005), pore water pressures (Li and Aubertin 2009a, 2009b) and non-uniform stress distribution along the width (Li and Aubertin 2008, 2010) have also been introduced in modified expressions. Other 3D formulations have also been proposed by Van Horn (1964) and Pirapakaran and Sivakugan (2007a). These solutions use the lateral earth pressure coefficient, K, as an equation parameter. The value of K can significantly affect the calculated stresses within a backfilled stope. There is however an uncertainty related to how K should be defined for such backfilled openings. This issue is addressed in the following sections, considering the stresses along the central line of vertical stopes obtained from numerical simulations.

RÉSUMÉ: ical investigation of the lateral earth

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Mohamed Amine Sobhi; Li Li; Michel Aubertin (2014) Numerical investigation of the lateral earth pressure coefficient along the VCL of vertical backfilled stopes in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper115,author = Mohamed Amine Sobhi; Li Li; Michel Aubertin,title = Numerical investigation of the lateral earth pressure coefficient along the VCL of vertical backfilled stopes,year = 2014}