Dimensionamento de corte e aterro

Dimensionamento de corte e aterro

(Parte 1 de 4)

Backfill σv

Span Open Stope

Sill Mat Pillar

- Cohesion -Friction Angle

-Tensile Strength

- Density

•Vertical Loading • Stope Geometry Influences

•Failure Modes

Seis mic σh βββ-Modulus, Poisson Ratio

Design Requirements •Lateral Loading

•Seismic Effects • Support Implications

• Strength Properties •Fill Placement

•Other (Air Gaps/Cold Joints) • Stope Closure

Backfill σvσv

Span Open Stope

Sill Mat Pillar

- Cohesion -Friction Angle

-Tensile Strength

- Density

•Vertical Loading•Vertical Loading • Stope Geometry Influences• Stope Geometry Influences

•Failure Modes•Failure Modes

Seis mic σhσh βββ-Modulus, Poisson Ratio

Design Requirements •Lateral Loading•Lateral Loading

•Seismic Effects•Seismic Effects • Support Implications• Support Implications

• Strength Properties• Strength Properties •Fill Placement •Fill Placement

•Other (Air Gaps/Cold Joints)•Other (Air Gaps/Cold Joints) • Stope Closure• Stope Closure

Figure 1: Mining underneath consolidated backfill.

Design Spans – Underhand Cut and Fill Mining Presented at 107 CIM-AGM Toronto, April 2005

Rimas Pakalnis, Cristian Caceres, Kathryn Clapp, Mario Morin University of British Columbia

Tom Brady, Ted Williams Spokane Research Laboratory, NIOSH

Wilson Blake Consultant

Mary MacLaughlin Montana Tech, University of Montana

Abstract

This paper describes a focus of work presently being conducted at the Rock Mechanics Research Group at the University of British Columbia. The underhand method under consolidated fill ensures a high recovery under an engineered back that is comprised of cemented rock fill and/or cemented paste fill. This method of mining is generally necessary either due to a weak rock mass comprising the immediate back and/or high induced back stresses. A major concern in the design of sill mats is the loading and strengths associated with the overlying sill mat. This paper reviews past practice coupled with present observations and measurements from over ten(10) mines throughout North America. It outlines areas of concern in terms of design requirements.

1. Introduction

Backfilling in North America has been practised since the turn of the century. Souza et al. (2003) has summarized the advancements in backfill with the introduction of hydraulic fills in the 1950’s and the addition of cement in the 1960’s. This coupled with cemented rock and paste fills being introduced in the 1980’ and 1990’s respectively resulted in the implementation of mining methods that require extraction under a consolidated back largely comprised of fill rather than timbered mats/cables (Marcinyshyn, 1996). The increased use of consolidated fills in the late 1990’s to present under engineered conditions with a high degree of reproducibility in terms of strengths and predicted behaviours has enabled man-entry methods such as underhand cut and fill to be implemented under greater controlled spans resulting in a safe and economic alternative to conventional cut and fill mining, (Mah, 2003). A database of twelve (12) underhand cut and fill operations was compiled as part of this project through mine visits. The operations were located throughout North America and summarized in Table 1.

The placement of consolidated fill either cemented rock fill or paste requires one to understand the overall factors affecting design. Figure 1 graphically summarizes some of the parameters that are being investigated in terms of their implication on developing a design span enabling man entry access. A sill for this study is defined as a consolidated layer of previously placed fill immediately above the mine opening that is being excavated. This sill may be comprised of one large vertical height placed by bulk mining as shown in Figure 2a or by single lifts as placed by conventional drift and fill and/or underhand cut and fill as shown in Figure 2b. The major difference is that in Figure 2a one is largely operating remotely from the immediate filled back, whereas in Figure 2b one is mining by man-entry methods. This necessitates that the factor of safety for the man entry be substantially greater than the non-entry approach.

Figure 2: Mining under a consolidated back.

2. Design Constraints

Figure 1 shows the factors that have to be accounted for in terms of mining under an engineered back. These will be outlined in this paper from a general perspective with focus on the analytical, numerical assessment of span and applied loading conditions.

2.1 Design Load

A critical factor is estimating the design loads onto the sill mat. A recently completed MASC thesis by Caceres (2005) employing the Musselwhite mine of Placer Dome as a case study had looked at the loading conditions that exist on a cemented rock fill mat as shown schematically in Figure 2a. Knowing the loads is critical to determining the strength required of the sill mat for the given stope geometry. Under-estimating can cause a premature failure of the sill mat once mining exposes the mat whereas overestimating can result in unnecessary expense due to the cost of the cement in place. Knowing the vertical loading is not a trivial solution as many factors affect the overall loading conditions as evident from the many theoretical derivations that are available as per Janssen (1895), Terzaghi et al.(1996), Reimbert (1976) and Blight (1984) all of which have significant assumptions in terms of coefficient of lateral earth pressure “K” as detailed by Marcinyshyn (1996). The value for “k” describes the ratio between the horizontal and vertical stresses in the fill and indirectly the ability of the degree of load transfer by arching. When fill is placed initially, very little shear resistance is mobilized through grain interaction, the coefficient of earth pressure at rest (Ko). Subsequent placement of fill results in the fill mass to settle and compact, increasing the shear resistance and transfer load to the abutments through arching. As mining underneath the sill pillar progresses, a void is created toward which the fill mass will tend to move if unconsolidated with transfer of vertical stresses laterally through arching. This condition is described by the passive earth pressure coefficient (Kp) where full shear resistance is mobilized. This is analogous to classical embankment theory where the walls are moving into the fill. The effect of K employing individual fill load formulaes is shown in Figure 3a. The analytical methods shown in Figure 3a assume vertical stope walls which is generally a conservative estimate of vertical loading as shown by Caceres (2005). The typical geometry was modelled employing FLAC2D (Itasca, 2005) which did not have the constraints the analytical methods had in terms of ‘K” and stope inclination. An analytical approximation as shown by eqn. 1 was derived by Caceres (2005) relating the numerical simulation to an equivalent relationship as shown in Figure 3b.

CRF Sill Mat Pillar

Backfill 10m +

10m + a) Longhole mining under a cemented rock fill sill mat (Caceres, 2005) b) Underhand cut and fill under a paste back.

PREVIOUSLY BACKFILLED ~3m LIFT

2 3(low)

2 3(low)

Figure 3: Estimation of vertical loading onto sill mat. Eqn. 1:

The above was derived for cemented rock fill, however, the analytical solution would be similar to that of paste as the input parameters would define the loading conditions.

(Parte 1 de 4)

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