EXPERIMENTAL TESTS ON A SMALL-SCALE MODEL OF A MINE STOPE TO STUDY THE BEHAVIOUR OF WASTE ROCK BARRICADES DURING BACKFILLING 

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Backfill uses in mining operations

Mining aims to extract valuable minerals from ground resources, which usually creates voids. The primary function of mine backfills is to help manage the mining operations that create voids. Backfilling is one of the techniques used to improve the flexibility of ore extraction strategies and mostly allows better recovery of orebodies (Potvin et al., 2005; Belem & Benzaazoua, 2008). The use of various backfill types, their specific functions, and their technical aspects are intimately associated with methods of extraction, extraction strategies, and sequences of the extraction process. In practice, the choice of an exploitation method depends largely on the characteristics of the deposit and the rock mass. Mining methods are divided into surface mining methods (e.g., open-pit mining, strip mining) and underground mining methods (Hartman & Britton, 1992; Darling, 2011). Brady & Brown (1985) proposed to divide mining methods into three main categories, as shown in Figure 2-1.

Types of mine backfills

As explained above, safety and environmental factors have led mining companies to consider backfilling mine wastes to prevent the mine from collapse during later and deeper mining phases, ground subsidence during mine abandonment, and environmental pollution. Mine waste surface disposal (i.e. waste rock and tailings) can be minimized by backfilling, which can help prevent acid mine drainage generation (Aubertin et al., 2002; Bussiere, 2007; Belem et al., 2000; Benzaazoua et al., 2004; Belem & Benzaazoua, 2008).
Underground extraction of economic minerals creates voids of various shapes (termed stopes). These voids create risks of instability for the mining of adjacent pillars that contain beneficial minerals. Underground voids also pose the risk of infrastructure subsidence on the ground during development or after the abandonment of mines. During the history of mining technology, various techniques have been developed to fill underground cavities. Common practical methods for waste disposal are rock backfill, hydraulic backfill, and cemented paste backfill. The implementation of any such approach depends on financial considerations and other goals, such as mining development or abandonment.

Rockfill

Rockfill (RF) consists of waste rock from mining development or surface quarry (e.g., Yu, 1989& Dismuke & Diment, 1996; Farsangi, 1997; Potvin et al., 2005). A truck or a conveyor transports RF to fill underground stopes. Rockfill acts as a working floor (when side and base exposures are not required) and provides ground stability and active support when exposed during the extraction of adjacent stopes (Hassani & Archibald, 1998; Liston, 2014). Cemented Rockfill (CRF) is mixed with binders, which can be Portland cement, slag, and fly ash, with a percentage ranging from 1% to 8% (solid weight). The RF can also be modified by optimizing the grain size (such as grinding or adding fine-grain materials) depending on its applications (Hassani & Archibald, 1998; Kuganathan, 2005b).

Physical properties of rockfill

It is not easy to obtain an accurate particle size distribution of waste rock in piles where heterogeneity, segregation, and the presence of large rocks make measurement difficult (Maknoon, 2016). Various methods can be used to estimate the particle size distribution, such as sampling and laboratory measurements, in situ determination, and image analysis of photographs (McLemore et al., 2009). Typical waste rock shows a wide gradation curve (from silt to boulders size) with a coefficient of uniformity (CU =D60/D10) of 20 or more (Barbour et al., 2001; Aubertin et al., 2002). Maknoon (2016) collected different grain size distributions of waste rock (measured in the laboratory or in situ). The distributions showed that the size of particles ranged from 75 μm to about 10 cm and above (Fig. 2-5). The typical angle of repose of waste rock is near 37°(±3°), which may differ with the physical characteristics of the waste rock (Farsangi, 1997; Aubertin, 2013).
The Talbot grading has been adopted to design the optimal particle size distribution for RF, which gives the maximum in-situ density (Kuganathan, 2005b). The typical RF particle size used can vary from 1 to 100 mm and should be optimized to minimize the void ratio and maximize the density in situ. Thus, the addition of binder and/or fine-grained materials increases the strength of the filling which can be associated with a decrease in the final void ratio (or porosity). However, RF optimization remains difficult, and the particle size distribution varies from one operation to another. Segregation usually occurs during the RF filling operation when the RF is placed in a stope by trucks or raises (Hassani & Archibald, 1998; Potvin et al., 2005; Belem et al., 2018). Different factors produce RF with different layers, such as ejection velocity, the trajectory of movement, and settlement effects between coarse and fine aggregates. The degree of segregation varies depending on the filling method, elevation height and length, aggregate size, stope geometry, and free-fall height (Yu, 1989; Annor, 1999; Belem et al., 2018).

Mechanical properties of rockfill

The angle of friction of the uncemented rockfill material can vary from 35°to 55°, depending on the relative density (Kuganathan, 2005b). Large-scale tests indicated that the internal friction angle of the waste rock is between 21°and 62°, with typical values ranging from 34°to 45°(Aubertin, 2013). It is of critical importance that the waste-rock material modulus is carefully and effectively analyzed in underground backfill mining. Zhang et al. (2019) conducted a series of laboratory-based compression tests on waste-rock samples in the Tangshan coal mine (China) and compared the results to field tests and numerical modelling using FLAC. They stated that waste material used in underground backfill mining has a granular texture and, when compressed, acquires non-linear deformation characteristics (Fig. 2-6). They also reported that the deformation modulus of waste rock determined by a laboratory compression test differs significantly from the real deformation modulus in the field due to the full confining impact of the loading steel cylinder. The waste rock deformation modulus exponentially increased with an increase in axial strain. The exponential equation to fit the relationship between the deformation modulus and the strain in the field as follows: = 3.6915 18.94 (2-1).
It has been stated (by Lee et al., 2009) that the stress-strain behaviour of crushed rock is nonlinear, inelastic, and stress-dependent during large-scale direct shear tests. The stress-strain characteristics of crushed waste-rock material are generally dependent on the grain size, gradation, degree of saturation, uniformity, breakage, etc.
Generally, the fill elasticity modulus increases with the content of the binder (Pierce et al., 1998). The elastic modulus of cemented rockfill varies between 2.0 and 3.8 GPa according to some in-situ and laboratory tests (Yu & Counter, 1983). Usually, the cemented rockfill elastic modulus is much higher than that of cemented hydraulic fill and paste fill (Zhu, 2002).
The unconfined compressive strength (UCS) of RF is close to zero because it is a non-cohesive material, and it can be much higher with the addition of binders and fine-grained materials. The in-situ UCS of CRF can range from less than 1 MPa to more than 10 MPa (Hedley, 1995; Annor, 1999).

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Hydraulic properties of rockfill

The hydraulic conductivity represents the average velocity when water flows through the fill mass under constant hydraulic gradient. It is affected by particle size distribution, particle shape, void ratio, and so on (Potvin et al. 2005). Infiltration tests on the waste rock pile at Tio mine showed that the hydraulic conductivity ranges from 4×10-5 to 3×10-3 m/s (Lessard, 2011).

Hydraulic fill

Hydraulic fill (HF) consists of classified mill tailings and/or natural sand and contains a large amount of water that drains after placement. HF is slurry, with low solids content (Cw-f = 60% to 75%), transported to underground stopes through boreholes and pipelines (Hassani & Archibald, 1998; Potvin et al., 2005). It is usually prepared with about 70% of solid content for traditional transport (Hassani & Archibald, 1998; Grice, 2001). A portion of the fine materials is eliminated, usually by hydrocyclone (Fig. 2-7a), to avoid inadequate drainage (particle size ≤ 10 µm does not exceed 10%). Below, the figure 2-7b represents a typical particle size spindle of tailings that can be used for the hydraulic fill preparation. Normally, the particle size of HF varies from 1 to 1000 µm (with an average of around 100 µm). The principal advantage of hydraulic backfill is its simplicity and low cost of production and delivery (Pirapakaran & Sivakugan, 2007).

Physical properties of hydraulic fill

Rankine (2005) indicated that the HF relative density could change from 2.8 to 4.5, and the density index ( ) between 50% and 80%. The settled fill porosity ( ) varies from 0.37 to 0.48, and the void ratio ( ) of 0.58 to 0.93.
The uniformity coefficient ( = 60/ 10) is usually used to describe the spread of the particle size distribution. Bussiere (2007) reported that the of full-tailings samples can range from 8 to 18. For HF, normally varies between 5 and 10 (Kuganathan, 2005a), pointing to a relatively narrow spread. Segregation can happen within the HF, where fine particles (including binders) can be driven by water flow and concentrated in layers, resulting in a heterogeneous mass (anisotropy) with strength zoning (Belem et al., 2018).

Hydraulic properties of hydraulic fill

The laboratory tests are used to measure HF’s hydraulic conductivity. The constant head infiltration test is commonly used with coarse-grained materials. The falling head infiltration test is also used for relatively low permeability fills (Rankine, 2005). Figure 2-8 shows a set up used for the constant head permeability test. The sample is subjected to a constant water head (the difference between its ends). The water flow rate through the sample of fill is measured. The constant head permeability of the fill is given by: ℎ = ln ℎ0 (2-2).
Where k = coefficient of permeability; = area of the burette; = length of fill sample; = area of the fill sample; ℎ0 = initial height of water; h1 = final height of water = h0 – Δh; t = time required getting head drop of Δh.
High permeability is important for slurry backfill to achieve rapid drainage and self-weight consolidation and to prevent the potentially dangerous build-up of hydrostatic pressure in underground stopes (Sveinson, 1999). The in-situ hydraulic conductivity of uncemented HF usually ranges from 10-6 to 10-5 m/s (Grice, 2001). Rankine (2005) reported that the hydraulic conductivity of Australian hydraulic fills typically ranges from 2.78 ×10-6 to 8.33 ×10-6 m/s. Alternatively, some functions, based on the Kozeny-Carman equation for granular materials, can be used to predict the hydraulic conductivity of HF indirectly from grain size curves (Aubertin et al., 1996; Mbonimpa et al., 2002; Chapuis & Aubertin, 2003).

Table of contents :

INTRODUCTION.
1.1 Problem statement
1.2 Originality
1.3 Thesis objectives
1.4 Contributions
1.5 Methodology
1.5.1 Physical modelling
1.5.2 Numerical modelling
LITERATURE REVIEW 
2.1 Backfill uses in mining operations
2.2 Types of mine backfills
2.2.1 Rockfill
2.2.2 Hydraulic fill
2.2.3 Cemented paste fill
2.2.4 Summary of backfill properties
2.3 Design and construction of fill barricades
2.3.1 Types of barricade
2.3.2 Construction of waste rock barricades
2.3.3 Barricade design methods
2.4 In-situ measurements
2.5 Small-scale models of mine stopes
2.6 Numerical simulations for vertical backfilled stope stress evaluation
EXPERIMENTAL TESTS ON A SMALL-SCALE MODEL OF A MINE STOPE TO STUDY THE BEHAVIOUR OF WASTE ROCK BARRICADES DURING BACKFILLING 
3.1 Introduction
3.2 Small-Scale Model and Equipment Description
3.3 Material Characteristics
3.3.1 Mine Tailings
3.3.2 Waste Rock Barricade
3.4 Experimental Setup and Program
3.4.1 Setting up the waste rock barricade
3.4.2 CPB Mix Design and Preparation
3.4.3 Program experimental
3.5 Results
3.5.1 Stress State after the Placement of the Backfill in the Small-Scale Model
3.5.2 Stability and Failure Mechanism of the Waste Rock Barricades
3.6 Concluding Remarks
ASSESSMENT OF STRESS STATE IN PASTE BACKFILL DURING PROGRESSIVE BACKFILLING: EXPERIMENTS AND MODELLING
4.1 Introduction
4.2 Experimental & numerical program
4.3 Physical & numerical models
4.3.1 Physical model
4.3.2 Numerical model
4.4 Results
4.5 Conclusion
5.1 Introduction
5.2 Experimental tests
5.3 Results
5.4 Concluding and remarks
6.1 Discussion
6.2 Conclusions
6.3 Recommendations for further study
REFERENCES

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