Approach to Numerically Model Edz in Vcr Mining

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Another phenomena that needs to be appropriately considered in simulation of wave propagation is the strain rate effect. At very low strain rates, a few large fractures dominate the fragmentation process, and only a few large fragments will be formed. Under high strain rates, on the other hand, numerous small fractures simultaneously grow and interact, resulting in many small fragments (Fig. 1)[2]. In the present study, an isotropic continuum damage model based on equivalent tensile strain is suggested. Damage to rock medium is defined as the probability of fracture at a given crack density with respect to the equivalent tensile strain. The damage scalar in the model is determined by three parameters, namely threshold strain and other two material constants that will be discussed later. The threshold strain is modelled by an equivalent static tensile strain, and the other two damage parameters are determined based on those obtained from uniaxial tensile experiments with constant strain rates. The present damage model and strength model can be implemented by modifying the user subroutine of AUTODYN. Damage to rock mass induced by an explosion in an underground chamber can be simulated by using the AUTODYN with the suggested damage and strength model.

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1.2 OBJECTIVES, SCOPE OF WORK AND RESEARCH

OBJECTIVES

PRIMARY OBJECTIVE

• Discovering how to implement numerical modelling to determine the explosive damage zone created by blasting in VCR mining.

SECONDARY OBJECTIVES

- Understanding the basics of numerical modelling to better determine the most suitable technique to be used.

- Understanding different criteria to be established to determine the explosion induced damage zone.

SCOPE

Once a modelling method for explosion damage zone in Vertical Crater Retreat mining is developed it will be very productive and efficient as it will help minimise dilution, increase productivity and improve stability.

- After deciding the most suitable modelling method and rock material model, actual modelling of a mine can be done

- Results can be obtained using data from blasting done in the past to re-check and then the model can be applied to future use.

PROCEDURAL FLOWCHART TO EXECUTE THE MODELLING PROCESS

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DEFINING DOMAIN OF NUMERICAL MODEL[5]

Dongguashan Copper Mine is located in Tongling, Anhui, China. It has the ability to produce around 4.3 million tons of copper ore annually, and its service life is 28 years. As the largest copper mine of down-hole pit mining in Asia, its mining depth is more than 1000 meters, which ranks the first among the nonferrous metal mines in Asia. The deposit is as long as more than 1800 m in trend, more than 500 meters wide, and 20–70 m thick. It is divided into panels every 100 m, and there is an 18 m wide barrier pillar in each pair of adjacent panels. A panel is 100 m wide, whose length and height equal the width and the thickness of the deposit, respectively. Every panel consists of 20 stopes, which are arranged along the trend of the deposit and 18 m wide

VCR mining method was adopted for underground mining in Dongguashan Copper Mine. According to the reality, large-diameter deep-hole blasting is introduced. The blasthole diameter is 165 mm, the charge length is 1.5 m to 10.5 m, and the stemming length is 1.2 m to 2.0 m. In this study, a three-dimensional model is established as in Figure 2, which is measuring 20 m in X-direction, 20 m in Y-direction, and 10 m in Z-direction. 10 blasting holes are equally divided into 2 rows. The distance between two rows is 3 m, and the distance between 2 adjacent holes in the same row is 2.8 m. Only the bottom surface is free, and the remaining surfaces are applied with nonreflecting boundary.

Due to the model which is built symmetrically, a quarter of the model is modelled using the software Strand 7 to reduce the size of the research object. So the model was simplified as a 10 m cube, and the charge length of 6m is shown in Figure 3. The bottom surface is free; the front surface and the right surface are applied with normal displacement constraint. The remaining surfaces are applied with nonreflecting boundary.

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CHAPTER 2

ROCK MODELS

2.1 CONTINUUM DAMAGE MODEL[1,3]

The isotropic continuum damage model assumes rock mass as an isotropic, homogeneous and continuous medium. It models the overall effects of crack system by using an equivalent material stiffness. The damage growth is governed by a loading surface of equation expressed in strain space which defines a unique relationship between the size of damage surface and the accumulated damage. In this study, a damage loading surface is defined based on an equivalent tensile strain,

Fd(εij,KD,D)= ε’ – KD(D) ≤ 0 (1)

Where, ε’ is the equivalent tensile strain and

ε’ = √(1∑3(‹εi›+)2 ; (‹x›+ (|x| + x)/2, εi are principal strains) (2)

The threshold strain KD(D) takes the largest value of the equivalent tensile strain ε’ ever reached by the material at the considered point during the previous loading cycles. The above suggested loading surface can be determined by rate-dependent uniaxial tensile strain experiments since the equivalent tensile strain ε’ here is just the uniaxial tensile strain. For damage process, it is assumed that damage is caused by the activation and growth of pre-existing micro cracks, which is accumulated over time and is irreversible. When a rock material is subjected to tensile stress, it will not fail unless the value of the stress is larger than its static strength. This is accounted for by setting a critical value for the equivalent tensile strain. Since blast damage is accumulated as a function of time and applied stress, crack density is expressed as a function of the equivalent tensile strain and time which is similar to the model defined by Liu and Katsabanis.[1]