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Interpretation Of Acoustic Emission In-Situ Stress Measurements For Underground Mine Design And Stability In Zone 5

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Stress is an enigmatic quantity which, according to classical mechanics, is defined at a point in a continuum and is independent of the constitutive behavior of the medium (Amadei & Stephanson, 2012). Natural materials such as rocks and soil are subjected to natural (virgin) stresses called In-situ stresses. In-situ stress magnitudes and orientations play a very important role in geological engineering, and they are the most basic parameters inputs in design of rock structures (Peng & Zhang, 2007). One of main functions of rock mechanics is to determine In-situ stress. The research proposes analysis of In-situ stress of rock mass for purpose of underground mine design and stability, specifically for decline holes in sandstone terrain in Zone 5, Boseto. An intensively studied method which is relatively new to rock mechanics as an In-situ stress measurement technique Western Australia School of Mines Acoustic Emission stress measurement is used. The method samples the Kaiser Effect elucidated by Villaescusa et al (2003). The Kaiser Effect is an expression of the immediately preceding maximum stress to which a specimen of rock has been subjected, and provides a method of estimating the recent stress history of a core sample recovered from a borehole (Barry & Brown, 2007). The research will analyse the results from this method, inherently analyzing competence of the rock mass to engineering work.

Many deep underground excavation practices show that the size and distribution of in situ stress are the main factors resulting in the deformation and instability of the surrounding rock structure. In situ stress is one of the most important factors affecting the stability of underground excavation engineering it is important to understand the effect of horizontal in-situ stress on the failure mechanism around underground openings excavated in isotropic, elastic rock zones (Komurlu et al). In situ stress is the fundamental force causing the deformation and destruction of mining, water conservancy and hydropower, civil engineering, and other underground rock excavation engineering. It is a necessary prerequisite to determine the mechanical properties of engineering rock mass, analyze the stability of surrounding rock, and realize the scientific design of rock excavation and decision-making (Cai, He, and Liu, 2002). With the continuous expansion of the scale and depth of mine excavation, in situ stress has become more prominent and the occurrence of many disaster phenomena is closely related to the in situ stress (Mazaira and Konicek, 2015). Therefore, it is necessary to determine the distribution of the in situ stress before the mining project. The rational layout of the roadway can not only improve the safety and avoid the occurrence of disaster but also reduce the cost of support and maintenance.

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At present, there are many in situ stress measurement methods, which can be divided into two categories: direct measurement method and indirect measurement method (Cai and Qiao, 1995). In the direct method, only the hydraulic fracturing method and the acoustic emission method do not require pre-excavation of the chamber, which is relatively more convenient and quick. The hydraulic fracturing method is to estimate the stress field of the measuring point on the spot after drilling to the measuring point. This method is expensive due to the bulky hydraulic fracturing vehicle and expensive measuring instruments. The acoustic emission method only needs to obtain the rock core of the corresponding measuring point, and the Kaiser effect of the rock can be used to estimate the in situ stress. Due to a simple and convenient operation, low price, less on-site operation, and suitability for indoor large-scale batch testing, the acoustic emission method is more widely used in mines (Meng et al, 2016).

Statement of the problem

The design of underground openings at depth requires knowledge of the in situ stress state. In-situ stress stresses control the stresses around underground openings such as tunnels, mines, shafts or caverns. Stress concentrations in the excavations walls may be large enough to overstress the rock, mobilize the strength of the rock mass locally or at large, and create failure (Amadei & Stephansson, 1997). Zone 5, Boseto is proposed to be an underground mine, however the rock mass In-situ stress is unknown. Therefore In-situ stress measurement will be carried out to determine virgin stress of persistent formations that significantly influence the design, support and stability of two declines. The two lithologies are Sandstone and Limestone of Ngwako Pan Formation. Virgin stress of the aforementioned lithologies will be determined using Western Australia School of Mines Acoustic Measurement Stress Measurement which is relatively cheaper and less tedious.

Objectives:

  1. To determine the mechanical properties of engineering rock mass and analyze the stability of surrounding rock.
  2. To understand whether the rock types will withstand the stress associated with mine design and development infrastructure along the decline positions.
  3. To understand the principle of Kaiser Effect (KE) and how it can be used to estimate the in situ stress of the rock.
  4. Use the results obtained to provide basic data for the mining design, and stability evaluation of the surrounding rock in the later stage of mining and provide the guarantee for high efficiency and safe production of the mine.

Significance & Contribution to the Discipline

The study explores a relative new In-situ stress measurement technique to the field of rock mechanics, Western Australia School of Mines Acoustic emission stress measurement method. The results from the study will provide analyses to this method and, verification and validation of the results will contribute to the long-term research programme henceforth provide confidence to the aforementioned stress measurement method. The method repertoire of In-situ stress measurement method is limited at present. There have been only two basic methods, overcoring and hydraulic fracturing. A major drawback of overcoring stress measurement is the costly and tedious process whilst hydraulic is less complicated, but it works best in relatively simple geologic conditions and stress regimes (Lehtonen, 2008). Acoustic Emission is relatively cheap and less tedious in a sense that is performed in accurately oriented sub cores from diamond drilling inconsequential of excavations. The Ghanzi Group consists of the Ngwako Pan, formation of interest. It also is made of D’kar and Mamuno Formations. The Ngwako Pan Formation (approximately 2000 m thick) consists of basal poorly exposed high mudstone-matric sandstones (wackes), overlain by better-sorted red sandstones interbedded with pebbly layers and granulestones (Master, 2010). The research will provide insight of In-situ stress in the Formation, substantially. The objective of the research is to determine In-situ stress of persistent lithology in the formation consequently, the research will aid in elucidating rock mass properties of the lithology in relation to the natural stresses prior and after engineering work.

Research Design and Methodology

The project will be focused on a qualitative method of research. This is because the research is aimed at the development of theories and understanding. The research studies the phenomena of Acoustic emission in a natural settings. The objective of qualitative research is to promote better understanding as well as come up with a better way and approach to solving an existing problem Qualitative methods that will be used to collect data in this project include direct observation, document analysis and overview, participant observation, and open-ended unstructured interviewing. The Acoustic Emission stress measurement methodology has been developed over the years by several researchers with the objective of providing a practical technique for retrieving the Kaiser Effect. Kaiser effect-based stress measurement is not a novel idea, and its possible use for stress determination has been researched for decades. Most of the pioneering research on Kaiser Effect was conducted in Japan and United stated, and spread to China and Australia (Lehtonen, 2008). The research paper will focus on literature review of the Kaiser Effect based stress measurement. Several papers and reports on the topic will be reviewed and juxtaposed to gain more knowledge of the Kaiser Effect in relation to In-situ stress measurement.

Acoustic Stress measurement technique is carried out on accurately oriented core samples from diamond drilling. Two boreholes are drilled using Hanjin D&B 30 Multi drilling (Diamond) machine. The Two boreholes are designated HA-059b-G and HA-056b-G for sample 1 and sample 2, respectively. Samples will be collected at depth of 705 m to 710m and 643 m and 648 m for sample 1 and sample 2, respectively. Details of the two boreholes is included in the Appendix A. Note, the two samples are taken from mid-point of sample length and desurveyed hole. The minimum total length of core samples required for each measurement site consist of one full tray of orientated core (Appendix B). The core fracture frequency will not exceed 4 breaks per metre and no fractures of any kind parallel to the axis. In respect to orientation, three core orientation runs in a row should coincide whilst the direction of drilling must be clearly indicated with an arrow at the bottom of the hole. The prepared samples are sent to the lab immediately after preparation to avoid time-dependent decay of Kaiser Effect. Analysis of the test results will be carried to determine the In-situ stress of both lithologies. A close study of the graph of cumulative acoustic emission will be carried out to note the turning points of AE stress points of both cores visually. The change in the rate of AE will then be noted. AE stresses values will be resolved from the data. The main objective of the analysis will to be understand if the rock mass will be able to withstand stress and loading accompanied by engineering work along proposed decline positions. Furthermore, analysis support methods to be used if at all the rock mass is not competent to withstand the load.

The primary goal of this project is to understand the principle of acoustic emission in relation to the Kaiser effect in studying and understanding in-situ stresses In zone 5 for underground mine design and stability. The research is intended to focus on understanding if the Ngwako Pan formation will be competent enough for underground mine development and will not result in any deformation and instability of the rock structure. In situ stress is one of the most important factors affecting the stability of underground excavation. The advantages of the technique are its low cost and no requirement for underground access. Sample selection sufficiently far away from existing excavations is a pre-requisite to ensure the in-situ stress has not been disturbed prior to the stress measurements.

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