Results

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In this paper, we present a procedure for high pressure resin impregnation of microporous rock. This procedure produces the high-quality pore casts that reveal the fine details of the complex pore space of micritic carbonates. We carefully test our resin impregnation procedure and demonstrate that it renders the high resolution, 3D confocal images of pore casts. In our work, we use silicon micromodels as a reference to validate the key parameters of high-pressure resin impregnation. We demonstrate possible artifacts and defects that might develop during rock impregnation with resin, e.g., the resin shrinkage and gas trapping. The main outcome of this paper is a robust protocol for obtaining the high-quality epoxy pore casts suitable for rock imaging with Confocal Laser Scanning Microscopy (CLSM). We have implemented this protocol and provided the high resolution, three-dimensional (3D) imagery and description of microporosity in micritic carbonates.

Abstract

The distribution and evolution characteristic of abutment pressure is a key factor to controlling surrounding rock-mass in underground coal mining. Aiming to analyze the formation mechanism and dynamic evolution characteristic of abutment pressure, different surrounding rock-mass models of stope at corresponding multi-time-space conditions were established based on mechanics of materials. It is demonstrated in this study that the whole life cycle of a fully-mechanized mining face in flat seams, including open-off cut, regular advancing zone and finish line, could be divided into four stages which include: Initial stage (I), Steadily growing stage (II), Dynamic stage (III) and Final stage (IV), where the Final stage (IV) could be further divided into the higher pressure situation and lower pressure situation based on different surrounding rock-mass structures. The open-off cut zone and first collapse of immediate roof are within the Initial stage (I) and Steadily growing stage (II) respectively. The abutment pressure of Initial stage (I) is a static load and the abutment pressure of Steadily growing stage (II) is steadily growing with the advancing of mining face; the value of abutment pressure before periodic weighting is larger than that after periodic weighting when it enters the Dynamic stage (III); the roadways are easily controlled in lower pressure situation of Final stage (IV) and it may avoid the support crushing in higher pressure situation of Final stage (IV). According to the dynamic evolution of abutment pressure featured in life cycle of a mining face, it could make the prediction of high stressed zone, and reinforcement support or pressure-relief methods could be done in advance, thus the occurrence of dynamic hazards of surrounding rock-mass could be effectively prevented.

1. Introduction

The stability of roadway surrounding rock-mass in underground mining plays a significant role in safety and high production, and can be easily influenced by the distribution and dynamic evolution of abutment pressure (Tan, 2012; Zhang, 2014). Qian et al. (1995, 1996) adopted key stratum theory and “voussior beam” hypothesis to study the interaction between overlying stratums movement and mining induced pressure. Xia et al. (2017) conducted numerical simulation of the whole process of longwall mining on stope pressure in underground mining using FLAC^3D. Lai et al. (2014, 2016) studied the transformation mechanics of advanced abutment pressure through physical simulation. Hosseini et al. (2012, 2013) used passive seismic velocity tomography to analyze the variations of abutment pressure around the panel during coal mining. Ren et al. (2014) studied the dynamic feature of advanced abutment pressure in shallow mining through physical simulation，numerical simulation (FLAC^3D) and in-situ observations. Xia et al. (2011) analyzed the wave form and inversion of the in-situ micro-seismic data obtained from a fixed mining face to determine the interaction between micro-seismic activities and advanced abutment pressure. Zhu et al. (2016) adopted micro-seismic monitoring technical for abutment pressure monitoring in coal mining. Jiang et al. (2002, 2006) analyzed the peak value of abutment pressure in coal mining through mechanical model on stope. Zhao et al. (2006) adopted ADINA finite element to discuss the range and peak value point of abutment pressure. Li et al. (2005) hold the view that the abutment pressure was consisted of static pressure and dynamic pressure. Zhou et al. (2016) used hollow inclusion strain cells measurement technique to get the evolution law of abutment pressure with mining face advancing. Zhang et al. (1994) studied the distribution law of abutment pressure with advanced support resistance. Pan et al. (2014, 2015) took the coal seam and immediate roof in front the mining face as elastic basement firstly, then used mechanical model to analyze the mechanical behavior of roof before periodic weighting.

Abstract

Underground structures are proving to be the smart structures by not only resolving various problems associated with storage and transportation but also helpful in maintaining traffic due to scarcity of land. Due to heterogeneous behavior of rock-mass, urban tunnels are highly susceptible to failure. Massive deformation in the lining is encountered even when the tunnel is subjected to small static loading. It is important to safeguard these shallow structures against static as well as other impact loads. Impact load influences the behavior of tunnel lining if induced deformation exceeds the capacity of lining and rock-mass. If tunnel is situated deep down, horizontal and vertical stresses are more balanced as compare to tunnel at primary layers. In this paper physical modeling of shallow tunnels is discussed to understand the in-situ behavior of peripheral tunnels when subjected to high rate of impact loading/ projectile penetration. In order to obtain better understanding, experimental modeling of tunnel in different synthetic rock-mass is carried. Three different cover depths are casted for rock-tunnel model and different impact loads are applied at centre by means of drop hammer arrangement. Deformations are measured by means of linear variable differential transducer (LVDT) in the tunnel lining. Experimental data is validated by numerical analysis using commercially available FEA software Abaqus.

1. Introduction

Rock and rock mass at higher depth are stress controlled, while at shallow depth are geology controlled. For such weak rockmass it is difficult to determine the strength through routine laboratory test. Thus, some simple and inexpensive index test methods have been developedto estimate the mechanical properties of such rock mass indirectly (ISRM 2007).Ahmed and Iskander (2011) investigated the effect of ground movements induced by tunneling, on structures (above ground and underground) through physical modeling. Most of such experimentations are performed on the soils (Meguid et al. 2008), however the underground structures are constructed in weak rock mass and soft rocks as well. In such fragile conditions the underground structures are subjected to different type of loading (for e.g. earthquake loading, blast loading, impact loading, loading due to increase in the litho-static pressure) for which the study of strength-deformation behavior of rockmass and its effect on both lined and unlined underground structures are very essential.Several researchers have successfully carried out static and dynamic impact tests on rock in order to determine the failure mechanism of rock (Mishra et al., 2016).

Abstract

Bench blasting is the most common method of rock excavation in productive quarries. Properties of the rock mass and the discontinuities are factors that influence rock fragmentation but cannot be controlled. In this study, the remote sensing techniques known as Terrestrial Laser Scanning (TLS) and Close Range Photogrammetry (CRP) using Unmanned Aerial Vehicles (UAV) have been utilised to generate 3-dimensional (3D) models of rock surfaces using Topcon ScanMaster and Agisoft Photoscan software. From the data obtained, rock mass can be classified using rock mass rating (RMR) and geological strength index (GSI); the blastability index (BI) is then obtained for blast design. A 3D model of the blasted rock surface is established based on pre and post-point cloud data. It can be concluded that the TLS and CRP methods, using UAV, can be used as complementary methods when classifying the rock mass and establishing the 3D finite element model of a quarry face.

1. Introduction

The aim of blasting is to extract the largest possible quantity of rock at minimum cost in the safest manner possible whilst minimising side effects like flyrock, noise and ground vibration. Mackenzie (1966) mentioned that blasting is the cheapest way to fragment rock. Rock fragmentation has been the subject of much research because of its direct effects on the costs of drilling and blasting as well as the efficiency of the subsystems, such as loading, hauling and crushing in mining operations (Goodman &; Shi, 1985; Dershowitz, 1993; Faramarzi et al., 2013). In order to get good fragmentation during blasting, geometrical information such as discontinuities in the rock mass, needs to be clearly identified as this can help when designing the blasting pattern etc. Adhikari &; Gupta (1989) mentioned that rock mass properties are important parameters in a blast design and understanding the influence of geological discontinuities and the physico-mechanical properties of the rock is fundamental.

Characterisation of the rock mass requires the gathering of information through geological fieldwork, conventionally being collected manually using a compass-clinometer and a tape measure. The conventional methods are simple and effective, however the collection of data via fieldwork can be a hazardous, time consuming process and data quality may be affected by the user’s level of experience (Slob et al., 2010). Tannant (2015) highlights some of the drawbacks of hazard assessment of rock faces, such as: (i) safe access to the rock face to carry out geological mapping often does not exist, (ii) it is difficult to measure the orientation and geometry of large geological structures such as faults by simply measuring an orientation where a scanline crosses the fault, and (iii) mapping with a compass at the base of a steep slope exposes people to the risk of harm from rock falls.

Abstract

With the increasing of mining activities in China, a series of hazards such as landslides, rockbursts, large deformations occur frequently. At present, the challenges for these hazards consist in finding appropriate countermeasures. They can be classified in three types: monitoring of landslide, rockburst control, and use of innovative mining technology. This paper will first introduce a study on the theory of double-blocks mechanics including the measurement of the Newton force, which is the necessary and sufficient conditions for initiating a landslide disaster due to the block motion, using the so-called constant-resistance and large-deformation cable with negative Poisson’s ratio effect. Secondly, the mechanism of rockburst and its control will be introduced. As an important method for rockburst control, the use of a novel energy-absorbing bolt which features with a constant-resistance under impact loading and large-elongation for containing large deformations of the rock mass at burst-prone conditions. Finally, a new longwall mining method the so-called the 110 mining method (1 working face, 1 excavation roadway and no pillar), including directional pre-splitting cutting will be introduced. Compare with the traditional longwall mining method the so-called as 121 mining method (1 working face, 2 excavation roadways and 1 remaining pillar), using the new 110 mining method, 50% of the roadways are no longer needed to be excavated; instead, they are formed by a controlled roof collapse. By reducing the need for roadway excavation, mining accidents and consequently costs can be significantly decreased.

1. Introduction

At the beginning of the 21st century, as the demand of more energy, the shallow resources are decreasing and the intensity of mining and infrastructural project are increasing. Domestic and foreign mines have successively entered the state of deep resource exploitation. With the increasing of mining depth, nonlinear dynamic mechanical phenomena and geological disasters, for instances, landslides, rockburst, gas outburst, rock nonlinear rheology, and water outburst, etc., occur with high frequency. The problems of rock mechanics caused by the deep mining engineering are the focuses in the fields of mining engineering and rock mechanics. Based on the studies of the particular geomechanical environments of deep engineering rock mass and its nonlinear mechanical characters, this paper introduces the current research status from three aspects: monitoring of landslide, rockburst control, and use of innovative mining technology.

Firstly, a novel monitoring method for landslide was developed. Landslide is a major geological disaster causing huge casualties and economic losses each year. In 2015, a total of 5616 landslides occurred, accounting for appropriately 68.3% of the overall geo-disasters recorded within China. This article outlines the ideas and related findings on how to predict the landslide geo-disaster using the unconventional in-plane Newton force measurement technique (He 2009; He et al. 2009, 2011, 2014) and mechanics of the double block system (DBS).

Abstract

Crush pillars are used extensively in the platinum mines of South Africa as part of the stope support in intermediate depth tabular mines. Effective crush pillar design ensures that the pillars crush when formed at the mining face. This behaviour occurs when the pillars have a width to height ratio of approximately 2:1. Once crushed, the residual stress state of the pillars provide a local support function. In many cases, effective pillar crushing is not achieved, resulting in pillar seismicity. As this mining area produces approximately 70% of the world’s platinum group metals, it is critical that layouts and pillar design are optimised to ensure safety and sustainable production. The objective of the research was to determine the parameters which influence crush pillar behaviour. A limit equilibrium constitutive model was proposed to investigate the behaviour of the pillars. The model, implemented in a displacement discontinuity boundary element code provided insights into the stress evolution of a crush pillar. The results indicated that the stress on the pillar depends on its position relative to the mining face, size, the impact of geological structures, layout, rock mass parameters and mining depth.

A comprehensive underground mining trial was conducted to quantify the behaviour of crush pillars. A numerical model was used to back analyse the pillar behaviour at the underground trial site which consisted of a mined area of approximately 22 000 m² containing 55 crush pillars. Both the observed and measured behaviour of the crush pillars in the trial site could be replicated by the model. The findings validated the use of the limit equilibrium model to simulate the behaviour of crush pillars on a mine-wide scale.

1. Introduction

Mining practices are aimed at maximising the extraction of a particular orebody without compromising safety. Crush pillar mining appears to be a method unique to South African hard rock mines. These pillar systems are used in shallow and intermediate depth platinum stopes. It allows for a higher extraction than what can typically be achieved with a conventional non-yield pillar system. The crush pillar system must be used in conjunction with a regional pillar system. Crush pillar dimensions are generally selected to give a width to height ratio (w:h) of approximately 2:1 (Ryder and Jager, 2002). This w:h ratio should ensure that the pillars fail while being cut at the mining face. Once the pillar has failed in a stable manner, the residual strength of the pillar contributes to the support requirements by carrying the deadweight load to the height of the uppermost parting on which separation is expected to occur (can be as much as 45 m above the reef). The pillars therefore prevent the occurrence of large scale collapses (backbreaks). Closely spaced support elements are typically used between rows of crush pillars to provide additional in-panel support.

Abstract

Strainburst refers to local small seismic events generating shallow spalling with violent ejection of fragments in an active development heading. This rockburst category may affect worker safety and mine productivity. This paper conducts a preliminary study investigating properties of large-scale geological features, mine operational context, and both aseismic and seismic responses generating strainbursts. Using the LaRonde mine as a case study, key parameters influencing strainburst occurrence and severity are defined and highlighted. The distance to a lithological contact and the orientation of the drift are parameters that affect strainburst potential and severity at LaRonde. The analysed bursts were seismically triggered or self-initiated. The analysed seismic events generating strainbursts had a local magnitude of −0.7 ± 0.5 on average and were located between 3 and 58 meters from the damage. Finally, strainbursts mostly occur within five days after a development blast.

1. Introduction

Mining in deep, hard rock mines poses many challenges for engineers. An increasing engineering challenge associated with higher stress conditions at great depths is the potential for strainbursts, which can have considerable adverse effects on mining activities. In this study, strainburst refers to local small seismic events that generate shallow spalling with violent ejection of fragments in an active mine development heading (Ortlepp, 1992).

Many authors have studied and described strainburst mechanisms (Ortlepp, 1992; Ortlepp and Stacey, 1994; Kaiser et al., 1996; Kaiser and Cai, 2013). However, strainburst parameters such as time of occurrence and its link to more remote seismicity are uncertain and not fully understood, leading to difficulties in designing and implementing risk management strategies. Seismic risk management should rely on all available data at the mine to help understand rockmass responses to mining. Seismic and aseismic responses to mining are influenced by many parameters, such as contrasts in rock mass competency and stress conditions, which are partially controlled by the proximity to large-scale geological features such as faults, lithological contacts, and foliation. Hence, it is of great importance to characterize geotechnical environments and both seismic and aseismic responses properly in order to better understand strainburst occurrences.

This paper presents the development of a geotechnical model based on the integration of geological and geomechanical characterization data along with seismic events. This model provides useful tools for understanding the rockmass response to mining. More specifically, this paper focuses on determining seismic parameters and the properties of large-scale geological features that affect strainburst occurrence and severity. Properties of large-scale geological features refer in this case to the distance from the damage to geological contacts and faults, and the angle of interception of the damaged drift with the foliation. Analysis of the seismic sources includes the quantification of the distance from the damage to the linked seismic event, the magnitude of the linked seismic event, and the time of occurrence of the event in relation to the previous blast in the sector.

The study uses strainburst data from the LaRonde mine, a deep seismically-active hard rock underground mine located in the mining district of Abitibi-Témiscamingue, in Quebec, Canada. As mining progressed deeper, this mine experienced an increased frequency of strainbursts, providing a unique opportunity to demonstrate the applicability and usefulness of the developed methodology.

Abstract

The failure behaviour of a plaster beam when coated with a thin layer of polymeric liner was studied both experimentally and numerically. Plaster beams without any liner coating were tested to failure in a four-point bend test scenario and were found to fail in tension at the mid span of the beam. To assess the support mechanisms of thin spray-on liners when adhered to a rock surface, the plaster beams were coated with a 5 mm thick fibreglass reinforced polymer liner. These beams were tested to study the effect the polymer liner has on restricting tensile failure of the beam. Results of flexural tests on plaster beams with a reinforced polymer liner showed failure originating near the support points and extending to the loading points with some delamination of the polymer liner at higher loads. This ability of a thin polymeric liner to resist crack propagation at the interface of the liner was simulated numerically using a cohesive zone model. The model developed predicted the failure behaviour accurately and the numerical results obtained were comparable to the experimental results.

1. Introduction

Thin spray-on liners (TSL) form a composite skin layer and support the rock after application (Stacey, 2001). It has performance characteristics that lie between those of shotcrete and mesh. Liners like TSL and shotcrete, which are well adhered to the rock surface can restrict small movements of already fractured and loosened rock mass. However, the TSL being more flexible than shotcrete can generate more support resistance over a full range of rock deformations (Tannant, 2001). Laboratory tests were undertaken to quantify the strata skin reinforcement using a polymer based TSL. The polymer liner was adhered to plaster beams and then flexural tests were conducted on the polymer-plaster composites. Similar tests were also done on plaster only samples. The support resistance provided by TSLs was demonstrated by comparing the flexural strength of the polymer-plaster composite with that of plaster only beams.

The results of flexural tests on plaster only and a TSL coated plaster composite were simulated numerically using finite element modelling (FEM) and the failure behaviour was compared with experimental results. Cohesive zone interaction, available in Abaqus library (Abaqus, 2014), was used to model the interface between the plaster and polymer layers. The interface properties required for defining the cohesive zone interaction were from the work carried out by earlier researchers (Qiao et al., 2015, Shan, 2017) at the University of Wollongong.

2. Experiments

To simplify sample preparation, hydrostone, gypsum plaster and 5% Portland cement, was used to simulate the substrate instead of rock. Plaster beams having dimensions 160 mm by 40 mm by 40 mm were cast and textured to mimic rock beams. All hydrostone samples were prepared by mixing in a ratio of 3.5:1 by weight of plaster to water. The samples were then allowed to cure at 40° C in an oven for two weeks.

Abstract

Planning teams need confidence that the numerical tools they use to estimate potential for mine instability are sufficient to quantify the hazards. The tools must be quantitative, field validated, and the hazardous phenomena must emerge in these models as a function of the governing physics.

In seismically active mines, this essential task is more difficult as high rates of deformation and dynamic loading are still comparatively infrequent events. Reliable simulation of support and reinforcement for dynamic loading is even more complex.

The goal of this paper is to summarize a framework that has been used for reliable numerical rock mechanics simulation at several mines for forecasting of potential for induced seismicity.

1. Introduction

From an engineering perspective, the mining engineer designing ground support and excavations in a mine must undertake two broad tasks:

– Estimation of the potential for dynamic deformation and seismic events, including appreciating the mechanisms by which damage may occur to an excavation, including both rapidly (dynamically) increasing dilation of the rock mass due to increasing stress as well as the effects of seismic waves.

– Simulation of the response of installed support and the discontinuous rock mass around the excavation.

The only way to simulate significant excavation loading, deformation and energy change is to properly capture the physics of rock damage. If the extent and magnitude of the damage in the model is wrong, the displacements and energy changes, including estimates of seismic potential will also be wrong. The complete stress-strain behaviour of rock must be correctly captured.

A tool that can correctly forecast the rock mass behaviour on different length scales should include:

– The ability to incorporate explicit defects down to the length scale of interest.

– The strain softening, dilatant behaviour of the rock mass.

– The simulation of post-failure response of rock and support.

The modelling framework discussed in this paper attempts to satisfy these requirements. It consists of a constitutive model that describe the stress-strain behaviour of rock masses and structures and the application of an explicit Finite Element (FE) algorithm to represent the rockmass as a three-dimensional discretized volume with discontinuities and heterogeneous material domains. The main ingredients are:

– The continuum regions of the rockmass are modelled as strain-softening dilatant materials. This means that as strain increases the material softens, weakens, and dilates. All parameters can vary at different rates with respect to strain changes, and this allows approximation of complex stress-strain behaviour of real rock masses. A generalization of the Hoek-Brown yield criterion (Hoek et al., 1992) is used for the continuous regions of the rockmass derived from a more generic and versatile formulation of (Menetrey&;Willam, 1995).

– The behaviour of explicit discontinuities is approximated using cohesive elements. The constitutive behaviour of the cohesive elements can be defined using the presented constitutive model, or a constitutive model specified directly in terms of traction versus separation.

– The seismic potential of the modelled rock mass can be assessed by considering the modelled Rate of Energy Release (RER), which is the maximum instantaneous rate of energy release within a unit volume during a period of the simulation.