A Structural Geology Matrix for Geotechnical Design in Hard Rock
Abstract
The stability of excavations in hard rock mining environments is often influenced by zones of structural weakness in the rock mass. Consequently, the need for structural geology input for open pit and underground mine design are well established. However, the appropriate structural geology information, on both the major and minor scales, is often not clearly and sufficiently communicated in context to the geotechnical design process. In particular, uncertainty in the interpretation of the structural geology is poorly communicated. Geologists often struggle with presentation of a realistic and yet simplified, practical representation of the structures, when they are without understanding of the geotechnical consequence of various fault systems and discontinuity sets. This is especially true for greenfield programmes where structural geology information is often limited and uncertainty is high. This paper establishes a methodology to transparently define key major and minor structural discontinuity information for use in geotechnical mine design. The developed 'structural matrix' is a straightforward, yet highly versatile, communication tool to aid mining practitioners in the analysis and design of mine excavations. The matrix has evolved over dozens of geotechnical investigations to clearly define geometry, properties, timing relationships, data sources, and confidence for each identified and interpreted structural feature. Application of the matrix from conceptual studies through to operational assistance projects has created the opportunity for the evolution and application of the matrix. The output from the matrix permits users to define reliability and risk rankings for each structural feature ranging from major faults to secondary discontinuity sets. Each structure can then be considered and, when necessary, incorporated into the excavation design based on reliability and risk rankings. The matrix may also identify areas of high risk and low confidence, where additional rock mass characterization may be warranted to increase confidence in the design.
... It is well understood that the risk in mining and civil projects would be higher if the deposits are characterized based on limited and low-confidence data. The safety and productivity of a mine excavation depends on how reliably the geotechnical and structural model of the deposit is characterized in the early state of mine planning (Campbell et al., 2014;Venturini et al., 2019). To enhance the accuracy in structural data collection and eliminate bias, many attempts are made to assign different confidence levels in the data collection and interpretation framework. ...
... To enhance the accuracy in structural data collection and eliminate bias, many attempts are made to assign different confidence levels in the data collection and interpretation framework. For instance, through the term "structural matrix" Campbell et al. (2014) described various forms of characterization of major and minor structures. For efficient mine planning and design, a proper balance in data collection vis-à-vis accuracy is required to avoid unwarranted operational and safety risks. ...
The presence of hidden or concealed faults in and around mine openings and excavations has always been a unique challenge to mine management. Typical to Indian mining conditions, the limited availability of geotechnical and structural data during exploration results in a vast data gap and uncertainty in mine planning. To define the structural complexity of the mine and adequacy of
geotechnical data gathering, preliminary data from 200 coal blocks spread across different parts of India data were collated to determine the fault density, and an attempt was made to correlate its impact of mining operation and likelihood of fault induced operational and safety risks. Out of 200 coal block data assessed, it was observed that 109 coal blocks (55%) had no information about the faults in the borehole database gathered, indicating that either the deposits are free from major structural features such as faults or the borehole density adopted is not adequate to capture the existing structural complexities. To validate the challenges of
near fault zones, data from 09 large operational opencast mines complied, and it was observed that mines with higher fault density showed indications of several issues ranging from slope instabilities, water seepage, migration of coal fire and gases, ore dilution along with hindrance to mine operations such as the deployment of a surface miner, highwall mining, re-organization of mine excavation schemes and sterilization of resources, among others. An analysis of “Fault Density versus Level of Exploration” showed a clear correlation with an R2 value of 0.53, with a mean fault density value of 0.2/sq.km and 1.40/sq.km for regionally explored and fully explored coal blocks, respectively. Fault density was very low in regional exploration compared to the detailed one, but operational mines even showed a higher number of faults, indicating significant gaps in early-stage structural database gathering.
Keywords: Fault and Shear zones, degree of complexity, fault density, coal mines
... Unless the interpreter provides adequate communication of the model and its reliability, the model is likely to be mis-used and/or under-utilized. Communication should include data sources, data versus interpretative assumptions/decisions, necessary simplifications from reality, and what the model is specifically meant to represent (Campbell et al., 2014). ...
The Large Open Pit project (LOP3) commissioned the development of a white paper, Guidelines for Structural and Geological Models, that provides structural geologists and geotechnical engineers working in open pit mines with guidance on the creation and evaluation of 3-D structural geology models, and the integration of such models with lithological and alteration models. This paper summarises key components of the guidelines, which includes a suggested methodology to quantify the reliability of domains within a litho-structural model, using Venetia Mine, as a case study.
... Major geological units and a total of 21 faults were interpreted and modelled in 3D using Leapfrog™. The fault continuity and cross-cutting relationships were interpreted through evaluation of the data sources using similar workflows to that outlined by Campbell et al (2015), presented in Figure 6. In some cases, the faults defined the contacts between geological units, and represented the limits to observed slope displacements ( Figure 5). ...
Operating open pits are exposed to geotechnical stability risk throughout the life of mine. These risks are considered part of normal operating conditions and are managed by mining teams. Following the completion of mining, stability risks can continue to exist as pit slopes can deteriorate and exhibit progressive failure mechanisms. In some case these failure mechanisms can impact existing or planned mine infrastructure that is needed to achieve mine closure objectives (i.e., water treatment infrastructure, diversion channels, waste rock dumps, tailings facilities). This paper presents a Canadian perspective for open pit closure as it applies to a case study at the Faro Mine, a former lead-zinc operation in The Yukon Territory, Canada. Provincial and territory guidelines for evaluating post-operational open pit slope stability conditions are summarized and compared against those used to evaluate the East Wall of the Faro Pit. The case study demonstrates some key components for the geotechnical stability evaluation of a weak rock slope that has exhibited obvious deterioration during and following operation. The slope is located directly below a diversion channel that is required to convey water around the mine complex and be positioned outside of the long-term slope break-back. The case study summarizes the field investigation, monitoring, geological/structural geology model, stability evaluation and acceptance criteria/considerations.
... To avoid misconceptions and errors in design analyses based on misunderstandings of what is and is not reality in such models, there must be better communication of likely certainty and accuracy limits passed through to the engineers with delivery of such models (Campbell et al. 2014). A 3D data point cloud of colour-coded spheres representing hard factual data points should be provided along with the 3D interpretation model so that there is clarity on where actual data exists and where it is absent (Savage et al. 2013). ...
Despite major improvements over the past several decades in computer methods for analyzing and synthesizing exploration and site investigation geological data for engineering purposes, which should have decreased project risk, unexpected failures and construction problems still occur. Often, this unexpectedness can be tied back to inadequate structural geological understanding of actual site-specific conditions. Part of this diminished appreciation of structural geological controls on rockmass behaviour can likely be attributed to inappropriate use of computational geostatistics, block modelling and geomechanics computer codes, without significant structural geological input. Part may be because of inadequate domaining in the raw data analysis phase, such that no differentiation was ever made between uncertainties due to true natural variability versus those due to data inadequacy. Modelled data error issues cannot easily be resolved once mixing and smearing of parameters have occurred as a result of earlier inappropriate synthesis of an uncontrolled database. The increased use, industry-wide, of inadequately calibrated, often totally unverified, geomechanics computer modelling for design of civil or mining projects is a worrying trend that needs reversing. Much more attention needs to be paid to properly sub-dividing project sites into realistic geological entities through use of rigorous geological structural domaining techniques, such that much better understanding is gained of project-specific geological risk. This in turn will lead to improved reliability and representativeness in project geomechanical parameter selection for use in design. This paper attempts to set out some guidelines to help practitioners undertake more rigorous domaining, as an aid to parameter selection for design.
... Despite the inherent uncertainty of the model, it is a necessary tool for making informed design decisions. Campbell et al. (2015) introduced a basic tool referred to as the "structural matrix", a style of recording physical fault properties, which allows geologists and engineers to clearly define the relevant aspects of the structural geology and their uncertainty, and track the development of the structural model from its inception. ...
The East Ertsberg Skarn System is a large skarn and porphyry deposit in Papua, Indonesia that formed between the northern contact of a large diorite intrusion and the southern flank of a regional synclinal-type fold within carbonate-rich sedimentary sequence. PT Freeport Indonesia is currently mining the fourth and deepest phase (1,500 m) of the Deep Mill Level Zone (DMLZ) mega skarn column, after depleting the upper sections of the orebody. The induced vertical stresses combined with the competent nature of the rock mass in the Ertsberg diorite intrusion present challenges for cave propagation and rock fragmentation, as compared to the exoskarn rock mass hosted in adjacent carbonates. The depth of the DMLZ creates a geologic condition where the rock mass is more competent. Joint and weak vein densities are low. Non-continuous, segmented, tight faults have reacted to high excavation stresses, focusing seismicity during the mine development and initial block caving. These challenges have pushed the team to keep enhancing the quality of data collection and modeling. Caving is currently being assisted through a comprehensive hydraulic fracturing program. The mine is in a tectonic accretionary belt between the Australian and Pacific plates that formed a fold and thrust belt over the last 25 million years ago (Ma). Characterizing and modeling the faults in the DMLZ mine are one of the most critical tasks to support the mining operation, by providing detailed structure, vein and fracture frequency models as input for geotechnical analysis. The nature of structures within the 2.6 Ma diorite are distinguished from those in the older sediments. This paper presents the results of the study defined to better understand fault systems in the DMLZ. The objectives are to provide the most robust fault model and fault characterization matrix that will serve as input into the geomechanical design models as well as into fragmentation and caveability models.
... Trubetskoy, S. Alarie, M. Gamache [1], C.N. Burt, L. Caccetta [2,3], R.A. Carter [4,5], V.A. Temeng [6,7] and others contributed greatly to the theory and practice of automotive mine machinery usage at open-pit facilities. In their papers, they determined the sphere of haul truck efficient use in open-pit mining [1,3,5,[8][9][10], summed up the experience of its operation [2,4,7,[11][12][13][14], highlighted the issues of techno-economic studies devoted to automotive mine machinery usage efficiency [1][2][3][4][5][6][7][15][16][17] Belarusian Automobile Plant(OJSC «BELAZ», Republic of Belarus)is a major world manufacturer of heavy dump trucks used in mining and construction. Mainly its produce consists of mine dump trucks with a payload capacity of 30 to 360 t. ...
The study presents theoretical and methodological fundamentals of minehaul trucks optimal design. It provides methods based on the systematic approach to integrated assessment of truck technical level and methods for optimization of truck parameters depending on performance standards. The results of these methods application are given.
Including the structural geology of most open pits is a critical input into the geotechnical stability analyses and ultimately in the design of the pit. We are seeing large 3D numerical models being constructed with multiple fault structures, which are often only represented by one or two different Mohr-Coulomb strengths and not really considering the potential changes to the fault characteristics in 3D. The nature and characteristics of structures can change substantially depending on such aspects like fault geometry and intersections, lithology type, geological terrain and alteration influences. It is essential to be able to evaluate the variability of these structures in 3D and to also be able to discuss the reliability of the interpretation and the potential range of conditions that could be experienced in areas with less mapping or drill hole information. Not only is this information important for the numerical modelling, but it is also important for understand local performance on a multi-bench and inter-ramp scale, as well the rock mass performance above and below critical infrastructure items such as ramp accesses. 1 Introduction In the context of open-pit stability analyses, it is essential to incorporate a reliable and representative interpretation of fault structures, particularly their shear strength and stiffness. These factors are critical inputs into geotechnical stability assessments and ultimately influence the design of the pit. Currently, the industry is moving toward extensive 3D numerical models that include numerous fault structures. However, these faults are often characterized by a limited range of Mohr-Coulomb strength parameters, which do not adequately take into account the variations in shear strength and stiffness in three dimensions. Typically, more generalized parameters are applied when evaluating detailed local slope areas, rather than tailoring the analysis to the specific nature and variability of structures in the immediate area, informed by local fault mapping and drill core data.
The knowledge of the open pit scale structural geology is a critical input into the geotechnical domaining, stability analyses and ultimately slope design. There are several challenges to appropriately define structural geology features in a manner that captures a range of plausible stability controls that requires evaluation. In present times, a computer modelled wireframe representation of the structural system is most commonly used to communicate interpretation. Particularly challenging for the geotechnical community is the evolving understanding of segmented structural models, that try to reflect reality more closely than historical inputs into analyses. The paper discusses both the challenges and possible solutions to applying the structural understanding to the pit stability analysis through the stages of pit design and development. Three types of structural models are presented. The models are discussed in the context of application to the slope design process. The models include. Type 1-Data Constrained models that try representing reality as closely as possible. Type 2-Pattern Constrained, conservative models that represent the structural patterns or rock fabric of the pit, within defined domains. Type 3-Statistically Constrained models in which it is necessary to create a hypothetical representation of the structural system for obscured or undeveloped parts of the pit beyond the data coverage.
The standard account of the reasoning process within geology views it as lacking a distinctive methodology of its own. Rather, geology is described as a derivative science, relying on the logical techniques exempli- fied by physics. I argue that this account is inadequateandskewsourunderstandingof both geology and the scientific process in general. Far from simply taking up and ap- plying the logical techniques of physics, geo- logicalreasoninghasdevelopeditsowndis- tinctive set of logical procedures. I begin with a review of contemporary philosophy of science as it relates to geol- ogy. I then discuss the two distinctive fea- tures of geological reasoning, which are its nature as (1) an interpretive and (2) a his- torical science. I conclude that geological reasoning offers us the best model of the type of reasoning necessary for confronting the type of problems we are likely to face in
Geoscience may be regarded as an uncertain science, as it is often based on the interpretation of equivocal data. Analysis of multiple interpretations of a single dataset has shown that conceptual uncertainty can result in a wide range of interpretational outcomes. Many geological models based on a wide variety of concepts were developed by different geoscientists for the same dataset. In this paper we suggest methods to improve the effectiveness of interpretation workflows based on understanding of how geoscientists apply concepts to equivocal datasets, the processes they use, the effects of their previous experience, and their use of broader contextual information. We argue that understanding the influence of conceptual uncertainty on interpretation of equivocal data and modification of current workflow practices can improve risk management.