The geological model
The scope of this article includes a description of the components of a geological model, data collection methods, and how each component relates to mine design. The geological model characterizes the rockmass from a geological perspective. This characterization includes defining rock type, mineralization, hydrogeology, geological structures, their spatial distribution, and the geological history of the rock.
Mine design is influenced by geology due to impact on five categories of rockmass properties. These geologically controlled categories are intact rock properties, discontinuities, rockmass stresses, hydrogeology, and time controlled properties. (Hudson & Harrison, 1997)
A geological model is typically used in mine design to delineate geotechnical domains, define parameters for design methods, and incorporate the geological history of the rock into understanding of future behavior.
The geological information required includes the rock type, mineralogy, chemical composition, and the spatial distribution of each property. Typically rock type is defined using historical terms specific to the local area, or according to the standard charts. An example of a historical local term is “Sudbury Breccia” used in the Sudbury Basin to identify the impact breccia. This would generally be called pseudo-tachylite according to technical definitions (The Sudbury Structure, n.d.). The following chart is a typical one for defining plutonic and volcanic rocks. These rocks are defined by mineralogy and grain size.
(Rocks and Minerals, n.d.)
Sedimentary rock are classified based on particle composition, such as siliciclastic or carbonate, and by the size and distribution of the particles. An example of a classification diagram for a conglomerate is shown.
Metamorphic rocks are defined based on texture, initial rock type, and degree of metamorphism. Types of metamorphic textures include foliation and lineation (Rocks and Minerals, n.d.).
The initial step for determining rock type is to review historical mapping. In Canada the local Provincial Geological survey and the Geological Survey of Canada have large scale maps of much of the Canadian bedrock. Some regions, where mines have been developed or potential mineral deposits have been identified, may have small scale surface mapping. This local small scale surface mapping can then be done, or expanded, to define rock types and delineate surface contacts in regions of interest. Sampling programs, in conjunction with mapping programs, are used to determine mineralogy, chemical composition, and to conduct thin section petrography of the region of interest. Surface geology is then extended underground using borehole logging. Core is retrieved and logged by geologists. Software is then used to interpret the spatially located logs into a geological model.
Rock type can be used as an initial estimate for intact rock strength. The following table relates rock type to a typical range of Hoek-Brown intact strength parameter mi.
Rock type can be used to delineate the boundaries of geotechnical domains. Mine design in each domain, in terms of ground control and excavation dimensions, relies on identifying these
Geological contacts can be used as boundaries for the extrapolation of rock properties.
Geochemical data is collected in conjunction with field sampling programs. Geochemical markers, such as the amount of trace elements, provide information for characterizing the rockmass. This is more useful for exploration purposes rather than design but has implications for downstream processes. For example, arsenic influences nickel recovery in mineral processing. This affects the mine valuation and therefore mine planning.
Structural geology is the study of the natural deformation of the rockmass. In the major rockmass classification systems used in assessing rockmass quality for design, such as the GSI and RMR systems, structural parameters dominate strength calculations. This makes the determination of the rockmass structural geology essential for mine design. This includes small scale discontinuities, large scale structures, and metamorphism features (Hoek, 2006). Changes in structural conditions may be significant enough to require additional geotechnical domains.
Small scale features include joints and minor shears, which are joints along which movement has occurred. Large scale structures include dykes and faults. Metamorphic features, as discussed above, include rock textures such as foliation.
Joints are naturally occurring discontinuities which occur in rock. Joints can be formed by stress relief due to unloading, high stresses due to tectonic forces, or contraction due to lava cooling. Parameters which should be determined include joint spacing, persistence, roughness, and infilling properties.
(Hudson & Harrison, 1997)
Foliation creates strength and stiffness anisotropy because the rock is weaker in the direction of the foliation. This is illustrated in the following figure where uniaxial compressive strength is a function of the orientation of applied load. The orientation of foliation has major implications for mine design in terms of the failure modes to be expected. For example, tunnels driven parallel to foliation could experience bulking in the tunnel walls as dilation occurs between the foliation planes.
Dykes are an igneous intrusive rock that cuts across the older rockmass. Due to their relative youth they tend to have fewer discontinuities, higher GSI, and consequently higher rockmass stiffness. In high strength rock, under high stress conditions, this could result in rockburst risk. The location and condition of these structures should be determined in order to design ground control to accommodate this risk and design the mine to minimize exposure.
Faults are geological structures along which displacement has occurred. They are formed in geological time by tectonic activity. Fault zones have significant variability in fracture width, may have irregular orientation, and have high variability in strength.
There may be differential displacement along the fault compared with the rockmass. This strain difference results in rotation of the stress field. If the fault is stiffer than the rock, in the case of well healed structures, the stress parallel to the fault will increase. For weak faults stress parallel to the fault will be reduced. An example of complex stress rotation is shown. This example illustrates how the stress field may be significantly different on either side of a major fault. This is one possible reason for delineating the boundaries of geotechnical domains.
The tectonic history of the rockmass is a tool used to infer information about the rockmass. One example of this is to use the major tectonic regime to estimate the orientation of the principle stresses relative to faulting. The three major tectonic regimes are normal faulting, thrust faulting, and strike-slip faulting. These are visually represented by the block models below. These simple models are often used as an indication of the stress regime in the rock.
Strike slip regimes indicate that the maximum stress is parallel to the fault direction, intermediate stress is vertical, and the minimum stress is perpendicular to the fault direction.
Normal fault regimes indicate that the maximum stress is vertical, intermediate stress is parallel to the fault direction, and the minimum stress is perpendicular to the fault direction.
Thrust fault regimes indicate that the maximum stress is perpendicular to the fault direction, intermediate stress is parallel to the fault direction, and the minimum stress is vertical.
These principle stresses for each regime are illustrated in the following diagram.
These relative stresses are only an indication for the stress regime at the time of fault formation. If the tectonic stress regime has changed this may not be an accurate estimate.
Structural data is collected by reviewing historical mapping, field mapping of structures on the surface, and logging of structures in core. Oriented core allows for better definition of structure orientation.
Sampling bias can be a major issue in structural geology. Boreholes are typically laid out for the purpose of delineating ore and defining geological contacts. Standard practice is to drill perpendicular to contacts and perpendicular to the orebody in order to determine true width. Structures roughly perpendicular to the orebody, and the boreholes, are difficult to identify. Also, mining levels are horizontal which means that vertical structures are more often encountered and more easily delineated. As mentioned above, structural geology makes up the majority of the input parameters for rockmass classification. GSI is estimated by the discontinuity density and a qualitative assessment of the structures. A GSI table for a blocky rockmass is shown below.
The distribution of mineralization within the rockmass has a significant control over the mine design. Mining methods (LINK) differ based on high grade, low volume deposits and low grade, high volume deposits. The relevant parameters are grade, volume, and distribution of the mineralization within the rockmass. Coupled with mine economics (LINK) a cutoff grade is used to design the mine layout. Optimization of the mine design relies on accurate interpretation of sampling data.
Genetic models are used to describe the formation of the ore body. They provide information that can be useful for mine design such as the depth of formation, distribution of mineralization, and exploration geology also makes use of the genetic model to explore for additional resource. The genetic model is determined based on experience and the collected geological, geochemical, and structural data.
One of the major advantages of using genetic models is an improved interpretation of point data. Often there are many possible interpretations of geological data. The model allows geologists to use experience to make better interpolations between data points. Depth of formation, and the corresponding erosion estimate, can also be used to estimate locked-in stresses in the rockmass.
The USGS has a compilation of genetic models for many different types of ore bodies which can be found here:
The geological history gives important information for estimating engineering properties. This includes deposition and erosional history, diagenesis, glaciations, and tectonics.
Diagenetic processes, such as cementation of sedimentary rocks, have implications for the intact rock strength. Conversely, weathering reduces the cementation of intact rock and reduces the strength of discontinuities. This is reflected in the RMR, GSI, and Q rockmass classification systems.
Erosional history and glaciation can influence the stress field in the rockmass, creating locked in high horizontal stresses close to the surface. As the overburden weight is removed vertical stresses are relieved, but due to confinement, the horizontal stresses are maintained. Tectonic history, as discussed in the previous section on structural geology, influences the stress field and can be used to make inferences about stress ratios in the rockmass.
An example of this are the pop-ups that occur in Southern Ontario quarries. High locked-in horizontal stresses cause sudden failures as the overburden load is removed.
Hydrogeology is fundamental to engineering in soils. Fluid within the soil exert pressure on the soil particles, carrying load, and reducing the stress on the soils. (Hudson & Harrison, 1997)
In rocks, the permeability of the intact rock blocks is orders of magnitude lower than the permeability of the fractures. Therefore, the structures dominate permeability. There are two methods for analyzing flow in the rockmass, using discrete elements and discrete discontinuities to model flow or using an effective permeability. The effective permeability treats a fractures rockmass as an intact medium with an averaged permeability.
Fracture flow is a coupled problem that depends on aperture, infilling, and pressure gradients. Coupled thermal-hydro-mechanical-chemical models are extremely complex. They are required for projects where fluid flow is critical, such as reservoir engineering, or the time scale of the project is very long, such as a nuclear waste repository.
Hydrogeology is relevant for mine design though the concept of effective stress in the rockmass and for operational concerns such as pumping.
Geological information can be very useful as boundaries for geotechnical domains. Possible boundaries include major structures, geological contacts, or changes in structural domains.
Within each domain, geological features and tectonic settings are typically used as first estimates for rockmass properties. In early mine design when testing data is limited, this is useful approximation.
Finally, the past behavior of rock is an indication of future behavior. Geological history, rock formation, and tectonic history provide indications of how the rock will react to mining.
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