John Read
4.2 Model components
4.2.1 Major structures
Major structures include the folds and faults that are continuous along strike and down dip across the mine site, and features such as the laminated structures associated with metamorphic rocks like slate, phyllite and schist. The basic terminology used to describe these features is outlined below.
4.2.1.1 Folds
Folds are one of the most commonly occurring structures in deformed rocks. They are formed when planar features such as bedding and schistosity are deflected into wavelike curviplanar or curvilinear structures. They may develop in single or multi-layers and occur mostly by bending and buckling. They may also occur by gravity slumping and may have a wide variety of geometries and sizes.
Bending is a flexuring induced by a compression acting at a high angle to the layering, as illustrated in Figure 4.2a.
Buckling is a flexuring induced by compression acting at a low angle to the layering (Figure 4.2b). Bending can also occur in the form of a drape fold when, for example, sediments that cover a more rigid basement flex in response to components of vertical movement along basement faults (Figure 4.3). As the name implies, gravity
MODELS
DOMAINS
DESIGN
ANALYSES
IMPLEMENTATION Geology
Equipment
Structure Rock Mass Hydrogeology
Geotechnical Model
Geotechnical Domains
Structure Strength
Bench Configurations
Inter-Ramp Angles
Overall Slopes
Final Designs
Closure
Capabilities
Mine Planning
AssessmentRisk
Depressurisation
Monitoring Regulations
Blasting
Dewatering Structure
Strength Groundwater
In-situ Stress
Implementation Failure Modes
Design Sectors
Stability Analysis
Partial Slopes Overall Slopes
Movement
Design Model
INTERACTIVE PROCESS
Figure 4.1: Slope design process
Figure 4.2: The orientation of the principal compression for (a) bending and (b) buckling of planar layers
Source: Blyth & deFreitas (1984)
Figure 4.3: (a) and (b) Block diagrams of hypothetical drape-folds, the result of normal faulting in the basement. (c) Drape-fold geometry associated with block faulting in the basement. (d) Drape-folds over reverse faults in the basement
Source: Blyth & deFreitas (1984)
Figure 4.4: Terms used to describe the geometry of a fold profile:
h = hinge; I = inflection point; c = crest; t = trough; a = interlimb angle; L = wavelength; A = amplitude
Source: Blyth & deFreitas (1984)
Figure 4.5: (a ) and (b) Wavelength (L) and amplitude (A) of a fold. (c) Diagram showing the dependence of the fold outcrop pattern on the orientation of the plane of erosion
Source: Blyth & deFreitas (1984)
Figure 4.6: Types of asymmetric folds with differing limb lengths and positions of the hinge surface
Source: Blyth & deFreitas (1984)
Figure 4.7: (a), (c) and (e) Upward-closing folds. (b), (d) and (f) Downward-closing folds. Arrows indicate direction of younging.
Plan views of (g) eroded anticline and (h) syncline Source: Blyth & deFreitas (1984)
Figure 4.8: Antiform and synform in upright open folding, with corresponding degrees of acuteness in folding and the hinge of folding
Source: Blyth & deFreitas (1984)
slumping involves the sliding of a mass down a slope under the influence of gravity and is most common in a
submarine environment.
The basic terminology used to define folds is outlined by example in Figures 4.4 and 4.5. The most common different fold forms are outlined by example in Figures 4.6 to 4.11.
Three-dimensional representations of these different styles of folding using the stereonet (section 4.4.2) are illustrated in Figures 4.12, 4.13 and 4.14.
As outlined by Lisle and Leyshon (2004), Figure 4.12 shows how the symmetry of fold can be recognised by the orientations of the normals to the folded surface taken at a succession of locations across the fold. If the fold is
symmetrical, when plotted on the stereonet the poles of the normals to the fold will lie close to a single or best-fit great circle known as the profile plane. In turn, the pole of the profile plane gives the direction of the fold axis. If the poles cannot be fitted to a great circle, the fold is not symmetrical.
Figure 4.13 illustrates typical distributions of the poles on the profile plane for different degrees of openness and curvature of the fold. Typically, the degree of completeness
Figure 4.9: Fold forms. (a) Parallel. (b) Chevron. (c) Similar .(d) Upright. (e) Inclined. (f) Recumbent. (g) Curved axial surface Source: Blyth & deFreitas (1984)
Figure 4.10: Fold symmetry. (a) Symmetric. (b) Asymmetric Source: Blyth & deFreitas (1984)
Figure 4.11: Diagrams illustrating plunge. (a) and (b) Synclinal. (c) and (d) Anticlinal. (e) Block diagram of eroded anticline and syncline, with hard beds (brick pattern) forming surface features on eroded surface
Source: Blyth & deFreitas (1984)
Figure 4.12: Stereonet representation of a symmetrical fold Source: Lisle & Leyshon (2004)
of the great circle reflects the tightness of the fold, with the range of orientations for a tight fold (Figure 4.13i) being greater than for an open fold (Figure 4.13c). In the same manner, planar limbs of a fold (Figure 4.13a, d and f) show two clusters of poles whereas open folds (Figure 4.13c and f) show more diffuse patterns. If the limbs of the fold have unequal lengths one cluster of poles on the profile plane is likely to be more pronounced than the other.
Figure 4.14 shows differing fold classes based on plunge (Figure 4.11) and the dip of the axial surface, both of which are independent of the openness or degree of curvature of the fold (Figure 4.13). Classifications based on plunge can range from non-plunging to vertical. Classifications based on the dip of the axial surface can range from to upright (Figure 4.9d) to recumbent (Figure 4.9f).
4.2.1.2 Faults
The dictionary definition of a fault is a fracture surface or zone along which appreciable displacement has taken
place. The word ‘appreciable’ raises the question of how much is appreciable. For engineering purposes, however, any movement is a fault, recognising that even a minor (small-scale) fault may have considerable engineering significance in terms of strength reduction.
For slope design purposes, a suggested scale is given in Table 4.1. The components of displacement of a fault are measured in terms of throw, heave, total slip and shift (Figure 4.15).
Fault classification systems recognise a parent
hydrostatic state of stress in the Earth’s crust such that the magnitude of the horizontal stresses at any given depth in the crust is equal to the vertical geostatic stress induced at depth by gravitational loading. The magnitude of the horizontal stresses (s2 and s3) relative to that of the vertical stress (s1) can change in three ways. If the differential stress is sufficiently large these variations will give rise to three main faults – normal, thrust (reverse) and strike-slip (Figures 4.16 and 4.17).
Figure 4.13: Stereonet representation of different styles of folding
Source: Lisle & Leyshon (2004) Figure 4.14: Stereonet representation of differing fold orientations Source: Lisle & Leyshon (2004)
1 A normal fault is a lateral extension where both the horizontal stresses decrease in magnitude, but not by the same amount (i.e. s1 > s2 > s3). Normal faults can occur in any geological environment. They form grabens (Figure 4.17b), and in outcrop or drill hole exposures result in an apparent loss of strata.
2 A thrust fault results from compression. Both horizontal stresses increase in magnitude, but not by the same amount (i.e. s2 > s3 > s1). Thrust faults are typical of thrust and fold belt environments and result in the repetition of strata (Figure 4.18). Where the inclination of the fault surface is greater than 45° the term ‘reverse fault’ is used.
3 Strike-slip faults (transcurrent, tear, wrench or transform) occur where the fault plane is
approximately vertical and movement is in the strike direction (left or right lateral). One horizontal stress increases in magnitude while the other horizontal stress decreases in magnitude (i.e. s2 > s3 ≥ s1).
4.2.1.3 Metamorphic structures
Metamorphic rocks such as slate, phyllite and schist exhibit a planar fissility that at mine scale can have a major effect on the stability of the inter-ramp and overall pit slopes. The terminology used to describe the fissile texture of these metamorphic rocks can be confusing; it is clarified below:
■ slate – a fine-grained rock with perfect schistosity;
■ phyllite – a fine-grained schistose rock, sometimes with incipient segregation banding, with a lustrous sheen of mica and chlorite along the schistosity surfaces;
■ schist – a strongly schistose, usually well-lineated rock, generally with well-developed segregation layering. It contains abundant micaceous minerals. The grain size is sufficient to allow easy identification of the main component minerals in hand specimens.
A feature of these descriptions is the distinction between schistosity (or foliation), segregation banding (or layering) and lineation, which can be described as follows:
■ schistosity – a planar fissility in rock caused by the orientation of the mineral crystals in the rock with
Table 4.1: Suggested scale of fault magnitude
Length (m) Description
<1 Minor (small-scale)
1–10 Bench
10–100 Bench to inter-ramp
100–1000 Inter-ramp to overall
>1000 Mine scale to regional
Figure 4.15: Components of fault displacement (a, c and d lie on the fault surface, PQRS)
Source: Blyth & deFreitas (1984)
Figure 4.16: Stress directions for normal, thrust (reverse) and strike-slip faults
Source: Blyth & deFreitas (1984)
Figure 4.17: Relationship of faults to axes of principal stress. (a) Thrust. (b) Normal. (c) Strike-slip.
Source: Blyth & deFreitas (1984)
Figure 4.18: Development of (a) thrust and (b) overthrust, with repetition of strata
Source: Blyth & deFreitas (1984)
their greatest dimension subparallel to the plane of schistosity. Note that s-surfaces are synonymous with schistosity, but have a broader connotation in that the term is applied to any set of parallel surfaces, of metamorphic origin or not, that can be seen in the fabric of a metamorphic rock (e.g. bedding);
■ segregation banding – a laminated structure resulting from the segregation of simple mineral assemblages of contrasted composition during metamorphism into alternating layers parallel to the schistosity;
■ lineation - parallel alignment of linear elements in some direction within the schistosity, e.g. prismatic crystals of hornblende or epidote, rod-like aggregates of quartz, or the axes of microfolds.
4.2.2 Fabric
The bench scale structural fabric within the major domains can include micro-bedding and folding, minor faults, joints, schistosity and cleavage. The principal features of some common minor fold structures and joints are outlined below.
4.2.2.1 Minor fold structures
Common minor fold structures include fracture cleavage, tension gashes, boudinage structures and slickensides.
■ Fracture cleavage consists of a series of parallel fractures (or conjugate shears) formed in an incompe-tent bed (e.g. shale) in response to the folding of an enclosing competent bed (e.g. sandstone), as illus-trated in Figure 4.19 and the stereonet representations in Figure 4.20. Tension gashes may form by extension in the enclosing or other nearby brittle rocks in response to the folding. If the cleavage is parallel or subparallel to the axial plane of the associated fold, it is known as axial-plane cleavage. Because the amount and direction of the strains around the fold may vary,
the axial-plane cleavage may converge (Figure 4.20c) or diverge from the inner arc of the fold. When this occurs, the poles of the cleavage planes will show a
Figure 4.19: Fracture cleavage in a weaker rock folded between stronger beds, with relationship between tension gash and shear stresses
Source: Blyth & deFreitas (1984)
Figure 4.20: Stereonet representation of folds and cleavage Source: Lisle & Leyshon (2004)
Figure 4.21: (a) Tension within competent bed. (b) Boudin structures with quartz (q) between boudins. (c) Lineations Source: Blyth & deFreitas (1984)
greater spread, following a great circle perpendicular to the fold axis (Figure 4.20c). As noted by Lisle and Leyshon (2004), the bedding-cleavage intersections will, however, remain aligned parallel to the fold hinges (Figure 14.20c and d).
■ Boudinage structures are formed by extension during the flexuring of a brittle material, totally fracturing the layer into rod-like pieces (Figure 4.21).
■ Slickensides are lineations reflecting the direction of movement of adjacent beds or structures during folding or faulting (Figure 4.21).
4.2.2.2 Joints
Joints develop in response to three main geological processes:
■ deformation resulting from orogenic processes;
■ deformations resulting from epeirogenic (broad uplift and downlift) processes;
■ shrinkage caused by cooling or desiccation.
Joints in sedimentary rocks reflect the relief of stress that remained in the rocks after (epeirogenic) deformation.
The basic jointing is orthogonal with sets oriented perpendicularly to the bedding and normal to each other.
However, other sets may also be present, depending on subsequent deformation events. Joints in igneous rocks can reflect contraction cooling, the contraction being taken up in extension (opening of tension joints), or deformation processes after cooling has taken place.