The structural geology is a very important factor that significantly influences the stability of rock slopes. It refers to naturally occurring breaks in the rock, like bedding planes, joints, and faults, generally termed as discontinuities. The properties related to discontinuity in relation to stability are orientation, persistence, roughness, and infilling. The importance of discontinuities lies in the fact that they represent planes of weakness in the much stronger, intact rock; therefore, failure tends to occur preferentially along these surfaces. The discontinuities themselves may directly influence stability—these are the slopes shown in Figure 1. In Figure 1(a) the face is formed by the bedding planes, which are continuous over the full height of the cut; this condition is termed a plane. Alternatively, slope failure may occur on two discontinuities that intersect behind the face, as illustrated in Figure 1(b)—this condition is termed a wedge failure.
Alternatively, discontinuities may only indirectly influence stability where their length is much shorter than the slope dimensions, such as an open pit mine slope where no single discontinuity controls stability. However, the properties of the discontinuities will affect the strength of the rock mass in which the slope is mined. Almost all studies of rock slope stability require an assessment of the site's structural geology, and these studies include the two steps outlined below. First, determination of the discontinuity properties, which includes mapping outcrops and existing cuts, if any, and examination of diamond drill core, as appropriate for a given site. Second, check how the discontinuities affect stability with respect to their orientation relative to the face. This is called kinematic analysis, whereby all the possible modes of slope failure are identified.
The overall purpose of a geological mapping program is to define a set or sets of discontinuities, or a single feature such as a fault, which will control stability on a particular slope. For example, bedding may dip out of the face and form a plane failure, or a pair of joint sets may intersect to form a series of wedges. It is common that the discontinuities will occur in three orthogonal sets which are mutually at right angles with possibly one additional set. It is suggested that four sets is the maximum that can be incorporated into a slope design and that any additional sets that may appear to be present are more likely to represent scatter in the orientation of the sets. Infrequent discontinuities in the rock mass are unlikely to have a significant influence on the stability of the overall slope and so may be discounted in design. However, it is important to identify a single feature, like a through-going, adversely orientated fault, which may turn out to be a major controlling feature for stability.
There are certain geological conditions where the discontinuities may be randomly orientated. For example, basalt that has cooled rapidly while still flowing may have a "crackled" structure in which the joints are not persistent, curved and have no preferred orientation. Also, some volcanic rocks exhibit sets that only extend over a few meters on the face, and then different sets occur in an adjacent area.
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| Figure 1 (a) Figure 1 (b) Figure 1: Rock faces formed by persistent discontinuities: (a) plane failure formed by bedding planes parallel to face with continuous lengths over the full height of the slope; (b) wedge failure formed by two intersecting planes dipping out of the face In the design of civil project rock slopes, it is usually advisable that the full length of each slope be designed with a uniform slope angle. It is not practical to excavate a slope having different slope angles since it complicates surveying and the layout of blast holes. This will require that in applying geological data to slope design, the dominant geological structure, such as bedding or orthogonal joint sets, should be used for the design. An exception to this guideline may be where there is a significant change in rock type within the cut, and it may be appropriate to prepare a separate design for each. However, even in these cases it may still be less expensive overall in terms of construction cost to cut the entire slope at the flatter of the two slope configurations, or to provide support in the less stable material. Another problem in planning a geological mapping program is to determine the number of discontinuities that must be mapped to define the design sets. By inspection of a natural face, or often of an existing cut, it is normally possible to ascertain whether the structure occurs in sets or is randomly oriented. For areas of good rock exposure and uniform structure, as few as 20 measurements may be adequate to suggest the orientation of the sets, while another 50–100 measurements give typical properties such as persistence, spacing and infilling. Those conditions under which more than this number of discontinuities should be mapped include faulted or folded structure or contacts between different rock types. In these cases, hundreds of features could be required to be mapped to define the properties of each unit. A detailed procedure for determining the number of joints to be mapped is described by Stauffer (1966). Mechanism of joint formation All of the rocks we see in outcrops and in excavations have had a long history of change over time scales of hundreds of millions, or even billions of years. The sequence of change that a sedimentary rock, for example, might commonly go through is deposition at the surface, gradual burial to depths as great as several kilometres with imposition of heat and pressure, and then uplift to the surface (Figure 2). In this sequence, the rock may also undergo deformation that incorporates folding and faulting. Such sequential processes are often known to raise the rock stress several times higher than its strength, hence fracturing and eventually eventually forming joints and faults. In the case of sedimentary rock, bedding planes coincidental with breaks in the continuity of sedimentation will also form. .png) Figure 2. Development of jointing due to burial and uplift of
rock: (a) stress changes in rock during burial; (b) Mohr diagram showing conditions
for rock fracture; (c) inclination of joints with respect to stress direction
(adapted from Davis and Reynolds (1996)). Figure 2(a) shows how the
stresses in rock increase with burial, assuming that there are no water,
thermal or tectonic pressures acting in the rock (Davis and Reynolds, 1996).
The vertical stress, which is the major principal stress σ1, is equal
to the weight of the overlying rock and is given by σ1 = γrH .......(1) where γr is the unit weight of the rock and H is the
depth of burial. The horizontal stress, which is the minor principal stress σ3,
also increases with the depth of burial due to the effect of the Poisson’s
Ratio µ, and any temperature increase that occurs. In ideal conditions σ3
is related to σ1 as follows: where E is the modulus of deformation of the rock, ε is the coefficient of thermal expansion and rT is the temperature rise. The first component of equation (2) gives the value of the horizontal stress due to gravitational loading; if the Poisson's ratio is 0.25 for example thenσ3 = 0.33σ1. If the rock were not free to expand, and a temperature rise of 100◦C occurs in a rock with a modulus value of 50 GPa and an ε value of 15 × 10−6/◦C, then a thermal stress of 100 GPa will be generated. This latter is an unlikely scenario in reality, since tectonic action causing the deformation of rocks by say folding and faulting will modify the values of the principal stresses. On Figure 2(a), the value of σ1 is defined by equation (1) and the value of σ3 varies with depth as follows: the value of σ3 is tensile at depths less than 1.5 km, where the sediments have not been consolidated into rock, and below this depth, σ3 increases as defined by equation (2) assuming no temperature change. The formation of joints in rock during the burial–uplift process as illustrated in Figure 2.2(a), will depend on the rock strength relative to applied stresses. One way to establish conditions that will lead to rock fracture is with a Mohr diagram, as illustrated in Figure 2(b). Figure 2(a) shows how, under compressive stress, the rock strength varies as a straight line, while under tensile stress it is curved, because within any rock there are always present small micro-fractures which act as stress concentrators and reduce the strength if tensile stresses are applied. In the Mohr diagram, circles drawn as shown in Figure 2 (b), represent the σ1 and σ3 stresses at different depths, and where the circle cuts the strength line, failure will take place. The stress conditions show that at depth 2 km, with σ1 = 52 MPa and σ3 = 0 MPa, fracture will occur because there is no horizontal confining stress acting. On the other hand, at depth 5 km, with σ1 = 130 MPa and σ3 = 25 MPa, the rock is already highly confined and will fail to exhibit fracture because the stress does not reach the rock's strength. Under low or tensile conditions of σ3, failure occurs more readily than it would at greater depth where both σ1 and σ3 are compressive. Effects of discontinuities on
slope stability %20(1).png) Figure 3: Effects of joint properties on slope stability: (a) persistent J1 joints dipping out of face forms potentially unstable sliding blocks; (b) closely spaced, low persistence joints cause raveling of small blocks; (c) persistent J2 joints dipping into face formpotential toppling slabs. For example, Figure 3 illustrates three slopes excavated in a rock mass containing two joint sets: set J1 dips at 45◦ out of the face; set J2 dips at 60◦ into the face. The stability of these slopes differs as follows. In Figure 3(a) widely spaced set J1 with persistence greater than the slope height forms a potentially unstable plane failure over the full height of the cut. While in Figure 3 (b) both sets J1 and J2 have low persistence and are closely spaced so that while small blocks ravel from the face, there is no overall slope failure. In Figure 3(c), set J2 is persistent and closely spaced, and it forms a series of thin slabs dipping into the face, which creates toppling failure. The important lesson of Figure 3 is that although an analysis of the orientation of joint sets J1 and J2 would identify the same conditions on a stereonet, there are other characteristics of these discontinuities that must also be considered in design. Orientation of discontinuities  Figure 4: Terminology defining discontinuity orientation: (a)
isometric view of plane (dip and dip direction); (b) plan view of plane; (c)
isometric view of line (plunge and trend). 1 Dip is the maximum inclination of a discontinuity to the horizontal (angle ψ). 2 Dip direction, or dip azimuth, is the direction of the horizontal trace of the line of dip, measured clockwise from north. The dip/dip direction system is particularly useful for field mapping, plotting stereonets and for the analysis of discontinuity orientation data. Strike is an another way of defining the orientation of a plane and is defined as the trace of the intersection of an inclined plane with a horizontal reference plane:. The strike is at right angles to the dip direction, and the relationship between the strike and dip direction is shown in Figure 4(b) in which the plane has a strike of N45E and a dip of 50SE. In terms of dip and dip direction the orientation of the plane is 50/135, which is a simpler nomenclature that also allows for the use of stereographic analysis. Provided one always writes the dip as two digits, and the dip direction as three digits, such as 090 for 90, there can be no confusion as to which set of figures refers to which measurement. Strike and dip measurements can be readily converted into dip and dip direction measurements should this mapping system be preferred. In defining the orientation of a line, the terms plunge and trend are used (Figure 4(c)). The plunge is the dip of the line, with a positive plunge being below the horizontal and a negative plunge being above the horizontal. Trend is the direction of the horizontal projection of the line measured clockwise from north, and corresponds to the dip direction of a plane. While mapping geological structure in the field, there is distinction to be made between the true and apparent dip of a plane. The true dip is the steepest dip of the plane, and thusalways greater than the apparent. The true dip can be found by:. If a pebble or a stream of water is run down the plane, it will always fall in a direction which corresponds to the dip direction; the dip of this line is the true dip. |
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