2.6 Identification of modes of slope instabilityDifferent types of slope failure are associated with different geological structures and it is important that the slope designer be able to recognize potential stability problems during the early stages of a project. Some of the structural patterns that should be identified when examining pole plots are outlined on the following pages.Figure 2.16 shows the four types of failure considered in this book, and typical pole plots of geological conditions likely to lead to such failures. Note that in assessing stability, the cut face of the slope must be included in the stereo plot since sliding can only occur as the result of movement towards the free face created by the cut. The importance of distinguishing between these four types of slope failure is that there is a specific type of stability analysis for each as shown

in Chapters 6-9, and it is essential that the correct analysis method be used in design.The diagrams given in Figure 2.16 have been simplified for the sake of clarity. In an actual rock slope, several types of geological structures may be present, and this may give rise to additional types of failure. For example, in Figure 2.11, a plane failure could occur on joint set A, while the bedding could form a toppling failure on the same slope.

In a typical field study in which structural data have been plotted on stereonets, a number of significant pole concentrations may be present. It is useful to be able to identify those that represent potential failure planes, and to eliminate those that represent structures that are unlikely to be involved in slope failures. Tests for identifying important pole concentrations have been developed by Markland (1972) and Hocking (1976). These tests establish the possibility of a wedge failure in which sliding takes place along the line of intersection of two planar discontinuities as illustrated in Figure 2.16(b). Plane failure shown in Figure 2.16(a) is also covered by this test since it is a special case of wedge failure. For a wedge failure, contact is maintained on both planes and sliding occurs along the line of intersection between the two planes. For either plane or wedge failure to take place, it is fundamental that the dip of the sliding plane in the case of plane failure, or the plunge of the line of intersection in the case of wedge failure, be less than the dip of the slope face (i.e. f < ff) (Figure 2.17(a)). That is, the sliding surface daylights in the slope face.The test can also differentiate between sliding of a wedge on two planes along the line of intersection, or along only one of the planes such that a plane failure occurs. If the dip directions of the two planes lie outside the included angle between ai (trend of intersection line) and af (dip direction of face), the wedge will slide on both planes (Figure 2.17(b)). If the dip direction of one plane (A) lies within the included angle between ai and af, the wedge will slide on only that plane (Figure 2.17(c)).2.6.1 Kinematic analysisOnce the type of block failure has been identified on the stereonet, the same diagram can also be used to examine the direction in which a block will slide and give an indication of stability conditions. This procedure is known as kinematic analysis. An application of kinematic analysis is the rock face shown in Figure 2.1(b) where two joint planes form a wedge which has slid out of the face and towards the photographer. If the slope face had been less steep than the line of intersection between the two planes, or had a strike at 90 to the actual strike, then although the two planes form a wedge, it would not have been able to slide from the face. This relationship between the direction in which the block of rock will slide and the orientation of the face is readily apparent on the stereonet. However, while analysis of the stereonet gives a good indication of stability conditions, it does not account for external forces such as water pressures or reinforcement comprising tensioned rock bolts, which can have a significant effect on stability. The usual design procedure is to use kinematic analysis to identify potentially unstable blocks, followed by detailed stability analysis of these blocks using the procedures described in Chapters 6-9.An example of kinematic analysis is shown in Figure 2.18 where a rock slope contains three sets of discontinuities. The potential for these discontinuities to result in slope failures depends on their dip and dip direction relative to the face; stability conditions can be studied on the stereonet as described in the next section.2.6.2 Plane failure

In Figure 2.18(a), a potentially unstable planar block is formed by plane AA, which dips at a flatter angle than the face (^a < f and is said to daylight on the face. However, sliding is not possible on plane BB which dips steeper than the face (^b > f and does not daylight. Similarly, discontinuity set CC dips into the face and sliding cannot occur on these planes, although toppling is possible. The poles of the slope face and the discontinuity sets (symbol P) are plotted on the stereonet in Figure 2.18(b), assuming that all the discontinuities strike parallel to the face. The position of these poles in relation to the slope face shows that the poles of all planes that daylight and are potentially unstable, lie inside the pole of the slope face. This area is termed the daylight envelope and can be used to identify quickly potentially unstable blocks.The dip direction of the discontinuity sets will also influence stability. Plane sliding is not possible if the dip direction of the discontinuity differs from the dip direction of the face by more than about 20. That is, the block will be stable if |a af | > 20, because under these conditions there will be an increasing thickness of intact rock at one end of the block which will have sufficient strength to resist failure. On the stereonet this restriction on the dip direction of the planes is shown by two lines defining dip directions of (af + 20) and (af 20). These two lines designate the lateral limits of the daylight envelope on Figure 2.18(b).

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3.6.1 Core orientation

For conditions where there is insufficient design data on discontinuity orientation from surface mapping, it may be necessary to obtain this data from drill core. This will require that the core be oriented.The first step in orienting core is to determine the plunge and trend of the drill hole using a down-hole survey tool. One such tool comprises an aluminum (non-magnetic) drill rod that contains a dip meter and a compass, both of which can be photographed at specified time intervals. The orientation tool is lowered down the hole on the end of the drill string, and is held stationary at the times specified for the camera operation. At each time interval a photograph is taken of the dip meter and compass, and the depth is recorded. When the tool is recovered from the hole, the film is developed to show the hole orientation at the recorded depths. Other hole orientation tools include the Tropari single shot instrument, and gyroscopes for use in magnetic environments (Australian Drilling Industry, 1996).Most methods of orienting core involve marking a line down the core representing the top of the hole. Since the orientation of this line is known from the hole survey, the orientation of all discontinuities in the core can be measured relative to this line, from which their dip and strike can be calculated (Figure 3.13). Figure 3.13 shows that a plane intersected by the core has the shape of an ellipse, and the first step in the calculation process is to mark the down-hole end major axis of this ellipse. The dip (S) of this plane is then measured relative to the core axis, and a reference angle (a) is measured clockwise (looking down-hole) around the circumference of the core from the top-of-core line to the major axis of the ellipse. The dip and dip direction of the plane is calculated from the plunge and trend of the hole and the measured angles S and a. The true dip and dip direction of a discontinuity in the core can be determined by stereographic methods (Goodman, 1976), or by spherical/analytical geometry methods (Lau, 1983).In a few cases, the core may contain a distinct and consistent marker of known orientation,such as bedding, which can be used to orient the core and measure the orientation of the other discontinuities. However, minor, and unknown variations in the orientation of the marker bed are likely to lead to errors. Therefore, it is usually preferable to use one of the following three methods to determine the top-of-the-core.The clay impression method to orient core involves fabricating a wireline core barrel with one side weighted so that the barrel can rotate and position the weight at the bottom of the hole (Figure 3.14) (Call et al., 1982). A piece of clay is placed at the lower end of the barrel such that it protrudes past the drill bit when the core barrel is lowered down the rods and locked into place. A light pressure is then applied to the rods so that the clay takes an impression of the rock surface at the end of the hole. The core barrel is then removed and the clay impression, with top-of-core reference line, is retrieved. The next drill run proceeds normally. When this length of core is removed, the top-of-the-core is matched with the clay impression and the top-of-core line is transferred from the clay to the core run. The discontinuities in the core run are then oriented relative to the top-of-core line using the method shown in Figure 3.13.The advantages of the clay impression core orientation method are its simplicity and low equipment cost. However, the time required to take an impression on each drill run slows drilling, and the method can only be used in holes inclined at angles flatter than about 70. Also, the orientation line will be lost at any place where the core is broken and it is not possible to extend the top-of-the-core line past the break.A more sophisticated core orientation tool is the Christiensen-Hugel device that scribes a continuous line down the core during drilling; the orientation of the scribed line is determined by taking photographs of a compass in the head of the core barrel. The advantage of this method is that a continuous reference line is scribed so there is no loss of the line in zones of broken core. However, this is a more expensive and sophisticated piece of equipment than the clay impression tool.The most recent advance in core orientation is the use of the scanning borehole camera. A camera developed by Colog Inc. takes a continuous 360 image of the wall of the hole as the camera is lowered down the hole. The image can then be processed to have the appearance of a piece of core that can be rotated and viewed from any direction, or can be unwrapped (Figure 3.15). In the core view, planes intersecting the core have an elliptical shape, while in the unwrapped view the trace of each discontinuity has the form of a sine wave. The dip and dip direction of planes intersecting the core can be determined from the plunge and trend of the hole, and the orientation of the image from the compass incorporated in the camera. The dip direction of the plane is found from the position of the sine wave with respect to the compass reading, and the dip of the plane with respect to the core axis can be determined by the amplitude of the sine wave. The software with the camera system allows orientation data indicated by the sine waves to be plotted directly on a stereonet.The significant advantages of Colog camera system are that the camera is run down the hole at the completion of the hole so there is no interruption to drilling. Also, the image provides a continuous record of rock conditions, including cavities and zones of broken rock that may be lost in the recovered core. The disadvantage of the system is the cost and the need for a stable hole with clean walls, that is either dry or is filled with clean water.