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Characteristics of a planar rock slide: Hamersley range, Western Australia
Engineering Geology, 22 (1986) 335--348
335
Elsevier Science Publishers B.V., Amsterdam - - P r i n t e d in The Netherlands
CHARACTERISTICS OF A PLANAR ROCK SLIDE: HAMERSLEY RANGE, WESTERN AUSTRALIA
KARL-HEINZ WYRWOLL
Department of Geography, University of Western Australia, Nedlands, W.A., 6009 (Australia) (Received February 14, 1985; accepted after revision February 2, 1986)
ABSTRACT Wyrwoll, K.-H., 1986. Characteristics of a planar rock slide: Hamersley Range, Western Australia. Eng. Geol., 22: 335--348. A large planar rock slide in the Harnersley Range, Western Australia, is described and analysed for possible causes of failure. The general characteristics of the slide are determined by the major joint sets. The rock mass consists predominantly of alternating massive chert members and argillaceous horizons -- giving the rock mass a distinctive flaggy appearance. The failure plane dips at 17 ° and is coincident with a shale horizon. Because of the low inclination of the failure plane factors of safety less than one cannot be obtained from limiting-equilibrium hindcast estimates, unless high joint-water pressures and]or seismic loading are invoked. But while the operation of these two factors cannot be totally ruled out, the field evidence suggests that the necessary conditions for failure were due to the weathering out of shale horizons which could have led to "adjustments" of the rock mass, giving rise to dynamic loading conditions along potential failure planes. INTRODUCTION
The aims of this article are: (1) to describe the occurrence of a planar rock slide in the Hamersley Range, Western Australia; (2) to outline the kinematics-mechanics of the slide; and (3) to investigate the causes of initial failure. Rock slides of dimensions similar to the Hamersley slide are usually associated with high relief areas. The overall geomorphological setting of rock slides in these areas, their scale and the catastrophic nature of the failure process, often obscure m a n y slide characteristics and create difficulties in any discussion of the initial cause of failure. Such difficulties are highlighted by the frequent findings t h a t rock slides occur in situations for which apparent factors of safety calculations of >>1.0 have been obtained (e.g. discussion in Pariseau and Voight, 1979). To explain failure under such stability conditions, high joint water pressure or dynamic loading through seismic activity are often invoked -- n o t always convincingly. Without these factors, very low frictional resistance on the sliding mass is required for failure to occur. The Hamersley slide poses a similar problem but because of its size and geomorphological--geological setting the slide is accessible to study, and readily 0013-7952/86/$03.50
© 1986 Elsevier Science Publishers B.V.
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allows a qualitative consideration of the likely causes and the nature of hillslope failure. THE S E T T I N G A N D D E S C R I P T I O N OF THE SLIDE
The rock slide occurs south of Wittenoom in the Mount Bruce area of the Hamersley Range of northwestern Australia (Fig.l). The range is composed of openly folded Proterozoic sedimentary rocks with some associated volcanics and metasediments. The details of the regional geology are provided by De la Hunty {1965) and Trendall and Blockley (1970). The range is thought to be a dissected Tertiary plateau surface (MacLeod, 1966), bordered by well defined scarps and fringing pediments. The relative relief of the range is of the order of 200--300 m. The general characteristics of the slide are shown in Fig.2 and its geomorphological position in Fig.1. The slide has a failure scar length of 300 m, a width of 360 m and in thickness tapers from 12 m at the crest to 1--2 m at the base of the slope, and therefore involves some 0.8 to 1.0 X 10 ~ m ~. A total displacement of 215 m has occurred (measured from the back of the failed mass to the back of the failure scar). A number of other failures have occurred to the east of the main slide. Here a number of large blocks have daylighted out of the slope (Fig.2). The failed mass of the slide is divided into a back section which consists of large, intact but loosely aggregated joint blocks. The front of the slide has overridden an opposing hillside, and has shattered into "plates", which now make up the riser which mantles the opposing ridge crest (Figs.2 and 3). As shown in Fig.3, the eastern edge of the slide shows a clean separation from
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Fig.1. General l o c a t i o n and g e o m o r p h o l o g i c a l p o s i t i o n - - characteristics o f the Hamersley slide.
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Fig.2. Vertical air p h o t o g r a p h o f t h e H a m e r s l e y slide and associated failure area.
the rock mass with vertical faces marking the boundary of the failure scar. But the western edge of the failure scar has largely collapsed forming an aggregated arrangement of talus marking the slide boundary with a generally "dragged" appearance (Fig.2). The failed mass fills a small valley which formerly separated the two ridges. This valley must have markedly incised the toe of the now failed slope, as is apparent from the valley's present "ravine"-like appearance when traced eastward. This eastward extension of the valley branches, and along the
Fig.3. Offset profile view of t h e slide. T h e failure mass ( o u t l i n e d in w h i t e ) is seen t o have partially o v e r r i d d e n an o p p o s i n g e s c a r p m e n t , f o r m i n g a rise o n t h e ridge crest.
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northward branch the hillslope has also been incised, following a major structural lineament. In the case of all the failed rock masses evident in Fig.2, loss of rock mass support through basal and lateral stream erosion was clearly an important process in leading up to failure. The age of the rockslide is not known but weathering along the exposed failure surface suggests an age well outside historical times. R O C K M A S S CHARACTERISTICS
The slide has taken place on the southeast facing dipslope of a ridge of the Proterozoic Brockman Iron Formation. The Brockman Formation is a classic Precambrian Banded Iron Formation, it consists of finely banded jaspilite -alternating bands of magnetite, hematite, martite and limonite with a fine grained quartz mosaic -- interbedded with quartz and shale horizons. Surface exposures of this formation generally weather to a reddish brown shaly outcrop. The fissile shale layers are up to 30 cm in thickness (Fig.4A) and weather preferentially. The shale horizons contain " p o d s " of finely banded chert, which are left as weathering residuals on joint planes (Fig.4). Shale layers form the main failure plane of the slide, as well as minor planes along which other rock mass movements have taken place. The alternation of the lithologies gives the rock mass a blocky-flaggy
Fig.4. Series of photographs showing the characteristics of the joint surface/shale horizons. Weathering o u t o f fissile shale horizons resulting in open joints. Isolated chert " p o d s " are found as residuals along s o m e of the bedding horizons.
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character as shown in Figs.4 and 5. The flaggy character is produced by three main joint sets. The geometry of these joint sets and their relationship to the hillslope are shown on the equatorial equal-area stereonet given in Fig.6. Most dominant is the bedding plane set (J1) which dip at an angle of 17 °, with a strike-azimuth of N 65 °. The spacing of this joint set is variable with a range of 1--23 cm, with the separation of the major argillaceous horizons being of the order of 50--100 cm. The two other joint systems are clearly evident in Fig.2 from the way they delimit the sides of the failed rock mass. Both sets are nearly vertical with strike-azimuths of N 51 ° (J2) and N 150 ° (J3), respectively. The joint set J2 has an irregular spacing varying from 2 to 8 m. Joint set J3 has a spacing of 8--10 m. In addition to the major joint sets, more irregular joints also occur; these are generally coincident with sets J2 and J3, and in combination with the bedding set J,, divide the larger joint blocks into smaller slabs. The joints are generally open with variable roughness characteristics. Joint surface roughness is readily described by the JRC index (Barton, 1973). A visual comparison of the surfaces of the three joint sets with the typical roughness profile for various JRC ranges (Barton and Choubey, 1977, fig.8) suggests that set J1 surfaces have a JRC value of 2--4, and sets J2 and J3 have values in the range 16--18. A JRC value of 2.8 characterises a smooth, plane unweathered joint, while a JRC value of 16.7 applies to arough, irregular joint surface (Barton and Choubey, 1977, table 4). Schmidt rebound hammer tests (see Barton and Choubey, 1977) were used to obtain an estimate of joint wall compressive strength. Joint set J, -which constituted the failure plane -- was found to have an average joint wall compressive strength of the order of 15 MPa. U p p e r b o u n d estimates of the resistance offered by the failure plane to sliding -- the joint friction angle (¢ + i), where i is a measure of the roughness of the joint plane (see Patton, 1966) - - w e r e obtained from field tilt tests of joint blocks. These tests gave a mean value of ¢ ÷ i = 37 °. Likely values of the residual friction angle ( ~ ) of the failure plane, of 19--22 °, were obtained from Lilly's (1982) work on the uppermost member of the Mt. McRae Shale (see Table I). The Mt. McRae Shale underlies the Brockman Iron Formation and is made up of shale, siltstone and dolomitic shale, with jaspilite and chert horizons (De la Hunty, 1965). The lithology of the upper member of the formation is similar to the Brockman Iron Formation being a cherty banded iron formation consisting of beds of shale and thick chert (Trendall and Blockley, 1970, p.87). Geomechanically the Mt. McRae Shale is a weak rock, with uniaxial compression strengths in the range of 4--30 MPa with a mean of about 12 MPa, similar to the joint wall compressive strength of the failure plane of the slide as indicated by the Schmidt hammer results. The McRae Shale is characterized by closely spaced (1--5 cm) bedding lamination; these would appear to be similar to those found in the argillaceous horizons of the slide rock mass. Lilly u n d e r t o o k shear tests on the bedding planes, and the strength parameters obtained are given in Table I. The failure surface of the slide, where exposed, is polished, slickensided,
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Fig.5. Side wall o f the rock mass in t h e u p p e r part o f t h e slide scar. The a l t e r n a t i o n s o f lithologies result in a rock mass w i t h a flaggy g e o m e t r y . The side wall is a p p r o x i m a t e l y 12 m high (circle delimits for scale t h e faint o u t l i n e o f a person).
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JOINT SYSTEM (dl) (d2) (d3}
DIP AMOUNT
DIP DIRECTION
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N 155 ° N329 ° N60 °
Bedding Planes Back Wall Side Wall
ORIGINAL SLOPE Average Slope Angle Slope Orientation
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Fig.6. Great circle diagram showing the joint system orientations with respect to the slope. TABLE I Parameters in the equation r = c + a tan ~ ; joint surfaces which are clay coated are designated as "1" surfaces, bedding surfaces are described as "2" surfaces (after Lilly, 1982) Joint surface type
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tan o (degrees)
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19.8 24.2
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69.82 80.29
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(C) Small ridges
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57.50 75.50
0.53 0.66
27.9 33.4
342 chemically unweathered and with no evidence of a gouge infill. Consequently, joint surface types A and B of Table I most closely resemble t h e failure surface. Taking the mean of these measurements yields a Cr value of 24 °. Following Barton (1973, 1976) the shear strength of a joint surface can be expressed as: r = on tan
[J
--JCS~¢b] RC log10 Un
(1)
where r = peak shear strength; on = effective normal stress; JRC -- joint roughness coefficient; JCS = joint wall compressive strength; Cb = basic friction angle (usually obtained from residual shear tests on flat unweathered surfaces). The expression inside the square brackets of eq. 1 approximates the angle of shearing resistance (~) of the joint surface before failure. Using Cb = 24, On = 250 kPa (obtained from the geometry of the slide with a unit weight = 2.85.103 kg m3), JRC = 3 and JCS = 15 MPa, yields a value of @of 30 °. Lilly (1982) has been able to show that the JRC values provide friction values which overestimate the actual friction value prevailing along the joint plane. In light of this a value of ¢ in the middle to high twenties is probably a more realistic estimate. If, however, the lowest value for Cb (19.8) given by Lilly (Table I) is used in eq.1 an estimate of ¢ = 25 ° is obtained for the failure plane. In light of earlier comments this estimate can be reduced to a value in the low twenties. SLIDE KINEMATICS After failure of the " b l o c k " slide, movement was in the direction of maximum dip. This resulted in the slide being slightly offset towards the west, with respect to the failure scar. Because of this deflection the western slide boundary dragged along the rock mass, resulting in the subsequent collapse of the rock mass margin. It is assumed that the slide moved essentially as a block. It cannot be established whether there was any relative velocity difference between the major joint domains of the block, as recognised in the model studies of Rengers and Mfiller (1970). The slide characteristics changed dramatically on reaching the opposing hillside. On impact, the slide was partly fragmented and deflected up the opposing slope. The manner in which this deflection took place is evident at the eastern base of the slide (Fig.7). Here the failed mass consists of folded, imbricated and overthrust large slabs. On impacting with the opposing hillside, the slide began to act as a loose aggregate of slabs and smaller plates. These were deflected up the hillside, with the following plates overthrusting the plate beneath and mounting the hillside. The slide mass has partially overridden the ridge, and now forms a wall of about 10 m high on the crest of the ridge (Fig.3). The well defined margin of this "wall" indicates that the slide came to an abrupt halt. Only a few "plates" have fallen beyond the margin, some of these may have been
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Fig.7. U p t h r u s t i n g o n r e a c h i n g t h e o p p o s i n g hillside of t h e l o w e r p a r t of t h e failed mass.
launched in the "ski-jump" manner. Because the r u n o u t (FahrbSschung, see Hsfi, 1975) of the slide was obstructed by the opposing ridge, and because of the rather limited distance of the unimpeded travel path, it is difficult to obtain realistic estimates of slide velocity. But from the way the rock slide has partly overridden the opposing hillside it is clear that, if the hillside had not obstructed its path, the slide would have had a "FahrbSschung" of significant length. MECHANICS AND CAUSES OF FAILURE
Fig.8 gives all the forces that can act on an idealised block of unit width. At equilibrium: Z1
X 1
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(2)
o
By ignoring the thrust from the rear and all joint water effects, eq.2 yields the limiting equilibrium condition: tan 0 = tan ¢ This is the standard minimum stability criterion for this problem which ignores all lateral restraints. Consequently, with a failure plane inclination of
344
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Fig.8. Free b o d y diagram for a rock-block being pushed f r o m the rear d o w n an inclined plane 0. The block is propelled by the total stress ax + P applied to the rear and the comp o n e n t of its weight parallel to the slope. In the diagram Pb is the bulk density o f the rock a n d pj fluid pressure on the failure plane (adapted f r o m H u b b e r t and.Rubey, 1959).
17 °, an angle of shearing resistance of less than 17 ° had to be realised before failure could occur. But the discussion of likely values for the angle of shearing resistance along the failure plane indicates a likely range from 20--27 °. Evidently a significant reduction in the value of @needs to occur before failure can take place. Furthermore, the extensive summary (for a range of lithologies and stress conditions) of joint surface friction angles provided by Einstein and Dowding (1982} indicates that a joint surface friction angle of 17 ° is very low. These considerations place focus on a central problem of the study: why did a slope with such a relatively low potential inclination fail? In principle it is a simple matter to obtain significant reductions in shear strength necessary for failure to take place. A form of eq.2 may be used with the joint water parameter retained, the value of which can be obtained from back-calculations and rock mass geometry. The slide is located in an area with a mean annual precipitation of approximately 300 mm. Most of this rainfall is received in the summer months, associated with high intensity rainfall events. Tropical cyclones are also an element in the precipitation regime of the area. It can be argued that failure could have taken place at some time during the Late Quaternary when the precipitation regime of the area was especially conducive to the development of high joint-water pressures. While our present understanding of the Late Quaternary climatic history of this area would make this unlikely (Wyrwoll, 1979), it is also unnecessary to take this line of argument. The present high intensity--low frequency precipitation regime is favourable for the development of high joint-water pressures. However, the geomorphological position of the slide and the structural geology of the rock mass makes it difficult to envisage high joint-water pressures developing. Another mechanism that could be invoked is acceleration of the rock mass due to seismic activity. While seismic activity in this area is low (Doyle et al., 1968), a possible role of seismic "loading" cannot be discounted. But while neither joint-water pressures nor seismic loading can be dismissed, there is more tangible evidence that processes internal to the rock mass may have led to failure.
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Discussion of strength reduction and failure of load bearing shale horizons Clearly, a major control of rock mass stability is the shale horizons which are interbedded with the more massive " c h e r t " members. This is highlighted by the fact that the overall failure of the hillslope took place along one of these shale horizons. In addition, it is evident that more minor rock mass displacements have taken place along other shale horizons (see Fig.4A). This evidence points to the conclusion that failure has taken place as a result of a reduction in the strength of a shale horizon. The initial failure of such loadbearing shale horizons must be an extremely complex process, and due to a combination of failure modes. The stability of steep unsupported slope faces in the Brockman Iron Formation demonstrates that some cohesive strength must act along the shale horizons. On weathering, the stress distribution within a shale horizon must change. Such changes in the stress field can result in compression failure due to the formation of tensile cracks, and their interaction through flexure and shear. The intact horizons appear to behave largely as a brittle material, which could fail in this manner. No evidence was f o u n d of time-dependent viscoplastic behaviour. However, weathering leading to gouge formation is evident in the rock mass and such material is likely to exhibit time-dependent deformation characteristics. But strong slickenside development on the unweathered surface of the failure plane suggests brittle behaviour during failure. There may also have been some "shearing o f f " of chert pods along the failure plane (Fig.9). In a mechanical sense, joint discontinuities define the rock mass by affecting the stress transfer through the rock mass, and thereby breaking the smooth flow of stress trajectories, often resulting in stress concentrations. Consequently, in an open jointed rock mass any adjustment of the rock mass through local failure following weathering along a shale horizon, may result in significant stress re-distribution within the body of the rock mass. Prior to catastrophic failure, any rock mass adjustments would have led to a reduction of cohesion, accompanied by a concomitant reduction in the angle of shearing resistance from peak to residual values -- a process illustrated by the laboratory data of Lajtai (1969). Once such displacement-dependent, peakresidual strength transitions attained a critical minimum value, subsequent rock mass adjustments could directly result in overall failure. With more significant displacements, other factors can now operate which further reduce the angle of shearing resistance along a potential failure plane. In elementary physics the significant differences that can occur between the angle of static and mobile friction are well known. The frictional behaviour of joint surfaces under conditions of dynamic loading, on the scale of natural hillslopes, are complex and not understood. Available studies make it clear that frictional resistance along large failure planes in rocks is much more complex than simply adopting a characteristic " s t a t i c " coefficient of friction for the failure plane (for example Dietrich, 1979a, b; Kosloff and Liu, 1980). Some indication of the possible influence of dynamic loading on potential rock mass failure also comes from the work of Crawford and Curran (1982).
346
Fig.9. Surface characteristics of the exposed failure plane of the slide. Strong slickenside development, polishing and partial "shearing off" of chert pods. These authors have been able to show that a velocity-dependent strength reduction effect on the stability of a rock mass is of equal importance to that of the better known displacement-dependent resistance reduction. From this work it follows that with significant relative displacements during rock mass adjustments, angles of shearing resistance along a potential failure plane should be lower than those indicated by static conditions. In the overall failure of the rock mass the failure mechanism could well be augmented by a changing stress field, which can exaggerate the importance of what initially were only minor stress adjustments. The rock masses of many slopes are a " l o o s e " amalgam of joint blocks, and any adjustment in the rock mass is accommodated by joint block re-arrangement. Even when playing with children's building blocks, it is often evident that apparently insignificant displacements of individual blocks can have catastrophic consequences. Combined, these dynamic processes can result in a failure environm e n t quite different to t h a t c o m m o n l y depicted by static models. CONCLUSIONS
In the context of the scope and resources of the present study it is impossible to rule out likely roles for joint water pressures and seismic loading in the failure process. However, it is envisaged that the conditions leading to
347
overall failure of the Hamersley slide occurred through the weathering " o u t " of the shale horizons, accompanied by the collapse of joint bridges and asperities, which then lead to magnified and potentially catastrophic rock mass adjustments. With significant rock mass adjustments, static shear stresses along the failure plane were augmented by dynamic loading conditions. Therefore, any limiting equilibrium stability criterion must take into consideration the substantial kinetic energy imparted by the rock mass on the failure plane and the new equilibrium conditions imposed by a changing stress field. If such processes operate, static limiting equilibrium criteria, such as those of eq.2, are misleading. While general considerations of rock mass characteristics p r o m p t this conclusion, it remains an inference which should be tested in the more controlled setting of open-pit mines and laboratory models. ACKNOWLEDGEMENTS
I thank K. Wyrwoll, G. Kuchling, W. Wilson and J. Glover for their help. The paper was initially c o m m e n t e d on by Dr. J.N. Jennings shortly before his death. I wish to acknowledge the help I received from him. I am grateful to the two anonymous referees and Professor W.R. Judd for their helpful criticisms. REFERENCES
Barton, N., 1973. Review of a new shear strength criterion for rock joints. Eng. Geol., 7: 287--332. Barton, N., 1976. The shear strength of rock and rock joints. Int. J. Rock Mech. Min. Sci. and Geomech. Abstr., 13: 255--279. Barton, N. and Choubey, Y., 1977. The shear strength of rock joints in theory and practice. Rock Mech., 10: 1--54. Crawford, A.M. and Curran, J.H., 1982. The influence of rate- and displacement-dependent shear resistance on the response of rock slopes to seismic loads. Int. J. Rock Mech. Sci. and Geomech. Abstr., 19: 1--8. De la Hunty, L.E., 1965. Mount Bruce, W.A., West. Australia Geol. Survey, 1:250,000, Geol. Series Explan. Notes. Dietrich, J.H., 19793. Modeling of rock friction, 1. Experimental resultsand constitutive equations. J. Geophys. Res., 84: 2161--2168. Dietrich, J.H., 1979b. Modeling of rock friction,2. Simulation of preseismic slip.J. Geophys. Res., 84: 2169--2175. Doyle, H.A., Everingham, I.B. and Sutton, D.J., 1968. Seismicity of Australian Continent. J. Geol. Soc. Aust., 15: 295--312. Einstein, H.H. and Dowding, C.H., 1982. Shear resistance on deformability of rock discontinuities. In: Y.S. Touloukian, W.R. Judd and R.F. R o y (Editors), Physical Properties of Minerals and Rock. McGraw-Hill, N e w York, N.Y., pp.177--219. Hsii, K.J., 1975. Catastrophic debris streams (Sturzstroms) generated by rockfalls. Geol. Soc. Amer. Bull., 86: 129--140. Hubbert, M.K. and Rubey, W.W., 1959. Role of fluid pressure in mechanics of overthrust faulting, I. Mechanics of fluid-filled porous solids and its application to overthrust faulting. Geol. Soc. Am. Bull., 70: 115--166. Kosloff, D.D. and Liu, H.-P., 1980. Reformulation and discussion of mechanical behaviour of the velocity-dependent friction low proposed by Dietrich. Geophys. Res. Lett., 7: 913--916.
348 Lajtai, E.Z., 1969. Strength of weakness planes in rocks. Int. J. Rock Mech. Min. Sci., 6: 499--575. Lilly, P.A., 1982. The shear behaviour of bedding planes in Mr. McRae Shale with implications for rock slope design. Int. J. Rock Mech. Min. Sci. and Geomech. Abstr., 19: 205--209. MacLeod, W.N., 1966. The geology and iron deposits of the Hamersley Range area, Western Australia. West. Australia Geol. Survey Bull., 177. Pariseau, W.G. and Voight, B., 1979. Rockslides and avalanches: basic principles, and perspectives in the realm of civil and mining operations. In: B. Voight (Editor), Rockslides and Avalanches, 2. Elsevier, Amsterdam, pp.1--86. Patton, F.D., 1966. Multiple modes of shear failure in rock. Proc. Congr. ISRM, 1st, Lisbon, 1: 509--513. Rengers, N. and Miiller, L., 1970. Kinematische Versuche an geomechanischen Modellen. Rock Mech. Suppl., 1: 20--31. Simmons, J.V. and Cmden, P.M., 1980. A rock labyrinth in the F r o n t Ranges of the Rockies, Alberta. Can. J. Earth Sci., 17: 1300--1309. Trendall, A.F. and Blockley, J.G., 1970. The iron formations of the Precambrian Hamersley Group, Western Australia. West. Australia Geol. Surv. Bull., 119. Wyrwoll, K.-H., 1979. Late Quaternary climates of Western Australia: evidence and mechanism. J. R. Soc. West. Australia, 62: 129--142.
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Elsevier Science Publishers B.V., Amsterdam - - P r i n t e d in The Netherlands
CHARACTERISTICS OF A PLANAR ROCK SLIDE: HAMERSLEY RANGE, WESTERN AUSTRALIA
KARL-HEINZ WYRWOLL
Department of Geography, University of Western Australia, Nedlands, W.A., 6009 (Australia) (Received February 14, 1985; accepted after revision February 2, 1986)
ABSTRACT Wyrwoll, K.-H., 1986. Characteristics of a planar rock slide: Hamersley Range, Western Australia. Eng. Geol., 22: 335--348. A large planar rock slide in the Harnersley Range, Western Australia, is described and analysed for possible causes of failure. The general characteristics of the slide are determined by the major joint sets. The rock mass consists predominantly of alternating massive chert members and argillaceous horizons -- giving the rock mass a distinctive flaggy appearance. The failure plane dips at 17 ° and is coincident with a shale horizon. Because of the low inclination of the failure plane factors of safety less than one cannot be obtained from limiting-equilibrium hindcast estimates, unless high joint-water pressures and]or seismic loading are invoked. But while the operation of these two factors cannot be totally ruled out, the field evidence suggests that the necessary conditions for failure were due to the weathering out of shale horizons which could have led to "adjustments" of the rock mass, giving rise to dynamic loading conditions along potential failure planes. INTRODUCTION
The aims of this article are: (1) to describe the occurrence of a planar rock slide in the Hamersley Range, Western Australia; (2) to outline the kinematics-mechanics of the slide; and (3) to investigate the causes of initial failure. Rock slides of dimensions similar to the Hamersley slide are usually associated with high relief areas. The overall geomorphological setting of rock slides in these areas, their scale and the catastrophic nature of the failure process, often obscure m a n y slide characteristics and create difficulties in any discussion of the initial cause of failure. Such difficulties are highlighted by the frequent findings t h a t rock slides occur in situations for which apparent factors of safety calculations of >>1.0 have been obtained (e.g. discussion in Pariseau and Voight, 1979). To explain failure under such stability conditions, high joint water pressure or dynamic loading through seismic activity are often invoked -- n o t always convincingly. Without these factors, very low frictional resistance on the sliding mass is required for failure to occur. The Hamersley slide poses a similar problem but because of its size and geomorphological--geological setting the slide is accessible to study, and readily 0013-7952/86/$03.50
© 1986 Elsevier Science Publishers B.V.
336
allows a qualitative consideration of the likely causes and the nature of hillslope failure. THE S E T T I N G A N D D E S C R I P T I O N OF THE SLIDE
The rock slide occurs south of Wittenoom in the Mount Bruce area of the Hamersley Range of northwestern Australia (Fig.l). The range is composed of openly folded Proterozoic sedimentary rocks with some associated volcanics and metasediments. The details of the regional geology are provided by De la Hunty {1965) and Trendall and Blockley (1970). The range is thought to be a dissected Tertiary plateau surface (MacLeod, 1966), bordered by well defined scarps and fringing pediments. The relative relief of the range is of the order of 200--300 m. The general characteristics of the slide are shown in Fig.2 and its geomorphological position in Fig.1. The slide has a failure scar length of 300 m, a width of 360 m and in thickness tapers from 12 m at the crest to 1--2 m at the base of the slope, and therefore involves some 0.8 to 1.0 X 10 ~ m ~. A total displacement of 215 m has occurred (measured from the back of the failed mass to the back of the failure scar). A number of other failures have occurred to the east of the main slide. Here a number of large blocks have daylighted out of the slope (Fig.2). The failed mass of the slide is divided into a back section which consists of large, intact but loosely aggregated joint blocks. The front of the slide has overridden an opposing hillside, and has shattered into "plates", which now make up the riser which mantles the opposing ridge crest (Figs.2 and 3). As shown in Fig.3, the eastern edge of the slide shows a clean separation from
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Fig.1. General l o c a t i o n and g e o m o r p h o l o g i c a l p o s i t i o n - - characteristics o f the Hamersley slide.
337
Fig.2. Vertical air p h o t o g r a p h o f t h e H a m e r s l e y slide and associated failure area.
the rock mass with vertical faces marking the boundary of the failure scar. But the western edge of the failure scar has largely collapsed forming an aggregated arrangement of talus marking the slide boundary with a generally "dragged" appearance (Fig.2). The failed mass fills a small valley which formerly separated the two ridges. This valley must have markedly incised the toe of the now failed slope, as is apparent from the valley's present "ravine"-like appearance when traced eastward. This eastward extension of the valley branches, and along the
Fig.3. Offset profile view of t h e slide. T h e failure mass ( o u t l i n e d in w h i t e ) is seen t o have partially o v e r r i d d e n an o p p o s i n g e s c a r p m e n t , f o r m i n g a rise o n t h e ridge crest.
338
northward branch the hillslope has also been incised, following a major structural lineament. In the case of all the failed rock masses evident in Fig.2, loss of rock mass support through basal and lateral stream erosion was clearly an important process in leading up to failure. The age of the rockslide is not known but weathering along the exposed failure surface suggests an age well outside historical times. R O C K M A S S CHARACTERISTICS
The slide has taken place on the southeast facing dipslope of a ridge of the Proterozoic Brockman Iron Formation. The Brockman Formation is a classic Precambrian Banded Iron Formation, it consists of finely banded jaspilite -alternating bands of magnetite, hematite, martite and limonite with a fine grained quartz mosaic -- interbedded with quartz and shale horizons. Surface exposures of this formation generally weather to a reddish brown shaly outcrop. The fissile shale layers are up to 30 cm in thickness (Fig.4A) and weather preferentially. The shale horizons contain " p o d s " of finely banded chert, which are left as weathering residuals on joint planes (Fig.4). Shale layers form the main failure plane of the slide, as well as minor planes along which other rock mass movements have taken place. The alternation of the lithologies gives the rock mass a blocky-flaggy
Fig.4. Series of photographs showing the characteristics of the joint surface/shale horizons. Weathering o u t o f fissile shale horizons resulting in open joints. Isolated chert " p o d s " are found as residuals along s o m e of the bedding horizons.
339
character as shown in Figs.4 and 5. The flaggy character is produced by three main joint sets. The geometry of these joint sets and their relationship to the hillslope are shown on the equatorial equal-area stereonet given in Fig.6. Most dominant is the bedding plane set (J1) which dip at an angle of 17 °, with a strike-azimuth of N 65 °. The spacing of this joint set is variable with a range of 1--23 cm, with the separation of the major argillaceous horizons being of the order of 50--100 cm. The two other joint systems are clearly evident in Fig.2 from the way they delimit the sides of the failed rock mass. Both sets are nearly vertical with strike-azimuths of N 51 ° (J2) and N 150 ° (J3), respectively. The joint set J2 has an irregular spacing varying from 2 to 8 m. Joint set J3 has a spacing of 8--10 m. In addition to the major joint sets, more irregular joints also occur; these are generally coincident with sets J2 and J3, and in combination with the bedding set J,, divide the larger joint blocks into smaller slabs. The joints are generally open with variable roughness characteristics. Joint surface roughness is readily described by the JRC index (Barton, 1973). A visual comparison of the surfaces of the three joint sets with the typical roughness profile for various JRC ranges (Barton and Choubey, 1977, fig.8) suggests that set J1 surfaces have a JRC value of 2--4, and sets J2 and J3 have values in the range 16--18. A JRC value of 2.8 characterises a smooth, plane unweathered joint, while a JRC value of 16.7 applies to arough, irregular joint surface (Barton and Choubey, 1977, table 4). Schmidt rebound hammer tests (see Barton and Choubey, 1977) were used to obtain an estimate of joint wall compressive strength. Joint set J, -which constituted the failure plane -- was found to have an average joint wall compressive strength of the order of 15 MPa. U p p e r b o u n d estimates of the resistance offered by the failure plane to sliding -- the joint friction angle (¢ + i), where i is a measure of the roughness of the joint plane (see Patton, 1966) - - w e r e obtained from field tilt tests of joint blocks. These tests gave a mean value of ¢ ÷ i = 37 °. Likely values of the residual friction angle ( ~ ) of the failure plane, of 19--22 °, were obtained from Lilly's (1982) work on the uppermost member of the Mt. McRae Shale (see Table I). The Mt. McRae Shale underlies the Brockman Iron Formation and is made up of shale, siltstone and dolomitic shale, with jaspilite and chert horizons (De la Hunty, 1965). The lithology of the upper member of the formation is similar to the Brockman Iron Formation being a cherty banded iron formation consisting of beds of shale and thick chert (Trendall and Blockley, 1970, p.87). Geomechanically the Mt. McRae Shale is a weak rock, with uniaxial compression strengths in the range of 4--30 MPa with a mean of about 12 MPa, similar to the joint wall compressive strength of the failure plane of the slide as indicated by the Schmidt hammer results. The McRae Shale is characterized by closely spaced (1--5 cm) bedding lamination; these would appear to be similar to those found in the argillaceous horizons of the slide rock mass. Lilly u n d e r t o o k shear tests on the bedding planes, and the strength parameters obtained are given in Table I. The failure surface of the slide, where exposed, is polished, slickensided,
340
Fig.5. Side wall o f the rock mass in t h e u p p e r part o f t h e slide scar. The a l t e r n a t i o n s o f lithologies result in a rock mass w i t h a flaggy g e o m e t r y . The side wall is a p p r o x i m a t e l y 12 m high (circle delimits for scale t h e faint o u t l i n e o f a person).
341
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JOINT SYSTEM (dl) (d2) (d3}
DIP AMOUNT
DIP DIRECTION
17 ° 80 ° 90 °
N 155 ° N329 ° N60 °
Bedding Planes Back Wall Side Wall
ORIGINAL SLOPE Average Slope Angle Slope Orientation
26 = NISO °
Fig.6. Great circle diagram showing the joint system orientations with respect to the slope. TABLE I Parameters in the equation r = c + a tan ~ ; joint surfaces which are clay coated are designated as "1" surfaces, bedding surfaces are described as "2" surfaces (after Lilly, 1982) Joint surface type
n
c (kPa)
tan o (degrees)
(A) Slickensided and polished
(1) (2)
10 35
74.89 110.74
0.36 0.45
19.8 24.2
(B) Very flat, smooth
(1) (2)
53 108
69.82 80.29
0.41 0.57
22.3 29.7
(C) Small ridges
(1) (2)
78 131
57.50 75.50
0.53 0.66
27.9 33.4
342 chemically unweathered and with no evidence of a gouge infill. Consequently, joint surface types A and B of Table I most closely resemble t h e failure surface. Taking the mean of these measurements yields a Cr value of 24 °. Following Barton (1973, 1976) the shear strength of a joint surface can be expressed as: r = on tan
[J
--JCS~¢b] RC log10 Un
(1)
where r = peak shear strength; on = effective normal stress; JRC -- joint roughness coefficient; JCS = joint wall compressive strength; Cb = basic friction angle (usually obtained from residual shear tests on flat unweathered surfaces). The expression inside the square brackets of eq. 1 approximates the angle of shearing resistance (~) of the joint surface before failure. Using Cb = 24, On = 250 kPa (obtained from the geometry of the slide with a unit weight = 2.85.103 kg m3), JRC = 3 and JCS = 15 MPa, yields a value of @of 30 °. Lilly (1982) has been able to show that the JRC values provide friction values which overestimate the actual friction value prevailing along the joint plane. In light of this a value of ¢ in the middle to high twenties is probably a more realistic estimate. If, however, the lowest value for Cb (19.8) given by Lilly (Table I) is used in eq.1 an estimate of ¢ = 25 ° is obtained for the failure plane. In light of earlier comments this estimate can be reduced to a value in the low twenties. SLIDE KINEMATICS After failure of the " b l o c k " slide, movement was in the direction of maximum dip. This resulted in the slide being slightly offset towards the west, with respect to the failure scar. Because of this deflection the western slide boundary dragged along the rock mass, resulting in the subsequent collapse of the rock mass margin. It is assumed that the slide moved essentially as a block. It cannot be established whether there was any relative velocity difference between the major joint domains of the block, as recognised in the model studies of Rengers and Mfiller (1970). The slide characteristics changed dramatically on reaching the opposing hillside. On impact, the slide was partly fragmented and deflected up the opposing slope. The manner in which this deflection took place is evident at the eastern base of the slide (Fig.7). Here the failed mass consists of folded, imbricated and overthrust large slabs. On impacting with the opposing hillside, the slide began to act as a loose aggregate of slabs and smaller plates. These were deflected up the hillside, with the following plates overthrusting the plate beneath and mounting the hillside. The slide mass has partially overridden the ridge, and now forms a wall of about 10 m high on the crest of the ridge (Fig.3). The well defined margin of this "wall" indicates that the slide came to an abrupt halt. Only a few "plates" have fallen beyond the margin, some of these may have been
343
Fig.7. U p t h r u s t i n g o n r e a c h i n g t h e o p p o s i n g hillside of t h e l o w e r p a r t of t h e failed mass.
launched in the "ski-jump" manner. Because the r u n o u t (FahrbSschung, see Hsfi, 1975) of the slide was obstructed by the opposing ridge, and because of the rather limited distance of the unimpeded travel path, it is difficult to obtain realistic estimates of slide velocity. But from the way the rock slide has partly overridden the opposing hillside it is clear that, if the hillside had not obstructed its path, the slide would have had a "FahrbSschung" of significant length. MECHANICS AND CAUSES OF FAILURE
Fig.8 gives all the forces that can act on an idealised block of unit width. At equilibrium: Z1
X 1
f (ox + p ) d z + PbgZlXl sin 0 -- f (Oz - - P l ) tan ~' d x = 0 0
(2)
o
By ignoring the thrust from the rear and all joint water effects, eq.2 yields the limiting equilibrium condition: tan 0 = tan ¢ This is the standard minimum stability criterion for this problem which ignores all lateral restraints. Consequently, with a failure plane inclination of
344
o~
x
W=p W=p bb Xr Xr ~'j ~'j gg
''qll
Fig.8. Free b o d y diagram for a rock-block being pushed f r o m the rear d o w n an inclined plane 0. The block is propelled by the total stress ax + P applied to the rear and the comp o n e n t of its weight parallel to the slope. In the diagram Pb is the bulk density o f the rock a n d pj fluid pressure on the failure plane (adapted f r o m H u b b e r t and.Rubey, 1959).
17 °, an angle of shearing resistance of less than 17 ° had to be realised before failure could occur. But the discussion of likely values for the angle of shearing resistance along the failure plane indicates a likely range from 20--27 °. Evidently a significant reduction in the value of @needs to occur before failure can take place. Furthermore, the extensive summary (for a range of lithologies and stress conditions) of joint surface friction angles provided by Einstein and Dowding (1982} indicates that a joint surface friction angle of 17 ° is very low. These considerations place focus on a central problem of the study: why did a slope with such a relatively low potential inclination fail? In principle it is a simple matter to obtain significant reductions in shear strength necessary for failure to take place. A form of eq.2 may be used with the joint water parameter retained, the value of which can be obtained from back-calculations and rock mass geometry. The slide is located in an area with a mean annual precipitation of approximately 300 mm. Most of this rainfall is received in the summer months, associated with high intensity rainfall events. Tropical cyclones are also an element in the precipitation regime of the area. It can be argued that failure could have taken place at some time during the Late Quaternary when the precipitation regime of the area was especially conducive to the development of high joint-water pressures. While our present understanding of the Late Quaternary climatic history of this area would make this unlikely (Wyrwoll, 1979), it is also unnecessary to take this line of argument. The present high intensity--low frequency precipitation regime is favourable for the development of high joint-water pressures. However, the geomorphological position of the slide and the structural geology of the rock mass makes it difficult to envisage high joint-water pressures developing. Another mechanism that could be invoked is acceleration of the rock mass due to seismic activity. While seismic activity in this area is low (Doyle et al., 1968), a possible role of seismic "loading" cannot be discounted. But while neither joint-water pressures nor seismic loading can be dismissed, there is more tangible evidence that processes internal to the rock mass may have led to failure.
345
Discussion of strength reduction and failure of load bearing shale horizons Clearly, a major control of rock mass stability is the shale horizons which are interbedded with the more massive " c h e r t " members. This is highlighted by the fact that the overall failure of the hillslope took place along one of these shale horizons. In addition, it is evident that more minor rock mass displacements have taken place along other shale horizons (see Fig.4A). This evidence points to the conclusion that failure has taken place as a result of a reduction in the strength of a shale horizon. The initial failure of such loadbearing shale horizons must be an extremely complex process, and due to a combination of failure modes. The stability of steep unsupported slope faces in the Brockman Iron Formation demonstrates that some cohesive strength must act along the shale horizons. On weathering, the stress distribution within a shale horizon must change. Such changes in the stress field can result in compression failure due to the formation of tensile cracks, and their interaction through flexure and shear. The intact horizons appear to behave largely as a brittle material, which could fail in this manner. No evidence was f o u n d of time-dependent viscoplastic behaviour. However, weathering leading to gouge formation is evident in the rock mass and such material is likely to exhibit time-dependent deformation characteristics. But strong slickenside development on the unweathered surface of the failure plane suggests brittle behaviour during failure. There may also have been some "shearing o f f " of chert pods along the failure plane (Fig.9). In a mechanical sense, joint discontinuities define the rock mass by affecting the stress transfer through the rock mass, and thereby breaking the smooth flow of stress trajectories, often resulting in stress concentrations. Consequently, in an open jointed rock mass any adjustment of the rock mass through local failure following weathering along a shale horizon, may result in significant stress re-distribution within the body of the rock mass. Prior to catastrophic failure, any rock mass adjustments would have led to a reduction of cohesion, accompanied by a concomitant reduction in the angle of shearing resistance from peak to residual values -- a process illustrated by the laboratory data of Lajtai (1969). Once such displacement-dependent, peakresidual strength transitions attained a critical minimum value, subsequent rock mass adjustments could directly result in overall failure. With more significant displacements, other factors can now operate which further reduce the angle of shearing resistance along a potential failure plane. In elementary physics the significant differences that can occur between the angle of static and mobile friction are well known. The frictional behaviour of joint surfaces under conditions of dynamic loading, on the scale of natural hillslopes, are complex and not understood. Available studies make it clear that frictional resistance along large failure planes in rocks is much more complex than simply adopting a characteristic " s t a t i c " coefficient of friction for the failure plane (for example Dietrich, 1979a, b; Kosloff and Liu, 1980). Some indication of the possible influence of dynamic loading on potential rock mass failure also comes from the work of Crawford and Curran (1982).
346
Fig.9. Surface characteristics of the exposed failure plane of the slide. Strong slickenside development, polishing and partial "shearing off" of chert pods. These authors have been able to show that a velocity-dependent strength reduction effect on the stability of a rock mass is of equal importance to that of the better known displacement-dependent resistance reduction. From this work it follows that with significant relative displacements during rock mass adjustments, angles of shearing resistance along a potential failure plane should be lower than those indicated by static conditions. In the overall failure of the rock mass the failure mechanism could well be augmented by a changing stress field, which can exaggerate the importance of what initially were only minor stress adjustments. The rock masses of many slopes are a " l o o s e " amalgam of joint blocks, and any adjustment in the rock mass is accommodated by joint block re-arrangement. Even when playing with children's building blocks, it is often evident that apparently insignificant displacements of individual blocks can have catastrophic consequences. Combined, these dynamic processes can result in a failure environm e n t quite different to t h a t c o m m o n l y depicted by static models. CONCLUSIONS
In the context of the scope and resources of the present study it is impossible to rule out likely roles for joint water pressures and seismic loading in the failure process. However, it is envisaged that the conditions leading to
347
overall failure of the Hamersley slide occurred through the weathering " o u t " of the shale horizons, accompanied by the collapse of joint bridges and asperities, which then lead to magnified and potentially catastrophic rock mass adjustments. With significant rock mass adjustments, static shear stresses along the failure plane were augmented by dynamic loading conditions. Therefore, any limiting equilibrium stability criterion must take into consideration the substantial kinetic energy imparted by the rock mass on the failure plane and the new equilibrium conditions imposed by a changing stress field. If such processes operate, static limiting equilibrium criteria, such as those of eq.2, are misleading. While general considerations of rock mass characteristics p r o m p t this conclusion, it remains an inference which should be tested in the more controlled setting of open-pit mines and laboratory models. ACKNOWLEDGEMENTS
I thank K. Wyrwoll, G. Kuchling, W. Wilson and J. Glover for their help. The paper was initially c o m m e n t e d on by Dr. J.N. Jennings shortly before his death. I wish to acknowledge the help I received from him. I am grateful to the two anonymous referees and Professor W.R. Judd for their helpful criticisms. REFERENCES
Barton, N., 1973. Review of a new shear strength criterion for rock joints. Eng. Geol., 7: 287--332. Barton, N., 1976. The shear strength of rock and rock joints. Int. J. Rock Mech. Min. Sci. and Geomech. Abstr., 13: 255--279. Barton, N. and Choubey, Y., 1977. The shear strength of rock joints in theory and practice. Rock Mech., 10: 1--54. Crawford, A.M. and Curran, J.H., 1982. The influence of rate- and displacement-dependent shear resistance on the response of rock slopes to seismic loads. Int. J. Rock Mech. Sci. and Geomech. Abstr., 19: 1--8. De la Hunty, L.E., 1965. Mount Bruce, W.A., West. Australia Geol. Survey, 1:250,000, Geol. Series Explan. Notes. Dietrich, J.H., 19793. Modeling of rock friction, 1. Experimental resultsand constitutive equations. J. Geophys. Res., 84: 2161--2168. Dietrich, J.H., 1979b. Modeling of rock friction,2. Simulation of preseismic slip.J. Geophys. Res., 84: 2169--2175. Doyle, H.A., Everingham, I.B. and Sutton, D.J., 1968. Seismicity of Australian Continent. J. Geol. Soc. Aust., 15: 295--312. Einstein, H.H. and Dowding, C.H., 1982. Shear resistance on deformability of rock discontinuities. In: Y.S. Touloukian, W.R. Judd and R.F. R o y (Editors), Physical Properties of Minerals and Rock. McGraw-Hill, N e w York, N.Y., pp.177--219. Hsii, K.J., 1975. Catastrophic debris streams (Sturzstroms) generated by rockfalls. Geol. Soc. Amer. Bull., 86: 129--140. Hubbert, M.K. and Rubey, W.W., 1959. Role of fluid pressure in mechanics of overthrust faulting, I. Mechanics of fluid-filled porous solids and its application to overthrust faulting. Geol. Soc. Am. Bull., 70: 115--166. Kosloff, D.D. and Liu, H.-P., 1980. Reformulation and discussion of mechanical behaviour of the velocity-dependent friction low proposed by Dietrich. Geophys. Res. Lett., 7: 913--916.
348 Lajtai, E.Z., 1969. Strength of weakness planes in rocks. Int. J. Rock Mech. Min. Sci., 6: 499--575. Lilly, P.A., 1982. The shear behaviour of bedding planes in Mr. McRae Shale with implications for rock slope design. Int. J. Rock Mech. Min. Sci. and Geomech. Abstr., 19: 205--209. MacLeod, W.N., 1966. The geology and iron deposits of the Hamersley Range area, Western Australia. West. Australia Geol. Survey Bull., 177. Pariseau, W.G. and Voight, B., 1979. Rockslides and avalanches: basic principles, and perspectives in the realm of civil and mining operations. In: B. Voight (Editor), Rockslides and Avalanches, 2. Elsevier, Amsterdam, pp.1--86. Patton, F.D., 1966. Multiple modes of shear failure in rock. Proc. Congr. ISRM, 1st, Lisbon, 1: 509--513. Rengers, N. and Miiller, L., 1970. Kinematische Versuche an geomechanischen Modellen. Rock Mech. Suppl., 1: 20--31. Simmons, J.V. and Cmden, P.M., 1980. A rock labyrinth in the F r o n t Ranges of the Rockies, Alberta. Can. J. Earth Sci., 17: 1300--1309. Trendall, A.F. and Blockley, J.G., 1970. The iron formations of the Precambrian Hamersley Group, Western Australia. West. Australia Geol. Surv. Bull., 119. Wyrwoll, K.-H., 1979. Late Quaternary climates of Western Australia: evidence and mechanism. J. R. Soc. West. Australia, 62: 129--142.