Deformation Characteristics of Slate Slopes Associated with Morphology and Creep

Engineering Geology 178 (2014) 132–154

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Engineering Geology

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Deformation characteristics of slate slopes associated with morphology and creep

Chia-Ming Lo a, Zheng-Yi Feng b,⁎ a Department of Civil Engineering, Chienkuo Technology University, Taiwan, ROC b Department of Soil and Water Conservation, Chung Hsing University, Taiwan, ROC article info abstract

Article history: This study investigated the deformation characteristics of consequent slate slopes in the region between Cuifeng Received 14 September 2013 and Wuling in Taiwan. Onsite surveys, terrain analysis, and UDEC numerical models were used to describe the Received in revised form 17 May 2014 characteristics of gravity-driven deformation under various conditions and identify the process of slate deforma- Accepted 7 June 2014 tion as well as potential failure mechanisms. Our results demonstrate that valley erosion and slope toe soaking Available online 20 June 2014 mechanisms play key roles in the deformation of slate and accelerate the weakening of slate material. Comparisons

Keywords: of material strength of rock and foliation, the location of erosion gullies, and the inclination of the foliation indicate Slate rock that a reduction in the strength of rock material and foliation expanded the range of slate deformation. The incli- Numerical model nation of foliation is the most important factor in the deformation of slate and the location of erosion gullies has Deformation process relatively little inﬂuence. Slate deformation was shown to begin in the tension zone at the cliff top, wherein the Potential failure mechanism slope body slips along the highly inclined foliation, contributing to shear failure or composite failure near the eroded zone of weakness. The phenomenon of foliation opening was widespread within the area of deformation, enabling surface water and groundwater to seep in, thereby accelerating failure in the slate deformation zone. © 2014 Elsevier B.V. All rights reserved.

1. Introduction rock slopes is characterized by a tendency toward heterogeneity and strong foliation. Under the force of gravity, foliation often forms buck- Slate is a plate-like rock derived from shale or tuff through low- ling folds, which extend to the toe of the slope with the strata near the grade metamorphism. Slate often contains a dense foliated texture re- surface becoming folded. The characteristics of plasticity are more pro- ferred to as slaty cleavage. The ease with which this rock splits along nounced when creeping behavior penetrates more deeply, and because the direction of the slaty cleavage weakens its resistance to erosion the rock is inhomogeneous, most of the discontinuous deformation is and weathering. In dry conditions, slate slopes display hard texture created along the shear plane near the surface (Nemcok, 1972). Nemcok and high strength, for which they are considered more stable than also found that creeping deformation in rock ranges from just millimeters shale or interbedded sandstone and shale. However, slate presents to several centimeters per day. Deformation can extend to 250–300 m in strong foliation, such that the long-term influence of gravity, groundwa- depth. In 1978, Radbruch-Hall classified creep as a type of landslide, ter, rainwater infiltration, and weathering can cause weaknesses to moving downwards or outwards in a very slow manner. Without contin- form, resulting in deeply weathered deformations or colluvium. The uous fracture planes, slopes in regions of metamorphic rock often display geological structure of slate slopes can be severely compromised by toppling and creep due to gravity (Radbruch-Hall, 1978). Deep rock de- changes in material strength, the degree of weathering, and permeabil- formation, however, is associated with foliation. Chigira (1992) and ity, thereby accelerating the failure of slate slopes and threatening the Chigira and Kiho (1994) investigated the influence of foliation on the de- safety of residents and road users in the vicinity. formation of rock under the force of gravity; however, most of these re- A number of previous studies have investigated the deformation of search results are speculative, based solely on evidence from outcrops. slate slopes. Zischinsky observed the deformation of slopes in the Alps The depth of slate deformation is nearly impossible to confirm, differing and discovered the slow motion of large rock masses on discontinuity according to the inclination of foliation and resulting in varying degrees planes and a tendency toward of opposite changes in foliation in slopes, of failure. Few studies have discussed such issues. speculating that these were the results of downhill creep (Zischinsky, Considerable researches have been conducted on the failure mecha- 1966). Nemcok (1972) observed that the deformation of metamorphic nisms associated with the deformation of slate slopes. Broili (1967) discovered that large-scale deformation or creep could lead to catastrophic landslides. In a study on the long-term stability of rock slopes, Skempton ⁎ Corresponding author at: Department of Soil and Water Conservation National Chung Hsing University 250, Kuo-Kuang RD Taichung, 402 Taiwan, ROC. (1964) discovered that clay minerals produced by weathering can de- E-mail address: [email protected] (Z.-Y. Feng). crease the angle of friction to between 8 and 10°, making this a primary

http://dx.doi.org/10.1016/j.enggeo.2014.06.011 0013-7952/© 2014 Elsevier B.V. All rights reserved. C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154 133 factor behind a reduction in the strength of rock (Skempton, 1964). In an the deformation process and potential failure mechanisms associat- examination of slate along the Central Cross-Island Highway in Taiwan, ed with consequent slate slopes under the influence of gravity. Yang (1990) and Tsai (2002) found that an acceleration of weathering can increase the quantity of clay minerals, thereby reducing the strength of the slate by 30% to 50% and undermining the stability of the slopes 2. Terrain and geology of study area (Tsai, 2002; Yang, 1990). Zhu et al. (2004) performed mechanical tests to examine the water-weakening of slate specimens. Their results indi- The study region of this study is located in Ren-ai Township of cate that the uniaxial compressive strength and elastic modulus of Nantou County (Figure 1). Geological data included topographic maps slate decreases significantly with an increase in water content. In addi- from 1936 and 1996, aided by a 5 m × 5 m digital elevation model tion, with fixed water content in the slate material, uniaxial compressive (DEM) and aerial photos from 2010 to facilitate the explanation of strength and the elastic modulus also decrease to minimal values as the changes in topography and landscape. According to the aerial photos duration of soaking is extended. Therefore, time does not benefit the and geological data, the section of road along the Taiwan Provincial High- self-sustainment of slate slopes (Zhu et al., 2004). way 14A between Cuifeng and Wuling is approximately 12.5 km. From It is difficult to quantify the scope of deformation and failure in west to east, the important landmarks include Cuifeng (EL. 2463 m), weakened slate slopes using simple terrain analysis, geological surveys, Xinrengang (EL. 2725 m), Yingyingfeng (EL. 2780 m), Yuanfeng or mathematical models and little literature on this issue exists. Thus, (EL. 2800 m), Kunyang (EL. 3080 m), and Wuling (EL. 3275 m). The we conducted an in-depth investigation of consequent slopes of slate slate slopes in this section form gradients ranging between 35° and rock deformed by gravity between Cuifeng and Wuling in the Central 60°. The foliation and weak planes display an inclination between 30° Mountain Range of Taiwan (Figure 1). The Universal Distinct Element and 70°, most of which run along the inclination of the slopes, thereby Code Version 5.0 (UDEC 5.0, Itasca, 2011) was used for numerical simu- forming classical consequent slopes. In the region from Cuifeng to lations to further understand the deformation behavior in slate slopes Wuling near the source area, the consequent slopes of slate rock form under the long-term influence of gravity and material weakening. distinct hummocky surfaces, the edges of which display highly devel- The main focuses of this study include the following: oped erosion gullies (Figure 2(A)). This is key evidence supporting the existence of slate deformation. a. Examination of the deformation characteristics of consequent slopes According to a 3D aerial photo compiled in 2003 (Figure 2(B)), the of slate rock under various conditions with regard to rock material erosion gullies near the source area at Xinrengang and Yingyingfeng strength, foliation strength, erosion weakening, and foliation are extremely advanced, with groundwater seeping out from the slate inclination; near the erosion gullies, forming individual deformable blocks. On ac- b. The use of numerical simulations to provide detailed description of count of gravity and other factors, these individual blocks cause gradual

Fig. 1. Location of study area, major landmarks, and elevation map. 134 C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154

Fig. 2. (A) Hummocky surfaces and actual surface deformation in consequent slopes of slate rock in study area. (B) 3D aerial photo of study area (base map includes aerial map from 2003 overlapping DEM of 2003). C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154 135

Fig. 3. (A) Geological map of study area at scale of 1/50,000 (modiﬁed from geological map of Central Geological Survey, 2002). (B) Engineering geology maps with main geomorphological elements and cross-section, along the A–A′ and B–B′ trace. 136 C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154 deformation and creep in the slate slopes, resulting in obvious hummocky Tayuling Formation contains slate and siltstone seams, thick seams and surfaces. The hummocky surfaces around the erosion gullies developing blocks of sandstone, thick seams of slate occasionally lined with marl near the Yuanfeng source area are far more distinct that those in the lenses, and thin seams of interbedding sandstone and slate. source areas at Xinrengang and Yingyingfeng. Furthermore, substantial The region near the Jhuoshuei River is characterized by high moun- quantities of colluvial deposit can be found piled in the watercourse, tains and deep valleys. With regard to geological development, the re- blocking off the stream at elevations between 2200 m and 2600 m. This gion is relatively young. The mountain ridges on both sides of the may have been caused by sliding slate bodies many years in the past. River extend beyond 2500 m in elevation. Due to severe downcutting The region of Kunyang and Wuling is the source of the Jhuoshuei and lateral erosion in the river valley, most of the river valley and the River. Development of the water system has led to severe downcutting erosion gullies along the Jhuoshuei River comprise weak slate. The erosion at the toe of the consequent slate slopes. In conjunction with steepness of the slopes in the region also contributes to a pronounced gravity, this has resulted in gradual slope deformation toward the need for stress release, resulting in large-scale landslides and sediment river, squeezing the upstream watercourse. In a number of locations, deposits that increase the turbidity of the river water. Attitude measure- the slope toes have collapsed and the roadbed show signs of subsidence, ments of the slate slopes revealed that the cleavage of the slate runs thereby increasing the risk of slope failure. For this reason, the targets of roughly in the northeast–southwest direction, inclining mostly toward the historical topographical and geomorphological analysis of the con- the southwest at angles between 32° and 70°. This is consistent with sequent slate slopes between Cuifeng and Wuling should include the the effects of gravity and the direction of slate slope deformation in slope sources of the important landmarks and the areas of erosion the region. We can therefore conclude that most of the slate slopes in gully development. Analysis of previous terrain in the images helped the region are consequent slopes. To clarify the mechanisms associated to clarify the relationship between slate deformation at the slope source with deformation and failure in the slate slopes, we conducted onsite and the distribution of erosion gullies. surveys and onsite geomorphological comparisons in July of 2011, The geological map of the study area in Fig. 3 indicates that most of the February and November of 2012, and February of 2013. In conjunction exposed strata includes Lushan Formation from the Miocene era (about with the terrain analysis results, these observations aided in the descrip- 500 ka) and Tayuling Formation from between the Oligocene (about tion of slate deformation characteristics and provided reference data for 2500 ka) and Miocene era. The Lushan Formation is the most widely dis- the development of a numerical model. tributed, occupying approximately 95% of the entire study area. The Lushan Formation comprises mostly argillite, slate, and phyllite with sand- 3. Geomorphologic and geological features of slate stone interbedding. The Lushan Formation between Cuifeng and Yuanfeng slope deformation is approximately 700 m thick, containing sandstone metamorphosed from caesious ﬁne particles and dark gray sandy slate. The outcropping This section outlines our investigation of slate deformation charac- Lushan Formation near Kunyang is at least 1000 m thick, comprising teristics and geomorphological changes in the region between Cuifeng slate and phyllite with fully developed foliation and occasionally lined and Wuling, based on topographic maps from 1936 and 1996 as well with thin or thick seams of metasandstone. From top to bottom, the as onsite observations.

Fig. 4. (a) Terrain and geomorphological changes in region between Cuifeng and Yingyingfeng in 1936 and 1996. (b) Terrain and geomorphological changes in region between Yuanfeng and Kunyang in 1936 and 1996. (c) Terrain and geomorphological changes in region between Kunyang and Wuling in 1936 and 1996. C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154 137

Fig. 4 (continued).

(a) Cuifeng to Yingyingfeng: The topographic map of 1936 widespread hummocky surfaces as well as slope toes with ero- (Figure 4(a)) indicates that overall, the development of the sion gullies passing through in 1996. Onsite surveys revealed ev- water system presented a dendritic drainage pattern. Advanced idence of overhang, significant accumulation of lithic debris in erosion gullies at the slope source induced severe downcutting the watercourse, and water seeping through the slopes on both and side erosion in the slate slopes on both banks of the valley, banks of the river (Figure 5). In addition, slate slopes showed ob- making this a key contributor to subsequent deformation and vious signs of deformation (Figure 6), particularly in the toes collapse in the slate slopes. Terrain analysis in 1996 revealed sig- near the erosion gullies, which displayed kink bands. It appears nificant changes in the development of erosion gullies in the that valley erosion and the seepage of water in the slope toes area. Hummocky surfaces were also widely distributed near the on both banks are crucial factors in the subsequent occurrence slope sources, which deflected and blocked the water system of slate slope deformation. Observation of variations in slate foli- near the source, thereby influencing the progress of erosion. A ation in the region between Cuifeng and Yingyingfeng revealed comparison of the terrain between 1936 and 1996 revealed the following features: (a) the inclination of cliff top foliation densely distributed erosion gullies in 1936 that generated ranged between 30° and 42°, presenting the characteristics of 138 C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154

Fig. 5. Downcutting, water seepage, and lithic accumulation in valley near source area in study area.

Fig. 6. Example of slope and slope toe deformation in slate near Xinrengang source area (inclination angle of foliation: approximately 52°). C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154 139

stepped scarps; (b) the number of kink bands, where the inclina- the deformation zone and increase the infiltration of water. Con- tion of the foliation was approximately between 20° and 30°, in- sequently, the strength and self-sustainment of the slate rock creased as the length of the slope increased or as the distance materials are significantly weakened, permitting deformation in between erosion in the slope wings decreased; (c) severe frag- the slope body to persist until failure. In contrast, many of the mentation and weathering of slate near the kink bands enabled consequent slate slopes with steeper foliation (Figure 8(b)) surface water to seep into the cracks and soak the rock materials were accompanied by deformation in the form of sharply steep- at the slope toe, thereby weakening them. Furthermore, gravity ening foliation at the toe of the slate slopes. Such deformation increased the stress in the slope toe and drastically steepened fo- was less progressed than that found in consequent slate slopes liation, thereby increasing the likelihood of large-scale failures. with gentler foliation, albeit the steeper inclination of the folia- (b) Yuanfeng to Kunyang: A comparison of topographic maps tion increased the speed of deformation due to gravity. In the (Figure 4(b)) revealed that most of the hummocky surfaces in event that the slope angle less than the inclination angle of the fo- the area are situated where erosion gullies progress rapidly. In liation, the deformation was similar to the kink band slumping the event that the slope toes were cut and hollowed, gravity model in which the foliation changed from consequent to gradually deformed the slate slope body and obstructed the obsequent near the slope toe. Furthermore, slate slope toes development of erosion gullies at the slope source, thereby with erosion gullies passing through displayed sharply steepen- forming distinct hummocky surfaces. According to the results ing foliation or water seepage, both of which were the starting of onsite surveys, the inclination of the foliation in the slate point of deformation and weakening in the slate slopes. Due to slopes near Yuanfeng is relatively gentle (approximately 30°). continuing deformation in the slope toe, the amount of slip varied We obtained evidence of arch-shaped deformations in the region with the extent of deformation in the toe. In addition to the de- (Figure 7(a)), which produced obvious protruding deformations velopment of hummocky surfaces on both sides, the amount of on the outer surface of the slate slopes. Furthermore, the tension slip may also be a key factor of overall slope stability. However, ruptures that often form in the hinge zone of the arch deforma- estimating the scale of slate slope deformation and the depth of tions as well as the tension cracks that occur in slope faces enable failure is difficult, when merely observing surface deformation surface water to permeate the slope body and weaken the slate and the extent of erosion at the toe. For this reason, this study material, thereby facilitating the next stage of failure. Our survey used numerical simulations to investigate the slope deformation of Taiwan Provincial Highway 14A-28k indicated that the characteristics and potential failure mechanisms under a range of foliation of the slate slopes in that area is inclined at approxi- conditions with regard to foliation inclination angle, material mately 45°. Signs of the foliation changing from consequent to strength, and changes in the location of erosion weak zones. obsequent were apparent near the slope toe, and the overall (d) The topographic maps from 1936 and 1996 were used in this slate deformation characteristics were similar to those in study. However, since the topographic maps involved various the kink band slumping model proposed by Kieffer (1998) surveying scales, the error margins for these data might be great- (Figure 7(b)). As for the deformation mechanisms in the slope er than acceptable for research goals (Shen and Chang, 2003). body in this location, in circumstances where the consequent Typically, for this study the 1:25,000 scale of the topographic slate slope has a slope angle smaller than the foliation inclination maps (1996) was used, however, some 1:50,000 scale topo- angle, foliation with shorter toe lengths and slate materials with graphic maps (1936) were also employed. The 1:25,000 scale more weathered surfaces are gradually pushed outwards by the provides larger and clearer details than the 1:50,000, but it does slope body, resulting in obsequent foliation attitudes. Deforma- not cover the whole study area. The 1996 topographic map are tion resulted in weathering and fracturing at the junction of the definitely more detailed and accurate for the geomorphological deformation, which enabled surface water to seep in and weaken interpretation carried out in this study. However, the use of the slate material. This accelerated the progress of damage in the 1936 topographic map for the interpretation, apart from those deformation zone at the toe, cutting and hollowing out the toe large hummocky surface features (such as changes in the surface and substantially undermining the overall stability of the slope. area of more than 10,000 m2), makes it difficult to point out mor- (c) Kunyang to Wuling: Wuling and Hohuanshantungfeng are the phology changes in greater detail. Therefore, the precision and primary sources of the Jhuoshuei River. Headward erosion has accuracy of the interpretation are poor than the 1996 topograph- caused numerous failures in this region and escalated the defor- ic map; much of the information may have omissions or errors. mation in the slate slopes. Comparison of the topographic maps The interpretation of the results from the 1936 topographic (Figure 4(c)) shows that the headward erosion in Jhuoshuei map must be particularly careful, particularly for the assessment River induced two major collapses in Wuling in 1936. The river of the slope deformation. cut through the toes of the consequent slate slopes and eroded the slopes along the weak surfaces from the bottom, which gradually cut out the borders of the deformable slate bodies. Hum- 4. UDEC numerical simulation mocky surfaces were discovered at two locations between Kunyang and Wuling. The topographic map of 1996 indicates UDEC analyzes rock masses, using multiple block elements that can that the erosion gullies were blocked off or deflected in several be translated, rotated, and make soft contact in any direction. When ex- places, promoting the development of hummocky surfaces in ternal force is applied to the rock mass, the blocks may separate along five locations. Two of these locations were expansions of the orig- foliation or discontinuous surfaces. With regard to contact between inal hummocky surfaces observed in 1936. The hummocky sur- the block elements, the amount of overlap replaces the amount of defor- faces in the remaining three locations were also progressing mation. The principles of discrete element method are grounded on the toward the boundaries of the erosion gullies, thereby demon- force–displacement law, which is used alternatively with Newton's sec- strating the importance of erosion effects on the deformation of ond law of motion. Using the force–displacement law, contact force is slate slopes. Onsite surveys revealed that cases of significant de- derived from the amount of overlap (deformation) among the block el- formation in the region were all located within the hummocky ements through inverse calculation. The combined force influencing the areas. Foliation that was more gently inclined in the consequent motion of the block elements is then calculated using Newton's second slate slopes (Figure 8(a)) often constituted kink bands and law of motion to derive the location of block elements in the next time arched deformations, the former of which frequently contained step. Variations in the system in each time step are calculated using ex- fissures and fracture zones that may accelerate the weather of plicit finite difference. 140 C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154

Fig. 7. (a) Example of slope and slope toe deformation in slate near Yuanfeng source area (inclination angle of foliation: approximately 30°). (b) Example of slope and slope toe deformation in slate near Taiwan Provincial Highway 14A-28k (inclination angle of foliation: approximately 30°). C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154 141

Fig. 8. (a) Example of slope and slope toe deformation in slate near Kunyang source area (inclination angle of foliation: approximately 35°). (b) Example of slope and slope toe deformation in slate near Wuling source area (inclination angle of foliation: approximately 48°).

Mao et al. (2006a, 2006b) discovered in the constitutive model of parameter settings, which enabled the simulation of behavior such as block element materials that slate rock materials display deformation weak surface dilatancy, yield, slope failure, and slip displacement characteristics in stages, namely, instantaneous elastic strain, primary using parameters including the elastic stiffness, cohesion, and the ten- creep, and steady-state creep. Thus, this study used the Burgers–Creep sile strength of rock foliation. In addition, creep time is very important Viscoplastic Model in the UDEC program. The primary parameters in- for numerical model of slate slope deformation. We consequently clude density, bulk modulus, Kelvin shear modulus, Maxwell shear used 1/5000 topographic maps and aerial photographs which were modulus, Kelvin viscosity, Maxwell viscosity, friction angle, cohesion, taken between 1977 and 2012 to determine the extent of slope defor- and tensile strength. The parameters are more complex than those mations for cause study model. These data could assist for the calibra- required in the Mohr–Coulomb numerical model, but they more accu- tion of simulation results and deﬁne the extent of slope deformation rately reﬂect the deformation behavior of slate rock material. The defor- over 35 years. The schematic evolutionary sequence, reported in mation and interlayer slip characteristics of slate are associated with 1977–2012 gravitational deformation steps, neglects depositional foliation strength and friction characteristics; therefore, this study in- phases or minor erosion before 1977, since no geological constraints corporated these considerations into the numerical model. We adopted are available to hypothesize a more detailed geomorphological evolu- the Coulomb slip model with foliation as the joint area contact for tion of the slopes. 142 C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154

5. Modeling methodology and input parameters Table 1 Rock mass and foliation strength parameters used in this study.

The main factors involved in the numerical model of this study in- Experiment type Rock Foliation cluded rock material strength, foliation strength, the location of erosion Density (kg/m3)2.7E+3– gullies, and the inclination of the foliation. The basic parameters of the Bulk modulus (MPa) 8.0E + 3 – Burgers–Creep Viscoplastic Model in UDEC included density, bulk mod- Kelvin shear modulus (MPa) 1.67E + 3 – ulus, friction angle, cohesion, and tensile strength. Maxwell shear modulus (MPa) 1.97E + 3 – – Central Geological Survey of the Ministry of Economic Affairs Kelvin viscosity (MPa × s) 8.3E + 14 Maxwell viscosity (MPa × s) 8.3E + 15 – (2008) and Directorate General of Highways, MOTC (2010) had Friction angle (°) 17–47 12–37 conducted field investigation and mechanical experiments on the Cohesion (MPa) 0.05–0.26 0–0.05 metamorphic rocks. The uniaxial compression deformation test, direct Tensile strength (MPa) 0.01–0.04 – shear tests, and triaxial compression tests were carried out on slate rock samples of the Lushan Formation drilled in Taiwan Central Mountain. Thus, the study collected data regarding slate triaxial tests and foliation direct shear tests for the Lushan Formation (Central Geological Survey of the Ministry of Economic Affairs, 2008; top of the numerical model. This helped to avoid errors in the gravity Directorate General of Highways, MOTC, 2010) have been used as a transfer and slip deformation in the slate slopes caused by inadequate preliminary reference for the rock strength parameters. The Hoek– boundary conditions in the model. In the setting of boundary condi- Brown empirical strength criterion is based upon the assumption tions, we restrained the deformation of the numerical model in the X that the rock mass is isotropic and has widely been used for design and Y directions at the left boundary and bottom boundary to ensure and safety assessment of slope engineering. An update of the Hoek– that the deformation would be concentrated on the right side of the Brown empirical strength criterion was presented in 2002 that includ- slope. Zhu et al. (2004) and Mao et al. (2006a, 2006b) observed a signif- ed improvements in the correlation between the model parameters icant decline in strength in argillite and slate rock material with an in- and the geological strength index (GSI). The basic idea of the Hoek– crease in water content and soaking time. A series of uniaxial Brown criterion was to start with the properties of intact rock and compression tests of the 18 layers of argillite with different water con- to add factors (rock mass structure and surface conditions) to reduce tents are conducted in Rock Mechanics Rigidity Servo Testing System those properties because of the existence of joints in the rock. Apuani (RMT-150B). Mao et al. ever used the XTR01 electric fluid serving com- et al. (2007) and Bozzano et al. (2012) ever used the Hoek–Brown pression machine to compare the compressive strength of slate rock empirical strength criterion on numerical model of deep-seated grav- under different saturated conditions. The study observed that after itational deformation in the central Italian Alps (Apuani et al., 2007; soaking in water for 3–50 days, the strength of argillite and slate Bozzano et al., 2012). Especially, Bozzano et al. collected the engineer- rocks are only 1/2–1/17 of its original strength, which can lead to soft- ing geology data of numerical model to evaluate the ISRM (2007) sug- ening and disintegration (Zhu et al., 2004; Mao et al., 2006a, 2006b). gested indices Ib (block size index) and Jv (the volumetric joint count) These results show that the softening of compressive strength of argil- which have been measured by geomechanical scanlines on the out- lite and slate rock is not only closely related to its water content, but cropping rock masses (Bozzano et al., 2012). This study adopted the also the time of being soaked in water. A horizontal projection from research methods outlined by Bozzano et al. (2012) for case study the cliff top of Xinrengang to 295 m presented signs of erosion gully de- model (Xinrengang and Taiwan Provincial Highway 14A-28k). An velopment and groundwater permeation with severe slate deforma- equivalent continuum approach was adopted to attribute strength tion. This demonstrates that the seepage of groundwater at this and stiffness parameter values to the different classes of jointed rock location had weakened the slate rock considerably. As a result, we re- masses (Bozzano et al., 2012). duced the strength of the rock materials at locations of seepage and Wu et al. (2004) selected three drill cores of the slate rock to make erosion in the simulation (assumed reduce to 1/10 of the original simple beams for rheological test. Ten measuring points are chosen strength). These locations served as the starting points of deformation and strain gage is fixed on each point. Concentrated load acts on the in the slate slopes, eventually resulting in the formation of the hum- center of the bean and corresponding time-dependent strains of the mocky surfaces seen today. ten strain gages are measured. According to the measured strains, the In order to understand the parameters' influence of the numerical viscoelastical constitutive model of the slate is obtained by inversion model, we accord to the results of field investigation of slate deforma- and analysis shows that the model accords well with the real tion mechanisms from Cuifeng to Wuling to include friction angle, viscoelastical behavior of the shale. Mao et al. (2006a, 2006b) used the cohesion, and tensile strength as key rock material parameters in the XTR01 electric-fluid servo-compression machine to conduct triaxial numerical model. Based on the triaxial test results, we selected 17°, creep experiments of slate under different confining and axial pressures. 27°, 37°, and 47° for the friction angle of the rock, which also served The strain–time relation curves of different stress levels are obtained by as the basis for cohesion, for which we selected 0.05 MPa, 0.12 MPa, series of tests. During the creep process, decay creep and steady creep 0.19 MPa, and 0.26 MPa. In terms of tensile strength, we compared appear under different axial pressures, which can be described through 0.01 MPa, 0.02 MPa, 0.03 MPa, and 0.04 MPa to further our understand- the viscoelastical–plastic model (Mao et al., 2006a, 2006b). For the ing of how rock strength influences slate slope deformation. With creep parameters, we preliminary referred to the slate creep test data regard to foliation strength, we focused on the influence of weak sur- obtained by Wu et al. (2004) and Mao et al. (2006a, 2006b) with regard face strength on slate deformation and discussed the impact of slope to Kelvin shear modulus, Maxwell shear modulus, Kelvin viscosity, and strength weak zones and variations in the inclination angle of foliation Maxwell viscosity (Table 1). on the deformation characteristics of slate. Based on the results obtain- To determine the influence of rock and weak surface parameters, ed from a direct shear test, we experimented using 12°, 20°, 28°, we employed the slate rock deformation case in Xinrengang and the and 36° for the friction angle of foliation and 0 MPa, 0.017 MPa, UDEC numerical model for simulation analysis (Figure 9). The height, 0.034 MPa, and 0.051 MPa for foliation cohesion (Table 2). The param- width, and total slope length of the numerical model were 700 m, eters' influence of the creep parameters were not discussed in this 1020 m, and 913 m, respectively, and the angles of the slope and the in- study, as they are difficult to obtain, require a greater number of test clination of foliation were both 50°. After several simulation tests, we samples than were available, and involve complex stress–strain behav- sought to prevent the influence of boundary distance on gravity trans- ior, thus we just used creep time (1977–2012) to control the creep fer by placing a section of horizontal terrain (length of 430 m) at the parameters. C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154 143

Fig. 9. Basic numerical model of slate slope adopted in this study.

6. The parameters' influence of the numerical model in the numerical model using a group spacing of 2.5 m and an inclination angle of 50°. Foliation strength was established by fixing the pa- 6.1. Strength of rock material rameters for friction angle and cohesion at set values (Table 2). The results (Figure 10) indicate that under circumstances of low strength, The strength of rock material determines the compressive and ten- the range of deformation in the slate slope is far broader than that sile strength of intact rock in slate slopes. Changes in parameters such under high strength conditions. When the strength of rock material as the friction angle, cohesion, and tensile strength can have profound was gradually increased, the depth of the slate deformation progressed impact on the deformation of slate. Thus, this study fixed the foliation toward the slope face and the arched deformed surface (RMST-1)

Table 2 Rock mass and foliation strength parameters used in the parametric study.

Parameters Rock Foliation The location of erosion gully The inclination of the foliation ψ C T ψ C Relative elevation (°) (°) (MPa) (MPa) (°) (MPa) (m)

RMST-1 17 0.05 0.01 17 0.05 350 50 RMST-2 27 0.12 0.02 17 0.05 350 50 RMST-3 37 0.19 0.03 17 0.05 350 50 RMST-4 47 0.26 0.04 17 0.05 350 50 FST-1 37 0.19 0.03 12 0 350 50 FST-2 37 0.19 0.03 20 0.017 350 50 FST-3 37 0.19 0.03 28 0.034 350 50 FST-4 37 0.19 0.03 36 0.051 350 50 LEGT-1 37 0.19 0.03 17 0.05 530 50 LEGT-2 37 0.19 0.03 17 0.05 410 50 LEGT-3 37 0.19 0.03 17 0.05 290 50 LEGT-4 37 0.19 0.03 17 0.05 170 50 IFT-1 37 0.19 0.03 17 0.05 350 30 IFT-2 37 0.19 0.03 17 0.05 350 45 IFT-3 37 0.19 0.03 17 0.05 350 60 IFT-4 37 0.19 0.03 17 0.05 350 75

Note: (a) RMST: Rock Material Strength Test. (b) FST: Foliation Strength Test. (c) LEGT: the Location of Erosion Gully Test. (d) IFT: the Inclination of the Foliation Test. 144 C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154

Fig. 10. Simulation comparison of various rock material strength parameters (red dotted lines show depth of slate deformation).

formed by low-strength rock changed to slip deformation along the fo- deformation revealed softening and subsidence in the slate at the cliff liation (RMST-2 to RMST-4). The inclination angle of the deformed areas top of RMST-1. This is because the rock and foliation are both relatively near the erosion weak zone increased with an increase in the strength of weak, which meant that under the force of gravity, the slate did not slip the rock material. A comparison of results related to cliff top along the foliation. As the strength of the rock material was increased, C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154 145 interlayer slipping along the foliation at the cliff top becomes increas- weak zone is closer to the slope source, the scope of deformation in the ingly apparent and the angle of subsidence at the cliff top increased as slate slope is reduced and the degree of foliation deformation is less ob- well, resulting in irregularly step-shaped deformation. vious than in models with erosion weakening closer to downstream sec- A comparison of the results of slope toe deformation revealed a near tions. This indicates that distribution of erosion gullies influences the horizontal band of deformation at the erosion weak zone in RMST-1. The extent of slate deformation. By comparing deformation in the cliff tops edges of the deformation band present jagged z-shaped features that are and toes of the numerical models, we found that erosion weakening at less pronounced near the middle, gradually developing into round defor- lower elevations (closer to downstream sections) displayed cave-ins of mation surfaces at the cliff top. As the strength of the rock material was greater severity in the cliff top slate. Foliation in the slate at the slope increased, the deformation at the foliation of the weak spots gradually toe was also more deformed than that observed in models with erosion changed from jagged z-shaped to curvy s-shaped (inverted). Further- weakening nearer to the slope source. This promoted the generation of more, the angle at the bottom of the deformation surface increased, larger gaps and fractures at the front end of the deformation belt at the the depth of deformation decreased, and the deformation band warped slope toe. In terms of gravity, slate in eroded weak zones closer to down- into buckled foliation deformations. However, deformation surfaces stream sections is subject to greater gravitational forces. This enhanced with high angles also contributed to an increase in compressive stress the deformation at the slope toe, which expanded further into the in the foliation in the deformation band at the slope toe. As a result, slope body and increased the scope of cave-ins in the cliff top slate as the buckled foliation either widened the voids at the front edge of the de- well as the scope of overall deformation. With fixed material parameters, formation band or created fractures in the surface rock. changing the location of erosion weakening did not induce significant variations in the angle of the deformation belt at the slope toe, indicating 6.2. Foliation strength that it influences only the scope of deformationintheslateandthede- gree of deformation at the cliff tops and slope toes. When the inclination angle of the foliation in a rock slope exceeds the friction angle of the foliation, the strength of the rock material is controlled by foliation; otherwise, rock material determines the overall 6.4. The inclination of the foliation strength of the slope. Therefore, when the foliation angle of the slate slope at Xinrengang was greater than 50°, foliation strength exerted Chigira (1992) claimed that the attitude of the foliation influences the greater influence over the deformation characteristics of the slate mass rock creep structures of foliated rocks. We therefore fixed strength slope than did rock material strength. For this reason, we fixed rock of rock material, the strength of rock foliation, and the location of erosion mass strength and other relative conditions (Table 2) in the numerical gully to investigate the influence of foliation inclination (30°, 45°, 60°, model to determine the influence of foliation strength on the deforma- and 75°) on the deformation of a slate slope. Simulation results tion characteristics of slate. (Figure 13) indicate that under these fixed conditions, increasing the in- Fig. 11 presents the simulation results for various degrees of foliation clination angles of slate foliation increases the scope of deformation and strength, the results of which show that as foliation strength increased, the inclination angle of the deformation belt at the toe. Clearly, the incli- the range of deformation in the slate slope decreased significantly. nation of foliation is also a crucial factor the deformation of a slate slope. When both foliation strength and rock material strength were relatively Cliff top deformation results indicate that foliation of low inclination an- high, deformation existed only within a range of 20 m from the surface gles formed a slightly concave deformation at the cliff top. As the inclina- of the slope. Observation of cliff top deformation shows that under low tion angle of the foliation gradually was increased from 30° to 60°, the foliation strength conditions in the FST-1 model, the cliff top presented concave form of the cliff top slate changed to a tilted linear scarp. features of nearly uniform tilt subsidence. As the foliation strength was When the inclination angle of the foliation was increased from 60° to increased, these features gradually changed to step-shaped subsidence, 75°, the tilted linear cave-in at the cliff top of the slate changed to a slight- the height of which increased with foliation strength. A comparison of ly bulging deformation. The resulting deformation at the slope toes toe deformation results revealed that the inclination angle of the defor- shows that when the inclination angle of the foliation is 30°, hummocky mation belt at the slope toe increased significantly with foliation surfaces are easily formed at the slope toe. Foliation within the deforma- strength (increasing from 5° in FST-1 to 47° in FST-4). This demon- tion belt at the slope toe presents z-shaped deformations, and the front strates that foliation strength exerts a greater impact on the deforma- edge of the deformation belt is prone to developing a large void. These tion of slate with wide foliation angles than does the strength of rock deformation characteristics are similar to those displayed by the foliation material. In addition, the deformation belt at the slope toe gradually in gently inclined slate near the Yuanfeng slope source. As the inclination shrank with an increase in foliation strength. Furthermore, the extent angle of the foliation was widened, the deformation belt at the slope toe of foliation deformation within deformation belts under conditions of expanded and progressed more deeply into the body of the slope. The in- high-foliation strength was greater than under conditions of low- clination angles in the deformation belt also increased considerably, foliation strength, indicating that the friction angle and cohesion of foli- resulting in severe warping in the foliation at the slope toe due to gravity. ation in slate are key parameters influencing slope toe deformation. As for the slip characteristics of foliation surfaces, lower foliation strength enabled the deformation belts at the toe of slate slopes to slip along fo- 7. Deformation process of slate slope and potential liation surfaces more easily, which produced larger deformation belts failure mechanism and more severe distortion in the foliation. Large-scale deformation can occur more easily in deformation belts at the toe of slopes with a To understand the deformation process of consequent slate slopes, gentler angle than in slate with high foliation strength. this study conducted simulation analysis on slate slopes at Xinrengang and Taiwan Provincial Highway 14A-28k, this study selected simulation 6.3. Location of erosion gully (weak zone position) parameters (Table 3) by comparing the deformation characteristics observed in the onsite slate with current terrain profiles. We then used To determine how the location of erosion gullies influences slate de- strain variation in the numerical simulations to explain the deformation formation, we fixed the strength of rock material and the strength of characteristics of consequent slate slopes in each case in order to reveal rock foliation as the primary material parameters in our numerical possible failure mechanisms. Sequential modeling (Figure 9) was per- model (Table 2). The relative elevation of erosion weakening was then formed by simulating the main erosional stages of the Miocene evolu- altered to demonstrate its influence on the characteristics of slate defor- tion of study area (Cuifeng to Wuling) in conjunction with the slate mation. The simulation results (Figure 12) indicate that when the eroded slope. The simulated stages were: 146 C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154

Fig. 11. Simulation comparison of various foliation strength parameters (red dotted lines show depth of slate deformation).

(a) the deepening of the valley from 2500 ka to 500 ka; The simulation model was reported in the above listed steps, (b) the deepening of the valley from 500 ka to 1977; neglects depositional phases or minor erosions, since no geological (c) the activity of the slate slope deformation at source area in constraints are available to hypothesize a more detailed geomorpho- 1977–2012. logical evolution of the slopes. As a consequence it was only C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154 147

Fig. 12. Simulation comparison of various erosion gully locations (red dotted lines show depth of slate deformation). considered that the depth increased with times but not the width of widening. However, this simpliﬁed approach is supported by wide- the valley, and it was neglected that the burial and conﬁning condi- spread evidences, collected all along the Central Mountain area, of tions on the present failure zone had affected by this progressive very fast landslides, probably triggered by high intensity external 148 C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154

Fig. 13. Simulation comparison of various foliation inclination angles (red dotted lines show depth of slate deformation).

actions, which produce a sudden evolution of the slope shape toward shape after each main erosional stage can be regarded as a reliable a more regular and stationary topographic profile. Based on these ob- approximation to be assumed for a simplified evolutionary sequence servations, a scattered evolution of the slopes toward the present of the fluvial valley formation. C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154 149

Table 3 Rock mass and foliation strength parameters used in the case study.

(A) Rock mass and foliation strength parameters used in the case study

Parameters Rock Foliation The location of erosion gully The inclination of the foliation (°) ψ C T ψ C Relative elevation (°) (MPa) (MPa) (°) (MPa) (m)

Xinrengang 50 0.32 0.04 45 0.05 400; 450; 550 50 (Class A) Xinrengang 37 0.29 0.03 32 0.03 (Class B) Xinrengang 27 0.13 0.01 17 0.01 (Class C) 14A-28k 50 0.32 0.04 45 0.05 420 50 (Class A) 14A-28k 37 0.29 0.03 32 0.03 (Class B) 14A-28k 27 0.13 0.01 17 0.01 (Class C)

(B) The rheological parameters used in the case study

Parameters Kelvin Maxwell

Shear modulus Viscosity Shear modulus (MPa) Viscosity (MPa) (MPa × s) (MPa × s)

Xinrengang 1.90E + 3 4.5E + 15 1.96E + 3 4.5E + 16 (Class A) Xinrengang 1.67E + 3 8.3E + 14 1.96E + 3 8.3E + 15 (Class B) Xinrengang 1.67E + 3 8.3E + 13 1.96E + 3 8.3E + 14 (Class C) 14A-28k 1.90E + 3 4.5E + 15 1.96E + 3 4.5E + 16 (Class A) 14A-28k 1.67E + 3 8.3E + 14 1.96E + 3 8.3E + 15 (Class B) 14A-28k 1.67E + 3 8.3E + 13 1.96E + 3 8.3E + 14 (Class C)

7.1. Deformation process of slate slope in Xinrengang upwards in the form of an s-shaped (inverted) deformation. Folia- tion within the deformation belt also warped under the effects of Fig. 4(a) displays three erosion gullies that developed on the surface gravity, for which the internal strain in the deformation belt at the of the slate slope. The difference in elevation between the two erosion slope toe was most pronounced. gullies at the top and the bottom is approximately 150 m. Slate defor- d. 1967–1977 presents the ultimate state of deformation in the eroded mation in the erosion gully downstream is greater than that found up- weak zone downstream. In this stage (1977), the deformation of the stream, resulting in the formation of kink bands near the erosion entire rock mass gained stability, and the tension zone at the cliff top gullies. Moreover, clear protrusions are shown in the slate near the exhibited step-shaped cave-ins (the slump of which is approximately downstream area of the erosion gully. As a result, we installed eroded 45 m from the free surface of the cliff top horizontally). The strain at weak zones at the elevations of 400 m, 430 m, and 550 m in the models the slope toe was the highest, resulting in obvious bulging. based on the bottom-to-top development of erosion gullies to describe e. 1977–1986 presents the beginning of the deformation caused by the the process of slate deformation and their potential failure mechanisms. second eroded weak zone. Because the weak zone furthest down- The simulation process of slate slope deformation (Figure 14)isex- stream had an elevation of only 50 m, the strain zone at the slope plained in the following. toe progressively connected with the deformation belt furthest downstream. At the cliff top, a new tension zone formed at a horizon- a. Under initial conditions (2500 ka–500 ka), stress release in the slate taldistanceof37mfromthefreesurfaceoftheclifftop,whichisalso slopes causes the formation of a tension zone at the cliff top (pink the primary slip and deformation depth in this stage. area from 500 ka to 500 ka). From 500 ka to 1942, the swiftly f. As the eroded weak zone progressed upstream, the gravitational expanding tension zone induced cave-ins at the cliff top. Horizontal- forces causing the block to slip decreased. As the deformation slowed, ly speaking, the tension zone expanded from 32 m away from the the range of deformation at the slope toe (starting at 37 m from the cliff top free surface reaching 60 m. slopesurface)andthedegreeofprotrusionbecamelesspronounced b. As the cliff top gradually caved in, the slope body slid along the foli- than those observed in the ﬁrst eroded weak zone (deformation ation, and the eroded weak zone gradually became a pressure zone depth at approximately 45 m). However, the overall range of eleva- (reddish brown area from 1942 to 1950). As a result, the slate foliation of the strain zone caused by pressure was relatively broad, tion within the pressure zone gradually deformed into the slope de- which is likely due to the concentration of pressure on the weak formation belt previously described, forming an independent zone from the slumped blocks (1986–1993). deformable block with tension zone at the cliff top. g. 1993–1998 presents the beginning of the deformation caused by the c. Under the continuous force of gravity, the deformation belt created third eroded weak zone. Due to the close proximity of the eroded by the pressure zone at the slope toe extended inward at a tile weak zone to the cliff top and the decrease in gravitational forces, (1950–1967). The strain developed inwardly at an inclination the newly generated strain zone was considerably shallower angle of 17° to the end of the deformation zone before progressing (approximately 25 m from the slope surface). Moreover, the new 150 C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154

Fig. 14. Process of slate slope deformation and monitoring results in Xinrengang case.

tension zone at the cliff top and the range of the cave-in account for (approximately 50°), resulting in substantially different characteristics only half of the entire tension zone. As the deformation slowed in overall deformation. We therefore adopted this case as a separate down (1998–2012), the foliation within the strain zone displayed focus in the discussion of slate deformation. The simulation process z-shaped features, making the slope surface protrude slightly. The for slate deformation (Figure 15) is explained below. distortion of the foliation and the scope of strain were not as distinct as the two strain zones downstream. a. Under initial conditions (2500 ka–500 ka), a tension zone formed at h. Overall, the slate slope in the Xinrengang case can be roughly divided the cliff top due to stress release. From 500 ka to 1904, the cliff top into three deformation zones, the depths of which from the slope began caving inward with the entire tension zone approximately source to the downstream section are 25 m, 37 m, and 45 m. The 32 m from the free surface of the cliff top in the horizontal direction. cliff top displays a pronounced cave-in consisting of four steps. Kink This caused the slate within a depth of 32 m to gradually slip down- bands are found in the three eroded weak zones and the deformation stream along the foliation. of the foliation in the pressured strain zone becomes increasingly ob- b. As the slope continued slipping, a pressured strain zone (red area) vious from the inside outwards. The overall deformation of the folia- with inclination angle of approximately 33° began forming at ap- tion increases in severity from the source (z-shaped) to the proximately 50 m above the eroded weak zone from 1904. A tensile downstream section (inverted s-shaped), which is largely consistent strain zone with a depth of approximately 23 m formed in the erod- with the observations in Fig. 6. ed weak zone. c. 1904–1952 presents the signiﬁcant expansion of the pressured 7.2. Deformation process of slate slope at Taiwan Provincial strain zone under the continuous force of gravity. In this stage, the Highway 14A-28k pressured strain zone expanded swiftly inwards at an angle of 28° to the location of slip at the foliation, where it progresses upstream. The inclination angle of the foliation in this case is the same as that The tension zone near the eroded weak zone at the slope toe also observed in the Xinrengang case. However, the angle of the slope (ap- grew, the pressured strain zone of which develops toward the proximately 35°) is lower than the inclination degree of the foliation eroded weak zone. From 1952 to 1977, the pressured strain zone C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154 151

Fig. 15. Process of slate slope deformation and monitoring results in Taiwan Provincial Highway 14A-28k case.

advanced toward the slope surface upstream, transforming approx- inclination angle of the foliation increased from 50° to approximate- imately 3/4 of the entire deformation zone in the slate into a ly 70°. From 1990 to 1998, the foliation at the front end of the pressured strain zone. At this point, the foliation within the strain pressured strain zone was already changing from consequent to zone had already begun warping and deforming. obsequent. The foliation within presented an inverted s-shape that d. 1977–1990 displays the inverted s-shaped deformation of the folia- are even more warped. The entire pressured strain zone was also tion within the pressured strain zone under the continuous force of gaining stability, such that the cliff top displayed a more regular gravity. The foliation at the foremost point of the pressured strain cave-in along the foliation, and the overall range of deformation in zone gradually bulged outwards due to the pressure, and the the slate was greater than that in the Xinrengang case. 152 C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154

Fig. 16. Conceptual model of deformation and failure for consequent rock slope (Huang, 2007).

e. 1998–2012 was the ﬁnal stage of deformation. The depth of defor- leading to failure. In a study on the characteristics of failure in slate mation was at least 90 m, stretching from the eroded weak zone in- with foliation at varying inclination angles, Mao and Yang (2005) ob- wards at 28° to the foliation at a depth of 90 m and then developing served that when the inclination angle of the foliation is between upwards toward the tension zone to form a quadrilateral deforma- 18.77° and 82.34°, the slate only shows damage along the foliation. tion zone with the slope surface. Most of the foliation in the upper When that the inclination angle is less than 11.4° or greater than portion of the eroded weak zone presented obsequent slopes, 82.34°, the slate displays shearing damage; when the inclination angle whereas the foliation in the rear presented inverted s-shaped is between 11.4° and 18.77°, the slate presents composite damage. In ac- features. The zone of maximum strain in the deformation belt was cordance with these observations, this study analyzed two cases in mostly in areas where the foliation was warped the most (dark which the foliation cracked open with slate deformation (in other blue area), resulting in openings or fractures in the foliation. At an el- words, the normal and tangential stress in the foliation equals 0) to in- evation of approximately 160 m in the upstream portion of the erod- vestigate the potential failure mechanisms of consequent slate slopes ed weak zone, we can see apparent protrusions, which are after deformation. The process of foliation opening and the potential consistent with the observations in Fig. 7(b) and outlined in the failure mechanisms (Figure 17) are described below. kink band slumping model (Kieffer, 1998). a. In Fig. 17(a), we can see that stress release caused the formation of a 7.3. Potential failure mechanism tension zone at the cliff top, in which the foliation shows signs of cracking open. Moreover, the scarp began caving in, creating an ac- Huang (2007) presented the creep–bending–shearing model as the tive pressure zone. Under the force of gravity, the foliation near the primary mechanism in large-scale failures in consequent slopes. From eroded weak zone deformed and opened, resulting in a passive resis- a mechanics perspective, the model can be divided into an active zone tance zone. Foliation opening developed from the slope surface in- and a passive zone (Figure 16(a)). Under the force of its own weight, wards, and the foliation near the weak zone became increasingly the active zone undergoes interlayer slipping along the weak surfaces warped as the slope toe deformed, thereby increasing foliation in the slope body. Before the rock layers in the passive zone in the opening. Foliation opening near the cliff top progressed downward slope toe are cut and hollowed out, passive compression occurs, whereas the scarp caved in, albeit the progress was not as advanced as by the rock layers are only able to generate near-vertical weak surface that in the eroded weak zone. As the eroded weak zone moved up- deformations, buckling upward to coordinate with the forces in the stream, foliation opening was observed over most of the cliff top upper slope body (Figure 16(b)). When the compression intensiﬁes and slope toe, most severe at a depth of 25 m. the upward buckling in the passive zone, it is ultimately sheared off, b. As for the potential failure mechanisms in this case, the inclination C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154 153

Fig. 17. Distribution and development of foliation openings in Xinrengang and Taiwan Provincial Highway 14A-28k cases.

angle of the foliation in the slate was approximately 50° and as a re- c. Fig. 17(b) presents the analysis results of Taiwan Provincial Highway sult, the initial slate slope slipped along the foliation or failed. When 14A-28k case. Initially, foliation opening developed on the slope sur- the gravitational force was passed to the eroded weak zone, the in- face, which under the continuing force of gravity, progressed inside of the slate formed a slanted foliation deformation range due wards and toward the cliff top and slope toe. The entire slope body to the variation of material strength. As the foliation in the deforma- first slipped downwards along the foliation, which transferred pres- tion zone underwent severe warping, a composite failure surface sure to near the eroded weak zone. This led to the development of formed on the slope toe. As this deformation progressed, cracks deformation and foliation opening in the weak zone. This was and fractures of various scales occurred in the slate material, en- obstructed by shorter slate near the weak zone that did not deform abling surface water and groundwater to seep into the foliation easily, resulting in a deformation zone with a high inclination opening to a depth of 25 m and accumulate between the foliation angle transforming the foliation opening at the front end into an deformation zone and the underlying impervious surfaces (fresh obsequent slope. Foliation toward the middle and the rear displayed slate that had not deformed). This lengthened the soaking time inverted s-shaped features, and the range of foliation opening ex- within the deformation zone, thereby accelerating the weakening panded beyond that observed in the Xinrengang case with more se- of the rock materials, ultimately leading to overall failure. vere warping as well. 154 C.-M. Lo, Z.-Y. Feng / Engineering Geology 178 (2014) 132–154 d. As for the potential failure mechanisms in Taiwan Provincial High- tion, which enable surface water and groundwater to seep in. This sig- way 14A-28k case, the initial failure in the slope lies in the slippage nificantly weakens the strength of the material at the boundaries of of foliation. As the force of gravity was transferred to the eroded the deformation zone, such that toppling or shear failure occurs at the weak zone, a highly inclined deformation zone formed, bending slope toe, further extending the scope of failure. the foliation at the front of the consequent slope until it formed an obsequent slope. Furthermore, the angle between the deformation Acknowledgments zone and the foliation was approximately 17°, which increased the chance of a composite failure surface forming in the deformation The research is mainly supported by the National Science Council zone at the slope toe. When it rained, surface water and groundwa- Taiwan, Grant no. NSC 101-2218-E-270-001 and NSC 102-2625-M- ter seeped into the slope body through cracks, immersing the con- 005-009. The advice, comments, and help provided by the editor and tents of the deformation zone in water, which rapidly weakened two anonymous reviewers have significantly strengthened the scientific the material and promoted slippage. The obsequent slope at the soundness of this paper. Their kind assists are gratefully acknowledged. front end of the slate deformation zone was a passive resistance zone, susceptible to toppling or shearing failure under continuous References pressure from the active zone in the rear. This hollowed out the Apuani, T., Masetti, M., Rossi, M., 2007. Stress–strain–time numerical modelling of a slope toe, which then destabilized the entire deformation zone and deepseated gravitational slope deformation: preliminary results. Quat. Int. 171–172, induced large-scale failure. 80–89. Bozzano, F., Martino, S., Montagna, A., Prestininzi, A., 2012. 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