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Abstract: Recently, various toppling slopes have emerged with the development of hydropower projects in the western mountainous regions of China. The slope on the right bank of the Laxiwa Hydropower Station, located on the mainstream of the Yellow River in the Qinghai Province of Northwest China, is a typical hard rock slope. Further, its deformation characteristics are different from those of common natural hard rock toppling. Because this slope is located close to the dam of the hydropower station, its deformation mechanism has a practical significance. Based on detailed geological engineering surveys, four stages of deformation have been identified using discrete element numerical software and geological engineering analysis methods, including toppling creep, initial toppling deformation, intensified toppling deformation, and current slope

formation. The spatial and time-related deformation of this site also exhibited four stages, including initial toppling, toppling development, intensification of toppling, and disintegration and collapse. Subsequently, the mechanism of toppling and deformation of the bank slope were studied. The results of this study exhibit important reference value for developing the prevention–control design of toppling and for ensuring operational safety in the hydropower reservoir area. Keywords: Laxiwa Hydropower Station; Hard rock slope; Toppling; Deformation mechanism; Discrete element method; Slope failure

Introduction

Recently, several hydropower projects have been established in the mountainous areas of

Received: 21–Jun-2018 Revised: 16-Nov-2018 Accepted: 20-Dec-2018

Mechanism of toppling and deformation in hard rock slope:

a case of bank slope of Hydropower Station, Qinghai

Province, China

CAI Jun-chao1,2 https://orcid.org/0000-0002-7152-5803; e-mail: 458515589@qq.com

JU Neng-pan1,2* https://orcid.org/0000-0002-3159-1689; e-mail: jnp@cdut.edu.cn

HUANG Run-qiu1,2 https://orcid.org/0000-0003-2560-4962; e-mail: hrq@cdut.edu.cn

ZHENG Da1,2 https://orcid.org/0000-0003-1640-7190; e-mail: zhengda@cdut.cn

ZHAO Wei-hua1,2 https://orcid.org/0000-0003-3010-1841; e-mail:weihuageo@gmail.com

LI Long-qi1,2 https://orcid.org/0000-0003-0784-3791; e-mail: 287820171@qq.com

HUANG Jian1,2 https://orcid.org/0000-0001-8936-035X; e-mail: huangjian2010@gmail.com

*Corresponding author

1 State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Chengdu University of Technology, Chengdu 610059, China

2 College of Environment and Civil Engineering, Chengdu University of Technology, Chengdu 610059, China

Citation: Cai JC, Ju NP, Huang RQ, et al. (2019) Mechanism of toppling and deformation in hard rock slope: a case of bank slope of Hydropower Station, Qinghai Province, China. Journal of Mountain Science 16(4). https://doi.org/10.1007/s11629-018-5096-x

© Science Press, Institute of Mountain Hazards and Environment, CAS and Springer-Verlag GmbH Germany, part of Springer Nature 2019

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western China. Typical slope failure modes, such as toppling deformation and failure, are commonly observed in the bank slopes of China’s hydropower stations. Further, toppling failure has been observed at several locations, including the left bank slope of the Jinping-I Hydropower Station, the deformable slope of the Xiaowan Hydropower Station, the slope of the Miaowei Hydropower Station, the right abutment slope of the Huangdeng Hydropower Station, the right bank slope of the Wunonglong Hydropower Station, the left bank slope of the Longtan Hydropower Station, and the Xiluodu Reservoir; additionally, new topples are being continuously discovered and observed (Xu et al. 2004a,b; Yang et al. 2006; Yu 2007; Xie 2010; Li 2011; Wang et al. 1992; Zhang et al. 2015; Liu et al. 2016). Toppling deformation occurs on a global scale and has been investigated by several scholars in recent years (Cruden et al. 1994; Adhikary et al. 1997; Nichol et al. 2002; Tamrakar et al. 2002; Böhme et al. 2013; Goodman 2013; Mohtarami et al. 2014; Smith 2015; Alejano et al. 2018).

Goodman (2013) summarized the developments in the research related to toppling deformation and concluded that toppling can occur in foundations, tunnels, and underground chambers or on a small scale on any rocky landscape, where frost, creep, or water forces are observed. Subsequently, the toppling types for layered slopes were studied by Goodman and Bray (Goodman and Bray 1976), who divided the toppling deformation into the following three basic types: flexural toppling, block toppling, and block flexural toppling. Hoek and Bray (Hoek and Bray 1981) further classified secondary toppling into the following four types: slide-head toppling, slide-base toppling, slide-toe toppling, and tension crack toppling. All the aforementioned studies mainly focused on small-scale shallow toppling. Large-scale toppling was recognized by scholars as a deep-seated creep deformation; further, large-scale toppling was considered to not form a penetrative destructive surface and was considered to be a toppling deformation stage (Boris 1997).

Wang et al. (1992) and Huang et al. (1994) systematically studied the deformation, failure laws, and failure mechanism of the antidip layered rock masses, denoting that various factors, such as the dip, slope angle, and rock structure, control the failure type and the scale of the antidip layer rock

slope. Further, the time-dependent view of the deformation and failure of the antidip rock mass was presented. The toppling mechanism and various factors affecting the toppling were further studied by Han and Wang (1999), who believed that the existence of antidip structural planes decisively controlled the toppling deformation of the slope. Geological engineering analysis methods and numerical methods were employed to study the toppling mechanism of Jiefanggou at Jinping Hydropower Station (Xu et al. 2004b), the deformable slope at Xiaowan Hydropower Station (Yang et al. 2006), and Xiluodu Reservoir (Zhang et al. 2015) . Their results comprehensively explain the toppling phenomenon.

Researches from Huang (2007), Huang (2008) Zhang et al. (2009), Huang (2013), Huang et al. (2017) summarized the basic laws of large-scale toppling deformation and landslide occurrence in the antidip layer rock slope using amount of engineering cases. In this case, the toppling deformation is divided into the following two types: a brittle-fracture type and a ductile-bending type. Additionally, deep and shallow toppling models are established based on the perspective of the geological process and the deformation stability of the toppling failure. It is believed that the shallow-type topple occurs in thick hard rocks, where the deformation depth is generally shallow, within a certain range of the slope surface. Deep toppling occurs in a thin layer of soft rocks over a long development time and at a considerable depth.

According to previous studies, toppling of hard rocks generally occurs within a certain depth range of the slope surface, and no large-scale damage occurs. Further, the deformable slope on the right bank of the Laxiwa Hydropower Station is a typical hard rock slope, with a deep development and a large amount of rock mass. When the slope movement occurred, the toppling of the slope surface intensified the tensile collapse of the trailing edge, and the top of the slope formed a collapsed platform. The trailing edge formed a steep cliff of more than 30 m in length, which was different from the common hard rock toppling. A detailed geological survey was conducted to obtain the structural and deformation characteristics of the bank slope. The deformation mechanism of the bank slope was further analyzed in detail using the aforementioned survey along with the discrete

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element numerical method and the geological engineering analysis method.

1 Geological Setting

The deformable body on the right bank slope of the Laxiwa Hydropower Station, located at the main stream of the Yellow River in Guide County, Qinghai Province, China, is considered to be the study area (Figure 1). The Laxiwa hydropower station is the largest hydropower project in the Yellow River area. With an estimated volume of approximately 100 million cubic meters, the deformable body has developed 900 m upstream of the Laxiwa Arch Dam and poses a high risk to the operation of the hydropower station.

The river section of the study area exhibits a typical deep valley profile. The whole slope has an average inclination of 42°, and the maximum inclination is 50°. The lithology of the slope rock mass mainly consists of Indosinian granites with developed structural planes. Four sets of major control structural planes have been recognized,

including Jset1 (N30°E/NW ∠ 60°–80°), Jset2 (NNW–SN/NE–E ∠ 60°–70°), Jset3 (N30°–60°E/NW∠40°), and Jset4 (NE30°–N20°W/SE–SW∠20°–35°). Particularly in the mid-elevation, Jset4 (include the Hf104 fault) gently inclined into the slope was developed (Figure 2). There are a small number of quaternary deposits on the gentle sloping region and the platform.

2 Failure Patterns

The deep river valley and the ditch-and-ridge terrain created good conditions for unloading, deformation, and instability of the secondary blocks. According to the detailed field geological survey and the relevant hydropower standard (GB 50330-2013), the rock mass structures of the slope are divided into the following four types: granular structure, fragmented structure, blocky structure, and massive mosaic structure. At the top of the bank slope, there are four “tensile–collapse” bands controlled by toppling. From the trailing edge to the front edge, these can be referred to as LF56,

Figure 1 Location of the study area on the right bank slope of the Laxiwa Hydropower Station, in the Qinghai Province of Northwest China.

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LF53, LF54, and LF55, respectively (Figure 3a). A large number of antislope scarps occur

facing outward from the top of the platform (Figure 3b). The slope surface represents a significant toppling of the rock mass and the slipping and falling after toppling fracture (Figure 3c). Extremely strong toppling causes significant tensile–collapse deformations of the slope. According to the observed characteristics of the toppling deformation and structure, the toppling rock masses can be classified as intensified strong toppling (Zone A), strong toppling (Zone B), weak toppling (Zone C), and loose-tensile fracture rock mass (Zone D) (see Figure 2).

The rock mass appeared to be fragmented and blocky in Zone A, exhibiting a localized granular structure. Further, the rock slabs were fractured, and they further partially slipped and fell. Majority of them remained on the slope surface. The rock mass in Zone B was toppled, fractured, and even partially broken. In addition, this rock mass exhibits a blocky structure overall and a partial mosaic fracture structure. The topple angle of the rock mass in Zone C is low, and microsplitting deformation can be observed between the rock

slabs. The inside of the rock slabs appears to exhibit a tensile-rupture deformation, which is in the weak-unloaded tensile-relaxed state. The rock mass in Zone D is located on the outside trailing edge of the toppled rock mass. Elastic relaxation occurs with the toppling of the lateral rock mass. Therefore, local microfracturing deformations occur in all the existing joint surfaces (zones), and only elastic relaxation–rupture deformations occur with no toppling. Further, the distance between tensile cracks is generally 8–12 m, and the cracks are usually 3–5 mm in width. The rock mass is intact. These are the typical characteristics of the slabbing granite rock mass exhibiting outstanding elastic characteristics that is observed in the study area (see Figure 4).

3 Analysis of the Toppling Deformation Mechanism

Based on the detailed geological survey combined with the discrete element and geological engineering analysis methods, the deformation mechanism of the deformable body was analyzed in

Figure 2 Geological engineering A–Aʹ section at the right bank slope of the Laxiwa Hydropower Station.

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detail.

3.1 Calculation model

A calculation model was established based on the geological engineering section of the slope, as denoted in Figure 5.The range of the calculation model can be given as follows: the bottom elevation

of the calculation model is approximately 2170 m, the height of the left border is approximately 120 m, that of the right border is approximately 820 m, and the horizontal distance is approximately 1,300 m. The model assumes an ideal elastoplastic material, and the yield conditions follow the Mohr–Coulomb criterion. Further, the effect of water is used by a strength reduction method,

Figure 3 The tensile–collapse bands and deformation pattern of the right bank slope. (a) The four “tensile–collapse” bands at the top of the bank slope. (b) The antislope scarps of the top of the bank slope. (c) Left: topple break at the bottom of ridge #2. Right: topple at mid-lower ridge #3.

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applied to the rock mass and the structure plane below the water level. Because the actual structural planes of the slope are complex and extremely developed, the structure planes are simplified for establishing a calculation model. The development spacing of J2 is 6, 12, 24, and 48 m, respectively, from Zones A–D. Further, the development spacing of J4 and J3 is 12 m from Zone A, 24 m from Zone B, 36 m from Zone C, and 72 m from

Zone D, except for the fault zones and the fracture zones. The distance of J1 is 24 m from Zones A and B and 48 m from Zone C; further, the specific gradient spacing should be based on the geological engineering section determined by the field survey in accordance with the principle of the gradual transition of the joints of zones from the surface to the interior of the slope.

Considering the field test, laboratory test, and

Figure 4 Rock mass of the toppling zones on the right bank slope of the Laxiwa Hydropower Station. (a) toppling Zone A, (b) toppling Zone B, (c) toppling Zone C, and (d) toppling Zone D.

Figure 5 Calculation model of (A–Aʹ section) ridge #3 of the right bank slope.

Table 1 Physical and mechanical parameters of the rock mass in the study area

Rock mass

Density (g/cm3)

Poisson's ratio

Elastic modulus (GPa)

Bulk modulus (GPa)

Shear modulus (GPa)

Friction angle (Deg)

Cohesion (MPa)

Tensile strength (MPa)

Zone A 2.4 0.23 0.2 0.13 0.08 24.24 0.02 0.1 Zone B 2.5 0.24 0.5 0.33 0.2 26.58 0.1 0.7 Zone C 2.6 0.25 1.5 1.0 0.6 35.01 0.5 4.0 Zone D 2.8 0.25 10 6.67 4.0 45.02 1.2 7.0

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related engineering parameters, the parameters of the rock mass materials used in the calculation are presented in Table 1. Further, the structural surface parameters are presented in Table 2.

To further study the evolution mechanism of the slope, special monitoring points were set up at key deformation sites to analyze the deformation characteristics and laws. Detailed information about the locations of the monitoring points is presented in Figure 5.

3.2 Toppling deformation mechanism of the slope

Based on the study of the geological engineering conditions and the rock mass structure characteristics, the slope is a brittle-fracture-type deformable body with slabbing granite (Figure 4) exhibiting typical toppling deformation characteristics.

The sharp cutting and epigenetic reformation of the river valley resulted in significant lateral unloading and rebound of the rock mass in the slope as well as the development of tensioned joints having a steeply dipping angle in the shallow unloading area. The rock mass exhibited a slabbing structure. This epigenetic reformation effect is particularly prominent in the hard rock masses dominated by elasticity. The “structural-bending” deformation of the near-vertical block-slabbing granite exhibits the “topple and break” feature under gravity. In this feature, a deep uniform sliding surface will not be present; further, collapse will occur at a certain depth and progressively move toward the inner side of the slope. Progressive deformation continues to occur until the slope stabilizes. Because of the unique structural features of the slope, multiple fracture zones are present in the upper and lower parts of the slope, especially in a set of joints (represented by Hf104) that gently dip into the slope and dominate the slope deformation. This tiny compression can create sufficient space for the initiation of deformation over the entire slope. When the water level rises, the sensitive deformable regions of the slope, especially the lower part and at the foot of the slope, begin to warp, similar to the key blocks presented in the key block theory (Shi 1985). These tiny deformations can cause large accumulated deformation of the

entire slope. The greater the relative height of the slope, the larger will be the total accumulated deformation (Figure 6).

Based on the geological engineering analysis and the discrete element method of slope deformation, we conclude that the slope deformation process undergoes the following four stages: toppling creep, initial toppling deformation, intensified toppling deformation, and current slope formation. The mechanism and deformation characteristics of the deformable rock masses present at the same site also show a phased feature in space–time.

3.2.1 Stage of toppling creep

The rock mass at the front edge of the slope is weakened because of the impoundment, and its physical and mechanical strengths are reduced. Further, the shallow surface rock mass at the front edge of the slope initially undergoes shear deformation along the structural plane because of its own weight. The trailing edge of the slope generates tensile cracks, and the trailing edge rock masses are outwardly pressed along the off-slope joints. Some local blocks roll off and fall at the

Table 2 Physical and mechanical parameters of the rock mass structure planes in the study area

Structural plane N-sti. (MPa)

S-sti. (MPa)

FA (Deg)

Cohesion (kPa)

Bound1 (Zone A-B) 50000 30000 26.58 100 Bound2 (Zone B-C) 50000 30000 30.98 150 Bound3 (Zone C-D) 50000 30000 35.01 100 J1 40000 24000 24.24 50 J2 40000 4000 24.24 50 J3 40000 24000 24.24 50 J4 40000 4000 24.24 50 Notes: N-sti. = Normal stiffness; S-sti.=Shear stiffness; FA = Friction angle.

Figure 6 Effect of accumulated deformation in a slope.

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upper part of the slope, where the joints developed. Even though the nearby rock masses have a toppling tendency, no obvious toppling could be observed (Figure 7a).

3.2.2 Initial stage of toppling: weak toppling, slab dislocation, and relaxation

The slabs that deformed under their own weight began to cantilever and topple off the slope. That gradually develops from the shallow part of the slope and moves to the deep part of the slope. Further, the rock mass of the shallow surface at the front edge of the slope is dislocated (Figure 7b), and the tensile stress is relaxed. The original cracks develop antislope scarps that create favorable conditions for the subsequent toppling of the trailing edge. Subsequently, the rock mass of the slope surface swells. The cracks on the top of the slope are obvious to the casual observers, and large vertical cracks are gradually formed on the inner slope. Because this is only the initial stage of toppling, macrorupture does not usually occur.

3.2.3 Intensified toppling stage: strong toppling, slab stretched, fractured, or sheared

Strong toppling occurs at the shallow end of the slope surface. When the cumulative tensile stress reaches or exceeds the tensile strength of the slab, the tensile fractures or tensile deformations along the existing structural plane are observed to occur within the slab (Figure 7c). There may also be breaking or shearing of the slab as well as falling slabs. Large tension cracks develop further, and the toppling at the upper slope intensifies.

3.2.4 Current slope

The shallow surface of the slope collapses. The local rock mass remains partially intact, because the collapse mainly occurs in the ridges, where the tension is released. The rock masses depict extremely intense toppling and tensile breaking. The overall slope denotes the fragmentation structures and features resulting from unloading and is prone to local collapse and sliding. The slope surface appears to be slack and overhead (Figures 3,

Figure 7 Displacement contours of the calculation model during the toppling process of the right bank slope. (a) Toppling creep during the initial deformation stage. (b) Initial toppling deformation during which weak toppling occurs. (c) Intensified toppling stage during which strong toppling occurs. (d) Current shape of the right bank slope.

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Figure 7d). The current surface of the slope is formed by the large-scale toppling and strong “tensile collapse” at the top of the slope.

In case of slopes comprising granites with prominent elastic properties, the aforementioned toppling results in the occurrence of a significant loose deformation zone within a large depth range. The rock masses are relatively fractured from the joint development and dislocate each other. In addition, water from the rainfall enters these fractures and tensile cracks, causing serious alterations of the rock mass and decreasing the slope stability.

3.3 Monitoring data analysis of calculation model

The data curves of all the monitoring points at ridge #3 are depicted in Figure 8.

The acceleration reflects the response of deformation at the monitoring sites during the unstable slope deformation process, and the displacement reflects the degree of deformation. According to the displacement curves of ridge #3, the displacements of the monitoring sites J1, J2, and J3 are large in both the X- and Y-directions. Further, the displacement of J4 is higher than that of J5; this is to ensure that the depth of its deformation reaches the root near Hf104 and the boundary between Zones C and D. The acceleration curves, the large deformation response, and the signs of deformation at Hf104 confirm the weakening effect at the lower part of the slope, resulting in an intensive deformation. In particular, the impact on the Hf104 Zone is noteworthy. The rock masses that have been toppled at the upper end of the slope exert pressure on the lower end of the slope and promote lower end of slope

Figure 8 Deformation curves of the monitoring points of the calculation model. (a) X-displacement of the monitoring points (J1–J5) of the calculation model; (b) Y-displacement of the monitoring points of the calculation model; (c) X-acceleration of the monitoring points of the calculation model; (d) Y-acceleration of the monitoring points of the calculation model.

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deformation. There is a parallel pattern in acceleration; when the acceleration of J2 is at its maximum, the accelerations of J1 and J3 also exhibit a large numerical peak. As the time step increases, the deformation tends to become stable.

The top of the slope has the largest displacements in both the X- and Y-directions. The displacement curve of J3 indicates that the position with the largest deformation is the top and that the displacement in the X-direction is larger than that in the Y-direction. This indicates that the deformation mode is bending and toppling. The Y-displacement of tensile collapse at the top of the slope is approximately 18 m, and the deformation level of the monitoring data is in accordance with that measured at the site.

The field monitoring point QC7 is the nearest to J3, its coordinate is (956.330,2934), and the displacement trend is similar to the results obtained using the calculation model. The displacement can reflect the trend of deformation displacement of the slope. Furthermore, the horizontal displacement is greater than the vertical displacement at the top of the deformation body, indicating that the deformation is mainly toppling deformation (Figure 9).

4 Conclusion

(1) The slabbing rock masses formed by the rapid cutting and the epigenetic reformation of the valley over time provide material conditions for the toppling of the rock slabs.

(2) The structure of the rock mass of a slope plays a role in controlling the deformation of the slope. The fracture zones of the fault exhibit a gently inclined slope, and the tensile–collapse bands of the slope top control the toppling of the entire slope. The toppling deformation of the slope is different from the common hard rock shallow toppling because of the special structure of the slope.

(3) The comprehensive discrete element method and geological engineering analyses denote that toppling was divided into the following four stages: toppling creep, initial toppling deformation, toppling intensification deformation, and the current pattern of the slope. The spatial and time-related deformation characteristics of the same site

presented the following four stages: initial toppling, toppling development, toppling intensification, and disintegration and collapse.

(4) The deformation curves of the characteristic points denote that the surface rock mass near the top (elevation: 2880 m) and the lower (elevation: 2630 m) part of ridge #3 is the first to exhibit tensile deformation. The largest acceleration peak indicated that the intensive joints of the slope significantly affect the slope’s deformation. The intensive joints and the large-scale “weakening” effect of the reservoir water can be considered responsible for the widespread destruction at ridge #3. Further, the accumulated effect of deformation is significant in a high slope. Over time, the deformation exhibits progressive destruction. A timely treatment could restrain the development of this deformation and provide security for the later operation of the hydropower project.

Acknowledgement

We are thankful to Professor LI Yu-sheng from the State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Chengdu University of Technology, for the fruitful discussions and comments during the preparation of this article. We would also like to thank LI Yu-sheng, LI Kan, WANG Jue, GUO Qian, YE Song, HUANG Yan-song, and the Northwest Engineering Corporation Limited for their support during the field investigation. Further, we appreciate the linguistic assistance provided by Enago during the

Figure 9 The displacement curve of the monitoring point QC7.

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preparation of this manuscript. This study is financially supported by the National Key R&D Program of China (2018YFC1504905), the Funds

for Creative Research Groups of China (41521002), and the National Natural Science Foundation of China (41772317 and 41372306).

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