Keywords

1 Introduction

The Qinghai-Tibet Plateau is uplifted by the collision and compression of the Indian and Eurasian plates. Many large-scale landslides have occurred in the deep-cutting rivers, which genetic mechanism has attracted more and more attention [1,2,3]. The formation of the landslide is closely related to the evolutionary history of the river. The formation mechanism of giant rock ancient landslides is studied by combining the method of field investigation and numerical simulation. The relationship between the occurrence of landslides and the evolution of the valley is analyzed. The formation process and mechanism of the toppling deformation bodies transformed into landslides are studied. The results show that the weak interlayer parallel to the slope direction is the main cause of the landslide. It is also affected by the factors such as the strength of the rock mass, the degree of the bank slope unloading, and rainfall infiltration [4,5,6]. The down-cutting of the river leads to the slope unloading toward the empty direction. The front edge of the toppling deformation body is eroded and loses part of the anti-sliding force, which causes the steeply dipping rock layer to gradually topple. The slip zone evolved from the toppling fracture zone is the decisive factor for the formation of giant ancient landslides [7, 8]. Construction excavation and continuous rainfall will further induce landslides [9, 10].

During the continuous uplift of the Qinghai-Tibet Plateau, many rivers cutting deeply, such as the Lancang river, Nujiang River, and Jinsha River [8, 11, 12]. Under the interaction of internal and external dynamic geological processes, the topography of the study area has changed significantly. Climate, lithology, valley deep-cutting, and other factors are all important factors influencing the occurrence of large-scale landslides. Existing studies have shown that the shape of the river and the profile of the river play a key role in the process of large-scale landslide formation. During the continuous and rapid uplift of the Qinghai-Tibet Plateau, the complex geological environment and climate change have caused large-scale geological disasters along the river [13,14,15].

With global climate change, the frequency of extreme weather events has gradually increased, leading to frequent occurrences of geological disasters. Due to the rainfall infiltration effect, the landslides induced by rainstorms occur frequently. With the increase of pore water pressure during rainfall, the effective stress of rock and soil mass decreases significantly. The shear strength of the slope is weakened [16,17,18]. Most of the landslides occur that attributed to rainfall. The cracks caused by strong rainfall and bank slope unloading are considered to be important interlayer surfaces of bedding landslides [2, 19, 20]. Slope ridges are affected by hydrostatic pressure, and the rise of groundwater level on the sliding surface is a favorable trigger for the occurrence of landslides. It has been found that most of the landslides are closely related to human engineering activities such as engineering excavation and mining [21,22,23]. Large-scale landslides along the Lancang river and other rivers have received widespread attention.

In the Lancang river, there are also several large-scale landslides of different scales, such as Gendakan landslide, front toppling deformation body, Meilishi 4# landslide, Meilishi 3# landslide, etc. [24, 25]. It is very effective to make full use of river profile and river topography to infer the influence of internal and external dynamic geological action on a landslide. Existing studies on the causes of large-scale landslides along the river show that the landslide process has an important relationship with geological environmental conditions, geological age, and valley evolution. However, there are few studies on the relationship between the giant ancient landslide that evolved from the toppling deformation and the evolution of the river [3, 7, 26, 27].

The purpose of this study is aim to investigate the formation process of landslides, river cutting process and discuss the relationship between landslides and valley evolution. Through field investigation and numerical analysis, the boundaries of landslides, the causes of landslides, and the formation mechanism of giant ancient landslides are determined. At the same time, the controlling factors of landslides are studied. The research results are helpful to describe the landslide process related to the evolution of rivers. The flow chart of the research method is as follows: The “Regional Geological Background” section describes the geological background of the landslide area. The “Zhenggang Landslide” section describes the basic characteristics and deformation characteristics of the landslide. “Numerical analysis of the formation and evolution process of Zhenggang landslide” describes the toppling deformation process of the slope in the process of valley down-cutting. In the “Discussion” section, the incubation process of the toppling deformation of steeply inclined layered rock mass and the evolution process and genetic mechanism of giant ancient landslides are discussed in depth. “Conclusion” Partially concluded (Fig. 1).

Fig. 1.
figure 1

Research methodology flowchart.

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2 Regional Geological Background

The study area is located on the left bank of the Lancang river upstream, about 3–4 km long and 1–2 km wide. The strong unloading of rock masses occur during the rapid down-cutting process of rivers. The valley is in the shape of a “V” (Fig. 2). The elevation of the mountain top is about 4000–6000 m. The maximum height difference is about 1500–3200 m. The terrain is high in the north and low in the south. The mountains spread out in a north-south direction as a whole. The slopes on both sides of the Lancang river are relatively steep, generally ranging from 20° to 45°. Some sections of the river are relatively wide, with well-developed terraces, and generally ranging from 10–25°. The three major rock types, sedimentary rock, magmatic rock, and metamorphic rock, are exist in the study area. The strata are mainly Devonian (D), Carboniferous (C), and Triassic (T). The lithology is mainly basalt sandwich slate, andesite, quartz sandstone, metamorphic sandstone, mudstone, and a small amount of thin limestone.

The study area is located in the Hengduan Mountains in the southeast of the Qinghai-Tibet Plateau. The dam site of the Gushui Hydropower Station is located in the earthquake intensity area of VII. The annual average temperature in the study area is 4.7 °C. The annual average rainfall is 633.7 mm. According to on-site investigations, the following four-level terraces have developed in the Lancang river section in the area:

  1. 1)

    First-level terrace: It is the modern riverbed of the Lancang river (the annual water level is 2070–2078 m above sea level), mainly composed of sand and gravel deposits on the floodplain.

  2. 2)

    Second-level terrace: The elevation difference from the current river surface is about 15 m, developed on the left bank of the upstream of the landslide. The terrace elevation is 2080 m (front edge)–2100 m (rear edge). The exposed width is about 40 m. It is composed of sand gravel layer and silt fine soil. A base terrace formed by a dual structure, the middle and rear part of the terrace passes through 214 National Road.

  3. 3)

    Third-level terrace: The elevation is about 2130 m. The height difference from the current river surface is about 60 m. It is the most complete terrace, and it is more developed on the opposite bank of the landslide (the left bank of the river).

  4. 4)

    Fourth-level terrace: only remains in this landslide area, with a development elevation of 2250–2280 m.

The bank slope unloading along with the terrace formation during valley cutting is a crucial factor for the formation of Zhenggang landslide.

Fig. 2.
figure 2

The typical landslide in the study area (a. the location of Zhenggang landslide; b. Second-level terrace Pebble layer; c. First to fourth-level terrace; d. Full view of Zhenggang landslide).

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3 Zhenggang Landslide

3.1 Basic Characteristics and Zone

The Zhenggang landslide deposits are located in the east of the Zhenggang mountain beam, about 900 m away from the Gushui Hydropower Station dam site upper. The maximum vertical length of the landslide is about 1010 m. The maximum horizontal width is about 1100 m. The top elevation is about 2720 m. The bottom elevation is about 2170 m. The thickness of the landslide accumulation body is about 15–60 m. The average thickness is about 26.9 m. The volume is about 4750 × 104 m3. It is a giant rocky ancient landslide. The landslide accumulation body is affected by the erosion and down-cutting of Zhenggang gully. The old landslide accumulation body is divided into Zone I and Zone II (Fig. 3). The size of zone II is larger than that of zone I, all of which are long tongue-like with a wide bottom and a narrow top. The overall morphology is “M” shaped.

The landslide deposits in Zone I have an elevation of 2180 m–2650 m. The left side is bounded by the No. 8 gully. The right side is bounded by Zhenggang gully. The front edge is relatively steep, with a slope of about 40°, and an elevation of about 2000 m at the front edge. The middle part tends to be gentle, with a length of about 430 m, a width of about 390 m, and an overall slope of about 10°. The rear edge has a steep wall of landslide with a slope angle of about 42°.

The distribution elevation of the landslide deposits in Zone II is 2180 m–2730 m. The left side is bounded by the Zhenggang gully. The right side is bounded by Yagong gully. The front edge is steep, with a slope of about 40°. The middle part tends to be gentle, with a length of about 190 m, a width of about 450 m, and an overall slope of about 10°. The back scarp at the rear edge is obvious, with a slope of about 30°.

Fig. 3.
figure 3

Deformation characteristics and zoning of Zhenggang landslide (c, e. Tension cracks in the landslide accumulation zone I; a, d. Tension cracks in the landslide accumulation zone II).

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3.2 Material Composition

The material composition of Zhenggang landslide can be divided into residual slope accumulation layer, ice-water accumulation layer, collapse slope accumulation layer, bottom slip accumulation layer, and slip zone soil.

  1. 1)

    The residual slope layer is mainly crushed sandy soil and silt, with a small amount of rock. The content of crushed rock and rock is about 30%–40%. The diameter is about 1 cm–10 cm.

  2. 2)

    The ice-water accumulation layer can be divided into two layers, mainly composed of blocks, gravel, sand, and silt. The diameter of the blocks in the block and crushed rock layer is about 5 cm–25 cm. The diameter of the crushed rock is less than 5 cm. The gaps are filled with sandy silt and a small amount of clay. The overhead phenomenon is obvious and loose. The diameter of the gravel in the gravel sand and silt layer is about 2 cm–3 cm. The gaps are mainly filled and cemented by sand, silt, and clay. The degree of compaction is good.

  3. 3)

    The collapsing layer is mainly composed of gravel soil mixed with sandy gravel soil. The diameter of gravel is about 0.5 cm–2 cm. The content is about 20%–30%. The diameter of block stone and crushed stone soil are about 6 cm–30 cm. The content of block stones is about 45%–55%. The content of crushed stone is about 20%.

  4. 4)

    The bottom-slip accumulation layer includes blocks, gravel soil, and broken rock mass. The diameter of the block and gravel soil is about 10–40 cm, and the maximum can reach 100 cm–300 cm. Broken rock masses are mostly blue-gray metamorphic sandstone, gray-brown slate, and light gray limestone. Locally it has a layered structure, and the fractured rock mass at the trailing bed attitude is N25°–30°W, SW∠20°–40°.

3.3 The Landslide Structural Characteristics

According to the PD1704 survey results of the landslide accumulation body in Zone I, it can be known that 0 m–4.5 m is the residual slope accumulation layer; 4.5 m–9.0 m is the collapse slope accumulation layer; 9.0 m–32.6 m is the bottom slip accumulation layer; and 32.6 m–68 m is the bottom slip accumulation layer.

According to the PD144 survey results of the landslide accumulation body in Zone II, it can be known that: 0 m–4.3 m is the residual slope accumulation layer; 4.3 m–16.0 m is the collapse slope accumulation layer; 16.0 m–107.5 m is the bottom slip accumulation layer; and 107.5 m–133.3 m is the bedrock: the lithology is limestone, which is broken, and the bed attitude is S29°–35°E, SW∠0°–20°; 133.3 m–159.0 m is the bedrock: the lithology is layered slate, and the bed attitude is N32°W, SW∠5°; 159.0 m–176 m is the bedrock: the lithology is gray-green basalt, SN, W∠55°, which is relatively broken and in the shape of fragments.

3.4 Characteristics of the Slip Zone

  1. 1)

    The slip zone of the landslide accumulation body in Zone I is exposed at 73.7 m on the right wall of PD1704. The slip zone soil is gray and brown clay, which is plastic. The fine particles in the slip zone soil are sub-circular. Larger particles are sub-circular to sub-angular. The diameter of gravel is about 0.5 cm–1 cm. The composition is slate. The thickness is about 10 cm–20 cm, it is a dark gray clay interlayer. The bed attitude of the slip surface is N25°W/NE∠20°–25° (Fig. 4a).

  2. 2)

    The slip zone of the landslide accumulation body in Zone II is exposed in the PD144 right wall 70–80 m. The thickness is about 7 cm. The clay content in the zone is relatively high. There are some white calcite bands in the zone. The bed attitude is 140°∠14°. The upper part of the sliding zone is mainly breccia gravel soil with a particle size of about 1 cm–10 cm. The lower part of the sliding zone is crushed stone soil, which is gray and brown overall, with a coarse-grain content of about 20%–30%, and a particle size of about 6 cm–10 cm. The rock composition is sandstone and slate (Fig. 4b).

Fig. 4.
figure 4

Characteristics of the sliding belt (a. PD1702 at 26 m on the right wall; b. PD144 at 90 m at the adit).

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3.5 Zoning Deformation Characteristics

According to the results of on-site survey, combined with the structural and deformation characteristics of the landslide, the Zhenggang landslide I zone is divided into a strong deformation area at the front edge, a strong deformation area on the right side, a strong deformation area in the middle, and a strong deformation area at the rear edge. The Zhenggang landslide II zone is divided into the front edge and its left strongly deformed area, the rear edge and its right strongly deformed area, and the middle strongly deformed area.

Zone I deformation characteristics

  1. a)

    The strong degeneration area at the front edge of Zone I is dominated by the deformation and destruction of the overburden on the slope surface. The material composition of the overburden is broken stone soil. The lithology of the broken stone parent rock is mainly slate, limestone, and sandstone. The deformation zone is eroded by the Zhenggang gully and the Lancang river.

  2. b)

    In the strong deformation zone on the right side of Zone I, four tension fractures and one shear fracture develop at the right boundary of the middle part. Three tension fractures I-1, I-2 and I-3 are inclined to Zhenggang gully. The strikes are approximately 160°. The 1-4 tension cracks intersect with the Zhenggang gully. The I-5 shear cracks develop along the boundary line of the basement in the I area.

  3. c)

    Neither of the two tensile fractures I-6 and I-11 and two shear fractures is open which developed in the strong deformation zone in the middle of Zone I.

  4. d)

    The strongly deformed area collapsed down along with the gully at the rear edge of Zone I, and the elevation is 2510 m. The collapsed area is mainly the overburden of the slope surface. The material composition is broken stone soil. The collapsed section is steeply. It can be seen that the upper overburden and the lower sliding bed are obviously staggered. Compared with the recent strong deformation area, this area has been strongly deformed since the last time. Now it has entered into a relatively stable period (Fig. 3c, e).

Zone II Deformation Characteristics

  1. a)

    The front edge of zone II and the strongly deformed area on the left side are dominated by the overburden on the slope surface. The material composition is mainly crushed rock soil. With tensile cracks, II-1 developed and local collapsed II-9. Multiple nearly parallel secondary tension cracks.

  2. b)

    There are three shear fractures II-2, II-4, II-6, and pull trough II-3 in the back edge of zone II and the strongly deformed zone on the right side. II-2 is located at the rear edge of zone II near the right boundary bedrock ridge, with an elevation of 2500 m, extending in an arc shape, intersecting with the main sliding direction of the landslide, and an angle less than 30°. II-3 elevation is 2560 m, and the main sliding direction of the landslide is 45°. II-4 and II-6 are nearly parallel and perpendicular to the slope direction.

  3. c)

    There are many nearly parallel shear fractures II-5 in the middle of the strong deformation zone of Zone II, with an elevation of 2480 m, obvious signs of shear deformation, step-like, extending direction perpendicular to the main sliding direction of the landslide. The material composition is mainly gravel soil and cultivated soil which is loosely structured (Fig. 3a, d).

4 Numerical Analysis of Zhenggang Landslide Formation and Evolution Process

4.1 Model and Parameters

For the toppling deformation process of steeply inclined layered rock mass study, this paper uses UEDC4.0 version for modeling analysis. UDEC software adopts the discrete element method, which is often used in the research of jointed slopes and tunnel excavation. The modeling process is completed with the aid of Auto CAD software etc. The length of this model is about 1600 m. The height of this model is about 850 m. The boundary displacement is constrained. There is no horizontal displacement on both sides and no vertical displacement at the bottom. The influence of weak joints on the slope is considered in this model. Because the slope rock layer toppling deformation under gravity, the model stress field only considers gravity (Fig. 5).

Fig. 5.
figure 5

Two-dimensional discrete element model of Zhenggang landslide.

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According to the results of on-site investigation and indoor test, referring to similar projects, and comprehensive local experience values, the physical and mechanical parameters of the rock and soil mass of the calculation model are determined as follows (Table 1).

Table 1. Physical and mechanical parameters of Zhenggang landslide.
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4.2 Numerical Calculation Results

  1. (1)

    When the model calculates to 20000 steps, the layered rock mass in the middle of the slope and the front edge basalt show no obvious deformation. The anti-dipping rock layer at the foot of the slope is slightly deformed. The shallow anti-dip layered rock mass at the rear edge develops micro-cracks that are nearly parallel to the plane. The entire rear edge is in a weak toppling creep stage (Fig. 6).

Fig. 6.
figure 6

The degree of toppling deformation at 20000 steps.

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  1. (2)

    When the model calculates to 36000 steps, the tensile cracks at the rear edge of the slope begin to develop. And then the tensile-shear fracture surface formed. The toppling and creep of the anti-dipping rock mass further intensely. The middle layered rock mass begins weakly stretched. The cutting layer also begins to develop. The anti-dipping rock mass at the foot of the slope bends and topples strongly in the direction of the river. The massive basalt on the front edge produces a shear-slip along the existing structural inside planes (Fig. 7).

Fig. 7.
figure 7

The degree of toppling deformation at 36000 steps.

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  1. (3)

    When the model calculates to 82000 steps, there is an obvious relaxation phenomenon between the front basalt blocks. The shear-slip surface tends to be partially connected. The front slope surface develops local slumping. The middle and rear edges rock mass is violently toppled and broken. The fracture surface has tended to penetrate. The scale of the surface cracks has increased significantly. The overburdened rock mass is gradually formed, which belongs to extremely strong toppling (Fig. 8).

Fig. 8.
figure 8

The degree of toppling deformation at 82000 steps.

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  1. (4)

    When the model calculates to 280000 steps, an integrated slip surface has been formed in the bank slope. The whole overburdened rock mass downward slides gradually, and finally forms a landslide. The internal slip surface of the front-edge massive basalt is provided by the self-developed along-slope structural surface and gentle toppling crack. The internal slip surface of the middle-rear edge anti-dipping rock mass is developed from its toppling fracture surface. When the partial slip surface is penetrated, the entire slip surface of the landslide is formed. In addition, the middle and rear edge of the falling overburden developed partially connected fracture surfaces, which provided the possibility for the secondary sliding of the landslide in the later stage (Fig. 9).

Fig. 9.
figure 9

The degree of toppling deformation at 280000 steps.

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5 Discussion

5.1 The Steeply Dipping Layered Rock Mass Toppling Formation Process

  1. 1)

    Slight toppling deformation stage: it was in the early wide valley stage of the Lancang river, with relatively low and gentle valley slopes on both sides of the river.

  2. 2)

    Toppling-bending creep stage: as the river further undercuts, the stress release increases. The rock mass begins to topple and deform toward the empty direction. The layered rock mass that has undergone toppling deformation produces relative displacement along the horizontal direction of the potential shear-slip structure. The bending-tension deformation is intensified. The bending of the rock mass gradually increases and accumulates the tensile strain generated in the layer. Under the combined action of the gravitational bending moment and lateral slip, when the accumulated tensile stress exceeds the tensile strength of the rock mass, it is accompanied by the further dislocation of the interlayer rock mass. The shear fracture surface appears which intensifies the toppling-bending deformation. At this stage, the slope body appears layer-wise tensile failure, and apparently. There are multi-level toppling crack grooves and anti-slope sills.

  3. 3)

    The stage of strong bending and near-through fracture: after valley formation, the unloading effect of the rock formation is enhanced. The shear slip of the cut layer continues to develop. The self-weight bending moment continues to increase. The slope gradually undergoes gravitational deformation toward the horizontal and empty direction. Obvious tensile cracks began to appear in the stress concentration area at the middle and rear of the slope. The toppling deformation has developed to the stage of toppling and toppling fracture stage. The rock strata-undergoes strong shear-slip dislocation along with the layer. At the same time it combines with the dominant structural surface to form a shear-slip surface. Macroscopically, the tensile crack tends to penetrate, and resulting in shear-slip.

  4. 4)

    Slipping instability stage: when the rock strata bend at a large angle in the root, the slope will undergo creep bending and tearing under the action of gravity. The bending and fracture surface will penetrate under the effect of gravity. A penetrating slip surface outside the gently inclined slope will be formed. The slope will slip and lose stability, then forming a giant landslide.

5.2 The Giant Ancient Landslide Formation and Evolution Process

  1. 1)

    In the early stage of toppling deformation, the river is wide and the valley slope is small. The rock mass has almost no obvious toppling deformation.

  2. 2)

    Early toppling deformation, the acceleration of the crustal uplift speed. The intensified river downward cutting, forming high and steep valley slopes. The rock mass began to cause obvious toppling deformation toward the river.

  3. 3)

    The river cutting further accelerates and the rock mass toppling deformation is intensified, forming a series of toppling fracture planes in the rock mass. Strong toppling deformation and toppling overburden area are formed in a certain depth of the slope. The collapsed section of the site is further expanded and penetrated. Eventually, a continuous failure surface is formed, and a landslide occurs.

5.3 The Genetic Mechanism of the Toppling Deformation Body Evolving into a Giant Ancient Landslide

For the Zhenggang landslide, the underlying basalt with a thickness of about 400m should constitute the foundation of the entire slope, which is the resistance body of the overlying and deformed rock mass. However, the shear outlet of the zhenggang landslide is located inside the basalt. The investigation found that the basalt is many shear dislocation zones formed in the early structural process (Fig. 10).

  1. 1)

    The weak layer zone 1 outside the inclined slope is bluish-gray, about 0.5–1 m thick, composed of clay breccia, with a particle size of 1–3 cm, sub-angular, and its bed attitude is 20–30°∠25–30°.

  2. 2)

    The weak layer zone 2 outside the inclined slope is blue-gray, yellow-brown, about 30–40 cm thick, composed of clay breccia, with a particle size of about 0.5–2 cm, and sub-angular.

  3. 3)

    The weak layer zone 3 outside the inclined slope, the bed attitude is 20–30°∠25–30°. The thickness is about 20–30 cm. The upper layer is yellow-brown. The lower layer is blue-grey, composed of clay breccia. The particle size is 0.5–2 cm, second Angular.

  4. 4)

    The weak layer zone 4 outside the inclined slope is about 40–50 cm thick, composed of yellow-brown clay breccia, with a grain size of 0.5–3 cm, sub-angular, and the rock mass between the two layers is extremely broken and loose, with development in between 2–5 cm thick soft layer belt.

Fig. 10.
figure 10

Shearing dislocation zone of adit (a. The weak layer zone 3; b. The weak layer zone 4).

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The existence of the weak zone makes it possible for the overlying toppling and fractured rock mass to appear shear dislocation along the weak zone. However, because these weak layers are not exposed on the surface, their extension length is not completely connected. Therefore, only limited to this, the overturning rock mass may still be insufficient to remove the basalt to form a landslide. Further investigation found that the basalt unloading zone. In addition, two groups of gently-dipping structural surfaces are also developed in the rock body (Fig. 11). (1) The gentle slope outside of the unloading crack (bed attitude 45°∠10°–20°). (2) The gentle slope inside of the unloading crack (bed attitude 240°∠10°–30°). The two groups of unloading cracks all have a gentle dip angle. The tendency of the first group unloading cracks direction is the same as the main sliding direction of the landslide, which provides a good channel for the upper overburdened rock mass to be cut out from the front edge.

Fig. 11.
figure 11

a. The isometric map of the poles of the gently-dipping structural plane in the basalt; b. Rose diagram of basalt gently dipping structural surface strike.

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The formation mode of the giant ancient landslide in Zhenggang is as follows, the upper part of the slope is along the gradually penetrating toppled rock mass. The lower part is along with the soft layer in the basalt along the slope. The front edge is cut out along the gently-dipping unloading fissure. The formation mechanism of the giant ancient landslide in Zhenggang is as follows, the Lancang river is eroding rapidly and formation many high-steep bank slopes. The steep-dip layered rock mass of the bank slope bends and topples towards the valley under the action of gravity and the pressure of the overlying rock mass. With the continuous erosion of the valley, the layered rock mass bending and toppling phenomenon intensified. The rock mass at the rear edge of the bank slope falls strongly. When the toppling deformation develops to a through slip surface in the layered rock mass, the overburdened rock mass will slide along the through slip surface, and finally forms a giant landslide (Fig. 12).

6 Conclusion

Based on the above analysis, the following understanding can be drawn:

  1. 1)

    The giant rocky ancient landslide in the Lancang river is jointly affected by regional structure and valley down-cutting. The weak layer structure is produced by complex tectonic geology. During the valley down-cutting, the structural surface of the slope rock mass is produced by time-dependent creep effects. The front edge erosion leads to the collapse of the steeply inclined layered rock mass.

  2. 2)

    The multi-level down-cutting of the river is closely related to the evolution process of the river. The cutting force along the river makes the slopes on both sides steep, the slope stress redistributes, and the slope surface deformation. The deformation process of the toppling deformation body is divided into, the micro-pumping deformation stage, the toppling-bending creep stage, the strong bending, the cracknear-through stage, and the slipping instability stage.

Fig. 12.
figure 12

Schematic diagram of formation mechanism of Zhenggang landslide.

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  1. 3)

    The steeply inclined layered rock mass structure of the bank slope is the main internal factor for the toppling deformation body evolution into a giant ancient landslide. The down-cutting of the river is the main external factor. The through surface formed by the connection of the toppling fracture zone is the decisive factor for the occurrence of giant ancient landslides. The intensely valley erosion promotes the process of toppling deformation, which leads to the concentration of shear stress on the rock layer. Rainfall infiltration reduces the strength of the weak layer and accelerates the occurrence of sliding. The Zhenggang giant ancient landslide belongs to the slip-shear type.