Formation and evolution mechanism of complex accumulation slope

Quaternary loose deposits are widely developed and distributed in western mountainous areas of China, which is a typical product of internal and external dynamic coupling. This kind of loose deposit is a transitional type between soil and rock, and there was not much contact and research in the past. In particular, it is a special geological body with diverse causes, complex components, disordered structure and mixed accumulation of earth and rock, and its derived geological disasters are random, repetitive and multiple, which has aroused widespread concern in engineering geology, soil mechanics, rock mechanics and other disciplines, and has become a novel and special important research object (Liu Hengqiu et al., 2008). This accumulation body is also very rich in the left bank of Hutiaoxia and has important engineering significance. Before studying its genesis, the regional type and spatial distribution law of this complex accumulation body are discussed first.

4. 1. 1 accumulation body type and distribution law

4. 1. 1. 1 stacked body type

Large-scale complex loose accumulation bodies have various causes and complex components. Its genetic types mainly include landslide accumulation, collapse accumulation, residual slope accumulation, alluvial and flood accumulation, moraine and so on. However, in many cases, these deposits are all mixed together and formed in the same geological period, so it is difficult to distinguish them. In practical engineering activities, the combination of two or more types can not well reflect the differences in structure, degree of cementation, disasters and environmental effects. Therefore, according to the distribution and engineering geological characteristics of loose deposits, according to their formation conditions and characteristics, the deposits are divided into three basic types: valley type, basin type and original surface type (Table 4. 1. 1).

4. 1. 1.2 Development and distribution of valley-type accumulation bodies.

Compared with the other two kinds of accumulation bodies, the valley-type loose accumulation body is more prone to disasters and has greater impact on the environment, which fully shows that the engineering effect of the valley-type loose accumulation body is the most prominent. The formation of valley-type loose accumulation body is a supergene transformation phenomenon in the natural evolution of valley slope, and its development and distribution are closely related to regional crustal uplift, fault activity and lithologic combination.

(1) Relationship between Crustal Uplift and Loose Accumulation Body

The development and distribution of loose accumulation bodies are not isolated, but distributed on both sides of rivers in southwest China, which has its regional formation law. The fundamental reason is that the western region is strongly collided by the Indian Ocean plate and Eurasian plate in Himalayan region, and the power in the crust drives the rapid uplift of the Qinghai-Tibet Plateau, and at the same time promotes the development and strong undercutting of rivers near the plateau, resulting in strong supergene transformation and a large number of loose deposits on both sides of the valley, which provides a good material basis for the formation of landslides. According to incomplete statistics, 90% of the landslides in the Three Gorges reservoir area of the Yangtze River are caused by the sliding of loose accumulations, 67% of the landslides in western Tibet are accumulation landslides, and 70% of the landslides in Hutiaoxia Valley area of Jinsha River are loose layer landslides (He Jianmin, 2004; Liu Hengqiu et al., 2006). Almost without exception, these areas belong to areas with large terrain elevation difference and easy to suffer from strong erosion, and loose deposits are strongly developed.

Table 4. 1. 1 Basic types of large complex loose deposits

In addition, in the field investigation in Hutiaoxia area, it is found that the development degree of loose accumulation in the canyon area is much higher than that in the wide valley area at the upper reaches of the canyon, both in quantity and scale. The maximum cutting depth of Hutiaoxia Valley is 3,500m, and the average crustal uplift rate since 93,000 years in the early Late Pleistocene is about 2.25 ~ 3.09mm/year. The wide valley section has been in intermittent uplift state since 80900, with a small uplift range, and the historical average uplift rate is about 0.89 mm/a; Due to the difference of crustal uplift rate, the development scale and quantity of loose accumulation bodies in canyon section and wide valley section are very different. The loose deposits in the canyon section are huge (for example, the total volume of two family deposits is about 1.8× 108m3), which are basically distributed continuously along the river bank, while the loose deposits in the wide valley section are relatively small (up to several hundred cubic meters) and scattered. Large-scale loose accumulations (with an area of more than 0.5km2) in the Three Gorges reservoir area are basically distributed in the canyon area (Qutangxia-Wuxia section), and so are the banks of the Palong Zangbo River, indicating that the development degree (including scale and quantity) of loose accumulations is positively correlated with the crustal uplift rate, indicating that the higher the crustal uplift rate, the easier it is to form high and steep slopes under the action of rivers, and the stronger the lateral unloading effect of river erosion.

(2) The relationship between fault activity and loose accumulation.

In the actual investigation, it is found that loose accumulation bodies are often distributed along fault lines, and their spatial correspondence shows that fault activity controls the distribution pattern of loose accumulation bodies. Fault dislocation is the result of dynamic action in the earth's crust, which easily leads to local tectonic stress concentration, resulting in a certain size and change of displacement field and deformation field around the fault zone. With the continuous accumulation of deformation, the geological body will rupture and its continuity and integrity will be destroyed. The long-term activity of fault is the continuous power source of fault zone and its surrounding geological bodies. Therefore, the rocks where faults pass are generally very broken, which can provide a large number of material sources for the formation of loose accumulation bodies. For example, the new site of Wushan immigrants in the Three Gorges reservoir area, Baotaping in Fengjie and Hutiaoxia in Jinsha River all suffered from strong fractures, which led to the fragmentation of rock mass and the formation of large complex loose accumulations. However, more than 90% of the reservoirs in Hutiaoxia reach were formed within 500m from the fault (Liu Hengqiu, 2006).

(3) the relationship between lithologic association and loose accumulation body

Loose deposits are mainly developed in the slope zone, especially the compound slope with steep upper part and gentle lower part, which is most beneficial to the development and distribution of loose deposits. Generally, the lower part of the slope is below 25, and the steep part of the upper part is above 38, such as Bomi area of Sichuan-Tibet Highway, Longpan area of Jinsha River and Liangjiaren area. This is closely related to the types of rocks that make up the slope, and the lithologic combination of hard top and soft bottom is the best condition for forming this kind of slope and developing large loose accumulation bodies (Yin Yueping, 2000; Zhang, 200 1). On the one hand, the upper hard and brittle rocks, such as limestone or argillaceous limestone, are affected by fault dislocation, and the terrain is steep, and the rock mass is more easily broken. Different types of slope failure modes can form a large number of loose deposits; On the other hand, under the action of rivers (lateral erosion), rocks with weak lithology in the lower part are easy to form a wide and flat platform, which is conducive to the occurrence of a large number of foreign substances. For example, the upper part of the slope of the Jinsha River is upper Devonian marble, and the lower part is schist and phyllite; The "easy-to-slip stratum combination" of loose accumulation slope and Baotaping landslide in fengjie county in the Three Gorges reservoir area is soft (T2b2) and hard (T2b3), in which T2b2 is calcareous mudstone in the second member of Badong Formation, and T2b3 is medium-thick argillaceous limestone and dolomite limestone in the third member of Badong Formation.

4. 1.2 analysis of slope formation mechanism of typical accumulation body

4. 1.2. 1 Genetic mechanism of two families of large loose accumulations

(1) Basic geological background

The accumulation body is located on the left bank of Jinsha River Valley, near Liang Jiaren Village of Hutiaoxia, about 2km downstream of the proposed Shanghutou dam site. Here, the river flows to the northeast, with narrow channels and asymmetrical V-shaped valleys. The slope of the right bank is steep, and the terrain slope is greater than 60; The slope on the left bank is steep and gentle, and the upper cliff is controlled by faults. The slope is generally above 70, the lower part is gentle, and the average slope angle is about 25. The topography on the left bank is high in the west and low in the east. The elevation of Jinsha River at the east valley bottom is about 1800m, and the elevation of the first bank slope at the rear edge is above 3500m: the terraces on both banks are extremely undeveloped, which belongs to the typical gravity erosion landform of alpine valleys.

The accumulation position is in the south wing of Yulong-Baha 'i anticlinorium, and the underlying bedrock is composed of gray sericite schist, mica quartz schist, dark gray-black gray sericite schist and sericite phyllite with unknown age. It is a set of medium-range metamorphic rocks composed of sandstone and mudstone (M), and the rock mass occurrence is NW 350 ∠ 45; Above 2 100 meters above sea level, the upper Devonian white marble (D2) is developed with vertical joints and fissures. There is fault contact between marble and schist, which is a tiger leaping stone fault and a compression fault. The overall fault occurrence is Ne5 ~ 10 ∠ 65. Here, the slope layer structure at the edge of bedrock belongs to the dual structure of hard top and soft bottom, which is steep and anticline.

This area belongs to the continental plateau climate zone affected by monsoon, with distinct dry and wet seasons and significant vertical temperature changes. The area below 2000m is a dry-hot valley with annual precipitation less than 700mm, and the precipitation from May to flood season 10 accounts for 80% of the annual precipitation. The middle and high mountains at an altitude of 2000 ~ 3000m have a warm climate, with wet rainy season and sunny dry season. The alpine region above 3000 meters above sea level has a cold climate, and it often snows and freezes. The groundwater type is mainly basalt fissure water, and the recharge mainly comes from atmospheric precipitation and glacial meltwater.

(2) Morphology and substance composition

The two sides of the accumulation body are bounded by two gullies and dam gullies, which are distributed between 2000 and 2500 m above sea level, with the maximum height difference of nearly 600m m, and the plane shape is strip-shaped extending along the river valley direction (Figure 4. 1. 1), with a total area of about 0.69km2. The accumulation body is thin from top to bottom, with an average thickness of about 80m in the middle.

Fig. 4. 1. 1 Two families of loose accumulation bodies.

Fig. 4. 1.2 geological profile of two families of loose accumulation bodies Ⅰ-Ⅱ.

The accumulation body is composed of Quaternary loose or loose rock and soil, with gravel or debris mixed with fine-grained soil (Figure 4. 1.3) and scattered huge stones, and the rock-soil ratio is generally 3∶7 or 4∶6. The composition of crushed (blocky) stone is mainly limestone, which mainly comes from D2 hard crystalline limestone and marble stratum at the rear edge. The rock is slightly weathered to moderately weathered, with a diameter of 0. 1 to 1.0m, and the maximum diameter can reach 10m. Fine-grained soil is mainly clay.

Fig. 4. 1.3 Material composition of loose accumulations of two families

(3) Structure and deformation characteristics

The internal structure of the two accumulations has basically disintegrated, showing a state of broken rock accumulation, with pores between the stones, or local filling is not dense, with strong water permeability. There are scattered outcrops with large volume in the range of accumulation body, and they are all boulders in the accumulation body formed by highway excavation. The rock attitude of these stones varies greatly, and most of them are inconsistent with the attitude of the underlying bedrock. Surface water erosion is very strong, and many large gullies are developed on the surface of the accumulation body (see Figure 4. 1.4), which are generally V-shaped, with a width of 10 ~ 40m and a depth of 10 ~ 30m. The structure of this accumulation body is loose and disorderly, and the relative sequence relationship of limestone strata can be seen faintly in some areas, which shows the complexity of its genetic types. On the vertical section, there are layers from bottom to top, that is, the lower rock mass is mainly composed of huge rubble, even retaining the characteristics of bedrock; The upper rock and soil mass is mainly fine-grained soil containing gravel, which shows debris flow; The middle rock mass is between the two, reflecting the heterogeneity of material distribution (Figure 4. 1.2).

According to the field investigation, there is no obvious tensile crack on the rear edge and surface of the giant accumulation body of two families, no tensile crack on the wall of the house, no bulging deformation on the slope in the middle and lower part, and only signs of local sliding and collapse can be seen at the edge of the gully; The front edge of the accumulation body is along the road line, and the slope collapses and slides due to road excavation (Figure 4.1.5); Especially in rainy season, expressways are often blocked by collapses and landslides. It is worth noting that this local collapse and landslide is caused by human engineering activities. The structure of accumulation body is loose, and the shallow landslide reconstruction is frequent in the later stage. 1992 The deformation and failure after the rain in July led to landslides, with stones overhead in the sliding body, good steep slope conditions and relatively complicated hydrogeological conditions. Under the action of external factors such as rainstorm and strong earthquake, the accumulation body slides down and blocks the water-flowing section of Jinsha River (Xia Jinwu et al., 1997).

(4) Genetic types of accumulation bodies

The two family accumulations were formed under complex and special climatic, geological and geomorphological conditions. According to detailed field investigation and drilling data, the main manifestations are as follows: ① there is no continuous main sliding zone similar to landslide between accumulation body and bedrock; (2) On the profile, from bottom to top, there is a gravel-gravel-fine soil with gravel accumulation sequence; The surface rock and soil mass is very broken and strongly weathered, in the form of debris flow movement, belonging to weathered colluvial accumulation; ③ The relative sequence relationship of limestone strata can be seen in the local area of accumulation body, reflecting the characteristics of local decline; ④M schist has fault contact with D2 marble limestone. Affected by tectonic dislocation, the rock mass is broken and prone to gravity collapse and block displacement. Loosely accumulated blocks are the product of collapse, bedding disappears and the arrangement of blocks is disordered. ⑤ There are scattered river sand pebbles at the front edge of the accumulation body. As for the multi-solution characteristics of this genetic type, this paper holds that the large-scale loose accumulations of these two families are not simple sliding types, nor are they simply accumulations formed by gravity collapse or collapse, but composite geological bodies composed of colluvial deposits, landslide deposits, weathered slope deposits and river deposits (Liu Hengqiu et al., 2005).

Fig. 4. 1.4 schematic diagram of gullies developed in the accumulation body

Figure 4. 1.5 Collapse and landslide caused by highway excavation

(5) Genetic mechanism analysis: the regional neotectonic movement is strong, characterized by intermittent rapid uplift, which leads to the alternating enhancement of river undercutting and lateral erosion and becomes an important driving force for the evolution of river valley slopes. The lower part of this valley slope is composed of weak rock layers (including schist and crushed limestone in Hutiaoshi fault zone), which is easy to form a wide and gentle platform under the lateral erosion of rivers, providing favorable spatial conditions for the occurrence and accumulation of foreign substances, and at the same time, river scouring will form a fluvial sandy pebble layer, which will be partially preserved in the pile.

Integrating the bottom of the front increases the complexity of the genetic types of accumulation bodies. The precipice at the rear edge of the accumulation body is controlled by the Chubo-Baihanchang fault. Under the action of rapid crustal uplift, the fracture activity is intensified, which causes the rock mass near the fault zone to rupture and form a large number of joints or cracks, which leads to the decrease of rock mass strength and stiffness. Moreover, the appearance and increase of cracks are more conducive to weathering and softening of water, which accelerates the instability and failure of slopes. Because the limestone in the upper part of the slope belongs to the rock mass with medium-high modulus and high strength, its energy storage condition is very good, and its rebound deformation is large when stress is released, which is prone to deformation and fracture. In addition, the vertical displacement caused by fault activity leads to a great height difference between the fault zone and its surrounding terrain. With the continuous activity of faults, the slope is steeper (the current slope is above 70) and the free surface is higher (1000 ~ 1200m). As a result, the effect of gravity on the slope is strengthened, which leads to the deformation and failure of the slope. Under the dynamic geological action of precipitation, earthquake and so on, the broken rock mass on the steep cliff is prone to gravity collapse or rock mass toppling and sliding, and it is a loose accumulation body at the lower part of the slope.

Strata provides a large number of material sources, and after a long geological history evolution, a large and complex loose accumulation body has been formed. The two accumulations are huge in scale and the formation process is very long. In this process, internal and external dynamic actions such as crustal uplift, lateral river erosion, fault activity, earthquake, precipitation and weathering all affect and control the formation of accumulation bodies to varying degrees. Among them, crustal uplift plays an important role, which affects the way and intensity of other dynamic geological processes to a certain extent. For example, crustal uplift intensifies fault activity and the development of rock mass structural planes, thus contributing to weathering. Although each dynamic function plays a different role, in essence, this large accumulation body is the product of alternating or parallel internal and external dynamic coupling (Liu Hengqiu et al., 2005). Based on the basic environmental conditions and main dynamic geological processes of accumulation bodies, this paper holds that the formation of large loose accumulation bodies of these two families is basically controlled by three basic conditions, namely, intermittent uplift of the crust and lateral cutting of rivers, which is conducive to the formation of free surfaces in the direction of river valleys; The lithology is weak, and it is easy to form a wide and gentle platform under the lateral erosion of rivers, which provides favorable reservoir space for the formation of accumulation bodies; The mountain at the rear edge is steep, the rock mass strength is high, and the rebound deformation is large when the stress is released. Affected by fault activity, rock mass is broken, which can provide a large number of material sources and provide material basis for the formation of accumulation bodies. The genetic mechanism can be summarized as shown in Figure 4. 1.6. The two groups of accumulations are representative in the left bank of Hutiaoxia, and the study of their genetic mechanism is of reference significance to the formation and evolution of loose accumulation slopes in the canyon area.

Fig. 4. 1.6 Formation mechanism of loose accumulation bodies of two families

4. 1.2.2 Formation mechanism of slide accumulation landslide

Basic characteristics of (1) landslide

Huashiban landslide is located at the side of Daju Township, Haba Snow Mountain in Xiahutiaoxia, less than 2km downstream of the proposed dam site in Xiahutiaoxia, with geographical coordinates of east longitude 100 18 and north latitude 27 2 1. Average width of landslide 150m, axial length 300m, thickness 30m, total volume 135x 104m3. The last landslide occurred on1October 28th, 1996, 10. The landslide body falls more than 300 meters and rushes into the Jinsha River, forming a natural rockfill dam in the valley, resulting in the river channel cut off (Figure 4. 1.7), and some landslide debris bodies rush to the other side of the Jinsha River and climb nearly 100m (Tang Chuan et al.).

The terrain where the landslide is located is high in the northwest and low in the southeast. The height of the first crack point on the steep slope is about 2500m, and below 2000m, there is a gentle slope ditch, in which landslides accumulate (Figure 4. 1.9). The plane shape of the landslide is broom-shaped, and it contracts upward along the groove. The rear edge is about 2000m above sea level and extends to the valley to an altitude of about1650m ... The axial direction of the landslide is S70°E, and the overall slope is about 24 (Figure 4. 1. 10). The residual gravel and soil of the landslide can be seen on the steep wall at the trailing edge.

The material composition of landslide body is mainly rubble formed by landslide accumulation, which contains (contains) a large amount of gravel and a small amount of soil, and the soil-rock ratio is 3∶7 or 4∶6. Gravel composition is mainly limestone, followed by argillaceous limestone. Broken (block) stones are weathered to moderately weathered, ranging in diameter from a few millimeters to dozens of centimeters, usually a few centimeters. The material of landslide mainly comes from the broken stones falling from the steep wall of the cliff at the rear edge. Due to the high content of broken stones, low specific gravity of soil, loose slope structure and low degree of cementation. Controlled by gravity separation, the accumulation rhythm of fine top and thick bottom can be seen; Soil-rock mixed accumulation body has large porosity, strong water permeability and uneven. Surface water erosion is very strong, and longitudinal gullies are developed on the landslide surface, with a depth of about tens of centimeters.

Fig. 4. 1.7 Landslide leads to the cutoff of Jinsha River.

Figure 4. 1.8 Landslide and debris flow rushed to the other side of the valley.

Fig. 4. 1.9 development and distribution map of skateboard landslide

The bedrock exposed on both sides of the landslide belongs to two strata of different ages, which constitutes the lateral limit surface of the landslide. On the south side, it is Carboniferous gray marble with an occurrence of 92 ∠ 45. There are fractures in the rock mass, and the fracture surface is two groups of joints, with the occurrence of 70 ~ 80 ∠ 60 ~ 75 and 330 ~ 340 ∠ 70 ~ 80 respectively. On the north side is Permian gray marble, whose strike is basically the same as Carboniferous marble, with an inclination of about 20. There is fault contact between the two sets of strata, and the Permian marble overthrusts on the Carboniferous marble. Under the influence of fault compression, folds in rock mass develop and pinch out along the slope. The sliding layer is the interface between slope deposit and chlorite schist (Figure 4. 1. 1 1).

(2) Analysis of landslide formation mechanism.

1) Basic conditions

Quaternary loose deposits, such as colluvial deposits, weathering and unloading deposits, residual slope deposits and alluvial deposits, commonly exist along the banks of the river in southwest China. These loose deposits are the transitional medium between soil and rock, and become "sliding bodies" because of their low mechanical strength and poor stability (Liu Hengqiu et al., 20 10). As a typical example of loose accumulation landslide in this area, the basic conditions for its formation include:

A. the special terrain provides favorable conditions for landslides.

Intermittent uplift of the new structure, coupled with long-term slow river erosion, forms a special compound slope with steep uphill and gentle downhill on the left bank of the region, which is generally lower than 25 and steeper than 38. This special slope is beneficial to the development and distribution of landslide deposits and provides favorable conditions for the formation of loose accumulation landslides.

Fig. 4. 1. 10 plane schematic diagram of loose accumulation landslide of sliding slate.

Fig. 4.1.11Ⅰ-Ⅱ slide landslide geological profile.

B. Landslide is the main carrier of landslide.

Under the influence of fault dislocation, the steep rock mass on the cliff at the rear edge has developed cracks and strong weathering, and it is easy to collapse under its own weight and accumulate in the lower gentle slope zone. Due to the loose structure, the stability of the deposit itself is poor. After reaching a certain thickness, the deposit has enough sliding force due to its own weight. Therefore, the accumulation of landslide deposits is the premise of landslide, and a large number of loose deposits become the main carrier of landslide.

C. The sliding control surface of landslide is formed by soft rock.

Weak rock stratum is an unfavorable internal condition for landslide formation. The bottom plate of loose accumulation body of sliding slate is chlorite schist with weak lithology, which is easy to soften and argillaceous when it meets water, and its water permeability is extremely poor, so it is easy to slope, which is not conducive to the stability of loose accumulation body. The contact surface between schist and accumulation body is the collection and runoff zone of shallow groundwater, and the water pressure of slope increases rapidly during rainfall, which greatly reduces the shear strength of schist. Chlorite schist is the main weak structural plane in this area, and it is easy to form the control sliding plane of landslide, which is an important factor leading to slope instability.

D. control of fault structure on accumulation body formation

There is a reverse fault F on the slope of the sliding plate, and the Permian marble overthrusts on the Carboniferous marble (Figure 4. 1. 12). This fault belongs to the first structural plane in the study area. On the one hand, it plays a decisive role in the formation of local main geomorphic units, and combined with the scouring effect of water flow, it forms a free surface in the direction of river valley. High gravity potential energy creates basic motion conditions for rock caving. On the other hand, faults play the role of crushing rock mass, and the rock is loose and broken. Under the action of its own weight, a large number of broken rocks are continuously provided under the slope and accumulated in the gentle slope below, forming a loose accumulation layer, which provides a material source for landslides. Fault structure plays a direct role in controlling the formation of loose accumulation landslide, and its activity determines the form and scale of deformation and failure of loose accumulation in landslide.

Fig. 4. 1. 12 Thrust faults are developed on the steep slope at the upper edge of the accumulation body.

(2) Analysis of internal and external dynamic action of accumulation body sliding.

The Quaternary colluvium in the slope zone is loose in structure and easy to deform and destroy. It is very sensitive to internal and external dynamic effects (rainfall, earthquake and human engineering activities, etc.). ), and it is easy to change its stable state due to erosion, erosion, infiltration and earthquake caused by rainfall.

A. the triggering effect of rainfall on landslides

The topography of landslide area is very beneficial to the confluence and infiltration of surface water. The colluvium has loose structure and large porosity, and atmospheric rainfall can directly penetrate into the slope. Because the bottom plate of the accumulation body is a layered chlorite schist with poor water permeability along the slope, the groundwater is difficult to seep down, so it can only move in the slope body, forming accumulated water in the upper part, generating strong hydrodynamic pressure and pore water pressure, and the mechanical strength of the interface between the slope body and schist decreases, leading to the downward landslide of the slope body. There is a 5-month rainfall period before the landslide (from early June to1early October 10). According to the statistics of rainfall data of Iunco Meteorological Station (altitude1920m) from September 20th to June 27th+0996, the total rainfall is 165.2mm (Figure 4.66). Therefore, rainfall is the main factor causing landslides.

Figure 4.1.131996.9.20 ~1996.10.26 daily rainfall statistics.

B. Influence of earthquake on landslide stability

The latest earthquake activity in the study area was1Lijiang earthquake with MS=7.0 on February 3, 1996. The landslide area was about 25km away from the epicenter and the earthquake intensity was Ⅷ (han xinmin et al., 1997). Under the repeated action of short-term sudden earthquake force, the earthquake has obviously disturbed the structure of landslide accumulation. Although it has not obviously caused the accumulation body to slide, the stability of the accumulation body will be greatly reduced, and the whole accumulation body is in an unstable state near the slope.

Tang Chuan et al. (1997) called the skateboard landslide as an earthquake delayed landslide, but judging from the influence of rainfall and earthquake, the maximum daily rainfall in the month before the landslide was 60.5 mm (10.4), and the rainfall intensity was not very great. Therefore, we believe that landslides are induced by the dual effects of rainfall and earthquakes. The ground motion causes the slope of accumulation body to be deformed and destroyed, and the stability is reduced, which is close to the unstable state. When the rainy season comes, rainwater permeates along the pores, the additional load of the accumulation body increases, the water softens the schist, and the pore water pressure increases rapidly, resulting in the overall sliding of the accumulation body.

(3) landslide evolution process

According to the development characteristics of sliding slate accumulation landslide, the formation of this landslide has experienced the process of accumulation first and then sliding. The whole process of its evolution is extremely complicated, but it is essentially a contradictory and unified process of anti-sliding force F and sliding force F' (landslide caused by the balance destruction of sliding force and sliding force). Therefore, in order to simplify the conceptual model of the evolution process, the formation and evolution process of landslide is divided into three stages (Figure 4. 1. 14).

1) Gravel accumulation stage. The caving on the steep wall of the sliding cliff is loaded on the rear edge accumulation body, and some of them roll down the slope and accumulate in the lower landslide zone, and the thickness gradually increases; Because the colluvial accumulation body is a mixture of earth and rock, its structure is loose and it is in an unstable state. The greater the thickness of the accumulation body, the worse its stability. The sliding force f' of the potential sliding body increases gradually, but the overall anti-sliding force f is greater than the sliding force f'. Although some parts are deformed, the whole is still in a relatively stable state.

2) Natural resting state. The loose accumulation body reaches enough thickness, and the whole body is in an unstable critical equilibrium state, which is very sensitive to external disturbances. At this time, the anti-sliding force f is basically equal to or slightly greater than the sliding force f', and the potential sliding bodies accumulate on the slope with a natural angle of repose of 38 ~ 40.

3) Significant sliding stage. The shear strength of potential sliding surface will be reduced due to unfavorable factors (such as heavy rainfall), the sliding resistance will be reduced, and unfavorable factors such as water pressure will increase the sliding force. When the overall anti-sliding force f is less than the sliding force f', the accumulation body develops from peristalsis to accelerated movement, and when the displacement develops and accumulates to a certain extent, the overall sliding will suddenly occur. After sliding, the thickness of loose accumulation body becomes smaller, and the sliding force drops sharply due to the continuous adjustment of its own organization. After that, the sliding resistance is greater than the sliding force, and the slope body enters a new round of caving and accumulation stage in the gradual transition from instability to stability. After continuous and long-term accumulation, after reaching sufficient thickness or critical equilibrium state, similar damage and unstable sliding will occur under the change of environmental factors.

Fig. 4. 1. 14 Schematic diagram of the formation and evolution process of skateboard landslide