What are the causes of landslides?

1. Definition and composition of landslide

The phenomenon that the rock and soil on the slope slides down along a weak surface (or weak zone) due to various reasons is called landslide, commonly known as "upward sliding", "mountain collapse", "ground sliding" and "soil sliding".

The main components of landslide are:

Landslide refers to the whole sliding part of landslide, referred to as sliding body for short;

Landslide wall refers to the exposed wall-like interface after the trailing edge of landslide is separated from the static mountain;

Sliding surface refers to the interface where the landslide slides along the underlying immovable rock and soil, which is referred to as sliding surface for short;

Sliding zone refers to the fracture zone where parallel sliding surfaces are folded and sheared, which is referred to as sliding zone for short;

Landslide bed refers to the underlying immobile rock and soil attached when the landslide slides, which is referred to as sliding bed for short;

Landslide tongue refers to the tongue-shaped protruding part of the front edge of landslide, referred to as sliding tongue for short;

Landslide step refers to a stepped staggered platform formed on the surface of landslide body due to the difference of sliding speed of various rocks and soils when landslide body slides;

Sliding boundary refers to the boundary between landslide and surrounding immobile rock and soil on the plane;

Landslide depression ~ refers to the groove or closed depression with low middle and high sides formed by the landslide body and landslide wall being pulled apart during the sliding process;

Landslide uplift refers to the hill where the front of landslide rises due to resistance;

Landslide cracks refer to a series of cracks produced in the landslide body and its edge during the landslide activity. The cracks located in the upper (rear) part of the landslide are called tensile cracks. Located on both sides of the middle part of the sliding body, the boundary between the sliding body and the non-sliding body is called shear crack; Shear cracks are often accompanied by pinnate cracks on both sides, which are called pinnate cracks; The tension cracks formed by the uplift of the front part of the landslide due to sliding obstacles are called drum tension cracks; Fan-shaped cracks are located in the middle and front of the landslide, especially at the sliding tongue.

The above landslide elements only exist in newly developed landslides, not all landslides.

2. The main conditions of landslide and the influence of external incentives.

The main conditions of landslide are: first, geological conditions and geomorphological conditions; The second is the influence of internal and external forces (forces) and human actions. The first condition relates to the following aspects:

(1) Rock and soil type: Rock and soil are the material basis of landslide. Generally speaking, all kinds of rock and soil are likely to form landslides, among which the rock and soil with loose structure, low shear strength and poor weathering resistance will change their properties under the action of water, such as loose overburden, loess, red clay, shale, mudstone, coal measures strata, tuff and schist.

(2) Geological structural conditions: Only when the rock and soil forming the slope are separated into discontinuous states by various structural planes can it slide down. At the same time, the structural plane provides a channel for water flow such as rainfall to enter the slope. Therefore, slopes with various joints, cracks, bedding planes and faults, especially those with steep dip angles of parallel slopes and vertical slopes and gentle dip angles along slopes, are most prone to landslides.

(3) Topographic and geomorphological conditions: Landslides can only occur if they are located in a certain landform and have a certain slope. Generally, it refers to the slopes of rivers, lakes (reservoirs), seas and ditches, slopes with wide front edges, railways, highways and engineering buildings. They are all landforms that are prone to landslides. The slope is greater than 10 degrees and less than 45 degrees, steep downward, gentle upward and steep.

(4) Hydrogeological conditions: Groundwater activities play a major role in the formation of landslides. Its main functions are: softening rock and soil, reducing its strength, generating hydrodynamic pressure and pore water pressure, cutting down rock and soil, increasing its bulk density and generating buoyancy on permeable rock. Especially, the softening and strength reduction of sliding surface (belt) are the most prominent.

As far as the second condition is concerned, the area with frequent crustal movement and human engineering activities is a landslide-prone area, and external factors and functions can change the basic conditions of landslide occurrence, thus inducing landslide. The main inducing factors are: earthquake, rainfall and snow melting, scouring and soaking of surface water, and continuous erosion of slope toe by surface water bodies such as rivers; Unreasonable human engineering activities, such as excavation of the toe of the slope, stacking on the upper part of the slope, blasting, reservoir water storage (drainage), mining, etc. , can induce landslides, as well as tsunamis, storm surges, freezing and thawing.

3. Main factors of landslide dynamic strength

The active intensity of landslide is mainly related to the scale, sliding speed, sliding distance, accumulated potential energy and action of landslide. Generally speaking, the higher the position, the larger the volume, the faster the landslide moves and the farther the moving distance, the higher the activity intensity of the landslide and the greater the harm. Specifically, the factors that affect the intensity of landslide activity are:

(1) The greater the slope and elevation difference of the terrain, the greater the potential energy of the landslide and the higher the sliding speed of the landslide. The openness of the terrain in front of the slope has a great influence on the slip distance. The wider the terrain, the greater the sliding distance. The width has a great influence on the sliding distance. The wider the terrain, the greater the sliding distance.

(2) The higher the mechanical strength, the more complete the rock and soil mass that makes up the landslide, and the less landslides there are. The properties of rock and soil that constitute the sliding surface of landslide directly affect the sliding speed. Generally speaking, the lower the mechanical strength of the landslide surface, the higher the sliding speed of the landslide.

(3) The more geological structures cut and separate slopes, the larger the landslide scale.

(4) The stronger the external factors inducing landslide activity, the greater the intensity of landslide activity. For example, landslides induced by strong earthquakes and heavy rains are mostly large-scale high-speed landslides.

In a word, the activity intensity of landslide is the result of many factors.

4. Relevant factors affecting the temporal and spatial distribution of landslides and their laws.

The active time of landslide is mainly related to various external factors that induce landslide, such as earthquake, cooling, freezing and thawing, tsunami, storm surge and human activities. Generally speaking, there are the following rules:

(1) At the same time, some landslides move immediately after being affected by induced factors. For example, when there are strong earthquakes, rainstorms, tsunamis, storm surges and unreasonable human activities, such as excavation and blasting, a large number of landslides will occur.

(2) Lag phenomenon. Some landslides occur later than the inducing factors, such as rainfall, snowmelt, tsunami, storm surge and human activities. This lag law is most obvious in rainfall-induced landslides, which mostly occur after heavy rain, heavy rain and long-term continuous rainfall. The lag time is related to the lithology, structure and rainfall of the landslide. Generally speaking, the looser the landslide, the more developed the cracks and the more rainfall. The shorter the lag time. In addition, landslides that occur after artificial excavation at the foot of the slope, stacking and reservoir storage and drainage also belong to this category. The lag time of landslide induced by human activities is related to the intensity of human activities and the original stability of landslide. The greater the intensity of human activities, the lower the stability of landslides and the shorter the lag time.

5. The spatial distribution of landslides is mainly related to geological factors, climate and other factors. Generally, the following areas are prone to landslides:

(1), bank slopes of rivers, lakes (reservoirs), oceans, ditches, canyon areas with large topographic differences, slope sections of mountains, railways, highways and engineering buildings, etc. These areas provide favorable topographic and geomorphological conditions for the formation of landslides.

(2) Geological structural zones, such as fault zones and seismic zones, are usually in areas where the seismic intensity is greater than 7 degrees, and slopes with slopes greater than 25 degrees are prone to landslides in earthquakes; Rock mass fragmentation and fracture development in the fault zone are very conducive to the formation of landslides;

(3) The existence of loose overburden, loess, mudstone, shale, coal-bearing strata, tuff, schist, slate, phyllite and other easy-to-slip (slope) rocks and soils provides a good material basis for the formation of landslides;

(4) Rainstorm-prone areas or areas with unusually heavy rainfall. In these areas, abnormal rainfall provides favorable inducing factors for landslides.

The overlapping area of the above zones forms a densely developed landslide area. For example, China, from Taihang Mountain to Qinling Mountain, through western Hubei, Sichuan and Yunnan to eastern Tibet, is such a typical area, where landslides occur with high density and are very serious.

6. Main factors of landslide induced by human activities

Human activities that violate natural laws and destroy slope stability conditions will induce landslides. For example:

(1) Excavation of slope toe: construction of railways, highways, houses and factories built by mountains, etc. Because the lower part of the slope loses its support, it often leads to landslides. For example, some railways and highways in the southwest and northwest regions of China, due to vigorous blasting and forced excavation during construction, landslides occurred one after another, which brought harm to highway construction and operation.

(2) Water storage and drainage: overflow of ditches and ponds, discharge of industrial water and wastewater, agricultural irrigation, etc. It is easy to make water flow into the slope, increase pore water pressure, soften rock and soil, and increase the bulk density of the slope, thus promoting or inducing the occurrence of landslides. The sudden change of reservoir water level increases the hydrodynamic pressure of slope, and also causes landslides induced by slope and bank slope. It can't bear too much weight.

In addition, the blasting action of mountain mining can make the rock and soil on the slope be broken by vibration, resulting in landslides; Deforestation on the hillside makes the slope unprotected, which is conducive to the infiltration of rainwater and other water bodies, thus inducing landslides. If the above-mentioned human actions are combined with unfavorable natural actions, it is easier to promote the occurrence of landslides.

With the development of economy, more and more natural slopes are damaged by human engineering activities, so landslides have occurred more and more frequently in recent years, and there is a growing trend, which should be paid attention to.

7. Abnormal (precursor) phenomenon before landslide.

Different types, properties and characteristics of landslides will have different abnormal phenomena before sliding, showing signs (precursors) of landslides. Generally speaking, they are as follows:

(1) Before the landslide occurred, at the toe of the front slope of the landslide, the spring water that had been blocked for many years revived, or the spring water (well water) suddenly dried up, the water level of the well (borehole) suddenly changed and other similar abnormal phenomena appeared.

(2) In the landslide, transverse and longitudinal radial cracks appear in front of it, reflecting that the landslide has been pushed forward and blocked, and has entered a state of imminent sliding.

(3) Before the large-scale sliding, the soil bulged at the toe of the front slope of the landslide, which is an obvious forward-pushing phenomenon of the landslide.

(4) Before the landslide, there was a sound of rock breaking or being squeezed by shear. This phenomenon reflects deep deformation and rupture. Animals are very sensitive to this and have abnormal reactions.

(5) Before the landslide, the rock (soil) around the landslide will collapse and relax on a small scale.

(6) If there are long-term displacement observation data in the landslide, the horizontal displacement and vertical displacement have an accelerated change trend before the large slide, which is an obvious sign of impending slide.

(7) The cracks at the trailing edge of the landslide expand rapidly, and hot air or cold air gushes out from the cracks.

(8) Before the landslide, the animals in the landslide were in abnormal panic, and the plants were abnormal, such as pigs, dogs, cows, etc., fidgeting, unable to sleep, rats scurrying around, trees withered or skewed, etc.

8. Identification method of landslide stability

In the field, the stability of landslide can be roughly judged from a macro perspective according to some external signs and characteristics.

The old landslide with stable accumulation layer has the following characteristics:

(1) The back wall is high, covered with trees, without scratches, and stable;

(2) The landslide platform is wide and flat, with dense soil and subsidence;

(3) The front slope of the landslide is steep, the soil is dense, the trees are covered, and there is no loose collapse. The part of the leading edge facing the river has been washed by the river;

(4) At present, the river is far away from the tongue of the landslide, and even there are floodplains and terraces outside the tongue;

(5) The natural scouring ditches on both sides of the landslide cut deeply, even reaching the bedrock;

(6) There are clear springs flowing out from the tongue bottom of the landslide.

9. Unstable landslides usually have the following signs:

(1) The overall slope of the landslide is steep and extends for a long time, with uneven slope;

(2) There is a small landslide platform with gentle downward dip and unevenness;

(3) There are springs, wetlands and new gullies on the surface of the landslide;

(4) There are uneven local platforms on the landslide surface.

(5) The front of the landslide is loose, and small-scale collapse occurs from time to time, which is in danger of being washed away by the river;

(6) There are no huge upright trees on the landslide.

(6) There are no huge upright trees on the landslide.