"1" Zhaojiagou landslide in Gaopo Village, Zhenxiong County, Zhaotong, Yunnan Province

201311At 8: 20 am, a high landslide occurred on the south side of Zhaojiagou villager group in Gaopo Village, Zhu Guo Township, Zhenxiong County, Yunnan Province. The landslide is located in the northeast of Yunnan Province, and its geographical coordinates are 27 33' 5 "north latitude and104 59'15" east longitude. The total volume of landslide is about 2 1 1,000m3. After disintegration, the landslide moved rapidly. Most landslides slide to low-lying slopes and accumulate along the way. Part of the landslide moved to Zhaojiagou under the slope, resulting in 46 deaths, 2 injuries, 63 houses buried and more than 500 mu of cultivated land destroyed in Zhaojiagou village group 14 households.

2 Geological environment conditions

2. 1 Meteorology and Hydrology

Meteorology: Zhenxiong County is located at the junction of Yunnan-Guizhou Plateau and Sichuan Basin, and belongs to the monsoon climate of the southern temperate plateau. It is one of the regions with more rainy weather in China, with an annual average temperature of 1 1.3℃. Summer time is short, the temperature is high, and the highest temperature is above 35℃; The winter climate is cold and rainy, and the lowest temperature can reach below -5℃; The annual precipitation is 923.6mm, the maximum daily precipitation for many years is 153.4 mm, 5 ~ 10 is the rainy season, and the precipitation is 772. 1mm, accounting for 83.6% of the whole year, while the precipitation from June to August accounts for more than 60% of the total rainfall in the rainy season, with heavy rain and rainstorm every year. The dry season is 165438+ 10 to April of the following year, and the precipitation is 15 1.5mm, accounting for 16.4% of the whole year. 1973 to 20 13 years, the maximum rainfall over the years 1427.7mm( 1983), the minimum rainfall over the years 578.7mm(2009), the average rainfall 886.9mm, and the average number of rainy days over the years/kloc-0. Because of the long rainfall time, high frequency and great climate change (frost, long ice and snow time), it plays an important role in promoting the occurrence of geological disasters.

Hydrology: This area belongs to Wujiang River system, a first-class tributary of Jinsha River. The survey area is located near the watershed on the west bank of the Wujiang River system, and the surface water bodies in the area are not well developed. There are two gullies (G 1, G2) and one spring point (Q 1) exposed in the survey area. G 1 gully is located near the eastern boundary of the survey area, and the overall flow direction in the area is from south to north. The gully in this area is about 720 meters long. During the investigation, due to the melting of snow water, the flow of ditch water is about 12L/s, but it is clear and has high sulfur content, and sulfur-containing substances can be deposited at the bottom of ditch. The source of gully water is mainly composed of snowmelt water on hillside, pore water of slope body and fissure water of bedrock (Figure 650

Figure 1 annual average monthly rainfall distribution map

G2 gully is located at the outer northern edge of the survey area, and the general flow direction in the area is from south to north. During the melting period of snow water, the flow rate of ditch water is about 6 ~ 8 L/s, and the ditch water is clear, but the sulfur content is high, and sulfur-containing substances can be deposited at the bottom of the ditch. Q 1 spring point exposes Triassic Feixianguan Formation (T 1fb) sandstone in the front steep slope of landslide accumulation area, with exposed elevation of about 1635m and spring flow of about 1.5 ~ 2 L/s (Figure 2).

Figure 2 G 1 gully

2.2 topography

The survey area is located in the northern part of Wumeng Mountain in the northeastern Yunnan Plateau, which belongs to Zhongshan landform with low degree of structural erosion. The mountain range is consistent with the tectonic line and runs southeast. The terrain generally inclines from southwest to northeast. The survey area is at an altitude of 1450m ~ 1978.5m, with the highest point at Shanbao (surface watershed) in the southeast of the survey area and the lowest point at the east of the survey area. The average terrain slope is 20 ~ 25, which is a broken line with steep front and back edges and gentle middle (Figure 3).

Fig. 3 Q 1 spring point

The landslide area is developed on the natural slope on the southwest side of Zhaojiagou Village, and the slope is broken. 1535 ~ 1565 m slope bottom elevation section is the distribution area of Zhaojiagou Village Formation, which has been buried. This area is a gentle depression with a topographic slope of about 5 ~ 15 (Figure 4). The elevation of the front 1565 ~ 1605 m is steep, the terrain slope is about 35 ~ 45, and the bedrock is exposed. The middle part of the slope is a gentle slope platform, with the distribution elevation of 1605 ~ 1790 m and the terrain gradient of about 5 ~ 8 (Figure 5). Landslides are developed on the steep slope at the rear edge, with a topographic slope of about 35 ~ 45, and the distribution elevation of 1, 735 ~ 1, 900 m (Figure 6), and a large amount of colluvial silty clay mixed with gravel is accumulated.

2.3 Formation lithology

The exposed strata in the survey area are lower Triassic Longtan Formation (P2l), Changxing Formation (P2c), Feixianguan Formation (T 1f) and Quaternary System (Q) from old to new, which are described as follows:

Fig. 4 Panorama of landslide area

Fig. 5 Buried Zhaojiagou Villagers' Group

(1)P2 Mesozoic Upper Permian

Upper Permian Longtan Formation (P2l): The lithology is mainly gray argillaceous siltstone, siltstone and silty mudstone, mixed with fine sandstone, glutenite, mudstone and coal seam, and it is thin-medium thick layered. There are slopes and scarps exposed along the country road on the south side of Zhaojiagou Village, with a thickness of130.23 m.

Fig. 6 landform of gentle part in the middle of slope

Changxing Formation (P2c): It is a set of alternating facies deposits with complete rhythmic development. The lithology is gray, black mudstone, silty mudstone, fine sandstone mixed with argillaceous limestone, which is in integral contact with the underlying Longtan Formation. Sporadic outcrops in the south slope of the survey area, with a thickness of 37.23 ~ 46.45 meters.

(2) Lower Triassic in Mesozoic (T 1)

Feixianguan Formation (T 1f): This layer is widely distributed and exposed from west to east in the south-central part of this area, with a thickness of 198.59 ~ 3 15.58 m ... It can be divided into two sections according to lithology and fossil characteristics. Upper member (t1FB):112.5 ~182.6 m thick. Lithology is purple, grayish purple siltstone, fine sandstone and shale mixed with limestone, containing calcium, chalcopyrite and chalcocite, with oblique bedding, horizontal bedding, wavy bedding and banded structure. Lower segment (t1fa):16.0 ~ 203.0m thick. Lithology is gray-green and blue-gray siltstone, mudstone mixed with silty mudstone, containing pyrite and calcareous nodules.

(3) Quaternary (Q)

Residual slope deposits (): Quaternary residual slope deposits in the survey area are distributed in a large area, mainly in gentle slope areas, and are dark purple and brownish yellow silty clay with gravel, and the silty clay is mainly plastic. Gravel content is 15 ~ 25%, mixed with a small amount of stones. The stone is mainly composed of strongly weathered siltstone and fine sandstone, with angular ~ subangular shape, with a general particle size of 2 ~ 8 cm and a maximum of 60cm, and a loose structure. The thickness is generally 1~40m ~ 40m.

Colluvium (): colluvium () is distributed in the steep slope area where landslides develop on the south side of the landslide area. The colluvium and colluvium are mixed, and the lithology is silty clay mixed with gravel, with loose structure, large pores and strong water permeability. Gravel and boulder content is 30% ~ 40%, and the rock composition is mainly strongly weathered siltstone and fine sandstone, which is angular ~ subangular, with a general particle size of 2 ~ 20 cm and a maximum of 200cm. The structure is loose, with a thickness of 5 ~ 35 m, mixed with multiple layers of clay, with a thickness of 10 ~ 30cm, and distributed in interlayer along the slope.

Landslide accumulation layer (): mainly distributed on the natural slope on the south side of Zhaojiagou, with the accumulation elevation of1535 ~1918m, and the material composition is the same as that of landslide accumulation layer. Due to the sliding, disintegration and accumulation of landslide, the structure is loose, and the thickness of landslide accumulation layer is 2 ~ 7m.

2.4 Geological Structure and Regional Crustal Stability

2.4. 1 geological structure

The regional tectonic position belongs to the Neocathaysian tectonic system in northeast Yunnan, Yunnan-Guizhou local combined structure and Zhenxiong composite syncline. The general distribution direction of the structure is mainly NE direction, followed by N-S direction and nearly E-W direction (Figure 7). The survey area is located at the northeast inclined end of the northwest wing of Dachengkui syncline, and the whole structure is monoclinic. The stratum tends to the southeast, and the dip angle is 3 ~ 16, with an average of 5. The occurrence of strata changes little, and the dip angle is gentle.

Fig. 7 Landslide trailing edge landform

Dayoukui syncline is located on the line between Yu Dayou Kui and Gaopo. The axial direction is 65 northeast, which is a broad and gentle symmetrical syncline. The dip angle of strata in the northwest wing is 140 ~ 165, and the dip angle is 5 ~ 13. The dip angle of strata in the southeast wing is generally 310 ~ 340 and 2 ~15. The newly exposed stratum of the core is Feixianguan Formation.

regional crustal stability

(1) neotectonic movement

According to the Remote Sensing Investigation Report on Geological Structure and Regional Crustal Stability in Yunnan, the neotectonic movement type in the investigation area belongs to the stable rising area in eastern Yunnan. According to the intensity of neotectonic movement, it is further divided into fault block uplift area in northeast Yunnan. The neotectonic movement in this area is dominated by large-scale overall stable uplift, and the Pliocene planation plane is well preserved. The thickness of the crust is 36 ~ 38 km, which has increased by 1500 ~ 2000m since the late Cenozoic, and locally exceeded 2000m.

(2) Earthquake

According to the county annals of Zhenxiong County, since 1974, there have been many earthquakes in Zhenxiong County and its surrounding areas, with magnitude > 5 and above 6 times and less than 5 times. See table 1 for earthquake statistics around Zhenxiong. The neotectonic movement in this area is dominated by large-scale uplift and the active faults are not developed.

Table 1 seismic record table

2.5 engineering geological conditions

According to the geotechnical engineering geological characteristics, the rock and soil in the survey area can be divided into two types: loose soil (Ⅰ) and rock mass (Ⅱ), and the rock mass in the area can be divided into two subcategories according to the rock genesis, structural type, development characteristics and weathering degree. See Table 2 for details.

Table 2 Statistical table of characteristics of rock and soil mass types

sequential

2.6 Hydrogeological conditions

The survey area is located near the watershed, with large topographic relief and developed gullies. Topographic conditions are conducive to the runoff and discharge of atmospheric precipitation, but not to the recharge of aquifers. The survey area is generally located in the runoff area of groundwater recharge, which is mainly atmospheric precipitation.

2.6. 1 groundwater types and aquifer characteristics

According to the lithology, characteristics of water-bearing medium, occurrence form of groundwater and hydrodynamic conditions of exposed strata in the survey area, groundwater types in the survey area are divided into pore water and fissure water, with pore water as the main one.

(1) Quaternary (Q) porous aquifer with loose sediments

Water-bearing media are mainly rubble and rubble in colluvium (), landslide accumulation layer () and residual slope accumulation layer (), with loose structure and pore water. The aquifer is mainly recharged by atmospheric precipitation and a small amount of surface water. The abundance and water level of groundwater are obviously controlled by seasons, and basically dry up in dry season, with only a small amount of upper stagnant water distributed; In the precipitation season, the aquifer is replenished by precipitation, and the groundwater is shallow and moderately rich. There is a spring point exposed in front of the landslide accumulation body, and the spring water gushes out in strands, and the flow rate is about 1.5 ~ 2.0 L/s, and the spring point is formed after the landslide. The analysis shows that the spring water is melted by ice and snow, seeps into the ground, gathers through the interconnected pores in the aquifer, then flows down the slope, migrates, and is discharged centrally at the front of the landslide accumulation body with relatively low terrain (Figure 3).

(2) bedrock fissure aquifer

Aquifer (group) mainly includes siltstone, fine sandstone and argillaceous siltstone of Longtan Formation (P2l), Changxing Formation (P2c) and Feixianguan Formation (T 1f), with argillaceous limestone locally. The surface of aquifer (group) is strongly weathered, and the thickness of fully-strongly weathered layer can reach 30m. In the rainy season, groundwater mostly disperses and permeates from rock cracks, and the seepage flow is generally around 0. 10 L/s; Some low-lying areas are rich in weathered fissure water with good confluence conditions, and the maximum spring flow in rainy season is 0.50 L/s. The above aquifers (groups) are alternately distributed with mudstone, shale and other relative aquifuges. Restricted by lithologic combination and landform, the water abundance of fractured aquifer (group) is generally weak.

2.6.2 Groundwater recharge, runoff and discharge

The topography of the survey area is generally high in the southwest and low in the northeast, and the relative height difference is large, which is conducive to the discharge of surface water and groundwater. The water abundance of pore water and fissure water is generally weak, and groundwater is mainly replenished by atmospheric precipitation infiltration, which has the characteristics of centralized recharge in rainy season and discharge in different periods. Groundwater flows from southwest to northeast, and the runoff is slow. Most of them are scattered and overflowed in low-lying areas, and a small amount is discharged in the form of spring flow.

3 Geological characteristics and genetic analysis

3. 1 Geological characteristics

3. 1. 1 engineering geological characteristics

Landslides are developed on the southwest high slope of Zhaojiagou village, and their plane is approximately polygonal, and their distribution direction is consistent with the slope direction (northeast). This section is located in the transition zone of steep slope at the rear edge of natural slope, with low terrain and thick colluvium () accumulated on the slope. After the landslide, the collapse accumulation body covers the whole lower slope, with a total length of 778m, a width of 8 1~220m ~ 220m, an area of 0. 126km2, and a maximum drop of 223.3m m. After the rapid landslide, the boundary between the landslide area and the accumulation area is clear, and loose materials move a long distance under the action of gravity.

Morphological characteristics of (1) sliding zone

The slip zone is developed on the high slope in the southwest of Zhaojiagou Village, with the distribution elevation of 1768...2 ~ 1892.3 m, and the distribution direction is consistent with the slope direction, which is 50 northeast, axial 120m, width 105m and area of 0.0/kloc. After the landslide, a spoon-shaped groove was formed, which stood steeply in the east, west and south. A few tensile cracks are developed at the trailing edge and 3-7m away from both sides, generally 2- 10m long and 5- 15cm wide.

(2) Landslide boundary

After the landslide slides as a whole, its deformation perimeter is obvious, forming a gourd-shaped groove.

Trailing edge: development elevation 1838.4 ~ 1892.3 m, which is distributed in an arc in the northwest direction, with a length of 1 100 m, a sliding wall height of 34m and a slope of 77, and there are still small-scale collapses after the disaster.

Front edge: the shear outlet of the front edge of the landslide is distributed at the elevation of 1768.2 ~ 1783.4m, with a wave-like distribution of 1 15m long. The soil is cut along the natural slope toe, and the transition is steep, and most of the shear outlets at the front edge are buried by landslides.

West side: the strike is 23 ~ 32, and it is eroded by the landslide, forming a shear wall with a length of 125m, a slope of 50 ~ 70 and a height of 10 ~ 30m.

East side: strike 35 ~ 45, sheared by landslide, forming shear wall, length 128m, slope 50 ~ 70, height 5 ~ 30m.

(3) Landslide structure

Zhaojiagou landslide has a single lithology, consisting of colluvium () and silty clay mixed with gravel, with loose structure and poor stability in natural state. Gravel and rubble content is 30% ~ 40%, and the rock composition is mainly strongly weathered siltstone and fine sandstone, which is angular-subangular, with a general particle size of 2 ~ 20 cm, a maximum of 200cm and a thickness of 5 ~ 35 m (Figure 9). In addition, there are many clay interlayers along the slope direction, with the thickness of 10 ~ 30cm, which are wet-saturated, soft and plastic, and are weak structural planes in the slope.

Figure 8 Overview of regional structure

(4) Rock and soil characteristics of sliding surface (belt)

According to the on-site landslide disintegration investigation, the sliding surface is located in the clay interlayer in the slope, with the thickness of 10 ~ 30cm, wet ~ saturated, soft plastic, distributed along the slope direction, dip angle of15 ~ 25, and buried depth of 5 ~ 35m. Landslides mainly occur along this layer (Figure 10).

Fig. 9 Panorama of landslide sliding area

Figure 10 Sliding Body Structure

3. 1.2 Motion characteristics of sliding body

After the landslide slides rapidly at a high position, the landslide body slides to the low-lying part of the mountain in the form of debris flow, and most of it accumulates on the slope with the elevation of 1605 ~ 1780 m. The leading part of the debris flow accelerates twice at the scarp at the front of the slope, and then at 1540 ~ 1605 m (compared with Zhaojiagou landslide) According to the movement and accumulation characteristics of the sliding body, the zoning is described as follows (Figure 1 1, Figure 12).

Figure 1 1 sliding zone soil

Area I is 1540 ~ 1605 m elevation profile, with a length of 285m, a width of 150m, an area of 0.049km2, a thickness of 1 ~ 7 m, an average of 2m and a volume of 98,000m3. The slope of this section is of broken line type, in which the area of slope bottom elevation 1535 ~ 1565 m is the distribution area of Zhaojiagou Village, which has been buried, and this area is a gentle slope with a topographic slope of about 5 ~15; The elevation of the rear edge 1565 ~ 1605 m is steep, the terrain slope is about 35 ~ 45, and the bedrock is exposed. Area I is mainly the area where the debris flow faucet reaches after the landslide disintegration. After the debris flow flows to the steep slope near the altitude of 1605m, it forms a secondary acceleration and finally moves to the Zhaojiagou area at the bottom.

Area II is the elevation distribution section of 1605 ~ 1780m, with a length of 4 16m, a width of 265438+2~5m, an area of 0.074km2, an average thickness of 3m, and a volume of 1 12000 m3. The slope of this section is broken-line, the rear part is low and the front part is relatively gentle. Terrain slope is about 5 ~ 8, and the slope is 75 northeast. The landslide debris flow in this section moves 49 northeast at the low-lying section with the rear elevation 1730 ~ 1790 m, is blocked by the left mountain at the front elevation 1605 ~ 1730 m, turns 75 northeast under the centrifugal action, and then accumulates on the slope.

3.2 Analysis of influencing factors of landslide

3.2. 1 Terrain conditions

The landslide development section is located in the steep and gentle transition zone of the slope, which is a broken-line slope with steep upper part and gentle lower part. The slope is inclined to the northeast, and the terrain slope is about 35 ~ 45, which makes it a place where steep slopes collapse and accumulate. Moreover, the terrain of this section is relatively low, which is easy to collect surface water, and the slope zone has the conditions for breeding geological disasters.

Figure 12 Remote Sensing Image of Landslide Accumulation Area

3.2.2 Topography and lithology

As a steep transition zone of slope, its material formation and genesis are complex, and it is a mixed accumulation of multi-stage collapse and slope deposits. The lithology is mainly silty clay mixed, with loose structure, large pores and strong water permeability. The mountain is on the shady side, and the soil is in a state of wet saturation for a long time, so the stability is poor. In addition, there are many clay interlayers along the slope with a thickness of 10.

atmospheric precipitation

According to the statistical data of the day before the February 12 to 1 3 landslide, the average precipitation in Zhenxiong County is 15.83.7mm (in which February 12 is 12). It is lower than the maximum precipitation in the same period (1970,65438+February, 38.9mm), but according to Dashuixi observation station (about 20km west of the landslide), the precipitation is 70mm (including 65438+February 66.4mm,1early October 3.3mm), indicating that the local rainfall is much larger than the historical period. Therefore, according to the saturation characteristics of landslide deposits and surrounding rain and snow, it is inferred that since 20 13 12 1, the landslide has received 1 month groundwater infiltration recharge due to long-term rainfall and local rainfall. Precipitation infiltration increases the self-weight of the slope and reduces the strength of the geotechnical layer, especially the clay interlayer is easy to soften and become a sliding surface, which eventually leads to landslide.

Figure 13 movement zoning map of landslide accumulation body

3.2 Development trend

The whole landslide slides at a high place, forming a spoon-shaped groove, which stands steeply on the east, west and south sides. In the lateral profile of 3-7m on both sides, there are a few tensile cracks at the trailing edge and parallel sliding boundary, which are generally 2- 10m long and 5- 15cm wide. In particular, the collapse of the trailing edge and proluvial are still causing small-scale collapse. The elevation of area I 1565 ~ 1605 m, the terrain slope of the accumulation body is 35 ~ 45, and the self-stabilization ability of the accumulation body is poor, resulting in secondary landslide. The topography of accumulation body in area II is gentle, and debris flow on slope will be formed under the scouring of atmospheric rainfall. The above-mentioned potential geological disasters pose a serious threat to the following four village groups: Zhaojiagou, Zengjiazhai, Gaodong and Yankou, Gaopo Primary School 150 households, about 1000 residents, teachers and students, and the operation of rural roads.

Figure 14 Landslide Profile

(According to Yin Yueping 20 13)

4 Experience and Enlightenment

(1) Strengthen the popularization of catastrophe cases. On the basis of the knowledge training of geological disaster reduction and prevention, the case training of catastrophic disasters was carried out. Targetedly popularize the inspection points and disaster prevention and avoidance methods of high-grade remote landslides.

(2) Strengthen the planning and site selection of housing construction, avoid the construction of ravines in high mountains a few kilometers away, and plan safety zones before housing construction.

(3) In the flood season or the long rainy (snowy) season, in addition to the inspection in front of the house and behind the house, under the guidance of professional and technical personnel, strengthen the inspection in front of the mountain and take necessary engineering control measures for the hidden dangers found.

(4) Strengthen the investigation of high-speed and long-distance landslide disasters in southwest mountainous areas, and carry out detailed investigation of geological disasters in villages and towns and their surrounding areas by means of lidar, aerial remote sensing and mountain engineering.

refer to

Yin Yueping, Liu, et al., Study on Zhaojiagou landslide in Zhenxiong, Yunnan, 2013.2013.11. Journal of Engineering Geology, 2 1 (1): 6 ~ 65438.