Distribution and development law of landslide disasters in Wushan county, Chongqing, Three Gorges reservoir area

Gao Wenjun 1 Ye Xiaohua 1 Chen Zhongfu 2

(1 Sichuan 999 Construction Engineering Co., Ltd., Jiangyou, 621701; 2 Geological Environment Monitoring Station of Wushan County, Chongqing, 404700)

The natural geological environment conditions in Wushan county are complex, and geological disasters, mainly landslides, are developing, which have a serious impact on the relocation of immigrants in the reservoir area and the safety of people's lives and property. Through the special investigation and evaluation of monitoring and early warning of geological disaster prevention and control planning, the formation environment, types and distribution, formation mechanism, development law and harm status of landslides are basically found out, which provides basic information for the prevention and control of geological disasters in Wushan County.

Keywords:: the development law of landslide disaster in Wushan County, Three Gorges reservoir area

order

Wushan county, Chongqing is located in the eastern edge of the folded mountain area in Sichuan basin, in the east of Chongqing, in the hinterland of the Three Gorges of the Yangtze River, and is close to the "golden waterway" of the Yangtze River, and is known as the "East Gate of Chongqing". The natural geological conditions are complex, and geological disasters are developed, causing serious harm. With the gradual implementation of the Three Gorges Project construction and resettlement project, frequent human engineering activities have further enhanced the changes to the original geological environment, and the number, frequency, scale and harmfulness of geological disasters have further increased, further aggravating the impact on the Three Gorges Project construction, resettlement project implementation and the safety of people's lives and property in the reservoir area, and becoming one of the geological disaster-prone areas and hardest hit areas in the Three Gorges reservoir area. In March, 2004, our company was entrusted by the geological disaster prevention and control headquarters in the Three Gorges reservoir area to undertake the three-phase special investigation and evaluation of geological disaster prevention and control planning, monitoring and early warning in Wushan County, and basically found out the distribution and development law of geological disasters in Wushan County, providing important basic information for effective prevention and control of geological disasters in Wushan County.

1 Formation environment of landslide

1. 1 landform

The landform types in the reservoir area are erosion-erosion mountain landform and erosion-erosion hilly landform, which are distributed alternately in north and south, with erosion-erosion hilly landform as the main landform. The landform area in low mountains and hills is low and gentle, with large fluctuation, and the landform slope is generally 20 ~ 400. It is dominated by flowing water landform, with developed surface water system, long main ditch, gentle slope descent and wide valley bottom. Most of the mountains are deeply cut by gullies, and the slope types are mainly concave slopes and step slopes. The vegetation on the slope is generally undeveloped, mostly in agricultural areas, mainly in dry land. Landslides in this landform area are strongly developed and concentrated. The landform of Zhongshan is high and steep, with undulating ridges. The landform gradient is generally 30 ~ 600, and some of them are over 700. Mainly karst landform, weak surface water system, deep main ditch, short branch ditch, steep slope and narrow valley. The slope type is mainly convex and straight, and the vegetation on the slope is well developed, mostly forest land and grass irrigation land. Landslides in this landform area are weakly developed and scattered.

1.2 Formation lithology

Stratigraphic lithology is mainly sandy mudstone of Triassic Badong Formation, which is widely distributed in Daning River in Dachang, Futian and Shuanglong areas in the north of the reservoir area and the bank slopes of the Yangtze River in Sanxi, Wushan and Nanling areas in the middle, followed by the valley slopes in Guandu, Shibei and Shi Pei areas in the south. It is the main stratum in hilly landform area, with weak lithology, low mechanical strength and easy weathering and crushing. Quaternary residues and cracks are widely distributed on the slope. Triassic Jialingjiang Formation and Badong Formation are distributed alternately in strips, and Silurian to Triassic Dazhi Formation are scattered, with a small area. Together with Jialingjiang Formation, it constitutes the main stratum of Zhongshan landform area, and its lithology is carbonate rock mixed with clastic rock, which is firm, high in mechanical strength and strong in weathering resistance. The slope is high and steep, mainly rock slope, and loose earth and stone are distributed at the foot of the slope, forming a soil slope which is a landslide.

1.3 geological structure

Geological structure controls the pattern of topography and geomorphology, and also controls the distribution of stratum lithology. The main geological structures in the reservoir area are folds, anticlines and synclines which are regularly distributed alternately. Anticline is distributed in Zhongshan landform, mainly in hard rocks such as Jialingjiang Formation, and syncline is distributed in low mountains and hills, mainly in Badong Formation. Slope is beneficial to the formation of landslide, and the axis direction of anticline is basically the same as that of ridge, so slope is not easy to form landslide.

1.4 atmospheric rainfall and human engineering activities

The reservoir area is located in the middle of Wushan County, with an average annual rainfall of 1049.3mm, annual maximum rainfall of 1356mm, monthly maximum rainfall of 445.9mm (1September 979) and daily maximum rainfall of141.4mm (/. Rainfall is unevenly distributed in a year, with the main rainfall concentrated from May to September, accounting for 68.8% of the annual rainfall. Most of the landslides surveyed occurred in the rainy season or after a heavy rainfall process was delayed for a certain time. Especially in the middle and late period of rainstorm. Human engineering activities, such as steep slope reclamation, traffic and town construction, and resettlement, not only change the slope type and gradient, destroy the geotechnical structure, but also destroy the ecological environment, artificially increase the load, promote the formation and development of landslides, and at the same time stimulate the revival of old landslides.

2 Types, distribution and hazards of landslides

2. 1 landslide type

There are landslides 189 in this survey, which are classified according to material composition, scale and stability. All landslides are loose earth-rock landslides, and all collapses are rock collapses. Large and medium-sized landslides account for 9 1.5% of the total number of landslides, medium and small landslides account for 80% of the total number of landslides, and 57.2% of them are in potential instability or unstable state. All landslides are in potential instability state.

Table 1 Landslide and Collapse Disaster Classification Table

2.2 the distribution of landslides

Mainly distributed on both sides of the Yangtze River and Daning River, followed by Guandu and Baolong. According to statistics, the banks of the Yangtze River and Daning River account for 55.7% and 34.5% of the total survey respectively, and the Guandu-Baolong area only accounts for 9.8% of the total survey. From the distribution of landslides in topographic elevation and administrative areas, there are also obvious differences. There are 96 wading landslides and 98 non-wading landslides with the front edge below 175m (including 175m), taking the water level in front of the dam as the boundary, accounting for 49.5% and 50.5% of the total survey respectively. The number of landslides distributed in 2 1 township (town) in the reservoir area is Wuxia Town, 43 * *, followed by Quchi Township, dachang town and Daxi Town, 28, 27 and 25 respectively. The total number of landslides in the above four townships (towns) is 123, accounting for 63% of the total survey. Each township (town) accounts for 22. 16%, 14.4%, 13.9% and 12.9% of the total survey respectively. Longxi, Chuyang, Damiao and Du Ping have the least distribution, each of which is 1. According to cities (towns) and rural areas, there are 34 cities (towns), but most of them are in rural areas, with 160, accounting for 17.5% and 82.5% of the total survey respectively.

2.3 the hazards of landslides

2.3. 1 current situation of landslide disaster

Landslides cause the following direct hazards: serious harm to urban and rural engineering buildings; It has a destructive impact on shipping, highway traffic and transportation of the Yangtze River and Daning River; Bring huge losses to the lives and property of urban and rural people; Harm factories, mines, enterprises, science and education enterprises and institutions, and bring huge losses to state property; Increase the state's direct investment in the construction of the Three Gorges Project, urban relocation and resettlement.

There are the following indirect effects: increasing the psychological burden of urban and rural people and affecting social stability; It has an indirect impact on the development of local national economy; Lead to soil erosion, cultivated land loss, and deterioration of urban and rural people's living environment; Make the poverty in poor mountainous areas worse.

According to investigation and statistics, the hazards caused by landslides are mainly manifested in the destruction or collapse of houses, the collapsed houses and uninhabitable dangerous houses 2 1787m2, followed by death 1 person, local deformation and damage of ponds and canals, cracking and sinking of roads and collapse of subgrade slopes.

2.3.2 Landslide disaster prediction

In this survey, the number of people threatened by landslides is 39,589, and the threatened direct economic loss is1051580,000 yuan, mainly concentrated in Wuxia Town, Quchi Township, Daxi Township and dachang town. However, in seven townships (towns) such as Wuxia Town, guandu town, Daxi Township, Quchi Township, futian town, dachang town and Nanling Township, the number of people who may be threatened by landslides and collapses is 34,770, and the direct economic loss is 9105,000 yuan, accounting for 87.8% and 86.6% of the total respectively (Table 2).

Table 2 Prediction Table of Landslide and Collapse Disasters in Major Towns (Towns)

3 Formation mechanism, development characteristics and stability evaluation of landslides

3. 1 landslide formation mechanism

3. 1. 1 Analysis of landslide influencing factors

(1) Suitable terrain slope and terrain height difference provide space conditions for landslide formation. Under the action of river erosion and cutting, the valley slope is steep and empty, which is beneficial to the unloading of rock and soil on the side slope under the action of self-weight and creates spatial conditions for the sliding movement of rock and soil. Slope and slope shape are the main topographic factors. Landslides in the reservoir area mostly occur on slopes with a gradient of 20 ~ 40, and the slopes are concave or stepped.

(2) Soft rock and soil provide material conditions for the formation of landslides. The lithology of landslide is Quaternary loose earth and stone, silty clay or mixed accumulation of silty clay and gravel, with loose structure, and the contact surface with the underlying bedrock is the sliding control surface formed by landslide.

(3) Rainstorm is the main disaster-causing factor of landslide. Rainfall occurs every year, and the rainfall time and intensity are concentrated every year, which has obvious seasonal cycle characteristics. Every rainy season, a large amount of rainfall, especially heavy rain, infiltrates, erodes and softens the slope soil, reduces its stability and causes sliding deformation, which is the main factor for the formation or sudden landslide.

(4) Human activities are an important factor to promote the formation of landslides. Engineering activities such as manual slope cutting, excavation and loading have changed the shape and gradient of the slope, destroyed the stress structure and stress balance of the rock and soil, reduced the stability of the slope and promoted the formation and development of the landslide. Economic activities such as human land reclamation and deforestation loosen the earth and stone on the slope, destroy the ability of vegetation to fix soil and retain water, enhance the erosion of soil by rainfall, and promote the formation and development of landslides.

(5) Reservoir impoundment reduces the stability of landslide and intensifies the sliding deformation of landslide. The water content of rock and soil near the reservoir water level increases sharply. Under the action of saturation and scouring, the shear strength of landslide soil is greatly reduced, which is easy to cause instability, sliding deformation and destruction of local landslide, thus reducing the stability of the whole landslide and stimulating the revival of landslide.

3. 1.2 landslide deformation characteristics

Landslides in the reservoir area have experienced different degrees of active deformation in different periods. The initial time is generally from the late 1980s to the early 1990s, and the late 1990s is a period of high activity. Most landslides have experienced strong sliding deformation, and the landslide activity began to increase again for a period of time before and after the impoundment in 2003. The phenomenon of landslide deformation in this investigation mainly occurred in recent years, and some landslide deformation is obviously due to its recent occurrence or enhancement. Its main features are as follows:

(1) surface displacement and cracking. Due to artificial cultivation, burial and other reasons, the surface displacement cracks are mostly completely or partially destroyed, and the signs are unknown. The existing evidence shows that the surface tension crack length is generally 30 ~ 100 m, the longest is 200m, the horizontal displacement width is generally 1 ~ 10 cm, the widest is 100cm, and the vertical displacement height is generally 65438.

(2) Small-scale sliding failure in shallow layer. It mainly occurs on slope, valley slopes on both sides and steep slopes in front, and local ground collapses and damages are caused by strong local sliding deformation.

(3) Building deformation is a common deformation feature with obvious signs of stagnation. Mainly distributed in residential buildings, there are signs in roads, retaining walls, canals and other buildings. The main deformation features are cracking, inclination and collapse of residential walls, tensile cracking of foundation, subsidence of highway subgrade, tensile cracking and displacement of pavement, bulging and collapse of retaining walls and canals. Wall cracks are generally 2 ~ 10m long, 0.5~2cm wide and 5cm wide, with dislocation and inclination of 0. 1 ~ 1cm, maximum 10cm, 5 ~ 20m long and 0.2 ~ 1cm wide.

(4) There are deformation characteristics such as skewed trees, water leakage in ponds or paddy fields, depressions or wetlands on the ground, and ground uplift.

3. 1.3 landslide formation mechanism

According to investigation and analysis, the development and distribution of landslides are mainly controlled by stratigraphic lithology, geological structure, topography, atmospheric precipitation and human engineering activities. Stratigraphic lithology, geological structure and topography are the internal basic conditions for the formation of landslides, and atmospheric precipitation infiltration and human engineering activities are the main dynamic factors for inducing landslides.

Landslides in the area are all loose soil landslides, and the sliding body is silty clay mixed with gravel. The diameter of gravel varies greatly, and the roundness is generally poor. For landslides with mudstone as the parent rock, the diameter of gravel is between 2 ~ 2~ 150mm; In the landslide where the parent rock is hard rock such as limestone, the diameter of gravel is between 5 ~ 400 mm, the compactness of gravel soil is between loose and slightly dense, and silty clay and clay are hard ~ hard plastic ~ plastic, which is easy to soften when it meets water, and its plasticity is enhanced, and it is transformed into soft plastic, flowing plastic and easy to creep or flow plastic. The sliding surface is mostly the contact surface between Quaternary system and bedrock. And the slope is very steep, mostly between 20 and 40. After rainfall, surface water infiltrates into the soil, and the top surface of weathered rock or bedrock moves strongly, forming runoff along the slope surface, which softens the eroded soil and reduces its strength. Due to the influence of human engineering activities such as slope cutting and excavation, manual loading and reservoir storage, the occurrence and development of landslides are promoted.

3.2 Landslide development law

3.2. 1 terrain type difference and terrain gradient concentration

Landslides are concentrated in the hilly area in the middle of the reservoir area, with a total of 1 12, accounting for 59.3% of the total, followed by the hilly areas in the north and south, with 6 and 5 landslides in the middle and south, accounting for 3.2% and 2.6% of the total respectively. The slope of landslide is generally 20 ~ 40, and the slope of 30 ~ 40 is the most, reaching 102, accounting for 54.0% of the total.

3.2.2 Stratigraphic lithology concentration

Soft rock and soft soil are the main sliding layers. Slope rock and soil is the material basis of landslide formation, which controls the development and distribution of landslide. Slopes with weak lithology are prone to landslides. Most of the landslides are developed in the exposed area of Badong Formation, accounting for 90.5% of the total, 5.3% in the exposed area of Jialingjiang Formation and 4.2% from Silurian to Permian.

Differences in geological structure

Landslides in syncline structural areas develop strongly. Landslides are mainly distributed in Wushan syncline, Dachang syncline and Shuikou syncline, accounting for 58.2% and 22.2% of the total respectively, followed by Guandu syncline and Shi Pei syncline, accounting for 10.5% of the total respectively, and anticlines and fault structures are rarely distributed, accounting for 6.9% and 2. 1% of the total respectively.

3.2.4 Time seasonality

Landslides occur and develop in rainy season. According to the investigation, the influencing factors of landslide disaster show that rainfall, especially rainstorm, is the main inducing factor of landslide disaster. Wushan county has abundant rainfall, with an average rainfall of 1049.3mm for many years. Rainfall is concentrated from May to September every year, accounting for 68.8% of the annual rainfall. The rainfall intensity is high, with heavy rain and heavy rain, and the maximum daily rainfall reaches 1, 4 1.4mm, so landslide disasters in the reservoir area mostly occur in the rainy season every year, mainly in the rainy season from May to September.

3.3 landslide stability evaluation

According to the natural geological processes such as topography, stratum lithology, rainfall and the influence of human engineering activities such as excavation, loading and reservoir water storage, combined with the deformation characteristics and deformation development history of landslide, the stability of landslide is qualitatively evaluated.

3.3. 1 stability state evaluation

When evaluating the current stability of landslide, considering the similarity of influencing factors of landslide, the deformation characteristics of landslide increased in recent years, especially in recent years, are used as a semi-quantitative basis for evaluating the stability of landslide. The evaluation results show that there are 8 stable and basically stable landslides1,accounting for 42.8% of the total survey, and 57.2% of the total survey are potential unstable and unstable landslides 108. See.

Table 3 Statistics of landslide stability

3.3.2 Stability prediction and evaluation

When predicting the development trend of landslide, consider the current stability of landslide and the intensity and period of potential unstable factors (such as reservoir impoundment, rainfall, excavation, load, etc.). ) and its influence on landslide stability, and emphatically analyzes the influence of reservoir hydrological effect on wading landslide stability. In June 2003, the water level in the Three Gorges reservoir area reached the elevation in front of the dam 135 meters. According to the water storage plan, before the flood season from September 2007 to 2009, the water level reached the elevation of 156m in front of the dam, and then operated according to the water level plan of 175m in front of the dam. During the normal operation of the reservoir, the elevation of 145 ~ 175 m is a violent fluctuation area of the reservoir water level. The front of 96 wading landslides investigated in this survey are all in this fluctuation zone. When the water level rises, the soil on the bank slope is soaked by water, and the groundwater level rises, resulting in soil softening. When the hydrostatic pressure of slope increases and the water level drops sharply, it will lead to the difference of water head inside and outside the landslide and form hydrodynamic pressure. At the same time, when groundwater flows out, it will carry a lot of viscous substances and fine particles, which will make the slope unstable and collapse. Without preventive engineering measures, wading landslides will collapse to varying degrees, leading to local or overall sliding deformation and destruction of landslides. Therefore, due to the influence of reservoir hydrological effect, unstable landslides have greatly increased.

3.4 Main inducing factors of landslide

According to investigation and analysis, among many non-geological factors that cause landslides, such as rainstorm, erosion, reservoir water level, excavation, load and earthquake, rainstorm is the main natural inducing factor, and excavation load is the main man-made inducing factor. With the impoundment of 135m water level in front of the Three Gorges Dam, the reservoir water level has become an important factor affecting the landslide stability, and with the continuous rise and later changes of the water level, the impact on the landslide stability will become more and more serious.

3.4. 1 rainstorm is the main natural factor inducing landslide.

Landslides in the reservoir area are concentrated on the bank slopes of major tributaries such as the Yangtze River and Daning River. The landform type is low mountain and hilly terrain, with high population density, large amount of cultivated land, active agricultural economy, fragile ecological environment and poor vegetation conditions. After the slope rock mass loses vegetation protection, there will be heavy rain infiltration in rainy season, which will not only increase the bulk density of rock mass and reduce its shear strength, but also form an active zone of groundwater in the contact zone between soil and underlying rock mass, resulting in hydrostatic pressure and hydrodynamic pressure. Promote the development and formation of sliding surface, make the slope rock and soil develop in an unfavorable direction, and induce the formation and development of landslide. Every year in rainy season (May to September), a large number of landslides occur, and their sliding deformation or destruction is closely related to a large amount of rainfall, indicating that rainstorm is the main natural factor inducing landslides.

3.4.2 Excavation and loading are the main human factors that induce landslides.

Due to the relocation and resettlement of flooded towns, roads, residential areas, factories and other population and facilities in the reservoir area, human engineering activities are increasingly active, and the scope and intensity of influence are increasing. Frequent slope cutting and excavation and increasing the load on the slope not only change the slope shape, increase the slope gradient and height, but also destroy the internal structure and stress balance of rock and soil, and at the same time destroy the ecological vegetation, which causes changes and impacts on the geological environment to varying degrees, reduces the stability of the slope and promotes the deformation and destruction of the slope soil. In recent years, many new landslides have occurred in the relocation towns such as Xincheng District and Shuanglong Town in Wushan County, and landslides have occurred in the resettlement communities such as Miaowanzi in Chunzao Village, dachang town. All of them are closely related to human engineering activities such as slope cutting and excavation and increasing load.

3.4.3 The influence of reservoir hydrological effect on landslide stability is obviously enhanced.

After the reservoir area was impounded for the second time to the water level in front of the dam 135m, some low-lying landslides were completely or mostly submerged, and some landslides, such as Hongyanzi landslide in Wuxia town, Jiangdongzui landslide in Xiawan village in Shuanglong town and Xiangyangping landslide, etc. Before the landslide occurred, affected by this water level, the stability of the landslide decreased, and a new creep deformation phenomenon appeared. For example, Xiangyangping landslide is located on the right bank of Daning River, and the front of the landslide is submerged by the reservoir water level. In late July 2003, new cracks appeared in the concrete floor and brick wall of residential buildings. By April 5, 2004, it was found that the length of ground fissure was generally 2.6~6.9m, the longest was 19.2m, the width was generally 0.3 ~ 14cm, and the widest was 7m, and it sank.

The influence of reservoir impoundment on landslide stability is mainly through saturated immersion of rock and soil near water level, changing its hydrogeological conditions, increasing the bulk density of rock and soil, sharply reducing the shear strength, bank collapse caused by water level fluctuation, and hydrodynamic pressure caused by water level drop, which leads to local or overall sliding deformation or damage of landslide. With the reservoir water level rising to 175m in front of the dam, when the reservoir water level fluctuates between 145 ~ 175 m, the influence on the landslide stability in the reservoir area will be more significant.

4 conclusion

(1) The reservoir area in Wushan County has poor natural geological environment conditions, and geological disasters caused by natural geological processes are developed, mainly landslides, which have the characteristics of wide range, large scale and heavy harm. With the gradual implementation of the Three Gorges Project and the resettlement project, geological disasters induced by human activities continue to occur, and the harm is intensified, making Wushan County reservoir area one of the prone, frequent and hardest hit areas of geological disasters in the Three Gorges reservoir area.

(2) Landslides in this survey are all loose earth-rock landslides, with large and medium-sized landslides accounting for 9 1.5%, and landslides in potential instability or unstable state accounting for 57.2% under current conditions; They are mainly distributed in low mountain landform areas, and concentrated in Wuxia Town, Nanling Township, Quchi Township and Daxi Town along the Yangtze River in the central and western regions, as well as Dachang Town, futian town Town and Shuanglong Town along the Daning River in the north.

(3) Landslides are mainly formed on the exposed slopes of the Triassic Badong Formation. The slope with a terrain gradient of 20 ~ 40 is the dominant terrain of landslide. If the slope is too steep, it is easy to collapse. The slope shape of landslide is mainly concave, all of which are soil structure slopes. The contact surface between the accumulated soil layer and the underlying bedrock often forms a slip control structural plane. Due to the vertical infiltration of rainfall or surface water, the contact surface is lubricated and softened, the accumulated soil layer is saturated with water, the cohesion is reduced, and the shear strength between the accumulated soil layer and the underlying bedrock is greatly reduced, resulting in landslide or deformation.

(4) Rainfall and human engineering activities are the main factors inducing landslide collapse, and they are also more active than geological environment conditions. Under the influence of reservoir hydrological effect, the deformation and failure of landslide develop in the form of front edge collapse-traction retreat-temporary stability-collapse retreat until stability. Reservoir hydrological effect is the main inducing factor of resurrection, so the prevention and control of wading landslide should be strengthened.

(5) Landslide is the main geological disaster in Wushan County, with serious harm. Therefore, monitoring, prevention, emergency treatment and engineering management should be strengthened.