Analysis on the development characteristics and disaster-causing environment of geological disasters in the coastal areas of southern Fujian

(Fujian Geological Engineering Survey Institute, Fuzhou 350002)

Based on the investigation and analysis of geological disasters in the southern coastal areas of Fujian, this paper expounds the development characteristics of geological disasters in this area, analyzes the disaster-causing factors of various geological disasters, and analyzes the regional disaster-prone environment from three aspects: natural geographical and climatic conditions, geological environmental conditions and human engineering and economic activities, and points out that geomorphological units and geological units have different forms, structures and functions, and the regional climate and environmental characteristics have obvious influence on the formation of regional geological disasters.

Keywords: geological disasters; Characteristics; Disaster factors; Disaster-prone environment

Geological disasters are geological phenomena and events that damage human life, property and spirit under the interaction of internal and external forces on the lithosphere surface or under the dynamic geological action inside the crust. The gestation and development of geological disasters are controlled by regional natural conditions, regional geological environmental conditions and human activities.

1 Present situation and regional characteristics of geological disasters

The characteristics of regional natural geographical conditions, geological environmental conditions and the degree of human engineering activities make geological disasters relatively developed in some areas of the survey area. At present, the main geological disasters in the survey area are ***206 and so on, including collapse, landslide, debris flow and land subsidence. Among them, 95 places collapsed, accounting for 46.12% of the total number of geological disasters; 76 landslides, accounting for 36.89% of the total number of geological disasters; 2 mudslides, accounting for 0.97% of the total number of geological disasters; There are 33 land subsidence sites, accounting for 16.02% of the total number of geological disasters. See the attached table for details of geological disasters in the survey area. According to the statistical data of geological disasters, the geological disasters in the survey area have the general characteristics of geological disasters in Fujian Province, that is, many points, wide area, small scale, high frequency, great harm and great influence by human activities and rainfall [1].

2. Distribution scale, characteristics and disaster-causing factors of geological disasters.

The geological disasters in this area have obvious distribution laws. Collapse, landslide and debris flow are distributed in the hilly areas in the west and northwest of the survey area, mostly along the high and steep slopes formed by manual excavation in front of roads and houses. Land subsidence is distributed in coastal alluvial plain, and a small amount is distributed in uncompacted loose plain fill area; Debris flow is distributed in the slope area at the tail of hilly and gully, and various geological disasters have their own characteristics and disaster-causing factors.

2. 1 Landslide and collapse

2. 1. 1 landslide [2]

Landslide is a form of slope deformation and failure, which refers to a natural geological phenomenon that the rock and soil mass on the slope slides downward along a weak structural plane (belt) as a whole or dispersedly under the influence of river scouring, rainfall and other factors. Landslides are usually divided into blocks, which are in a relatively stable stage after sliding, and may be activated again and slide under the influence of other factors such as rainfall [3].

According to the material composition, thickness, formation reason and scale of landslide, the landslide in the survey area is classified and counted, and the slope before landslide is counted. The statistical results are shown in figure 1 and table 1 respectively.

Figure 1 forms a percentage map of landslide slope.

Table 1 landslide type division statistics table

There is no landslide in the rock, which shows that the landslide in the survey area is controlled by residual soil and strongly weathered rock, and the structural plane in the rock mass is not the main factor controlling the landslide in the survey area.

2. 1.2 crash

Collapse is a form of slope deformation and failure, and it is a slope under certain conditions. Due to the influence of artificial slope cutting, vibration and other factors, some rock and soil on the slope are loose and separated along a weak surface under the action of gravity, and suddenly collapse from the steep slope and the foot of the slope, forming a rock and soil pile with a certain natural angle of repose. At present, 95 collapses have been found in the survey area. According to the composition, formation reasons and scale of the collapsed materials, the collapse in the survey area is divided and counted. Table 2 shows the statistical data and characteristics of the survey area types.

According to the statistics of landslides with slope records, 64.55% of them have a slope greater than 40. Collapse caused by 30 ~ 40 years old accounts for15.19%; Collapse caused by 20 ~ 30 years old accounted for16.46%; Collapse caused by < 20 years old accounts for 3.80%. The scale of collapse caused by slope < 20 is only < < 200m3, which shows that the greater the slope, the easier it is to form geological disasters of collapse.

Table 2 Statistics and characteristics of collapse types

2. 1.3 Cause analysis of landslide and collapse

Landslide and collapse are affected by many internal and external factors, including topography, geotechnical properties and geological structure, etc. Rainfall and human engineering activities are external factors, among which rainfall is the most critical leading factor. Blasting vibration is another external cause of collapse.

2. 1.3. 1 landform

All landslides in the survey area are distributed in the hilly areas. Statistical analysis shows that the micro-geomorphic unit where most landslides are located is a convex slope. Moreover, with the increase of slope, the probability of inducing landslide also increases. When the slope exceeds a certain angle, the form of slope deformation and failure changes, from landslide to another mode of slope deformation and failure-collapse. The main factors affecting the collapse are slope type, slope and slope height, among which slope and slope height are the main factors. The higher the slope, the easier it is to collapse. The larger the slope is, the tensile stress range near the slope surface will also increase and strengthen, and the maximum shear stress in the stress concentration area at the foot of the slope will also increase. With the increase of slope height, the stress value of each part of the slope increases linearly, which proves that the high and steep slope formed by highway excavation slope and house back slope is easy to collapse.

2. 1.3.2 rock and soil properties

The nature of rock and soil is an important internal factor to control the deformation and failure of slope, and the landslide in the survey area is mainly determined by the thickness of residual slope deposits and strongly weathered rocks. Although the thickness of residual sediments is different, they are usually thicker. Residual and weathered bedrock, weathered bedrock and fresh bedrock are relatively weak zones, and activities such as groundwater are relatively active. Landslide is mainly controlled by this zone, and residual soil itself is relatively loose. Driven by rainfall, the slope with good free surface is easy to slide. It can be seen from the table that the residual rocks of intrusive rocks are prone to landslides, while in strongly weathered rocks, intrusive rocks, volcanic rocks and sedimentary rocks will all produce more landslides. See table 3.

Table 3 Statistical Table of Relationship between Landslide and Rock and Soil Types

Loose, poorly cohesive and easily disintegrating soil is prone to collapse, and slopes with developed joints and cracks are also prone to collapse. The landslides in the survey area are all residual soil type collapses, mostly intrusive rock areas. The main reasons are low clay content, poor cohesiveness, high gravel content, loose soil structure, easy to wet and collapse, and easy to collapse under the action of rain and precipitation. The development of joints and fissures provides a good channel for groundwater infiltration and runoff, and the cohesive force between broken rocks is poor, which is easy to induce collapse under the action of rainfall infiltration.

2. 1.3.3 Geological structure

The influence of geological structure on the landslide on the slope is manifested in the strong change of the structure of the rock and soil, which makes the rock and soil deformed and broken, increases the weathering degree of the rock and soil, forms a good structural weak surface in the rock and soil, and reduces the mechanical strength of the rock and soil. Tectonic movement also changed the slope gradient and slope shape, which played an indirect role in slope deformation and failure. The weak structural plane formed by geological structure in slope rock and soil provides good basic conditions for landslide. Once triggered by external factors, the slope is easy to slide and collapse along the weak surface of the structure, forming landslides and collapses. Eight of the 76 landslides in the survey area are closely related to the fault structure, accounting for 10.53% of the total number of landslides.

2. 1.3.4 Rainfall

Rainfall is the most sensitive and positive factor in landslide and collapse disasters, and almost all collapses are closely related to rainfall. Rainfall can change the mechanical properties of slope rock and soil, reduce the deformation resistance, change the stress state of slope rock and soil, and induce slope disaster. Under the action of high-intensity rainfall, some landslides with good catchment conditions may be further transformed into landslide and debris flow geological disasters.

In addition, the occurrence of landslides is closely related to rainfall, and almost all landslides are related to rainfall. Most landslides are distributed in areas where the annual average rainfall is greater than1200mm. According to the data, landslides may be induced in areas with rainfall greater than 100mm, but after a long period of rainfall, when the soil is basically saturated and the rainfall is greater than 50mm, landslides may also occur. When the rainfall exceeds 200mm, landslides usually occur.

2. 1.3.5 human engineering activities

There are 64 landslides related to human engineering activities in this area, accounting for 85.53%. The influence of human engineering activities on slope is manifested in slope cutting and slope filling. Cutting the slope changes the slope height and gradient, greatly changes the slope shape, makes the slope which was in relative stress balance lose its support and face the air, and destroys the stress balance of the slope. The slope height increases, the internal stress increases linearly, the slope increases, and the tensile stress near the slope increases. With the expansion of the range, the maximum shear force in the stress concentration area at the foot of the slope also increases. Therefore, with the increase of cutting slope and slope height, the more sufficient the conditions for landslide caused by slope deformation, the greater the probability of landslide. On the other hand, human engineering activities disturb and destroy the structure of slope rock and soil, reduce the strength of local rock and soil, and provide conditions for landslide. In human engineering activities, there is unreasonable filling, forming high and steep slopes. The geotechnical structure of the original fill has been destroyed, and it is easy to induce landslides when filling into high and steep slopes.

Human engineering activity is a more positive factor in the collapse disaster factors. Human engineering activities show that changing the shape of the slope actually changes the suspended state and stress field of the slope. Intense slope cutting activities make the slope in stress balance lose its free support and form a free surface, and the slope stress changes accordingly. The steeper the slope, the stronger the tension at the top of the slope, and the more it can promote the collapse under the action of gravity. Statistics show that the collapse caused by human engineering activities in disaster-prone environment is 7 1.58%.

The investigation shows that there are only three collapses caused by blasting vibration in this area, which are distributed in the quarry area. The main reason is that some rock and soil bodies are cracked and loosened by blasting, and their slopes have been excavated into steep slopes and cliffs. The elastic wave produced by blasting causes additional stress to loose rock and soil, which makes the structure of rock and soil on the slope loose, produces fracture surface, accumulates due to repeated action, and promotes the deformation, destruction and collapse of rock and soil.

2.2 Land subsidence

The land subsidence in this area is characterized by uneven settlement of buildings in soft soil area. Land subsidence in soft soil area is distributed in alluvial plain areas of Quanzhou, Xiamen and Zhangzhou, with 32 places.

Because the clay minerals that make up soft soil are kaolinite, illite and chlorite, their chemical compositions are SiO2, Al2O3, Fe2O3, etc. Its physical and mechanical properties mainly have the following characteristics: natural water content is large (w > 36%), it is soft plastic-fluid plastic, only partially plastic; Large void ratio (E > 1), high liquid limit (W 1 > 45.9%), low permeability, high saturation, high compressibility (A 1-2 > 0.7 MPa- 1) and low strength (fk = 60. Most of them are underconsolidated and have rheological and thixotropic characteristics. When groundwater is pumped by building foundation pit drainage, with the decrease of groundwater level, the effective stress of soil layer increases, which promotes the consolidation of soft soil and produces settlement. However, when the building is placed in the soft soil area, the additional stress in the soil layer increases greatly, which leads to the settlement of the highly compressible soft soil due to compression and consolidation. The settlement of soft soil in the survey area is mostly distributed in Holocene marine soft soil. Because of the uneven compression and consolidation of soft soil, uneven settlement of buildings often occurs, which leads to the inclination, cracking and destruction of buildings.

2.3 Debris flow

At present, only two debris flows have been found in this area, both of which are small in scale, and the earthwork volume is less than 1 000 m3, both of which are rainstorm type and gully slope type. Its actual model is landslide-debris flow. The material composition of debris flow is mainly residual cohesive soil, with a small amount of strongly weathered rock mass. The provenance is on a concave slope with a slope of 30 ~ 35. The circulation distance of debris flow is relatively short. The annual gully flow in the provenance is debris flow in natural state.

The debris flow source area is located on the concave slope, which is beneficial to the accumulation of surface water. The erosion ability of surface water collection is strong. The provenance is located in the thick eluvial layer, and the eluvial soil is easily saturated and disintegrated when it meets water. However, the low strength of loose strongly weathered rocks provides a good material source for the formation of debris flows, both of which are caused by long-term and high-intensity precipitation. The rock and soil mass on the slope first appears landslide phenomenon, and then the landslide mass flows under the slope under the action of surface water flow. Both mudslides are located in areas where human activities are weak and have little impact on human beings.

3 regional disaster environment analysis [4]

The distribution, occurrence and development of geological disasters are closely related to natural geographical conditions, geological environmental conditions and human engineering and economic activities. The survey area is located in the humid climate zone of the south subtropical maritime monsoon, and the geological environment conditions in the southeast coastal area of Fujian are complex, with strong human engineering and economic activities. Due to the different shapes, structures and functions of geomorphic units and geological units, as well as the characteristics of regional climate environment, the disaster-prone environment in the survey area has obvious influence on the formation of regional disasters.

3. 1 Physical geography and climatic conditions

Although the area is located in the humid climate zone of the south subtropical maritime monsoon, the landforms in the survey area are diverse, from coastal plains and platforms to hills and mountains in the northwest, which makes the rainfall distribution in the survey area uneven. The average annual rainfall of coastal plains and platforms is 900 ~ 1 100 mm, while the average annual rainfall of hills and mountains can reach 1500 ~ 1700 mm, while the local evaporation of coastal islands and peninsulas is roughly equal to the rainfall. The investigation area is located on the Pacific coast and is affected by tropical warm and humid airflow, which will be encountered every year. Abundant and unevenly distributed rainfall and short-term typhoon rainstorm rainfall strongly promoted the development of geological disasters in the survey area. Warm and humid climate conditions accelerate the weathering process of rocks in the survey area, providing a good natural environment for the occurrence and development of geological disasters. From the analysis of the causes of geological disasters in the survey area, it can be seen that process rainfall and rainstorm have an abnormal influence on the development of geological disasters in the survey area, and almost all slope deformation and damage are related to rainfall. This shows that the regional climate and environmental conditions in the survey area are the most direct and sensitive factors affecting the distribution, occurrence and development of geological disasters.

3.2 Geological environment conditions

Geomorphologically, this area is close to the coastal areas, and affected by tides and storm surges, with obvious erosion in some coastal areas, such as the east coast of Xiamen Island. However, most areas in this area are mountainous and hilly areas with developed valleys and gullies, with different topographical forms, with a topographic slope of 20 ~ 40, and some areas are steep, with an altitude of100 ~1000 m.

Geologically, this area is located between the fault uplift belt along the southeast coast of Fujian and the volcanic fault depression belt in the east of Fujian, and it is in an intermittent rising area as a whole. Active geological structure, developed fault structure, deeply influenced by Changle-Nan 'ao active fault structure, complicated geological structure and broken rocks. Areas with seismic fortification intensity of ⅶ and ⅷ. The complexity of geological structure provides a good environment for the development of geological disasters in the survey area.

Geologically, soft soil with high compressibility and low strength and saturated liquefied sand with different thickness are widely distributed in the plain areas of coastal areas, which makes these areas prone to soft soil settlement, earthquake settlement and saturated sand liquefaction. However, residual soil and various parent rocks with different thicknesses are distributed on the platform, hills and mountains, with complex lithology and different strength. The complex geotechnical conditions provide a good environment for the development of geological disasters, especially the residual soil of hilly platform in the survey area is deep, but it is greatly affected by rainfall, which makes the geological disasters develop in the survey area.

3.3 Human engineering economic activities

The survey area is the Golden Triangle area of Xiamen, Zhangquan and South Fujian, with extremely active regional economy, high population density and strong engineering activities. There are many highways at all levels, including Xia Quan Expressway, Xiamen-Zhangzhou Expressway, Zhangzhao Expressway, Zhang Long Expressway, yingtan-Xiamen Railway, Zhang Quan Railway, National Highway 324 and Jiulong River Water Diversion Project, Jinjiang Water Diversion Project, Houshi Power Plant, Jixing seawall and Gaoji seawall. Major projects and various provincial and county highways have greatly changed the geological environment. The economic activity in the survey area is vigorous, and the per capita GNP is more than 10000 yuan. Moreover, the population density in the survey area is relatively high, with the population density of 800 ~ 1800 people /km2 in coastal areas and 200 ~ 700 people /km2 in mountainous areas. There is less land available in mountainous areas, resulting in a situation of more people and less land, forcing residents to dig mountains and cut slopes when building houses, thus forming a large number of houses with high and steep back slopes. Dense population distribution and intense engineering economic activities have greatly changed the local geological environment, the forest vegetation coverage rate has decreased, local soil erosion has intensified, and geological disasters have occurred frequently. The human engineering economic activities in the survey area are active and intense, which provides a good environment for the development of geological disasters.

4 conclusion

There are several types of geological disasters in this area, such as collapse, landslide, debris flow and land subsidence, which have the general characteristics of many points, wide area, small scale, high frequency, great harm and great influence by human activities and rainfall. Due to the geographical location of this area, there is also a geological disaster-prone environment in the survey area. Abundant precipitation, strong typhoon influence, unfavorable topographic conditions, complex geological structure, complex geotechnical conditions, active and intense human engineering and economic activities are all aggravating the development of geological disasters.

refer to

He is Jin Yong. Characteristics, causes and countermeasures of geological disasters in Fujian. Geology of Fujian, 1995(4)

Yan, Yang Shun 'an, human. Landslide. Wuhan: China Geo University Press, 2003.

[3] Gao Tianjun, He. Geological disasters in Fujian coastal and sea areas and their prevention. Fujian Geology, 2000(2)

Chen. Study on environmental geology of Xinjiang section of New Eurasian Continental Bridge. Beijing: Geological Publishing House, 200 1

Development characteristics and formation mechanism of geological disasters along the coast of southern Fujian

Shi Wenyao

(Fujian Geological Engineering Exploration Institute, Fuzhou 350002)

Abstract: Based on the investigation and analysis of geological disasters, this paper expounds the characteristics of geological disasters, and analyzes the forming factors of geological disasters from three aspects of geographical climate conditions and geological environment conditions. The conclusion is that the shape, structure and function of geological units and geomorphic units, as well as climate characteristics, have obvious influence on the formation of geological disasters.

Keywords: geological disasters; Characteristics and forming factors of geological disasters; The environment that forms geological disasters