A preliminary study on the distribution and prevention of sudden geological disasters in Urumqi

Zhang Lan1 Yang Hongwei2

(1Geological Environment Monitoring Institute of Xinjiang Uygur Autonomous Region, Xinjiang, 830000; 2Master’s class of 2001 at China University of Geosciences, Beijing, 100083)

Abstract Urumqi City’s complex geological and environmental conditions, fragile ecological environment, and frequent human coal mining and other engineering activities have caused serious ground subsidence, collapse, landslides, and debris flow disasters in the area, and their distribution has a certain pattern. Based on the principle of giving priority to prevention and combining avoidance with prevention and control, measures such as monitoring and forecasting, avoidance, engineering management, and comprehensive planning of project construction layout are adopted to comprehensively carry out the prevention and control of the above-mentioned disasters in a planned and step-by-step manner to achieve the purpose of disaster prevention and reduction.

Keywords Distribution and prevention of sudden geological disasters in Urumqi

Foreword

Urumqi is located at the northern foot of the Tianshan Mountains and the southern edge of the Junggar Basin, surrounded by mountains on three sides. The Bogda Mountains in the northeast and the Tianshan Mountains in the south surround Urumqi on three sides from northeast to southwest. Only the plain in the northwest borders Changji City and Miquan City. Among them, the area of ??high mountains and mid-low mountains accounts for 53.1%, the area of ??low mountains and hills accounts for 20.6%, and the area of ??plains only accounts for 26.2%.

Urumqi City is located in the composite part of the Tianshan Eugeosynclinal Fold Belt and the Junggar Depression. The tectonic and neotectonic movements in the area are relatively strong, faults and folds are very developed, the landform types are complex, and the stratigraphic lithology changes greatly. , the geological environment is fragile. The mountains are high and steep, and the valleys are developed, forming landforms such as steep slopes, cliffs, deep ravines, and pedestal terraces. The structure and weathering have caused the development of joints and fissures in the rock layers and fragmented rock masses, providing basic conditions for the occurrence of collapses, landslides, and debris flows. The rainfall in the area is not very large, but the rainfall is concentrated and the intensity is high. Coupled with human activities, collapses, landslides, and debris flow disasters often occur. The area is rich in coal resources, and ground subsidence disasters caused by human coal mining activities are also very serious. .

With the acceleration of economic development, the intensity and scope of human engineering activities continue to increase, and the damage to the geological environment will become more and more serious, which will lead to the continuous intensification of geological disasters and the consequences for human life and property. Losses and threats will also continue to increase. The prevention and control of geological disasters has become one of the main tasks currently facing people.

1 Types, distribution and hazards of sudden geological disasters

According to the results of the "Urumqi Geological Hazard Investigation and Zoning" completed by the Xinjiang Uygur Autonomous Region Geological Environment Monitoring Institute in 2001, The main types of sudden geological disasters in Urumqi include ground subsidence, collapse, landslides, and debris flows, with a total of 159, including 102 ground subsidences, 20 collapses, 14 landslides, and 23 debris flows. Among the 159 geological disasters, 97 have occurred and 62 are potential points, all of which are hidden danger points.

1.1 Ground subsidence

Ground subsidence is mainly caused by roof collapse after coalfield goaf (101 places), followed by roof collapse of civil air defense projects (1 place). The city's ground subsidence area is 4.85km2. The collapses were mainly small (71), followed by medium (8) and large (2).

The distribution of ground subsidence is closely related to the distribution of coal seams. Jurassic coal-bearing strata are distributed in Hongguangshan-Lucaogou, the northeastern edge of Chaiwobao Basin, Xishan, Houxia Basin and Aiweiergou area. After years of mining, a large area of ??goaf has been formed. Top caving, blasting vibration, earthquakes or rainfall, infiltration of snowmelt water, and progressive damage to security coal pillars will cause the surface rock and soil to collapse and form ground subsidence. It mainly takes the form of two types of bead-shaped circular collapse pits and elongated collapse trenches on the surface. The diameter of a collapse pit is generally 10 to 20m, with a maximum of 30m, and a depth ranging from 5 to 15m; the width of a collapse trench ranges from less than 1m to 120m, and the depth ranges from less than 1m to 40m.

The main hazards of ground subsidence are: ① harming or threatening people, vehicles, and livestock that accidentally enter the goaf area; ② destroying the original ecological environment; ③ damaging farmland and groundwater systems, causing shallow wells to dry up and irrigation leaks ; ④ A large number of underground goafs that have not collapsed have brought great inconvenience and accident risks to ground construction, affecting urban construction and development; ⑤ Caused uneven settlement or cracks in building foundations. For example, in March 1972, there was a flood in Xiaohonggou, Dongshan District. People were trying to prevent floods above the goaf area of ??the coalfield, and suddenly the ground collapsed, killing 27 people.

According to recorded statistics, the ground collapse has caused 33 deaths, 2 injuries, and direct economic losses of 2.0832 million yuan.

1.2 Collapse

Collapses are divided into two forms: bedrock collapse (8 locations) and Quaternary accumulation layer collapse (collapse) (12 locations). The collapses were mainly small (19), and only one bedrock collapse was medium.

Bedrock mountainous areas are affected by folds and fault structures, and rock masses are broken. Bedrock collapses can be seen everywhere on steep slopes, but the scale is small. Mountainous areas are generally sparsely populated, and the possibility of disasters is low. In human engineering economic activity zones, such as mines, mountain roads, pastoral roads, etc., they are vulnerable to collapse hazards. There are several isolated low mountains and residual hills in the central urban area (such as Hongshan, Pingdingshan, etc.). Due to the narrow mountains, high and steep slopes, developed joints and cracks, and the dangerous rock masses are protruding, the slopes are often at risk of collapse. When a collapse occurs, blocks of different sizes roll down rapidly, forming a pile of rocks at the foot of the slope.

The collapse of accumulation layers is mainly distributed in high terraces and high steep slopes, and appears as loose scarps or cliffs on slopes. The collapse is mainly rapid collapse towards the foot of the slope, forming a loose accumulation cone at the foot of the slope. The scale of collapse is small, but the steep slopes or slope foot are often inhabited, which can easily lead to disaster events.

The main hazards of collapse are: threatening the life and property safety of residents at the foot of the slope, damaging and blocking roads, smashing transportation vehicles and engineering facilities at the foot of the slope, etc. For example, there is a dangerous slope at the top of Heishan Mountain in Pingding Mountain. The dangerous rocks at the top are abrupt and prone to collapse, posing a serious threat to the residents at the foot of the slope.

The 20 collapses in Urumqi caused no casualties and little property damage.

1.3 Landslides

Landslides in the area are not very developed. They are mainly soil (loose accumulation layer) landslides (11,), followed by bedrock landslides (3). All are small in size.

Soil landslides mostly occur on steep slopes and ridges in mid- and low-mountain hilly areas. The landslides are loosely accumulated residual slope layers, mainly cohesive soil and gravel soil, with a loose and bonded structure. It has small force and strong water permeability, and is obviously different from the underlying bedrock. It is prone to intra-layer and contact surface landslides. It is generally small in scale, ranging from more than ten meters to dozens of meters in length and width, with a maximum of no more than 100 meters, and a thickness of 1 to 3 meters. . Landslides occur with the nature of sliding first and then slipping.

There are three bedrock landslides in the area: one is the Heijiashan landslide; the other is the landslide at the mining face of the open-pit coal mine in Miaoergou Village, Shuixigou Township. Both landslides are related to artificial mining. Stone and coal mining engineering activities are closely related; the third is a landslide on the natural steep slope on the east bank of the upper reaches of the Toutun River.

The hazards of landslides are firstly burying and destroying residential areas and buildings at the foot of the slope, causing casualties and property losses; secondly, they are blocking roads and damaging channels. For example, in 1984, 1990, and 1996, the eastern, western, and middle sections of the south side of Heijia Mountain slid successively, resulting in 2 deaths, the collapse of 17 houses, and economic losses of more than 100,000 yuan. Currently, landslides in the middle and western sections still threaten residents. 161 people and more than 90 houses are safe at the foot of the Heijia hillside.

Landslides in the jurisdiction have killed 4 people and caused direct economic losses of 401,000 yuan.

1.4 Debris Flows

The debris flows in the area are all ravine type debris flows. The scale is mainly small (18), followed by medium-sized (5).

Debris flow formation areas are mostly limited to medium and low bedrock mountainous areas with an altitude of 1000 to 2500m. Mainly the Alagou area where the former Nanshan Mining Area Government was seated, Xiaosagou, Daxigou, Gangou and other places in Dabancheng in the Nanshan piedmont. There are developed gullies in this range, and the longitudinal slope of the gully bed is large, which has the terrain conditions for the formation of debris flow gullies. At the same time, the bedrock is exposed, weathered and broken, the surface vegetation coverage is low, and there is a large amount of loose material on the slopes, slope toes and gully beds. Debris materials can cause debris flows in the event of heavy rain or heavy rain, mostly from June to September when heavy rains are concentrated.

The hazards caused by debris flows mainly occur in accumulation areas. Most of them are caused by people's insufficient judgment of debris flow ditches and their lack of awareness of their hazards. They mistakenly move in disaster risk areas and become victims or threats of disasters. causing heavy losses. The main damage methods of debris flow include: ① Siltation hazard. Facilities in the accumulation area may be destroyed and buried. For example, on August 14, 1995, a mudslide in the Nanshan Mining Area buried workshops, workshops, warehouses, office buildings, stores and other buildings with an area of ??2,000 m2, causing a direct economic loss of 12.804 million yuan; ② Washout hazards. Debris flow has the characteristics of rapid force, fast flow rate and high energy, so it has huge scouring and impact capabilities, which directly threatens the safety of engineering facilities and people's lives and property in the downstream area of ??the debris flow. For example, on June 24, 1988, a sudden mudslide occurred at the location of Dongfeng Machinery Factory in Nanshan Mining Area, destroying an area of ??6,422 m2 of various factories, warehouses, offices, and civilian housing, resulting in a direct economic loss of 6.0778 million yuan. ③Hazards of blockage. Roads in mountainous areas are mostly built along rivers. When mudslides occur on both sides of the valley, it is easy to wash away or block the roads, causing traffic congestion. In addition, the occurrence of debris flows will also cause blockage and damage of water diversion channels, blockage of river ditches, etc. In 1997, the first mudslide occurred in the Qingnian Canal in Daxigou, which destroyed and buried 0.8km of the main water diversion channel and 0.5km of the flood discharge channel, resulting in a direct economic loss of 2.8 million yuan.

In summary, there are obvious patterns in the distribution of sudden geological disasters in Urumqi. The bedrock mountainous areas in the east and south are dominated by collapses and debris flows, followed by ground subsidence and landslides, and debris flows are the main hazards. The largest one; ground subsidence is the main problem in low mountains and hills, followed by landslides and landslides; sudden geological disasters are not developed in plain areas.

Various sudden geological disasters occurred in Urumqi, resulting in 37 deaths, 2 injuries, and direct economic losses of 25.1952 million yuan. It is predicted that 2,524 people will be threatened, and direct economic losses of 90.07 million yuan may be caused within the scope of the disaster.

2. Prevention and Control Plan for Sudden Geological Disasters

2.1 Prevention and Control Principles

(1) Highlight the "people-oriented" principle. Put the safety of people's lives first and minimize casualties and property losses caused by sudden geological disasters. Further popularize knowledge on geological disaster prevention and control, and improve the public’s awareness and ability of disaster prevention and reduction.

(2) Adhere to the principle of “prevention first, combining avoidance with governance”. Actively adopt forecasts and early warnings to prevent and control sudden geological disasters, especially man-made sudden geological disasters.

Give full play to the leading role of governments at all levels, mobilize the enthusiasm of society and the general public, adapt measures to local conditions, pay attention to reasonable avoidance, and select key governance areas.

(3) Adhere to the principle of comprehensive planning, highlighting key points, taking into account the general, and implementing step by step. According to the distribution pattern, development characteristics, risk and degree of harm of sudden geological disasters, combined with the local economic level, development plan and overall consideration, the prevention and control of sudden geological disasters shall be divided into light, heavy, slow and urgent. , to combine the short term with the long term, and combine local focus with environmental improvement.

(4) Adhere to the principle of combining engineering governance and non-engineering governance. On the premise of adhering to prevention and avoidance, implement engineering management of hidden danger points of sudden geological disasters that are particularly harmful, further improve the emergency monitoring and information systems for sudden geological disasters, and turn passive emergency disaster avoidance into proactive Disaster reduction and prevention achieve a virtuous cycle in the prevention and control of sudden geological disasters.

(5) Implement the "Regulations on the Prevention and Control of Geological Disasters", which stipulates that the government organizes the management of geological disasters caused by natural processes, and the beneficiaries bear reasonable burdens; for geological disasters induced by human beings, the person responsible shall be responsible.

2.2 Prevention and Control Plan

2.2.1 Monitoring and Forecasting Plan

Monitoring and forecasting is one of the important and indispensable means in the prevention and control of sudden geological disasters . Utilize the monitoring network system to fully mobilize the power of the masses and implement monitoring and forecasting of sudden geological disasters through a combination of professional monitoring and mass monitoring.

According to the characteristics of sudden geological disasters in Urumqi, the following simple methods can be adopted:

1. Ground subsidence

(1) Macrogeological survey method: Subtle changes in surface cracks and collapses are manually investigated and monitored.

(2) Monitor and record changes in the scope of the goaf area in a timely manner.

2. Collapse and landslide

(1) Earth deformation and displacement monitoring: Set up stable and fixed detection control points on the periphery of the collapse and landslide area, and arrange monitoring points inside the collapse and landslide body. The line-of-sight method measures the dynamics of displacement changes of monitoring points.

(2) Relative displacement monitoring of cracks: on both sides of the crack between the avalanche parent body and the avalanche body, as well as on both sides of the crack on the affected house wall, retaining wall, and the side wall of the mortar stone ditch, Set up monitoring points to measure the changes in cracks, or stick cement mortar sheets, paper, etc. on the cracks to observe the changes in cracks.

(3) Macrogeological survey method, using manual daily inspections to investigate and monitor various subtle changes in the landslide area.

3. Debris flow

The key to monitoring debris flow in the jurisdiction is mainly to monitor the triggering factor rainfall (heavy rain). The method of setting up fixed rainfall monitoring stations and temporary rainfall monitoring points can be used to monitor rainfall. monitor.

The above monitoring results should be reported to the superior authorities in a timely manner in order to organize analysis and make accurate and timely forecasts.

2.2.2 Avoidance plan

For most sudden geological disasters in Urumqi, avoidance is the best choice. For example, for most coalfield goafs, ground collapse disasters, because the goafs are mostly barren mountains, as long as people and vehicles are prohibited from entering, casualties and property losses can be avoided; on the other hand, due to the foot of most collapses and landslides, There are only some bungalows in the area and the mouth of the debris flow ditch, and the living conditions are poor. In addition, the cost of disaster management is very high. Under the condition of very limited prevention and control funds, relocation is necessary compared with the cost of project management. If the relocation cost is low, The options are appropriate.

2.2.3 Governance plan

Remediation is a disaster prevention countermeasure adopted when avoidance methods are not suitable. Further engineering investigation of the disaster body must be carried out before taking control measures. Different methods are adopted for different types of sudden geological disasters in Urumqi.

Ground subsidence: For underground goaf areas, forced roofing can be adopted and backfilled together with the original subsidence to restore the ecological environment; in important project construction or planning areas, grouting can be used to eliminate underground voids.

Collapses and landslides: Drainage projects (mainly surface drainage and anti-seepage); retaining projects (anti-slide stacks, anti-slide piles, anti-slide walls, anchoring, etc.); load reduction, removal, and solidification projects ; Slope protection, shotcrete and other engineering measures.

Debris flow: Adopt comprehensive management measures such as monitoring, engineering (blocking, drainage), biology, and the entire basin.

2.2.4 Reasonable planning of various engineering construction layout plans

In the prevention and control of geological disasters, reasonable planning of various engineering construction layouts is an important step in embodying the "prevention first" policy. One of the means. In the past, due to people's lack of sufficient understanding of the occurrence of geological disasters, on the one hand, the damage to the geological environment caused by the engineering construction itself led to the occurrence of disaster events; on the other hand, placing the engineering construction within the dangerous range of geological disasters led to the occurrence of disaster events and the loss of human lives. Casualties and property damage. Therefore, rational planning of the construction layout of various projects is also an important part of geological disaster prevention and control. To this end, various engineering construction layouts are carried out in accordance with the "Urumqi City Master Plan" and the "Urumqi Land Use Master Plan", and geological disaster prevention and control are effectively integrated with urban construction planning, mineral development and land master planning.

According to the requirements of the Ministry of Land and Resources' "Notice on Implementing Geological Disaster Risk Assessment of Construction Land", urban construction, engineering construction in geological disaster-prone areas and other engineering projects that may cause geological disasters are prohibited. , a geological disaster risk assessment must be conducted during the project site selection stage.

Based on the principle of “whoever induces it, who governs it”, implement the mine geological environmental protection plan preparation and review system and the new mine access system, strictly implement the mineral reserve overburden occupation system, and increase the supervision and management of mines. Ensure the safety of people's lives and property, protect mineral resources, protect and restore the ecological environment, and promote the sustainable development of the mining industry.

Strengthen the management of land use in areas prone to geological disasters, and while rationally planning the construction layout of various projects, prevent illegal use of land for various project constructions that may induce geological disasters; avoid improper use of land, As a result, engineering construction mistakenly enters the danger zone of geological disasters, thus triggering disaster events.

2.2.5 Bioengineering treatment plan

Bioengineering treatment is undoubtedly one of the important measures in the prevention and control of geological disasters, and the longer the time, the more significant the effect. However, for sudden geological disasters in Urumqi, it is currently not suitable for large-scale implementation due to the constraints of the natural geographical environment conditions in the area. One to two debris flow ditches in the Alagou area can be selected as pilot projects for implementing bioengineering control measures to obtain methods and experience in bioengineering control of debris flows in arid areas.

By comprehensively implementing the prevention and control of sudden geological disasters, we will maximize the safety of people's lives and property, minimize the frequency and losses of geological disasters, and promote rapid social and economic development.