"4" Landslide in Mawo Village, Oichi Township, Qixingguan District, Bijie City, Guizhou Province

20 13 On April 27th, a landslide occurred in Mawo Village, Oichi Township, Qixingguan District, Bijie City, Guizhou Province. The landslide volume is about 450,000 m3, and it is a push landslide. Eight people were killed in the landslide.

1 geological environment conditions

1. 1 topography and formation lithology

The landslide area is located in Oichi Township, Bijie City, northwest Guizhou Province, and is located in the transitional slope zone between the eastern Yunnan Plateau and the central Guizhou Plateau. The disaster point is a gully slope landform with low erosion and dissolution degree, with a slope of about 25 and a vegetation coverage rate of about 40%. The strata in the area occasionally appear on the slope, which are sequentially from top to bottom: Quaternary eluvial layer (), middle and upper Cambrian Loushanguan Formation (), and middle and lower Gao Tai Formation Qingxu Dongfu Formation, in which Quaternary layer () is yellow and yellowish brown gravelly clay and silty clay, with the maximum thickness of about 5m. The lithology of Loushanguan Formation of Middle and Upper Cambrian is gray and dark gray medium-thick dolomite, including chert dolomite and argillaceous dolomite, which is exposed in the northwest of the disaster site and widely covered by Quaternary. The lithology of Gao Tai Formation of Middle Cambrian is yellowish gray thin to medium silty dolomitic mudstone with siltstone bands, with the occurrence of 320 ∠ 32, and the thickness of surface strongly weathered layer is about 4.5m, among which this mudstone is the main material component of this landslide geological disaster. The Lower Cambrian Qingxu Dongfu Formation consists of gray medium-thick dolomite, limestone and argillaceous dolomite, which are distributed in the southeast of the landslide.

1.2 geological structure

The southern part of Qixingguan area is connected with Nanling east-west composite structural belt, and the northern edge is connected with "Central Guizhou Uplift". The northern part is connected with the "Sichuan Basin", the primary subsidence zone at the westernmost end of the Neocathaysian tectonic system. Since the Yanshan movement, the crust has changed intermittently for many times, forming a complex structural background and developing folds and faults. Oichi fault is the main fault structure in this area, which has the nature of reverse fault. Strike northeast, tend to southeast, dip angle is 50 ~ 75, length is 35km, fracture width is 2 ~ 10 m, and the material composition is mainly fault zone breccia. According to the Seismic Response Spectrum Characteristic Period Zoning Map of China (GB18306-2001) (1∶ 40,000), the characteristic period of the seismic response spectrum in this area is 0.35s, the peak acceleration of the earthquake is less than 0.05g, and the basic earthquake intensity is less than VI degree, so it is a crustal stability area.

1.3 Hydrogeological conditions

The landslide disaster occurs in the recharge area and runoff area of hydrogeological units, and there is no underground spring point exposed. The recharge is mainly atmospheric precipitation, and the runoff is mainly affected by topography and geological structure. Runoff generally passes through rock joints, fractures and fault zones, and is discharged into the gully erosion datum in the northwest and west in the form of falling springs.

1.4 engineering geological conditions

The engineering geological conditions of the site are simple. According to the engineering geological rock group, it can be divided into loose rock group, soft rock engineering geological rock group and hard rock engineering geological rock group. Among them, the loose rock group is mainly Quaternary residual, and the lithology is gravelly clay, with low strength and no practical engineering significance. The engineering geological strata of soft rock are mainly silty mudstone and dolomitic mudstone of Cambrian Gao Tai Formation. Because of its low mechanical strength, weak weathering resistance, soft texture and poor engineering mechanical properties, landslides and unstable slopes are prone to occur. Hard rock engineering geological rock formation is mainly Qingxu Dongfu Formation of Loushanguan Group of Lower Cambrian. The lithology is mainly dolomite, with high mechanical strength, strong weathering resistance and good engineering mechanical properties.

3 landslide disaster characteristics

According to the field investigation and analysis, the sudden landslide occurred on April 27th, 2003+2065438, and the specific analysis is as follows:

3. 1 Basic characteristics of landslide

The rear edge elevation 1300m, the front edge elevation 1230m, the vertical height difference of 70m, the longitudinal length of the landslide of 220m, the lateral width 150m, the average thickness 15m, and the landslide volume of about 450,000m3. The sliding body forms a three-stage back slope along the main sliding direction. The first threshold is the negative terrain of the trailing edge depression formed by the staggered thresholds at the trailing edge of the landslide. The second sill is the uplift and depression groove formed by the sliding of the ridge in the middle of the slope. The third stage is the rotation of landslide front with the help of ravines. The original topographic uplift and the trailing edge form a back slope ridge. The main sliding direction of landslide is 260. During the sliding process, the landslide filled the valley. The front edge is characterized by the uplift of the original landform on the left side of the landslide and the natural accumulation of landslide rock and soil on the right side according to the valley shape (Figure 1).

Figure 1 Schematic Diagram of Landslide Geological Environment

The landslide forms a staggered ridge with a slope height of 5 ~ 15m from both sides to the back wall, forming a staggered slope with a slope of about 70, a maximum horizontal distance of 55m, a crack strike of 350, a length of 30m and a width of 3 ~ 5cm. There are many feathered cracks on both sides of the landslide, and the cracks are generally trapped to form small-scale unstable blocks.

The landslide has caused 8 deaths. According to the field investigation, the landslide accumulation body is loose. After the landslide occurred, the largest staggered ridge was formed, with a height of 15m and a slope of 70, and cracks developed at the rear edge. Therefore, under the influence of rainfall, earthquake and other external forces, it is possible to form debris flow geological disasters along the valley, and small-scale landslides and landslides may occur at the rear edge of landslide accumulation and slope.

3.2 secondary landslide disaster

According to the field investigation, there are hidden dangers of landslide geological disasters on the right side of the landslide. Affected by the landslide geological disasters that have occurred, a crack with a strike of 230, a length of 12m and a width of 0.5 ~ 10cm is formed in the middle and lower part of the right mountain, and then the slope of the hidden mountain is 320 in the same direction as the occurrence of rock strata, which is prone to bedding landslide geological disasters. It is preliminarily estimated that the hidden danger of landslide geological disasters is about 65.438 billion m3. Once the landslide disaster occurs, it will cause great damage to the houses and land at the foot of the slope.

Fig. 2 Overall map of landslide

4 Analysis of the causes of landslide disaster

The basement stratum of the landslide area is argillaceous dolomite of the Loushanguan Formation of the Middle-Upper Cambrian, and there is a Oichi fault in the northwest of the area. Due to the influence of NE-trending fault structure, the slope rock mass is broken, which has no supporting effect on the stability of mudstone in the middle of Gao Tai Formation. "4.20 Sichuan Ya 'an Earthquake" and "4.25 Sichuan Yibin Earthquake" occurred, and the landslide area was obviously felt. The occurrence of earthquake intensifies the fragmentation of rock mass, reduces the mechanical strength of rock mass, and increases the possibility of soil changing from hard plastic to soft plastic. The human engineering activities here are mainly highway widening and brick factory production. Long-term engineering activities have transformed the original terrain of the slope, and the slope gradient has become steeper, which has aggravated the slope instability.

Fig. 3 Hidden danger of secondary landslide disaster

Comprehensive analysis shows that the slope of Bijie landslide geological disaster system on April 27 is affected by many factors, and the rock and soil mass is unstable under the action of self-weight.

5. Emergency prevention and control of geological disasters

After the disaster, party committees and governments at all levels in provinces and cities attached great importance to it and set up an emergency rescue headquarters, which consists of eight working groups, including on-site rescue group, surrounding people evacuation group, on-site monitoring group, on-site order maintenance group, medical rescue group, after-treatment group, logistics support group and publicity and reporting group. Timely rescue and disaster relief, at the same time delimit the scope of geological disaster danger zone, evacuate and resettle villagers in danger zone, set up warning signs and warning lines, and arrange special personnel to monitor and inspect. The Department of Land and Resources of Guizhou Province organized relevant technical personnel of Guizhou Geological Environment Monitoring Institute to form an emergency investigation team to investigate the current situation and analyze the reasons. On April 28th 18, the expert group of the Ministry of Land and Resources arrived at the scene to guide the rescue work.