Evaluation, prediction and evaluation of geological disaster types and their risks.

I. Types and characteristics of geological disasters

There are many geological disasters in Gansu, such as debris flow flood erosion, saline soil erosion, wind erosion and sand burial, goaf collapse, earthquake liquefaction and so on. The breakdown is as follows:

Debris flow and flood erosion

The distribution of debris flow in this section is small and the frequency of debris flow is low, but the disasters in some areas are serious. Sparse debris flow is mostly water-rock flow, which is often indistinguishable from sediment-laden flood, and the loss of engineering facilities and people's lives and property caused by flood scouring is more prominent. It is in line with the reality of this section to analyze it as a disaster.

Debris flow and flood erosion are mostly developed in the Qilian Mountain area on the south side of the corridor, as well as the gully beach and gully bed behind the mountain. Large-scale debris flows in the corridor are distributed in the central areas of heavy rainfall (rainstorm, heavy rainstorm), such as Linze Banqiao, the south of Yongchang-Wuwei and the northern foot of Changling Mountain in the east of Gulang. In the east of the corridor, the piedmont of Qilian Mountain in the south of Wuwei is a high-risk area, and the Fengliangzhai-Baidunzi section in the west of Wuwei is a high-risk area. Most of the western corridor and Beishan area were washed away by floods, and there were no reports of mudslides. Controlled by the monsoon, it mostly occurs in the rainy season in June, July and August every year.

The topographical conditions of debris flow formation mainly include basin area, hillside slope and gully bed slope. Debris flow gullies mostly originate from Qilian Mountain, and their drainage area is generally between 10 ~ 100 km2, with good water collection conditions. The steep slope and steep slope between gully and river bed in each basin are beneficial to the rapid collection of precipitation and the treatment of mud and gravel blocks. Solid matter mainly comes from weathered rock debris and flood alluvium. In Qilian Mountains, neotectonic faults are developed, rock mass is broken, physical weathering is strong, and there are many landslides, which provide a solid material source for debris flow. When the debris flow rushes out of the gully, a large number of mud and gravel blocks stop accumulating. At this time, when the water continues to flow downstream, if more floods come in, the flow will increase rapidly and then violently impact the ditch bed. Coupled with a large number of mud and gravel, it will once again evolve into a mudslide and expand the disaster. Strong surface runoff is the dynamic condition for the formation of debris flow. The main source of debris flow in this area is atmospheric precipitation. Although the annual precipitation in the corridor area is less than 200mm, the distribution of precipitation is uneven, with great interannual variation and local rainfall intensity. Moreover, the annual precipitation in Qilian Mountain area upstream of debris flow gully is much larger than that in corridor area, which can reach more than 500mm/a, further strengthening the water source conditions for debris flow formation. For example, in Dajing, Gulang County, the precipitation within 2.5 hours in August 1.977 was 154.5mm, and the average rainfall intensity was 61.8 mm/h. Debris flow broke out, burying 70,000 mu of farmland, destroying more than 5,000 houses, killing 100 people, cutting off traffic and communication, which is a super-large-scale debris flow disaster.

The perennial rivers in Heihe and Shule river basins in the west of the corridor area all have their own fixed riverbeds, with relatively developed vegetation on both banks and relatively fixed mainstream lines in flood season. Generally, mudslides do not break out. Sediment-laden flood in rainy season is the main disaster to the riverbed and both banks of the river, and it often damages linear projects such as roads and pipelines. However, many seasonal gullies are mostly sandy pebble beds, without floodplains and vegetation, and sparse debris flows may occur under the excitation of heavy rainfall. There are seven large river valleys and gullies in the Qiaowan-Yumen section of the evaluation area, and all pipelines pass vertically, which often leads to flood scouring disasters.

(2) Saline soil corrosion

Corrosion of saline soil is a slow chemical process, which is different from all other geological disasters. Saline soil along the line is mainly distributed in some low-lying areas between Hongliuhe and Jiayuguan in the west, and the groundwater level is small, even overflowing into springs. Due to the strong evaporation of groundwater, the depth of soil layer is generally less than 4m. The closer to the surface, the greater the salt content, and the maximum salt content is 23%. Belonging to sulfate chlorine saline soil. The evaluation area is mainly distributed in Badaogou-Qidaogou of Shule River, Heihe-Linze-Wujiang-northern Zhangye City, Gulang Baidunzi and other places.

(3) Wind erosion and sand burial

Mainly distributed in the southern edge of Tengger Desert in the east. The total length of the pipeline crossing the desert area is about 26km, mainly fixed and semi-fixed sand dunes, with few moving sand dunes. There are also small sand dunes in the west, but more are Gobi. Due to the movement of movable sand dunes, gas pipelines and ancillary facilities can be buried, increasing the static pressure on the upper part of the pipeline, affecting the normal operation of the pipeline or deforming it. On the contrary, strong wind erosion can remove the landfill layer of the pipeline, resulting in the pipeline being exposed on the ground.

(4) goaf collapse

There are two coal mines in the evaluation area, Shandan Coal Mine and Gushandun Coal Mine. Shandan coal mine is a state-owned coal mine, which is divided into two mining areas, and the evaluation area is the second mining area. In 1950s and 1990s, the mining horizon 150 ~ 300m deep has been basically mined, with only a few coal miners mining sporadically, and the mined-out area is 4. 15km2. Gushandun Coal Mine is a small coal mine, with the mining horizon of 120 ~ 130m, the main shaft roadway is 380m long, and the Wuwei-Dajing Highway passes through it, with an area of 0.53km2 At present, no ground collapse has been found in the two coal mines.

⑤ Earthquake liquefaction

Because of the special tectonic position, there have been many strong earthquakes in this section, but there are few records about earthquake liquefaction in modern history books. Earthquake liquefaction is mainly distributed in the area from Xinhua Township in Linze County to Dongle Township in Zhangye City. For example, during the Gulang M8 earthquake on May 23rd, 927, there were many records of ground fissures and black sediments.

Second, the risk assessment of geological disasters

On the basis of comprehensive consideration of environmental geological conditions, landforms, types, distribution and frequency of geological disasters, Gansu section is divided into 9 sections for evaluation.

(1) Hongliuhe to Qiaowan section (0 ~198km)

Flood scouring: There are 36 gullies with different sizes in this section, and the width is generally10 ~ 200 m. Because most of the sections are located in the downstream of railway subgrade, the flood in the upstream floodplain will flow into the downstream gullies through bridges and culverts after being blocked by subgrade protection works, and the sediment concentration is low, mainly scouring, so the danger is not great.

Saline soil corrosion: Sulfuric acid-chloride saline soil is scattered in the depressions 5 kilometers southeast of Koizumi and Xia Dong Station. The salt content at Koizumi's depth of 0. 1m is1.44%; The salt content at the depth of 0. 1m in the southeast of Xia Dong Railway Station is 8.24%, and the salt content at the depth of 1.6m is 2.86%. However, because the buried depth of the pipeline is ≥2.0m, the salt content of the soil under it drops sharply, so the danger is less.

(2) Qiaowan-Yumen Town (198 ~ 244km)

There are five large perennial rivers (Shule River, Shanshuigou, Wudaogou, Sandaogou and Wanjiahe) and two seasonal rivers (No.4 and No.6) in this section, among which No.3, No.4, No.5 and No.6 are the main flood discharge channels, and all these rivers pass through the evaluation area from south to north, with the main disaster mode of scouring and moderate risk.

The width of the above-mentioned river ditch is tens of meters, the largest of which exceeds kilometers, and the valley slope is less than 1%. The river channel is relatively fixed, and the riverbed is dominated by gravel and pebbles; River banks are mostly composed of silt, and the flood peak discharge is generally less than 150m3/s, and the maximum is 706m3/s ... The destructive power of floods to ground buildings is enormous. According to the document number. Jiuquan area 1999: 103,1August 4-7, 1999, the flood peak discharge at the head of changma canal was 32 1 ~ 355 m3/s, and the flood dike was divided into two branches after being washed away. Most of the water in front goes down to the west, heading straight for the main canal of Xinxi, and the Wang Yang is formed under the interception of its flood control dam. The water storage depth is about 8m, and the water volume is about 65,438+10,000m3. In the end, the old west main canal burst and poured backward, and the dam along Wudaogou River broke and destroyed the highway bridge. The flood * * * washed away the main canal 1.74km, the dam 1 seat and 78 houses, resulting in direct loss of 1.7 1.000 yuan.

Sulfur-chlorine saline soil is distributed in the barren area south of Shanshuigou West Railway. From south to north, the salt content of soil is from large to small. With the increase of vertical depth, the soil salt content decreased sharply; 0. 1m shallow layer with salt content of 23.2% belongs to dangerous saline soil; The salt content below 2.0m is 0. 1%, which belongs to saline soil and has low risk of corrosion disaster.

(3) Yumen Town-Jiayuguan (244 ~ 366 km)

In this section, except for the Baiyang River Valley in the east of Jinchi Town and Qingquan Town, there are several smaller gullies in other sections, which are partially scoured by floods, but judging from the scouring situation, it is less dangerous to underground facilities.

According to the data of Yumen Water Conservancy and Electric Power Bureau, on June 22, 2000, a rainstorm occurred in the mountainous area of Guan Dan (the cumulative precipitation reached 144.4mm), and the peak flow of the oil river reached more than 300m3/s (once in a hundred years), which washed out 48 downstream trunk and branch canals156.2.

This area belongs to windy area with wind erosion, but the risk of wind erosion is small because the surface is mainly gravel Gobi.

Saline soil is distributed in the spring flood zone of Yaoquanzi (downstream of the petroleum river) in Yumen City. Sampling analysis shows that it belongs to chloride-sulfate saline soil, with the salt content of 0.2 1% ~ 0.93%, and the erosion risk is small.

(4) Jiayuguan-Xinhua Township (366 ~ 535km)

The evaluation area in this section basically extends along the front edge of Qilian alluvial plain, and passes through Jiayuguan Fault, Beidahe River, Fengle River, Maying River Valley and some minor seasonal gullies from west to east.

Valley width 100 ~ 500m, slope 1% ~ 2%, and the riverbed is dominated by gravel and pebbles. The riverbank is 2 ~ 6m high, and the Beida River is 46m high, all of which are mainly gravels and pebbles. Once in 50 years, there is no obvious scouring and bank collapse, and the risk is small.

There are semi-fixed sand dunes to the east of Wutongquan Station and to the north of Mingshuihe Station, with small distribution area, thin sand layer and slight sand burial.

(5) Xinhua Township-Dongle Township (535 ~ 620km)

It belongs to one of the most densely populated areas in Gansu, and the evaluation area is mostly located in wasteland and farmland. The main geological disasters are earthquake liquefaction and flood erosion, as well as sand burial disaster and collapse of irrigated land.

According to the Seismological Records of Gansu Province, at 6: 32 on May 23rd, 1927, the doors and windows rang like a rainstorm, and the water in the small river ditch swayed to the shore and the trees swayed. The tulou in the city was destroyed by the earthquake for three or four meters. The earth patrol wall of the Western Wall collapsed. About 1% ~ 2% of flower walls, decayed walls and some decayed houses collapsed, leaving no injuries to people and animals. Outside the east gate, five or six cracks appeared in the small pool, and black mud and sand appeared. This is a clearly recorded earthquake liquefaction phenomenon in this section, and it can be judged from the black sand that its liquefaction layer should be muddy sand.

There are three perennial rivers from west to east, Liyuan River, Heihe River, Shandan River and some seasonal gullies. The valley width is about 1.20 ~ 200 m, and the slope is about 1%. The riverbed is dominated by gravel and pebbles. The steep bank of the river is 2 ~ 5m high, mainly silty soil; Because the river valley is flat and the vegetation is developed, the risk of river bed scouring and bank collapse is small. However, in the upper reaches of Heihe highway bridge, floods often destroy dams and inundate farmland and villages.

There are undulating sand dunes in some areas, surrounded by farmland and forest nets, and the sand dunes themselves have a small amount of vegetation, so their mobility is small and they are fixed and semi-fixed.

To sum up, the present situation of geological disasters in Xinhua Township-Dongle Township is less dangerous.

(6) Dongle Township-Shuimoguan (620 ~ 732km)

There is Shandan Coal Mine in the west of Shandan County, the evaluation area is the second mining area, the mining horizon is 150 ~ 300 m, and the mining time is 50 ~ 90 years. It went bankrupt because the ore body was basically mined and the output decreased. The existing individual coal miners carry out sporadic mining. Because there is no monitoring data, no ground collapse has been found so far.

According to the survey, there have been heavy rains and mudslides (sparse) in Liqiao, Laojun, Chenhu, Yinwa and Qijiadian in Shandan County. Qijiadian is located in the west of Shandan County, and Yinwagou is located in the northeast of Erlipu, which is within the evaluation area. On July 19 1 1, June 1923, July 1977, and July 2 1 day, torrential rain-type sparse mudslides occurred, which washed away houses, farmland and railways. Rainstorms and mudslides (sparse) occurred in Qijiadian on June 1977 and June 2 1 day, and 29 industrial, mining, government agencies and social teams suffered serious losses. The risk of debris flow disaster is moderate. There are two main rivers in this section. The runoff of Shandan River is small, and the riverbed gradient is about 0.6%, so the risk of scouring disaster is small. There is a reservoir at the exit of Xidahe River, the middle section is cut off, the downstream is replenished by spring water, and the valley is divided into several tributaries, with good vegetation development and low risk of geological disasters.

(7) Shuimoguan-Fengliangzhai section (732 ~ 806 km)

This section is located in a densely populated area, and most of the sections are villages and farmland. Alluvial fans passing through Dongdahe River and Huba River respectively from west to east. Mainly eroded by floods.

The East River was cut off for irrigation, and entered Jinchuan Gorge Reservoir through Jinchuan Gorge during flood, with no obvious scouring disaster.

Xiying Reservoir is built in the upper reaches of Huba River (Xiying River), with seasonal valleys in the middle reaches and springs exposed in the lower reaches. The river is 200 meters wide, and the river bottom gradient is about 2%. The riverbed and river bank are gravels and pebbles, which belong to the valley with basically balanced erosion and deposition.

According to the investigation, in August 1, 1960, in May 16, 1964, the flood washed away the river dam bridge of Gan Xin highway; On June 2, the Xiaoshahe Bridge of Gan Xin Highway was destroyed; On July 3rd1,65438, 0969, the flood peak flow reached 472m3/s, 22 flood control dams were washed away, and 4 people were drowned. On June 9th, a rainstorm occurred around 198 1 Xiying Reservoir, lasting for more than 20 minutes. Debris flow occurred in the front ditch of the spillway tunnel on the left abutment, blocking the spillway tunnel. 1at the end of June, 1984, the flood flow of Xiying River was 15 1m3/s, the flood dike in the irrigation area was 4250m, the channel was 13.5km, and the livestock were drowned 1253 heads. 1July 20, 989, the precipitation in the southern mountainous area reached 36mm, and the flood dike was destroyed by 2300 m m, which did little harm to underground facilities.

(8) section from Fengliangzhai to Baidunzi (806 ~ 959 km)

From west to east, there are perennial rivers such as Jinta River, Zamu River, Huangyang River and Gulang River and more than 10 seasonal flood gullies, among which 4 rivers and 22 gullies have experienced mudslides (Table 7-2), which is a regional debris flow-prone section.

Table 7-2 Statistical Summary of Major Debris Flow Gullies and Disasters in Fengliangzhai-Baidunzi Section

Among them: debris flow danger from Yangjiawopu compressor station to Changlin (33km); The debris flow between Fengliangzhai-Hedongbao Street (24km) and Shandan Shilibao-Dangjiatai Compressor Station (13km) is of moderate risk. The risk of debris flow in other sections is small.

To the east of Peijiaying, there are Songshan Gully and Xiaolonggou at the northern foot of Changling, whose topography, geology and other conditions are conducive to the formation of debris flow.

In addition, there are loess silt, wavy sand dunes and Gushandun coal mine in Dajingbei-Peijiaying area.

About 26km from Yangjiacaozi Highway in Tumen North to Dajing North and Wujiawan West is located in the southern edge of Tengger Desert, in which Gushandun Coal Mine-Wujiawan is a chain-like semi-fixed sand dune, and some are mobile sand dunes. Dune height 10 ~ 15m, slope 300, chain strike 2250. Due to desert control and low moving speed along the northern Ganwu Railway, the risk of wind erosion and sand burial is small.

Gushandun Coal Mine is a small coal mine, the mining horizon is located in the underground 120 ~ 130 m, and the main shaft roadway is about 380m long. Located beside the expressway, no land subsidence disaster has been found, which is less dangerous.

Luoquan Bay-Cao Shuang Daoban in the northeast of Tumen and Lijiawopu-Shipo-Shi Xia in the north of Peijiaying are loess-like silt distribution areas. It belongs to weak collapsibility or non-collapsibility soil, with little danger.

(9) Baidunzi-Gan Tang section (959 ~ 998km).

Saline soil is distributed in the length range of Baidunzi alluvial lake plain 10km, all of which belong to sulfuric acid-chloride type. The salt content of 1.0~2.0m ~ 2.0m is 0.94 ~ 1.72%, and the salt content of 3.0 ~ 4.om is 0.49 ~ 1.98%, so the corrosion risk is moderate.

This hilly area has the topographic conditions for flood formation, and there are gullies of different sizes in the evaluation area, but the overall scale is small and the risk of flood erosion is small.

The east is located on the southern edge of Tengger Desert. However, due to the better desert control along the railway, the vegetation coverage on sand dunes is high, and most sand dunes become fixed and semi-fixed, so the risk of wind erosion and sand burial is small.

From the above assessment, it can be seen that the overall distribution and change of geological disasters are controlled by landform, climate (rainfall) and geological conditions. There are few types of disasters in the west, and the risks are small, while there are many types of disasters in the east, and the risks are relatively large. Among them, the low-risk sections include Hongliuhe-Qiaowan, Yumen Town-Jiayuguan, Jiayuguan-Xinhua Township, Xinhua Township-Dongle Township, Dongle Township-Shandan Shilibao, Dangjiatai Compressor Station-Fengliangzhai, Hedongbao Street-Yangjiawopu Compressor Station and Baidunzi-Gan Tang, with a total length of 872km. Moderately dangerous sections are Qiaowan-Yumen Town, Shandan 20 Bao Li-Dangjiatai Compressor Station, Fengliangzhai-Hedongbao Street and Baidunzi Basin, with a length of 93km；; The dangerous section is Yangjiawopu compressor station-Changlin, which is 33km long.

Three, geological disaster risk prediction and evaluation

The prediction and evaluation is mainly to evaluate the risk of geological disasters after the completion of the gas pipeline itself and the possibility that the pipeline construction may induce and aggravate geological disasters.

(A) the pipeline itself may suffer from geological disasters risk prediction and assessment

1. Hongliuhe-Qiaowan section

This section is basically no man's land, and it is unlikely that other projects will be built in the next 50 years except railways, highways and pipelines. Geological environmental conditions, climate and hydrology will not change greatly, and geological disasters are still flood erosion, wind erosion and saline soil erosion. If proper protective measures are taken, these disasters can be basically avoided, so the risk of pipeline suffering from these disasters is very small.

2. Qiaowan-Yumen Town Section

With the implementation of the comprehensive development project of agricultural irrigation and resettlement in Shule River, the flood distribution pattern will change due to the retention of the main canal of Changmaxi River and the corresponding flood control dams and water-crossing bridges. In addition to the existing main flood discharge ditches such as No.3, No.4 and No.5, the interception of the west main canal and the flood control dam will turn the flood pool into a "lake" in the low-lying area on its upstream side. Once the flood dam breaks, the flood will gush out.

According to the Demonstration Summary Report on Rational Development and Utilization of Water Resources in the Middle Reaches of Shule River (Gansu Geological Survey Institute, 2000, 165438+ 10), it is predicted that after 10 years, the groundwater level in the poor areas west of Shanshuigou and south of the railway will be slightly lower than the present situation, and at the same time, this part of wasteland will be reclaimed and improved into irrigated farmland.

It is basically consistent with the above-mentioned saline soil distribution area. The groundwater level of the surface aquifer is 1.7 ~ 2.0m, and the salinity is 3.4 ~ 5.1g/L. After the pipeline is buried, it will be immersed in groundwater for a long time or part of the time. According to the corrosion evaluation standard of steel structure (steel pipe) by water, the corrosion evaluation result is medium. According to the above prediction, the groundwater level in this area will drop by about 1m in the next 30 years, and the pipeline will still be corroded by groundwater.

To sum up, the prediction and evaluation of geological disasters in Qiaowan-Yumen Town section is medium risk.

3. Yumen Town-Jiayuguan Section

The geological environment will not change greatly in the next 50 years, so the pipeline itself is less likely to suffer from geological disasters, just like the current situation assessment.

4. Jiayuguan-Xinhua Township Section

In the next 50 years, the precipitation and flood pattern in this section will not change obviously, the possibility of debris flow disaster is very small, the ratio of flood and sand burial disaster to the present situation will not change greatly, the dislocation distance of Jiayuguan active fault is less than < 10mm, and the pipeline line is less dangerous to be damaged by the above disasters. However, corresponding protective measures should be taken.

5. Xinhua Township-Dongle Township Section

According to the standard penetration test data of Gao Tai, Linze and Zhangye counties (all in the seismic intensity ⅷ degree area), the liquefaction index of each soil layer in the area is 0.02 ~ 3.90, which is a low-risk disaster.

There are three perennial rivers in this section, among which there are reservoirs in the upper reaches of Liyuan River and Shandan River. When they arrive at the assessment area, the flow is small and the valley is gentle, so there will be no major scouring disaster. Water-saving irrigation project is being implemented in Heihe River Basin. It is estimated that the downstream water volume of Zhengyixia will increase from the current 7.0× 108m3/a to 9.5× 108m3/a in 2004, that is, the flow of Heihe River will increase compared with the current situation. However, due to the wide and gentle valley, compared with the current situation, bank slope erosion and bank collapse disasters will not change greatly. However, it should be noted that some seasonal floods along the way are sudden and have great scouring harm, so prevention should be strengthened.

The risk degree of other disasters will not change greatly, that is, the risk of geological disasters in Xinhua Township-Dongle Township pipeline is not great.

6. Dongle Township-Shuimoguan Section

Shandan Coal Mine has basically stopped mining. In the original mining process, the goaf was partially backfilled, and no obvious collapse was found. Considering the possibility of future settlement, the risk level is medium. In Shandan County, there have been heavy rains and mudslides (sparse) in Liqiao, Laojun, Chenhu, Yinwa and Qijiadian. However, because the evaluation area is located at the downstream side of the above-mentioned debris flow occurrence area, the destructive power of debris flow has been weakened, and its destructive objects are mainly ground facilities. Even if debris flow occurs in the future, the destructive power of underground pipelines is weak, that is, the danger is relatively small, but protective measures still need to be taken.

As can be seen from the above, after the pipeline is buried, there is a coal mine goaf between Xi' erlibao and Dongxia in Shandan County, which is also a debris flow-prone area, which poses a certain threat to the gas pipeline and belongs to a medium-risk disaster area. The remaining sections are less dangerous.

7. Shuimoguan-Fengliangzhai Section

The piedmont of Dongdahe River and Hu Ba River in the area is a high-risk area of debris flow, but in the evaluation area, because of the gentle terrain, it is the downstream of debris flow accumulation area, and only the above two rivers and some seasonal gullies have flood erosion conditions. According to the statistics of survey data, major floods occur every 3 ~ 5 years, which have strong destructive power to ground facilities and less danger to underground pipelines.

8. Fengliangzhai-Baidunzi Section

This section is a high debris flow area, with 4 debris flow gullies 14 and more than 0 seasonal flood gullies 10. All debris flows in this area belong to hydrodynamic reasons. According to statistics, when the precipitation is ≥40mm, the debris flow disaster may occur in this section. According to the actual situation of the formation mechanism and influencing factors of debris flow in Hexi Corridor, the scoring standard is formulated, and the main debris flow gullies in this section are evaluated as follows (Table 7-3).

According to the data of Lanzhou Institute of Glacier and Frozen Soil, Chinese Academy of Sciences, the debris flow in this area mostly occurs from June to August, and the main debris flow gully occurs once every 6- 13 years.

Gushandun Coal Mine-Wujiawan is a chain semi-fixed sand dune, and some are mobile sand dunes. The moving direction is from north to south, with an average moving speed of 5 meters/year ... With the continuous sand control, disasters such as sand burial and wind erosion along the pipeline will be reduced, which will not cause harm to the buried gas pipeline, but attention should be paid to the restoration of sand dune vegetation during construction to avoid sand dune resurrection. It is predicted that the risk of wind erosion and sand burial is small.

Gushandun Coal Mine is located on the south side of the pipeline and belongs to a small coal mine. Because the ore body is located in the south and its mining direction is in the south, that is, the mined-out area is located in the south of the pipeline, and it will not expand in the direction of the pipeline, so the risk of causing disasters to the pipeline is small.

According to calculation, the collapsibility of loess silt south of Lijiawobao is 40 ~ 55 cm, which belongs to moderate danger. Attention should be paid to the compaction and waterproofing of excavated fill. The collapsibility of the loess silt in Shipo Village is 2 ~ 5 cm, which is less dangerous.

To sum up, the debris flow (sparse) is the most prominent in Fengliangzhai-Baidunzi section, followed by loess silt collapse, wind erosion and sand burial, which belongs to the most frequent geological disasters in Gansu. Prediction and evaluation of geological disasters: the danger between Yangjiawopu compressor station and Changlin is great; The danger between Fengliangzhai-Hedongbao Street and Shandan 20- Bao Li-Dangjiatai compressor station is moderate; The remaining sections are less dangerous.

Table 7-3 Risk Prediction and Assessment Table of Main Debris Flow Gulang from Shandan to Gulang

9. Baidunzi-Gan Tang Section

In the saline-alkali soil distribution area of Baidunzi Basin, the hydrogeological and climatic conditions will not change much in the next few decades, and the salt content of the soil will basically remain at the current level, with moderate corrosion risk to gas pipelines. The risk of flood erosion is similar to the present situation, and the risk is small. Under the continuous control of wind erosion and sand burial disaster, the danger will become smaller.

(2) Risk prediction and assessment of geological disasters induced by engineering construction.

Gas pipeline is a linear project. Most of Gansu section is located in alluvial plain and alluvial plain, and some sections are located in hilly areas with simple landform conditions. Pipelines are mainly excavated and buried. After the completion, the surface basically recovered to its original state. This kind of engineering construction has little influence on the environmental geological conditions of the evaluation area, and has no influence on the regional geological environmental conditions, that is, it will not induce and aggravate the occurrence of geological disasters such as debris flow, sand liquefaction and goaf collapse due to pipeline construction and operation. However, improper design and construction in some areas will induce some small-scale geological disasters, such as wind erosion and sand burial, collapsible loess and flood erosion.

In addition, the pipeline crosses the railway 8 times in Gansu section, the national highway 3 12 and other trunk highways 18 times, and the main canal 10 times. In the process of construction, the stability of local foundations of these buildings will be destroyed. If special treatment is not carried out, it may lead to uneven settlement, deformation of railway subgrade and rail, gully formation on highway pavement, settlement cracking and leakage at the bottom of main canal.

(c) Impact of human engineering economic activities on natural gas pipelines

Most of the Gansu section of the West-to-East Gas Pipeline Project is located in the Gobi Desert, but some sections are located in densely populated areas. There are frequent human engineering and economic activities in these areas, such as highway and channel construction, foundation excavation of large and medium-sized projects, drilling, blasting and mining. , which may cause damage to the pipeline. In addition, with the economic development and population growth, some existing wasteland will be reclaimed as cultivated land, such as Qidaogou area in Shule River Basin, where large-scale engineering activities such as land reclamation, canal repair and housing construction are being carried out. If management and prevention are not strengthened, it may also cause harm to the pipeline and affect its safe operation.