Natural geography and geological environment of the assessment area

1. Overview of physical geography

(1) Topography

The Gansu section of the gas pipeline runs through Beishan and the Hexi Corridor. The Beishan Mountains are denuded low hills and the pipeline is about 200km long. The Hexi Corridor is dominated by vast piedmont accumulation plains, with a pipeline length of about 800km. The eastern section of the pipeline passes through the southern edge of the Tengger Desert.

The low hills and hills of Beishan have an altitude of 1390-1600m. Due to long-term weathering and erosion, various bedrocks have become quasi-plain. The terrain is relatively open and flat, slightly tilted to the south, and gullies are relatively developed.

The Hexi Corridor extends from northwest to southeast. Its topography is characterized by: the piedmont flood plains such as Mazong Mountain, Heli Mountain, and Longshou Mountain in the north slope from north to south, and the Qilian Mountain piedmont in the south The floodplain slopes from south to north, and its terrain is high in the south and north, low in the middle, and high in the east and low in the west. The highest point in the corridor is the Xiuhua Temple, with a ground elevation of 2591.8m, and the lowest point is Bulungi, with a ground elevation of 1350.3m. To the northwest of Bulungi is the Beishan low mountain and hilly area, with the ground elevation remaining between 1500.0 and 1897.6m. The vast piedmont alluvial and flood plains constitute the main body of the corridor plain. The bedrock uplifts such as Kuantan Mountain, Yumu Mountain and Dahuang Mountain divide the corridor into a series of sedimentary basins such as Yumen, Jiuquan, Zhangye and Wuwei. Most of the areas where the pipeline passes are relatively flat.

(2) Climate and Meteorology

The assessment area is located deep in the hinterland of the continent and belongs to the temperate continental arid climate zone. The climate characteristics are: drought and little rain, strong evaporation, long sunshine hours, large temperature difference between day and night, strong winds, short and hot summers, and long and cold winters.

Temperature: The multi-year average temperature is 6.0℃~9.2℃, showing a gradual trend of high in the east and low in the west, high in the north and low in the south.

Precipitation: It gradually decreases from south to north and from east to west. Annual average precipitation: 46.7mm (Hongliu River) ~ 195.1mm (Shandan). About 70% of the annual precipitation is concentrated in summer. The maximum daily precipitation values ??are: Wuwei 62.7mm (June 3, 1985); Zhangye 46.7mm (August 16, 1954); Jiuquan 44.2mm (August 4, 1983). Although there are very few days with heavy rains with daily precipitation ≥50 mm, heavy rains are frequent in some areas and often cause large floods. For example, in Gulang Dajing, 154.5 mm fell in 2.5 hours on August 1, 1977. Floods formed within a short time after the rainfall. Severe floods.

Frozen soil: The maximum seasonal frozen soil depth is 116cm (Linze, Anxi) ~ 159cm (Yongchang).

Wind: There are more northwest winds in winter and more southeast winds in summer. Wind speed changes are higher in winter and spring, with average wind speeds of 2-5m/s and 3-5m/s in January and April; wind speeds are smaller in summer and autumn, with average wind speeds of 2-4m/s in July and October. The number of windy days (number of days with wind strength > Level 8) is highest in Anxi, generally 69 days, up to 105 days in most years, and around 45 days in other areas. Whenever there is a strong wind, the dust flies up and the sky becomes dark, forming a sandstorm.

(3) River hydrology

The Hexi Corridor is divided into three major inland river basins: Shule River, Heihe River and Shiyang River from west to east. There are 24 perennial rivers, among which There are 22 hydrological stations. These rivers all originate from the Qilian Mountains and Altun Mountains in the south, with a multi-year average total runoff of 6.025 billion m3/a; among them, 5 are in the Shule River Basin, with a runoff of 1.288 billion m3/a; 9 are in the Heihe River Basin, with a runoff of 3.208 billion m3/a; There are 8 Shiyang River basins, 1.529 billion m3/a. Most rivers have reservoirs at their mouths. About 70% of the annual runoff of each mountainous river is distributed between June and September, during which most rivers and ditches can form floods. The above-mentioned rivers are all perpendicular or obliquely intersecting with the pipeline. Except for the main stream, which has water all year round, the other tributaries of each water system are mostly seasonal valleys (Figure 7-1).

2. Geological environmental conditions

(1) Stratum lithology and distribution characteristics

The distribution of strata depends on regional sedimentary construction. The regional sedimentary structures in the Gansu section are relatively complex, and the pre-Quaternary can be divided into two categories: geosynclinal type and platform type. Among them, geosynclinal sedimentary structures are mainly composed of marine clastic rocks, argillaceous shale, siliceous volcanic rocks, fine porphyry, etc. Platform-type sedimentary structures are mainly composed of marine quartz sandstone, carbonate rock and continental clastic rocks. Clastic rocks and volcaniclastic rocks are mainly constructed, mainly distributed in the low mountain and hilly areas of the northern mountains of the western section and the middle and low mountains of the corridor. The Quaternary is mainly composed of alluvial, lacustrine, and aeolian conglomerate, gravel and pebbles, sand, silt, silty clay, loess-like silt, etc., and is widely distributed in various sedimentary basins, river terraces, and valleys in the corridor. (Figure 7-2 and Table 7-1).

Table 7-1 Brief table of strata in the geological hazard risk assessment area of ??the Gansu section of the West-East Gas Pipeline Project

(2) Geotechnical engineering properties

1 .Division of rock and soil types

The rock mass in the Gansu section can be divided into four categories: hard rock group, relatively hard rock group, weak rock group, and weak alternate rock group.

Soil can be divided into gravel soil, sandy soil, clay soil and special soil according to its type, origin and structure.

2. Engineering properties of rock mass

Hard massive intrusive rock mass, mainly granite intruded during the Variscan and Caledonian periods, is distributed in the low mountainous and hilly areas of the north. and the low mountains in the corridor.

Fresh rock mass is hard, with a measured dry compressive strength of 99.7-110MPa, but it is easily weathered.

The hard and relatively hard layered lava rock group is mainly composed of Paleozoic andesite and rhyolite, and is scattered sporadically in the low mountainous and hilly areas of the north. Rock strength is high.

A layered sandstone and limestone group of alternately soft and hard rocks, including Sinian-Carboniferous sandstone, limestone, phyllite, and slate, as well as Permian, Triassic, and Jurassic It is mainly composed of carbonaceous shale, siltstone, and limestone intercalated with coal seams, and is distributed in low hills and mountains in corridors. The dry compressive strength of rock is 20~32MPa.

Weak layered and thin-layered conglomerate, sandstone, and mudstone rock groups, mainly Tertiary mudstone, sandstone, and argillaceous sandstone. Due to the loose structure, it is easy to disintegrate when soaked in water, and is prone to weathering, erosion, and collapse. The permeability is uneven, which can cause disasters such as collapse.

3. Soil engineering properties

Gravely soil is the most widely distributed along the assessment area, and is dominated by Quaternary alluvial, alluvial or alluvial gravel. Except for the Yumen conglomerate, which has been basically cemented, other gravel and pebble layers have a loose structure, strong permeability, and high bearing capacity.

Sandy soil is of alluvial, diluvial and lacustrine origin. In the edge zones of various alluvial and diluvial fans, sandy soil often coexists with clay soil. The sandy soil itself has a high bearing capacity, but the groundwater level in this zone is generally shallow and there is a problem of earthquake liquefaction.

Cohesive soil is mostly alluvial-diluvial-lacustrine silt and silty clay. Most of the clay soils in the assessment area are older (Q1), are now in a state of erosion, are semi-consolidated, have low strength characteristics, and have relatively good engineering properties.

4. Special soil engineering properties

Milted soft soil, rich in humus, with developed plant root canals, has a sponge-like structure, high moisture content, loose structure, and high compressibility. It is in a soft, flowing plastic state. There is a subsidence problem in the project. The amount of subsidence is large and uneven subsidence is prone to occur.

Saline soil is mainly distributed in areas where the phreatic water level is less than 5m deep, and is of the sulfuric acid and chloride type. The lithology is mostly silt soil. As the salt content increases, the liquid and plastic limits of saline soil decrease.

Loess and loess-like soil, the lithology is mainly silt and silty clay, and has weak collapsibility.

Aeolian sand is a fixed or semi-fixed dune. Aeolian sand itself does not have any special engineering properties. Excavation during engineering construction is difficult and may also cause damage to engineering buildings.

In addition, the construction land for the Gansu section of the West-East Gas Pipeline Project has seasonally frozen soil from November to March of the following year. The spring thaw period can easily cause road muddying and building deformation.

(3) Geological Structure

The Hexi Corridor and the North and South Mountains still retain the basic structural framework formed by many major tectonic movements before the Mesozoic and Cenozoic, but most areas have Since the Cenozoic, it has clearly entered a unified development period of tectonic movement dominated by strong differential fault block movements. The southern Qilian Mountains are composed of several huge northwest-oriented fault block mountains. They have been strongly uplifted in recent times and the terrain is high and steep. The North Mountain Band of the corridor has been slowly rising recently, and the mountains are low. In areas of relative subsidence during the slow ascent process, some Jurassic or Paleogene-Neogene basins were formed.

The corridor plain belongs to a large-scale subsidence zone of recent geological structures. It is in contact with the south and north rising zones by large thrust faults. Since the Mesozoic and Cenozoic, a series of north-west, north-west and nearly east-west The Hexi Corridor is further divided into many structural basins of varying sizes by large faults in the direction and fault block differentiation produced along the fault zone. Controlled by the structural directions of the southern and northern mountains, the structural basins in the corridor are regularly distributed in two rows: south and north. The structural basins passing through the assessment area from northwest to southeast are: the eastern edge of the Anxi-Dunhuang Basin in the North Basin, the eastern Yumen-Tasheng Basin in the South Basin, and the central parts of the Jiuquan Basin, Zhangye Basin, and Wuwei Basin.

(4) Hydrogeological conditions

The assessment area is divided into two hydrogeological areas based on different hydrogeological characteristics.

Figure 7-1 Gansu regional geological map of the West-East Gas Pipeline Project

Figure 7-2 Regional geological map of the Gansu section of the West-East Gas Pipeline Project

1. Beishan Hydrogeological Region

The low hills and hilly areas in the north have become quasi-plain and are desertified low hills and residual mountains. According to the properties of the aquifer, it is divided into bedrock fissure water, Mesozoic and Cenozoic clastic rock interlayer water and Quaternary loose rock pore water. Groundwater resources are unevenly distributed and very poor. The water inflow in a single well is generally less than 500m3/d. The water quality is poor, and the salinity is generally 1 to 5g/L.

2. Corridor hydrogeological area

Huge thick Quaternary loose sediments have been deposited in each rift structural basin of the Hexi Corridor, which is a good carrier for groundwater storage and migration.

In the piedmont zone, rivers flowing out of the mountain supply groundwater through the alluvial sloping plains. The groundwater in this zone is dominated by a single phreatic aquifer with a large thickness. The water quality is excellent, the salinity is <1g/L, and the water richness is strong. The water inflow volume in a single well is 3000~5000m3. /d, up to 10000m3/d, and the groundwater level is buried deeper than 100m; after the groundwater flows northward to the alluvial and flood plains, the aquifer gradually changes into a phreatic layer of sand, gravel and medium-coarse sand composed of double or multi-layer media. Confined water aquifer group, the aquifer thickness is 50~100m, the water inflow volume of a single well is generally 1000~3000m3/d, the salinity of shallow water is >1g/L, the lower confined water is <1g/L, and the water level is buried deep 10 ~100m; further into the basin to the alluvial plain zone, the lithological particles of the aquifer become finer, the terrain becomes gentle, the depth of the groundwater level becomes shallower to 5~10m, and the groundwater overflows the surface in the form of springs, forming a group of parallel spring ditches; At the front edge of the fine soil plain, the depth of the groundwater level gradually decreases to <3m. Groundwater is mainly discharged by evaporation. The salinity is generally 1 to 3g/L, and locally >3g/L. Due to evaporation and concentration, the salinity of groundwater increases. , forming saline soil on the surface.

(5) Solid mineral resources

The Hexi Corridor is rich in solid mineral resources, among which Jinchuan Company’s large-scale copper, nickel, cobalt, selenium, gold, silver, antimony, etc. It is famous for its metal raw ores; in addition, there are metal deposits such as lead, zinc, silver, iron, and mineral resources such as coal, flux limestone, and clay. However, there are only small mines such as Shandan Coal Mine and Gushandun Coal Mine within the scope of the assessment area.

(6) Fault activity and earthquakes

The active faults in the Gansu segment are densely distributed. Affected by the strong compressive stress of the Qinghai-Tibet block, it is easy to breed strong earthquakes and regional strong earthquakes. The vibration frequency and intensity are both high. As far as the assessment area is concerned, the seismic intensity exceeding the 10% probability level in 50 years, except for the Beishan section with level VI and the peak earthquake acceleration of 50 gal, most areas are in the seismic intensity area of ??level VII or level VIII, including Qiaowan-Fengle Town. It is degree VII, and everything east of Fengle Town is degree VIII. Peak earthquake acceleration: Qiaowan-Tawanbao 100~150gal, Tawanbao-Linze 200gal, Linze-Shuimoguan 100~150gal, and east of Shuimoguan 200gal. Except for the 1927 Gulang magnitude 8 earthquake and the 1954 Shandan magnitude 7.25 earthquake, the other strong earthquakes that have occurred since 1901 are generally ≤6 magnitude. Before 1900, most strong earthquakes were of magnitude <7, but two earthquakes of magnitude >7 occurred near Linze (Figure 7-3).

3. The impact of human activities on the geological environment

The construction of railways, highways, reservoirs, canals and flood control, debris flow prevention and other projects have reduced the risk of flood-induced debris flow and other disasters. conditions, which can prevent or mitigate geological disasters.

There are three main aspects of human economic activities that have a destructive effect on the geological environment: First, the destruction of natural vegetation, indiscriminate reclamation, dredging of rivers, etc. destroy the surface structure of rock and soil, aggravate water and soil erosion, and increase This has increased the sediment content of floods and increased the possibility of inducing debris flows. In addition, artificial flood control facilities in important projects have changed the distribution pattern of floods and increased the chance of disasters. Second, various key construction projects are ongoing, such as "dredging". The development of a large amount of saline-alkali wasteland and the influx of tens of thousands of immigrants in the "Lehe Agricultural Irrigation and Resettlement Comprehensive Development Project" will inevitably have an impact on the natural geological environment; third, the goafs formed by coal mining have the possibility of inducing ground subsidence .

IV. Segmentation of complexity levels of geological environmental conditions

According to the "Technical Requirements for Geological Hazard Risk Assessment of Construction Land", the classification results of the complexity levels of geological environmental conditions in the Gansu section are as follows : 199km with complex geological environment conditions (Qiaowan-Yumen Town, Fengliangzhai-Baidunzi), medium 434km (Hongliuhe-Qiaowan, Xinhua Township-Dongle Township, Baidunzi-Gantang), simple 365.2 km (Yumen Town-Xinhua Township, Dongle Township-Fengliangzhai).

Figure 7-3 Regional active tectonics and strong earthquake epicenter distribution map in the Gansu section of the West-East Gas Pipeline Project