Physical geography and geological environment of the evaluation area

I. General situation of physical geography

(1) Terrain

Gansu section of natural gas pipeline passes through Beishan and Hexi Corridor. The pipeline is about 200km long in the denuded hills of Beishan mountain area. Hexi Corridor is dominated by the vast piedmont accumulation plain, and the pipeline is about 800km long. The eastern section of the pipeline crosses the southern edge of Tengger Desert.

The elevation of the low hills in Beishan Mountain is1390 ~1600 m. Due to long-term weathering and erosion, all kinds of bedrock have been quasi-flattened, with wide and flat terrain, slightly inclined to the south and developed gullies.

Hexi Corridor extends from northwest to southeast, and its geomorphological features are that the piedmont flood areas such as Mazong Mountain, Helishan Mountain and Longshou Mountain in the north are inclined from north to south, and the piedmont flood areas of Qilian Mountain in the south are inclined from south to north. The terrain is corridor-shaped, which is high in the north and south, low in the middle and high in the east and low in the west. The highest point of the corridor is Xiuhua Temple, with a ground elevation of 259 1.8m, and the lowest point is in the bronchus, with a ground elevation of1350.3m. The northwest of Bulongji is a hilly area of Beishan, with a ground elevation of 1500.0 ~ 1897.6 m, and a wide piedmont flood and flood accumulation.

(2) Climate and Meteorology

The evaluation area is located in the hinterland of the mainland and belongs to the temperate continental arid climate zone. The climate is characterized by drought and little rain, strong evaporation, long sunshine time, large temperature difference between day and night, strong wind, short and hot summer and long and cold winter.

Temperature: the average temperature for many years 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. Average annual precipitation: 46.7mm (Hongliuhe) ~ 195.5438+0 mm (Shandan). About 70% of the annual precipitation is concentrated in summer. The maximum daily precipitation value is: 62.7 mm in Wuwei (1June 3, 985); Zhang Ye is 46.7 mm (1August 95416); Jiuquan 44.2 mm (1August 4, 983). Although there are few days of rainstorm with daily precipitation ≥50mm, there are frequent rainstorms in some areas, which often lead to major floods. For example, Gulang Dajing 1 August, 9771,when the rainfall was 154.5mm within 2.5 hours, a catastrophic flood occurred in a short time after the rainfall.

Frozen soil: The maximum seasonal frozen soil depth is 1 16cm (Linze and Anxi) ~ ~ 159cm (Yongchang).

Wind: It is northerly in winter and southeasterly in summer. The wind speed changes greatly in winter and spring, and the average wind speed in June and April is 2 ~ 5m/s and 3 ~ 5m/s respectively. The wind speed is small in summer and autumn, and the average wind speed in July and1October is 2 ~ 4m/s. The number of windy days (the number of days when the wind force is greater than 8) is the largest in Anxi, generally 69 days, which can reach 105 days at most in a year, and the rest areas are about 45 days. Every time it is windy, dusty and dark, sandstorm weather is formed.

(3) River hydrology

Hexi Corridor is divided into three inland river basins: Shule River, Heihe River and Shiyang River from west to east. There are 24 perennial rivers, including 22 hydrological stations. These rivers all originate from Qilian Mountain and Altun Mountain in the south, with an average annual total runoff of 6.025 billion m3/a; Among them, there are 5 in Shule River basin, with runoff of 654.38+28.8 million m3/a; 9 in Heihe river basin, 3.208 billion m3/a; There are 8 Shiyang River basins with 654.38+529 million m3/a ... Most rivers have reservoirs at their outlets. About 70% of the annual runoff of each mountain and river is distributed from June to September, and most rivers and ditches here can form rapids. The above rivers are perpendicular to the pipeline or inclined at a large angle with the pipeline. Except for perennial water from the main stream, most of the other tributaries are seasonal valleys (Figure 7- 1).

Second, the geological environment conditions

(1) Formation lithology and distribution characteristics

The distribution of strata depends on regional sedimentary formation. The regional sedimentary formation in Gansu section is complex, and the pre-Quaternary can be divided into geosyncline type and platform type. Among them, the sedimentary formations of geosyncline are mainly marine clastic rocks, argillaceous shale, siliceous volcanic rocks and spilite porphyry. Platform-type sedimentary formations are mainly composed of marine sandstone, carbonate rocks, continental clastic rocks and pyroclastic rocks, which are mainly distributed in the hilly area of Beishan in the west and the middle and low mountains in the corridor. The Quaternary is mainly composed of conglomerate, gravel, pebble, sand, silt, silty clay and loess silt. , widely distributed in sedimentary basins, river terraces and river valleys in corridors (Figure 7-2 and Table 7- 1).

Table 7- 1 Brief Strata of Geological Hazard Risk Assessment Area in Gansu Section of West-East Gas Pipeline Project

(2) Geotechnical engineering properties

1. Classification of geotechnical types

The rock mass in Gansu section can be divided into four types: hard rock formation, hard rock formation, weak rock formation and weak rock formation.

According to its type, source and structure, soil can be divided into gravel soil, sandy soil, cohesive soil and special soil.

2. Engineering characteristics of rock mass

Hard massive intrusive rocks, mainly granites invaded in Variscan and Caledonian periods, are distributed in hilly areas and middle and low mountains in the north of the corridor. The fresh rock mass is hard, and the measured dry compressive strength is 99.7 ~ 1 10 MPa, but it is easily weathered.

Hard, hard layered lava rocks, mainly composed of Paleozoic andesite and rhyolite, are scattered in the northern hilly areas. Rock strength is high.

Soft and hard layered sandstone and limestone strata are mainly Sinian-Carboniferous sandstone, limestone, phyllite and slate, as well as Permian, Triassic and Jurassic carbonaceous shale, siltstone and limestone mixed with coal seams, which are distributed in low hills and corridors. The dry compressive strength of rock is 20 ~ 32 MPa.

Weak bedded, thin conglomerate, sandstone and mudstone strata, mainly tertiary mudstone, sandstone and argillaceous sandstone. Because the structure is loose, it is easy to collapse when it meets water, and it is easy to collapse due to weathering, denudation and uneven permeability, which can cause disasters such as collapse.

3. Soil engineering properties

Gravel soil is the most widely distributed in the evaluation area, mainly composed of gravel and pebbles deposited by Quaternary alluvial, diluvial or alluvial deposits. Except Yumen conglomerate, which has been basically cemented, other gravel layers have loose structure, strong permeability and high bearing capacity.

Sandy soil belongs to alluvial, diluvial and lacustrine origin, and often coexists with cohesive soil at the edge of alluvial-diluvial fan. The bearing capacity of sand itself is high, but the buried depth of groundwater level in this area is generally shallow, so there is the problem of earthquake liquefaction.

Cohesive soil is mostly alluvial-diluvial silty soil and silty clay. Most of the clayey soil in the evaluation area is old soil (Q 1), which is now in a denuded state, with the characteristics of semi-consolidation and low strength, and its engineering properties are relatively excellent.

4. Special soil engineering properties

Silty soft soil, rich in humus, with developed plant root canals, spongy structure, high water content, loose structure and high compressibility, is in a soft plastic state. There is a settlement problem in the project, which is large and prone to uneven settlement.

Saline soil is mainly distributed in the underground water table below 5m, belonging to sulfuric acid and chloride type. The lithology is mostly silt. With the increase of salt content, the liquid plastic limit of saline soil decreases.

Loess and loess-like soil are mainly silty soil and silty clay with weak collapsibility.

Aeolian sand belongs to fixed and semi-fixed sand dunes. Aeolian sand itself does not have some special engineering properties, so it is difficult to dig in engineering construction, and it will also destroy engineering buildings.

In addition, there is seasonal frozen soil in the construction land of Gansu section of West-to-East Gas Pipeline Project from June 165438+ 10 to March of the following year, which is easy to cause road frost boiling and building deformation during the thawing period in spring.

(3) Geological structure

The Hexi Corridor and the northern and southern mountains still retain the basic structural pattern formed by several major tectonic movements before Mesozoic and Cenozoic, but most areas have obviously entered a period of unified development of tectonic movements with strongly different block movements since Mesozoic and Cenozoic. Qilian Mountain in the south is composed of several huge NW-trending fault-block mountains, which have been strongly uplifted recently with high and steep terrain. Recently, the north corridor of the corridor is slowly rising and the mountain is low. Some Jurassic or Paleogene-Neogene basins were formed in the relative subsidence area during the slow rising process.

The corridor plain belongs to a large subsidence zone associated with modern geological structures, and it is in contact with the south and north rising zones through large thrust faults. Since Mesozoic and Cenozoic, a series of large faults in northwest, northwest and nearly east-west direction and the differentiation of fault blocks along the fault zone have further divided the Hexi Corridor into many structural basins of different sizes. Controlled by the tectonic direction of the north and south mountains, the tectonic basins in the corridor are regularly distributed along the north-south line. The tectonic basins passing through the evaluation area from northwest to southeast are: the eastern margin of Anxi-Dunhuang basin in the north, the eastern part of Yumen-Zhen Shi basin in the south, the central part of Jiuquan basin, Zhangye basin and Wuwei basin.

(4) Hydrogeological conditions

According to different hydrogeological characteristics, the evaluation area is divided into two hydrogeological areas.

Figure 7- 1 Gansu Regional Geological Map of West-East Gas Transmission Project

Figure 7-2 Regional Geological Map of Gansu Section of West-East Gas Transmission Project

1. Beishan hydrogeological area

The hilly area in the north is already a quasi-plain, which belongs to desertification low hills and residual mountains. According to the nature of aquifer, it can be divided into bedrock fissure water, meso-Cenozoic clastic interlayer water and Quaternary loose rock pore water. The distribution of groundwater resources is uneven and very poor. The water inflow of a single well is generally less than 500m3/d, and the water quality is poor. Salinity is generally1~ 5g/L. ..

2. Hydrogeological area of corridor

Thick Quaternary loose deposits are deposited in the Hexi Corridor fault basin, which is a good carrier for groundwater storage and migration. In the piedmont area, mountain rivers recharge groundwater through alluvial inclined plains. The groundwater in this area is mainly a single phreatic aquifer with large thickness, excellent water quality, salinity < 1g/L and strong water abundance. The water inflow of a single well is 3000 ~ 5000m3/d, and the maximum can reach 10000m3/d, and the buried depth of groundwater level is > 65438+. After the groundwater flows northward to the alluvial plain, the aquifer gradually becomes a gravel and medium-coarse sand phreatic water-bearing rock group composed of two or more layers of media. The aquifer thickness is 50 ~ 100 m, the water inflow of a single well is generally 1000 ~ 3000 m3/d, the salinity of shallow water is > 1g/L, and the lower part is confined water. Further into the basin to alluvial plain belt, the lithologic particles of aquifer become finer, the terrain becomes slower, the buried depth of groundwater becomes shallow to 5 ~ 1m, and groundwater overflows the surface in the form of spring water, forming parallel spring and gully groups; At the front of the fine soil plain, the buried depth of groundwater gradually decreases to < 3m, and the groundwater is mainly discharged by evaporation. The salinity is generally 1 ~ 3g/L, and locally > 3g/L.. Due to evaporation and concentration, the salt content of groundwater increases, forming saline soil on the surface.

(5) Solid mineral resources

Hexi Corridor is rich in solid mineral resources, among which Jinchuan Company is famous for its large polymetallic deposits such as copper, nickel, cobalt, selenium, gold, silver and antimony. In addition, there are metal deposits such as lead, zinc, silver and iron, as well as mineral resources such as coal, flux limestone and clay. However, there are only small coal mines such as Shandan Coal Mine and Gushandun Coal Mine along the evaluation area.

(vi) Fault activity and earthquakes

Active faults in Gansu section are densely distributed, and under the strong compressive stress of Qinghai-Tibet block, strong earthquakes are easy to occur, and the frequency and intensity of regional strong earthquakes are high. As far as the evaluation area is concerned, the seismic intensity exceeds the 50-year probability level 10%, and most areas are in the seismic intensity ⅶ or ⅷ zone, with Qiaowan-fengle town ⅶ and fengle town East ⅷ. Peak value of earthquake acceleration: Qiaowan-Tawanbao 100 ~ 150 gal, Tawanbao-Linze 200gal, Linze-Shuimoguan100 ~/50 gal, 200gal east of Shuimoguan. The strong earthquakes that occurred from 190 1 are generally less than or equal to magnitude 6, except for 1927 Gulang M = 8 earthquake and 1954 Shandan M = 7.25 earthquake. Most of the strong earthquakes before 1900 were less than 7, but there were two earthquakes > 7 near Linze (Figure 7-3).

Third, the impact of human activities on the geological environment.

The construction of railways, highways, reservoirs, canals, flood control and debris flow prevention projects has reduced the conditions for floods to induce debris flows and other disasters, and has the function of preventing or slowing down geological disasters.

There are three main aspects that human economic activities have a destructive effect on the geological environment: first, they destroy natural vegetation and dig in rivers indiscriminately, which destroys the surface structure of rock and soil, aggravates soil erosion, increases the sediment concentration of floods and increases the possibility of inducing mudslides. In addition, the artificial flood control facilities of important projects have changed the flood distribution pattern and increased the contingency of disasters; Second, the ongoing key construction projects such as "Shule River Agricultural Irrigation and Resettlement Comprehensive Development Project", the development of a large number of saline-alkali wasteland and the influx of tens of thousands of immigrants will inevitably have an impact on the natural geological environment; Third, the mined-out area formed by coal mining may induce ground collapse.

Four, the division of the complexity of geological environment conditions

According to the Technical Requirements for Geological Hazard Risk Assessment of Construction Land, the classification results of the complexity of geological environment conditions in Gansu section are: 199km (Qiaowan-Yumen Town, Fengliangzhai-Baidunzi) with complex geological environment conditions and 434km (Hongliuhe-Qiaowan, Xinhua Township-Dongle Township, Baidunzi-Gan Tang) with moderate geological environment conditions.

Figure 7-3 Regional Active Structure and Strong Earthquake Distribution in Gansu Section of West-East Gas Transmission Project