Division of water-bearing rock formation and compression layer in Beijing land subsidence area

(Beijing Geological Environment Monitoring Station, Beijing, 100037)

Based on the comprehensive basic geology and special investigation of land subsidence in Beijing, the hydrogeological and engineering geological conditions and the distribution status of land subsidence in the subsidence area were found out, and drilling and various hydrogeological and geotechnical engineering tests were carried out in typical land subsidence areas. According to the above results, the water-bearing rock group and compression layer group in Beijing land subsidence area are divided for the first time, and the geological model of Beijing land subsidence is initially established, which lays the foundation for the construction of the capital land subsidence website and the establishment of the land subsidence early warning and forecasting system. This paper makes a general introduction to this.

Keywords: Compressive layer of water-bearing strata in Beijing land subsidence

1 Introduction

1. 1 the purpose and significance of the research work

Land subsidence refers to an environmental geological phenomenon that the regional ground elevation decreases due to the compression of the surface soil of the crust under the action of natural and human factors. Land subsidence has brought many difficulties to urban buildings, road traffic, pipeline system, water supply and drainage, flood control and so on. In particular, some cities built in Quaternary loose accumulation plain areas are particularly affected by land subsidence disasters.

Land subsidence is one of the main geological disasters in Beijing Plain, and its scope and amplitude are expanding year by year. At present, the land subsidence area has reached 28 15km2, and the cumulative maximum subsidence is about 722mm. In addition to the subsidence in the eastern suburbs, the sea in Changping District in the outer suburbs is still developing. Luocheng in the south of Shunyi and Yufa in Daxing District have formed three new land subsidence areas. Ground subsidence has caused cracks in the walls of factories and residential buildings, ground subsidence and damage to more than 50 underground pipeline projects. At the same time, the seismic capacity of some buildings is reduced, and a large number of leveling points are inaccurate, which poses a threat to the capital city construction and people's property safety.

The purpose of this work is to establish a geological model of land subsidence in Beijing, and provide basic information for further study of land subsidence mechanism, construction of land subsidence monitoring website, prediction of land subsidence development trend and establishment of land subsidence early warning and forecasting system, and put forward prevention measures for land subsidence hazards, so as to provide basic information for capital planning and urban construction.

1.2 overview of research work and existing problems

Land subsidence in Beijing mainly occurs in the northern, eastern and southern plains of Beijing. The geological research in this area is relatively high, and a lot of regional geological work, hydrogeological work, engineering geological work, environmental geological work and disaster geological work have been completed.

The study of land subsidence in Beijing started late. 1984, Beijing Hydrogeology Engineering Geology Company, Beijing Institute of Surveying and Mapping, and Beijing Surveying and Mapping Office jointly compiled the Survey Report on Land Subsidence in Beijing. 1985 Engineering Geological Survey and Design of Land Subsidence in Beijing submitted by Beijing Hydrogeological Engineering Geology Company; The first land subsidence monitoring station in Beijing (Bawangfen land subsidence monitoring station) was built in 1990, which laid a foundation for studying the formation mechanism and development trend of land subsidence in the eastern suburbs of Beijing. In the same year, the Engineering Geological Survey of Land Subsidence in the Eastern Suburb of Beijing and the Report on the Establishment Stage of Bawangfen Monitoring Station were submitted. 1992 submitted the research report on the relationship between land subsidence and groundwater exploitation in the eastern suburbs of Beijing.

To sum up, the basic geology, hydrogeology and engineering geology of Beijing Plain have a high degree of research, but the preliminary work mainly serves for industrial and agricultural water supply and urban development and construction, and the degree of land subsidence research is low, especially the aquifer group and compression layer group in Beijing land subsidence area have not been divided, and the research on land subsidence mechanism and development law is relatively weak.

1.3 technical route and method of research work

The technical method adopted in this study is to select areas with serious land subsidence disasters and representative environmental geological conditions, and find out the history, present situation and development trend of land subsidence in Beijing Plain by means of ground survey, remote sensing interpretation and geophysical exploration. Specialized hydrogeological and engineering geological drilling was carried out in typical subsidence areas, and a large number of pumping tests and geotechnical tests were carried out. The stratigraphic structure, burial distribution characteristics of aquifer groups and compressible layers, hydrogeological parameters, physical and mechanical properties of compressible layers, soil mechanical parameters and pore water pressure of aquifer groups in subsidence areas were found out, which provided a reliable basis for dividing aquifer groups and compressible layers in subsidence areas.

2 Beijing Plain Geological Environment Background

2. 1 Meteorology and Hydrology

The climate of this city belongs to temperate continental monsoon climate, with annual average temperature 1 1.7℃, annual average precipitation in Beijing of 588.28mm, maximum annual precipitation 1406mm( 1959) and minimum annual precipitation of 256.2 mm (192).

Beijing's water system belongs to Haihe River Basin, with a developed river network, with more than 2700km of large and small rivers 100. These rivers belong to five major water systems, from west to east, they are Daqing River System, Yongding River System, North Canal System, Chaobai River System and Jiyun Canal System. The general flow direction of the river is from northwest to southeast, and finally flows into the Bohai Sea.

2.2 topography

The terrain of this city is high in the northwest and low in the southeast. The west and north are continuous mountains of Taihang Mountain and Yanshan Mountain, with a general elevation of 1000 ~ 1500m. The slope of the piedmont alluvial-diluvial fan 1 ‰ ~ 5 ‰, and the slope in most areas of the plain is less than 0.5‰. The landform is divided into three major units: western mountainous area, northern mountainous area and southeastern plain.

2.3 Geological Survey in Plain Area

2.3. 1 stratum

Except for Upper Ordovician (O3), Silurian (S), Devonian (D), Lower Carboniferous, Middle Carboniferous (C 1-2) and Upper Cretaceous (K3), all the other strata in Beijing Plain are distributed from Proterozoic to Quaternary. The strata from old to new are described as follows:

(1) Proterozoic (Pt) is mainly composed of siliceous dolomite, sandstone and shale of Great Wall System, Jixian System and Qingbaikou System, with slight metamorphism locally.

(2) The main stratigraphic lithology of Paleozoic (Pz) is Cambrian, Ordovician, Carboniferous and Permian carbonate rocks, clastic rocks and coal measures strata.

(3) The main stratigraphic lithology of Mesozoic is Jurassic, Cretaceous volcanic lava, pyroclastic rocks and coal measures strata.

(4) Eocene (E2) of Cenozoic Tertiary (Tr) is mainly composed of dark purple or pig liver-colored glutenite mixed with mudstone or sandy mudstone, which is in a semi-cemented state; The main lithology of Oligocene (E3) is gray, taupe and grayish green sandy mudstone, siltstone, breccia tuff mixed with black shale and grayish green hard sandstone; The main lithology of Miocene (N 1-2) is brownish red argillaceous sandstone and sandy mudstone, and brownish gray gravelly hard sandstone and hard sandstone mixed with fine conglomerate.

(5) The thickness of Cenozoic Quaternary (Q) in Beijing Plain varies greatly, ranging from tens of meters to 500-600 meters from the piedmont to the plain, and most of them are in parallel unconformity contact with the underlying Tertiary.

A the lower Pleistocene series (Q 1) is deposited by rivers and lakes, characterized by clayey soil mixed with gravel or clayey soil mixed with sand, and its thickness is100 ~ 300m.

The Pleistocene Series (Q2) in B is generally buried 50 ~ 70m below the surface, which is shallow in the west. The lower part is yellow-brown and reddish-brown sandy soil, containing coarse gravel and gravel layer. It is partially grayish black clay mixed with sand, and the bottom is a mixture of clay mixed with gravel, gravel and calcareous nodules, with a thickness of 70 ~ 1 10m.

C. Upper Pleistocene Series (Q3) is widely distributed in the piedmont plain area, and the piedmont lithology is yellow-brown loess silty clay and loess silty soil. Calcium-containing nodules, wormholes, pinholes and vertical joints are developed, with gravel layer in the lower part and calcium cementation in some parts, which is dense and hard; The strata in the plain area are mainly multi-layered structure, and the lithology is gravel layer or sand layer mixed with brown yellow and yellow gray cohesive soil. Gravel size gradually decreases from west to east, with a thickness of 20 ~ 90m.

D. The main lithology of Holocene (Q4) is generally cohesive soil, fine sand and gravel layer, mixed with swamp peat layer or organic silt layer, with a thickness of 5 ~ 10m and a thickness of 20 ~ 25m.

geologic structure

Beijing Plain belongs to the northwest corner of the North China fault depression of China-Korea paraplatform, and it is a subsidence area since Cenozoic. The periphery is often bounded by faults and adjacent areas, and it is further divided into three grade III structural units: Beijing superimposed fault depression, Daxing superimposed uplift and Dachang new fault depression (see figure 1).

Figure 1 Basement Structure and Quaternary Thickness Map of Beijing Plain Area

The main structure of Beijing Plain was formed in Mesozoic (Yanshan Movement) and has been further transformed by Himalayan Movement since Cenozoic. At the end of Mesozoic, many uplift zones and depression zones were formed in scattered arrangement, and a series of NNE and NNE faults were developed, and there were NW or NW tensile and torsional faults perpendicular or oblique to them. There are six active faults in the plain area, namely Babaoshan fault, Huang Zhuang-Gao Liying fault, Liangxiang-Qianmen fault, Nanyuan-Tongxian fault, Mafang-Xiadian fault and Nankou-Sunhe fault.

2.4 Quaternary hydrogeological conditions in plain area

2.4. 1 groundwater system and its characteristics

According to the basin, landforms, water-bearing medium structure of groundwater, groundwater occurrence conditions, hydraulic characteristics and hydraulic relations, Beijing Plain is divided into five systems, and the hydrogeological characteristics of each system are shown in Table 1.

2.4.2 Characteristics of groundwater recharge, runoff and discharge

The flow characteristics of Quaternary groundwater are the comprehensive embodiment of Quaternary groundwater recharge, runoff and discharge conditions. The recharge sources of Quaternary phreatic water and shallow confined water are mainly atmospheric precipitation infiltration, followed by mountain lateral runoff recharge, surface water and channel water leakage recharge, and farmland irrigation backwater infiltration recharge.

Table 1 Summary of Quaternary Groundwater System Characteristics

The discharge of phreatic water and shallow confined water is mainly artificial exploitation, followed by groundwater evaporation and lateral runoff discharge. The evaporation and discharge of groundwater in plain areas are mainly concentrated in areas where the groundwater level is below 4m. Overflow recharge of upper groundwater to lower shallow and middle-deep confined water is also a way of upper groundwater discharge.

Under natural conditions, the runoff direction of phreatic water and shallow confined water in plain area is consistent with the topographic change, that is, it moves from the piedmont to the plain. Affected by centralized mining, phreatic water and shallow confined water also move from the periphery of the falling funnel to the center of the funnel.

Because the middle and deep confined water has not been mined yet, the runoff field has not changed much, mainly horizontal runoff.

Groundwater Hydraulics

Annual dynamic characteristics of (1) groundwater

The dynamic change of groundwater in the study area is mainly meteorological-mining type, and the dynamic change of groundwater during the year is mainly affected by precipitation and artificial mining. In a hydrological year, the seasonal variation of groundwater level is obvious. The water level reaches its lowest value from April to June. The water level reaches its peak from July to September, and the fluctuation of water level can reach 5 ~ 10m.

Confined water is one of the main mining objects in plain area, and artificial mining is the most important factor affecting the dynamic change of confined water level. The dynamic type of shallow confined water is runoff mining, and the seasonal dynamic change of confined water is basically consistent with diving. In a hydrological year, there are also rising periods and falling periods, but the confined water head lags behind the time when precipitation reaches its peak. The lowest annual water level of confined water generally appears from May to July, and the highest annual water level generally appears from10 to February of the following year, with the annual water level changing range of 65438+.

(2) the dynamic characteristics of groundwater for many years

Figure 2 shows that before 1970s, there was little groundwater in Beijing, and the groundwater was basically in a natural state. After the 1970s, the groundwater level in the suburbs of the city dropped rapidly due to the substantial increase in groundwater exploitation. In 1980s, due to Beijing 1980 to 1984 arid and rainless climate (5-year average precipitation of 459.4mm), the groundwater recharge decreased, the exploitation increased, the groundwater level dropped rapidly, and the confined water level in concentrated mining areas in urban suburbs dropped rapidly. In the 1990s, the amount of groundwater exploitation was basically controlled, and there were four consecutive wet years from 1994 to 1998, and the groundwater level in the urban area increased. 1from the end of 1998 to 2003, due to the drought for five consecutive years, the groundwater recharge decreased. Compared with the water level at the end of 1998, the maximum decrease of groundwater level and confined water level is about 15 ~ 20m, with an average annual decrease of 3 ~ 4m/a. ..

Fig. 2 Dynamic curves of groundwater level in observation wells of Peking University (confined water) and Capital Normal University (diving).

2.5 Engineering Geological Conditions of Beijing Plain

Beijing Plain is located in the piedmont inclined plain of North China Plain, and the NNE active fault structure has controlled the basic pattern of the plain area since Cenozoic. Most of the plain areas are Quaternary loose continental deposits, which are distributed from lower Pleistocene (Q 1) to Holocene (Q4). According to its genetic types, it can be divided into alluvial facies, alluvial-diluvial facies, fluvial-lacustrine facies and piedmont slope diluvial facies strata; Formation lithology includes gravel, sand, silt and cohesive soil.

There are large and small diluvial groups, diluvial cones, loess plateau, residual hills and so on. At the edge of the piedmont, the width varies from 1 km to several kilometers. The lithology is mainly gravel, gravel and loess with sand lens, and the soil structure is complex.

The plain area is a fan-front plain mainly formed by the alluvial and diluvial actions of five major rivers, and the terrain at the junction of two adjacent fans is slightly lower, forming a depression between fans. This area is a multi-layer soil structure type with cohesive soil as the main body.

Classification of water-bearing strata and compression layers in Beijing land subsidence area 3

By 1999, the land subsidence area of Beijing is 2,865,438+50 mm, and the land subsidence area is 1826km2, which is distributed in the north and south areas. The northern area is mainly distributed in the urban area and the northern, eastern and southern suburbs, with an area of about 185 1km2, including the settlement areas such as Dongbalizhuang-Dajiaoting (the cumulative settlement center is 722mm), Laiguangying (the cumulative settlement center is 565mm) and Changping Shahe-Baxianzhuang (the cumulative settlement center is 688mm). The southern area mainly

Land subsidence in Beijing is closely related to the genetic type, lithology, thickness, structural characteristics and physical and mechanical properties of Quaternary strata, and groundwater exploitation is the main external cause of land subsidence. Therefore, it is of great significance to divide the aquifer group and compression layer in the subsidence area, analyze the distribution and burial conditions of groundwater aquifer and compression layer, and determine the contribution of main mining layer and compression layer to land subsidence.

3. 1 Principles and basis for dividing water-bearing rock formations and compression layers in subsidence areas

The principles and basis for the division of water-bearing rock group and compression layer group are as follows:

(1) According to Beijing geological records, Beijing regional geological survey report (1:50,000) and hydrogeological exploration data, combined with quaternary sporopollen and paleomagnetic data of Wangjing Station, Wangsiying Station and Tianzhu Station;

(2) According to the genetic type, age, lithology and burial conditions of Quaternary;

(3) According to the recharge and discharge conditions, groundwater flow characteristics and mining conditions of Quaternary groundwater in plain area;

(4) According to the physical and mechanical properties index, consolidation degree and in-situ test index of the compressed layer.

3.2 Division of Water-bearing Rock Groups

According to the above principles, the Quaternary aquifer in Beijing land subsidence area is divided into three water-bearing rock groups (see Table 2):

Table 2 Summary of Water-bearing Rock Groups Division in Beijing Land Subsidence Area

The first water-bearing rock group (phreatic water and shallow confined aquifer) is Holocene (Q4) and Pliocene (Q3) strata.

The second water-bearing rock group (middle-deep confined aquifer) is the Middle Pleistocene (Q2) stratum;

The third water-bearing rock group (deep confined aquifer) is the Lower Pleistocene (Q 1) stratum.

The burial conditions and hydrogeological characteristics of each water-bearing formation are as follows:

3.2. 1 First water-bearing rock group

Widely distributed in Beijing Plain, it is a phreatic aquifer with a single gravel structure at the top of various river alluvial fans, and the buried depth of the bottom plate is about 20m. The buried depth of shallow micro confined water is 20 ~ 40m, the buried depth of shallow confined water is 40 ~ 80m, and the buried depth of aquifer group floor is less than 100m, which is mainly caused by Holocene and Pleistocene diluvium. According to hydrogeological conditions, groundwater types and mining conditions, phreatic aquifers and shallow confined aquifers can be divided into two subcategories:

(1) phreatic aquifer subgroup

According to the different hydrogeological structure, this group can be divided into the top diving area of alluvial-diluvial fan and the middle and lower diving area of alluvial-diluvial fan.

A. phreatic area at the top of alluvial-diluvial fan: The aquifer is composed of sandy gravel, dominated by Pliocene (Q3) and Holocene (Q4) alluvial-diluvial facies, forming a single phreatic layer. The thickness of gravel in aquifer is 15 ~ 120 m, the pebble is round and sub-round, and the gravel diameter is generally 2 ~ 8 cm, up to 30cm. The permeability coefficient is 300 ~ 500 m/d, the aquifer is rich in water, and the water output of a single well is 5000 m3/d, which has been seriously over-exploited or over-exploited in most areas at present.

B. Submerged water area in alluvial-diluvial fan: The aquifer is deposited in Pliocene (Q3) and Holocene (Q4), and the lithology of the western and northern aquifers is mainly medium-coarse sand and gravel, with good water-abundance. The particle size gradually decreases to the east and south, the aquifer is mainly fine sand layer, and there is a small amount of gravel layer in some river sections, and the water abundance gradually becomes worse from northwest to southeast.

(2) Shallow confined water subgroup

The bottom depth of the aquifer is 80 ~ 100 m, mainly Pliocene (Q3) sediments, which are widely distributed in the middle and lower parts of Beijing Plain.

The aquifer in the middle and lower part of Yongding River alluvial fan is mainly composed of multiple layers of medium-fine sand and silty sand, with 1 ~ 3 layers of gravel locally, and the cumulative thickness of the aquifer is 20 ~ 35m. According to the experimental data of layered pumping, the permeability coefficient of shallow confined water aquifer in this area is generally 5 ~ 20 m/d, and the water yield of single well is 1500 ~ 3000m3/d, which decreases to 500 ~ 1500m3/d downstream.

In the middle and lower part of the alluvial fan of Chaobai River, the aquifer particles gradually become thinner from north to south, and the number of layers increases. Generally, it consists of two or three relatively stable gravel layers, and the cumulative thickness of the aquifer is 20 ~ 30m. According to the stratified pumping test data, the permeability coefficient of shallow micro confined water is generally 3 ~ 5m/d, the permeability coefficient of shallow confined water is generally 10 ~ 20m/d, and the water yield of single well is 3000 ~ 5000m3/d. ..

The aquifer in the middle and lower part of Wenyu River alluvial fan is 2 ~ 3 layers of gravel or sand, and the aquifer thickness is 5 ~ 10m. The cumulative thickness of aquifer is 20 ~ 30m, and the water yield of single well is 500 ~ 3000m3/d. ..

3.2.2 Second water-bearing rock group

Widely distributed in the middle and lower parts of alluvial and diluvial fans in Beijing Plain. The groundwater type is confined water in the middle and deep layers, and the buried depths of the top and bottom plates of water-bearing rock groups are 80 ~ 100 m and 300m respectively. The water-bearing strata are alluvial and lacustrine deposits of Quaternary Middle Pleistocene (Q2), and the lithology is mainly medium-coarse sand with a small amount of gravel. Aquifer is a multi-layer structure. According to the present mining situation and its dynamic characteristics, it can be divided into upper and lower parts of middle and deep confined water. The buried depth of the upper section is100 ~ 200m, and the buried depth of the lower section is 200 ~ 300m;

(1) Upper member of the second water-bearing rock formation

A. Yongding River alluvial fan. The buried depth of the floor of the water-bearing rock group is less than 150m, and the aquifer is composed of multiple layers of gravel with a cumulative thickness of 5 ~ 20m. According to the stratified pumping test data, the permeability coefficient of aquifer is generally 5 ~ 30m/d, and the water yield of single well is 500 ~1500m3/d. ..

B. alluvial fan of Chaobai River. The bottom boundary depth of this aquifer group is about 200m, and the aquifer is composed of multiple layers of gravel and sand, with a cumulative thickness of 30 ~ 50 m. According to the stratified pumping test data, the permeability coefficient of the upper aquifer is 20 ~ 25m/d, the middle aquifer 10 ~ 15m/d, and the lower aquifer 1 ~ 5m/d, and the single well flows out. ..

(2) Lower member of the second water-bearing rock group

A. Yongding River alluvial fan. At present, there are few data exposed by drilling holes in the lower part of the second water-bearing rock formation of Yongding River alluvial fan.

B. alluvial fan of Chaobai River. The bottom boundary depth of aquifer is less than 300 meters, which is mainly distributed in the depressions in the northeast and southeast of Beijing Plain. The lithology of aquifer is mainly medium-coarse sand and gravel, with a cumulative thickness of 30 ~ 50m. The water output of a single well is 500 ~1500m3/d.

3.2.3 The third water-bearing rock group

The rock group is mainly distributed in the central area of the depression in the northeast and southeast of Beijing Plain. The type of groundwater is deep pore confined water, and the buried depth of the water-bearing group roof is about 300m m. The lithology of the aquifer is Quaternary lower Pleistocene (Q 1) alluvial layer and lacustrine layer, and the lithology is mainly medium-coarse sand and gravel. The water-bearing formation has a multi-layer structure, and there is a cohesive soil aquifuge with a thickness of more than 30m at the top, which has poor hydraulic connection with the upper middle-deep confined aquifer.

3.3 compression layer division

According to the division principle, the compressible layer in Beijing land subsidence area can be divided into three compression layers: the first compression layer is less than 100m, the second compression layer is less than 300 m, and the third compression layer is more than 300m.

Physical and mechanical indexes of each compression layer are shown in Table 3.

Table 3 Comprehensive table of physical and mechanical indexes of compression layer in Beijing land subsidence area

3.3. 1 first compression layer

The first compressive layer is widely distributed in Beijing Plain, and the buried depth of this layer is less than 100m ... The stratum lithology is Quaternary Pliocene alluvial facies, alluvial lacustrine silty soil and cohesive soil layer, and the thickness varies from less than 50m to more than 80m (see Figure 3). According to its lithologic structure and compressibility, it can be divided into upper member and lower member:

(1) Upper part of the first compression layer:

0 ~ 10 m below the surface, the urban area is artificial backfill soil layer, and most areas are brownish yellow silt and silty clay layer, which is plastic ~ hard plastic, wet ~ saturated, moderately comprEssed, and the es value is between 8 ~ 15 MPa.

Below the surface 10 ~ 15m, the eastern, northeastern and northern parts of Beijing are silty clay and clay deposited by rivers and lakes, which are taupe ~ gray, contain organic matter, soft plastic ~ plastic, have poor compactness and high compressibility, and the es value is between 4 ~ 8 MPa, which is the main compression layer in this section. The southern part is alluvial silty clay and silty soil layer, brownish yellow, wet ~ saturated, plastic ~ hard plastic, medium ~ medium ~ dense, and the Es value is between 10 ~ 20 MPa.

25 ~ 40m below the surface in the east, northeast and southeast of Beijing are clay and silty clay deposited in still water environment, gray ~ grayish brown, plastic, moderately compressed, with Es value of 5 ~ 10 MPa, containing organic matter and snail shells, which is a soft soil layer with poor engineering geological properties. The southern part is alluvial silt and silty clay layer, which is brownish yellow, saturated, hard plastic and low compressibility, and the Es value is between 15 ~ 25 MPa.

(2) Lower part of the first compression layer:

40 ~ 50m below the surface is a stable clay and silt layer, which is widely distributed in the north, east, northeast and southeast of Beijing. The lithology is gray and brownish gray clay mixed with thick silty clay, which is generally plastic ~ hard plastic, medium dense, with large water content and medium compressibility, and the Es value is between 12 ~ 22 MPa. In the south of Beijing, the lithology is silty soil and silty clay layer, which is brownish yellow, saturated, hard plastic and low compressibility, and the Es value is between 18 ~ 28 MPa.

Fig. 3 Equal thickness zoning map of 0 ~100m underground compressed layer.

50 ~ 100 m below the surface is 3 ~ 4 layers of sand with 2 ~ 3 layers of silty clay and clay, which are widely distributed in the subsidence area. Silty clay and clay layers are mostly lens-shaped, with a thickness of 20-40m. Silty clay and clay layer are grayish brown to yellowish brown, saturated, locally containing organic matter, plastic to hard plastic, with medium and low compressibility, and the Es value is between 20 and 26 MPa.

3.3.2 The second compression layer

Widely distributed in the middle and lower parts of alluvial fans in Beijing area, the lithology is alluvial and lacustrine silt, silty clay and clay layer of Middle Pleistocene (Q2). In the southwest of Beijing, the buried depth of this group of floor is generally less than150m; In the east and north of Beijing, the buried depth of this group of floor can reach about 280 meters (see Figure 4). The ratio of compressed layer to total thickness is generally 0.6 ~ 0.8. With the buried depth of 200 meters as the boundary, it can be divided into upper section and lower section.

(1) Upper section of the second compression layer

The upper part of this section is about 10~30m ~ 30m silty soil, silty clay and clay layer, with a thin layer of silty sand. The eastern and northeastern parts of Beijing are alluvial-diluvial silty clay and clay layers, which are grayish brown to brownish yellow, saturated, hard plastic, compact in structure, locally mixed with grayish black silty soil and silty sand layers, with a water content of 25-34% and a compression modulus of 2 1 ~ 33 MPa. The southern part of Beijing is alluvial brown silty soil and silty clay layer, with compact structure, hard plastic-hard state, low compressibility, water content of 20-30% and compression modulus of 30-35 MPa.

Fig. 4 Equal thickness zoning map of underground 100 ~ 200m compressed layer.

The middle and lower part of this section is silty clay layer. Grayish brown, grayish yellow, saturated, hard plastic, low compressibility, and the compression modulus Es value is between 35 and 50 MPa. There are a large number of silt and muddy clay layers in some areas, which have high comprEssibility, and the compression modulus es value is between 20 and 25 MPa.

(2) the lower part of the second compression layer

The upper part of this section is a silty clay layer with a thickness of about 1.5 ~ 25m. The lithology is grayish black-grayish brown-grayish yellow silty clay and clay layer, which is saturated, hard plastic, compact in structure and low in compressibility, and the compression modulus Es value is between 50 and 70 MPa.

The middle and lower part of this section is grayish brown-grayish black silty clay layer, sandwiched with thin layers of grayish brown silty soil and silty fine sand, generally in a hard plastic-hard state, with compact structure and low compressibility, and the compression modulus Es value is between 50 and 50-70 MPa. Locally containing muddy clay and silt layer, the compressibility is high, and the compression modulus is only 27.7MPa.

3.3.3 The third compression layer

Mainly distributed in the central part of Beijing sag, it is grayish brown and gray silty clay and clay layer deposited by fluvial and lacustrine facies of Quaternary Lower Pleistocene (Q 1). The structure is dense, most of them are in a hard state, and the density is high, and the compression modulus is mostly greater than 70MPa. Most of the soil layers below 400m are consolidated and cemented, and the compressive modulus is mostly greater than 100MPa, so the compressibility is extremely low. The compression layer is composed of yellow medium-coarse sand and round gravel deposited in proluvial, with high density.

4 conclusion

(1) Groundwater in Beijing Plain is divided into five groundwater systems: Yongding River alluvial fan system, Chaobai River alluvial fan system, Juma River and Dashi River alluvial fan system, Wenyu River alluvial fan system and Jiyun River alluvial fan system. According to the genetic type, stratigraphic age, lithology and burial conditions of water-bearing media, the aquifer in Beijing land subsidence area is divided into three water-bearing rock groups:

The buried depth of the water-bearing floor of the first water-bearing rock group is less than 100m, and the gravel layer with single structure is mainly at the top or middle-upper part of the alluvial fan, and the groundwater type is mainly phreatic water. The middle and lower parts of alluvial-diluvial fans and lake plains are multi-layer structures, and the main types of groundwater are diving, shallow micro-confined water and shallow confined water.

The second water-bearing rock group is mainly distributed in the middle and lower part of alluvial fan and lake plain area, with multi-layer structure. The groundwater type is middle-deep confined water. The buried depth of alluvial fan floor in Yongding River is less than 150m in most areas, and the buried depth of alluvial fan floor in Chaobai River is 270 ~ 280m；.

The third water-bearing rock group is mainly distributed in the central area of the depression in the northeast and southeast of Beijing Plain. The groundwater type is deep confined water, and the buried depth of the roof is 270 ~ 280 m.

(2) According to the genetic type, stratigraphic age, lithology, burial conditions, physical and mechanical properties, consolidation degree and in-situ test index of soil, the land subsidence in Beijing is divided into three compression layers:

The first compression layer is widely distributed in Beijing Plain, and the buried depth of the floor is generally less than 100m ... Generally speaking, from west to east and from north to south, the compression layer gradually transits from alluvial silty clay to alluvial silty clay and clay layer, which is generally in a plastic-hard plastic state and is a normal consolidated soil.

The second compression layer is widely distributed in the middle and lower parts of alluvial fans in Beijing. The lithology is silty soil, silty clay and clay layer of alluvial-diluvial and lacustrine deposits in the Middle Pleistocene. The buried depth of this group of floor in the southwest of Beijing Plain is generally less than150m; The buried depth of the floor in the east and north of the plain can reach about 280 meters. The ratio of compressed layer to total thickness is generally 0.6 ~ 0.8, and cohesive soil is in plastic-hard plastic state, which is overconsolidated soil.

The third compressive layer is mainly distributed in the middle of Beijing Plain Depression, and the buried depth of the roof is more than 270m m. The compressive layer is mainly clay, which is hard overconsolidated soil.

The division of aquifer group and compression layer group in subsidence area and the basic geological parameters of each aquifer group and compression layer group have laid a solid foundation for the construction of land subsidence monitoring website and the establishment of land subsidence early warning and forecasting system.

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Tianjin Institute of Environmental Geology, Institute of Hydrology, Ministry of Geology and Mineral Resources. Research report on land subsidence mechanism and comprehensive management of prediction in Tianjin. +0998. 199889868666

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