1 Taiyuan basin
Taiyuan University of Technology Berlin Campus is located in the west of Taiyuan Basin, and the engineering geological conditions in the area are closely related to the formation history, geological structure and seismic investigation of Taiyuan Basin.
The formation history of 1. 1
The engineering properties of soil are closely related to the formation history of soil. Since Cenozoic, Taiyuan basin has been in a downward trend, while the mountainous areas on both sides have successively increased. The structure at the bottom of the basin is complex and the fault activity is frequent, and the structural stability of the site is controlled by the regional structure. Fenhe River runs through the middle of the basin from north to south, and the first and second terraces are formed on the east bank of Fenhe River, and the second terraces are connected with the Dongshan floodplain, while the west bank is connected with the Xishan floodplain.
1.2 geological structure
There are two sets of faults in Taiyuan basin, one is transverse fault, that is, NE-EW fault; The other group is a longitudinal fault, that is, a NNE-NNW fault, which belongs to Pliocene-Quaternary fault with strong activity but small scale. Because Taiyuan basin is located between rift basins, strong earthquakes in adjacent areas can affect Taiyuan basin, and the earthquake damage is also serious.
1.3 seismic activity statistics
Taiyuan basin is a multi-earthquake area, which is mainly characterized by many times and weak intensity. According to statistics, since 1304, there have been 22 earthquakes of magnitude above 4 in this basin, with the maximum magnitude of 6.5. In plane distribution, Taiyuan basin can be divided into three seismic zones: the western margin of the basin 1) seismic zone. Located in the piedmont fault zone of Xishan Mountain. 2) Seismic belt on the eastern margin of the basin. Located in the Dongshan piedmont fault zone, it is an area with more earthquakes and larger magnitude in the basin. Among the 22 earthquakes with magnitude above 4, the number of earthquakes in this area reached 10. The largest earthquake of magnitude 6.5 occurred in this area. 3) Seismic belt in the middle of the basin. It is mainly distributed in the area from Liujian to Nanwayao in xiaodian district, Taiyuan City, and in the areas of Qingxu and Xugou, and it is also a region with more earthquakes in this area.
2 Berlin campus
2. 1 terrain
Taiyuan University of Technology Berlin Campus is the old campus of Shanxi Institute of Mining and Technology, located in Wanbailin District, one of the six urban areas of Taiyuan City. The campus is 320 meters long from east to west and 340 meters wide from north to south.
The campus is located on the west side of Fenhe River, and the landform is the interactive area of I terrace on the west side of Fenhe River and piedmont diluvial inclined plain. Physically, there is Huyu River in the south and Yumen River in the north, with a relative distance of several hundred meters. Both rivers are seasonal rivers in Fenhe River Basin. Its source is the residual vein of Xishan, which discharges eastward into Fenhe River. The existing natural landforms evolved from the geomorphic units of the past fluvial facies series. The terrain of the site is gentle, and the ground elevation is between 800.03 and 804.438+0m. The underground water level in the area is 4.52~6.5m, and the water level elevation is 795. 1 ~ 798.23m, which is non-corrosive.
2.2 Vertical distribution of strata
According to the analysis of most previous engineering geological reports in this area, the strata within the exploration depth range are: Quaternary Holocene artificial deposits and Quaternary Pleistocene river alluvial and diluvial strata, and the lithology is mainly artificial fill, domestic garbage and construction garbage; Grade I non-gravity collapsible loess; Silty clay, silt, fine sand, clay, some soil layers are thin, and the thickness changes greatly, mostly interbedded.
2.3 Site Soil Types
The soil type of the site is mostly medium soft soil, and the thickness of the site covering layer is more than 80m, which is mostly Class III construction site.
2.4 seismic intensity and foundation liquefaction
According to the seismic fortification intensity map of Shanxi Province 1 ∶ 1 million, the seismic fortification intensity of Taiyuan City is 8 degrees. The range below the ground 15m in the site is saturated silt and sandy soil. According to the preliminary liquefaction judgment conditions in GBJ 1 1- 89 Code for Seismic Design of Buildings, liquefaction judgment should be made. Except the southwest side of the campus, that is, the site of high-rise residential buildings is slightly liquefied, other parts are judged as non-liquefied.
2.5 Uniformity of foundation soil
The slope of each soil layer in campus is often greater than 10%, which can be regarded as uneven foundation. At the same time, the weighted evaluation with compressive modulus also belongs to uneven foundation.
2.6 Stability of foundation soil
There are no natural adverse geological phenomena on campus. The civil air defense project built in the era of "deep excavation" belongs to man-made adverse geological phenomenon. Under the action of building load, stress concentration will appear around the air-raid shelter, forming a plastic zone. The further expansion of plastic zone may lead to local instability of foundation and affect the safe use of buildings. At the same time, it also brings many problems to the design and construction of foundation.
27# residential building in the campus is a six-story brick-concrete structure, which was started on 1997. In the process of foundation pit excavation, it is found that the western mountain wall of the building is just on the north-south air-raid shelter, and the top surface of the cave has reached the design foundation pit excavation elevation. Immediately, the shelter was excavated and the foundation was blocked. Because the bottom of the air-raid shelter is deep, flaky and graded sand will be compacted in layers after excavation. Under the condition that there is no abnormality at the end of the audit, the foundation treatment is replaced by Sanqi lime soil according to the original design, and the raft foundation is adopted. During the one-year construction, all parties involved in the construction are very concerned about the settlement, especially the western gables. Up to now, the settlement is even, and no cracks have been found in Xishan wall and other walls.
2.7 Collapsibility evaluation
According to the changes of water content of foundation soil, surface seepage and groundwater level, the collapsibility of collapsible soil is Grade I non-gravity collapsibility.
2.8 Bearing capacity of foundation soil
Bearing capacity of upper miscellaneous fill: 80 kpa ~100 kpa; Bearing capacity of silty clay:120 kpa ~160 kpa; Bearing capacity of fine silt: above 150 kPa.
The soil layer distribution of the 29-story high-rise residential building on campus is shown in the engineering geological profile (Figure 2.8), and the standard value fk of the bearing capacity of each layer of foundation soil is shown in Table 2.8.
3 Foundation treatment
There are two kinds of foundation treatment in this campus: soil replacement cushion method and pile foundation.
Standard value fk (kPa) of soil bearing capacity of each layer of high-rise residential building foundation
3. 1 soil replacement cushion method
It is often used for backfilling earthwork with large foundation pit area and large amount of excavated earthwork (and it will not affect the safe use of adjacent buildings during excavation), and is generally suitable for treating shallow soft soil foundation, collapsible loess foundation and miscellaneous fill foundation with low groundwater level.
In recent years, the six-story brick-concrete residential building on campus has adopted the replacement of Sanqi lime-soil and raft foundation.
3.2 Pile foundation
Suitable for multi-storey and high-rise buildings with high site requirements. At present, the commonly used pile types in foundation construction include bored pile, sunk pipe pile, static pressure pile, high-pressure jet grouting pile and manual digging pile. And superfluid plain concrete piles, multi-branch piles and bored piles developed in recent years.
3.2. 1 manual hole-digging belled pile
The characteristics of this pile type are: simple construction equipment and low engineering cost; Strong adaptability and little environmental pollution; Fast hole-forming speed and short construction period; The quality of finished pile is easy to control, and the bearing capacity of single pile is high. Due to the limitation of buried depth of groundwater level and geological conditions of strata (such as quicksand, silt, gushing water belt, etc.). ).
3.2.2 Immersed cast-in-place pile
This type of pile is rigid, and its main features are: high bearing capacity of single pile, fast construction speed and low project cost. However, it is limited by pile diameter, maximum sinking depth, stratum and groundwater conditions.
cast-in-situ bored pile
It is the most widely used pile foundation. The advantage is that the adaptability of pile type is wide, and it can adapt to almost all kinds of complex engineering geological and hydrogeological conditions. The disadvantage is that the project cost is relatively high, the mud displacement is large during construction, and there is certain noise pollution, which has a great impact on the environment during construction in busy urban areas, so it cannot be used as the first choice of pile type. Only when other pile types are not suitable or can not meet the bearing capacity requirements can this pile type be selected.
When choosing the pile type of the 29-story high-rise residential building on campus, because the adjacent buildings are very close and the groundwater is shallow, the bored reinforced concrete cast-in-place pile is used, and other pile types are not suitable for the site conditions. In principle, the selection of bearing stratum should be based on the Code for Geotechnical Investigation of High-rise Buildings (JGJ72-90) and the Technical Code for Building Pile Foundation (JGJ94-94), and the medium-low compressibility soil, silty soil and medium-dense-dense sand with stable thickness should be selected, and there is no soft soil layer or liquefiable soil layer under it. ⑦ The layers above silt can't meet these requirements, and the bearing capacity of gravel in the eighth layer is very high, but the buried depth of this layer changes greatly and the pile length is too long. Therefore, the bearing layer at the pile end is ⑦ layer of silt, with a length of 36m and a diameter of 800mm. ..
4 basic selection
The selection of foundation should be based on the geotechnical engineering conditions of the site and the importance of buildings, and comprehensively analyzed and compared from the aspects of foundation stability, bearing capacity, control of uneven settlement, construction period, construction difficulty and project cost.
There are three basic forms on campus:
1) strip foundation: in the early five-story and below brick-concrete structure, the strip foundation under the wall was often used.
2) Raft foundation: Considering the inhomogeneity of the site, this foundation form is mostly used for the later five-to six-story buildings.
3) Box foundation. The outstanding advantages of box foundation are good seismic stability, high stiffness and strong ability to resist uneven settlement. Because the general load of high-rise buildings is large, the additional stress on the foundation has a wide range of influence. Therefore, in order to reduce the excessive load on the ground, it is necessary to consider setting a basement under the ground of the building to unload the dead weight pressure of the soil, make it balanced and make the foundation more stable. At present, the height of Taiyuan urban area and high-rise buildings is 60 meters or even 100 meters, and there are 1 ~ 3 floors of basements underground, which not only increases the use space, but also solves the problem of foundation stability. Therefore, it is completely possible to bury the foundation of high-rise buildings 5 ~ 10m underground.
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