Super high-rise buildings have strong safety requirements, and their construction quality also has an important impact on surrounding buildings. Geotechnical engineering investigation, as an important preparation before foundation construction, is of great significance to the construction of subsequent projects. Geotechnical engineering investigation and analysis personnel must ensure the scientificity and accuracy of geotechnical engineering investigation with a high sense of responsibility.
1. Project overview
The proposed buildings of Cui Yi Jinsheng Jiayuan Project (Phase II of Area E) are: ① ~ ④ (Blocks A and B) * * 4 (8 buildings), all of which are 48-storey high-rise buildings with a height of H= 150m. The planned total land area of the residential area of this project is 18 125m2, the total green area is 6536m2, and the green rate is 36.06%. This project adopts pile foundation, frame-shear wall structure or frame structure. The project has two basements. The indoor floor of the design building is 5.00m absolute elevation (1985 national elevation benchmark, the same below), the absolute elevation of the basement roof is 5.00m, the absolute elevation of the basement on the first floor is -0.6m, the absolute elevation of the basement on the second floor is -4.50m ... The ground elevation of the site is 3. 19m~4.30m, and the excavation depth is 8. The purpose of this survey is to find out the type, distribution, thickness, buried depth and physical and mechanical properties of the rock soil layer in the foundation of the proposed site, analyze and evaluate the stability, uniformity and bearing capacity of the foundation, find out whether there are adverse geological effects and their harmfulness, and provide geotechnical engineering geological data for the foundation design and construction of the proposed building.
2. Geological characteristics of geotechnical engineering
In this project, the designer will arrange 40 boreholes in the site, including 7 control boreholes/kloc-0 and 23 general boreholes. Borehole spacing of buildings 10m ~ 18m, and borehole spacing of foundation pit and surrounding areas is 25m ~ 30m. According to this drilling, the foundation of this site is composed of Quaternary artificial fill (Qml), Quaternary alluvial soil (Qal) and Eocene Baoyue Formation (E2by) clastic rocks. The geotechnical types are complex. The lithology revealed by Baoyue Formation in Eocene is argillaceous siltstone and fine sandstone. There is a common phenomenon of "soft and hard" interlayer or interbedding in weathered bedrock. According to investigation and drilling, the engineering geological profile is divided into 9 layers, and then the layers are divided into 2 sub-layers.
3. Hydrogeological characteristics
The proposed site is located in the south of the Tropic of Cancer, and belongs to the subtropical monsoon climate zone. It is warm and humid with abundant rainfall and is regulated by the maritime climate. There is no surface water system near the plant site and near the plant site. After drilling the last hole, the measured initial groundwater level of each hole is1.50m ~ 2.60m; 24 hours after the final hole, the measured relatively stable water level is between 2.00 m and 2.60 m, with an average of 2.12 m; That is, the elevation is between 0.74m-2.20m, with an average value of1.34m.
The groundwater level measured this time is the water level during the survey, which cannot represent the long-term stable water level of the site, nor is it the highest water level that may be generated within the design service life of the building. The types of groundwater are mainly stagnant water in the upper layer of the fill layer, pore water in the Quaternary loose soil layer and bedrock fissure water. In the field investigation, the engineering water sample 1 was taken from borehole GZK 1 and GZK 18 respectively, and two samples were * * *. Analysis: pH = 7.02 ~ 7.04, belonging to neutral water; According to the total salinity = 231.67mg/L ~ 242.438+0mg/L, it belongs to fresh water. Hydrochemical types are: HCO3? Calcium sulfate water. According to the judgment results of water quality corrosivity, the comprehensive evaluation shows that the groundwater in the site has slight corrosivity to the concrete structure and slight corrosivity to the steel bars in the reinforced concrete structure. Anticorrosion shall be carried out in accordance with Code for Anticorrosion Design of Industrial Buildings (GB50046-2008).
4. Suggestions on excavation and support of foundation pit
The proposed building is a super high-rise building with two floors underground. The elevation of the foundation pit floor is-4.50m, and the excavation depth is 8.0m. Baishun Road is on the west side of the foundation pit, Lily Road is on the east side, and Hua Fu Road is on the south side. The environmental conditions are relatively simple. The soil layers within the excavation depth of foundation pit are mainly artificial fill, silty clay, muddy soil and medium sand. The groundwater level is relatively high. According to the results of field pumping test, when the dewatering depth of foundation pit is 6 ~ 8 m, the influence radius of dewatering reaches 50 ~ 75 m ... The engineering geological conditions are complex and the safety level of foundation pit is Grade II.
According to the surrounding conditions of the foundation pit site and the characteristics of the geotechnical layer of the site, slope excavation is not suitable for foundation pit excavation, and enclosure excavation is recommended. After the completion of earthwork excavation, the foundation pit should be closed immediately to prevent flooding and exposure, and the underground structure construction should be carried out in time. It is suggested that rotary bored piles or bored piles should be used for foundation pit support, and cement mixing piles should be used to stop water between piles. The site is rich in sandy soil and groundwater, so it needs to be combined with tube well point dewatering. The scheme consists of four parts: support system, support system, water stop system and drainage system. The supporting system adopts row piles, and the pile type is rotary bored pile or bored pile.
The supporting system is composed of surrounding sandalwood and supports, and the supporting form can be reinforced concrete inner support, steel structure inner support or combined use. Dewatering and drainage by tube well point. Ensure the smooth progress of foundation pit excavation. During foundation pit excavation, ground protection measures such as plastering mortar and setting drainage ditches should be taken to prevent rainwater from infiltrating into the foundation pit and affecting the stability of the foundation pit. Water interception measures should be taken at the bottom of foundation pit to prevent the bottom of foundation pit from floating and ensure construction safety. Dewatering and excavation of foundation pit have great influence on surrounding roads and underground pipelines. It is suggested that monitoring should be strengthened in the process of basement foundation pit construction, information should be fed back to the design and construction units in time, and the scheme should be adjusted in time to ensure the safe and reasonable construction of foundation pit.
5. Suggestions on foundation construction
(1) There is a large area of artificial fill on the surface, so the influence of negative friction on the bearing capacity and settlement of pile foundation should be considered. It is suggested to take measures to reduce negative friction, such as:
(1) Before construction, conduct compaction treatment;
② When determining the bearing capacity of a single pile, if there is artificial fill or muddy soil around the pile, the friction around the pile is not calculated;
(3) For the end-bearing pile foundation, because the bearing layer at the pile end is hard, after the pull-down load caused by negative friction, no settlement will occur or the settlement will be small. At this time, negative friction will act on the upper surface of the neutral point of the pile for a long time. Therefore, the pull-down load caused by negative friction above neutral point should be calculated, and the pull-down load should be taken as a part of external load to check the bearing capacity of pile foundation.
(4) When the load is uneven, the precipitation is uneven or the soil layer around each pile foundation is uneven, uneven settlement will occur, and the pull-down load and settlement caused by the negative friction of each pile will also be uneven. Therefore, negative friction should be considered to check the settlement of pile foundation.
(2) The medium sand in Layer (4) of the site is thick, mainly medium dense. If bored piles are used, the mud concentration should be moderate; To control the drilling speed, not too fast; Only in this way can the hole wall be protected and the hole collapse be prevented.
(3) In the construction of punching (drilling) bored piles, practical construction methods should be adopted when crossing hard interlayer or steep rock surface. Soft soil is thick in some sections, so necking should be prevented during pile foundation construction.
(4) There is a phenomenon of "soft and hard" interlayer in weathered bedrock, so please pay attention to it when designing and constructing pile foundation. When using prestressed concrete pipe pile, attention should be paid to the construction method to prevent the pile from breaking or the pile head from breaking due to sudden change of soft and hard, and to ensure the quality of finished pile.
(5) The influence of noise, vibration, waste water and spoil on the environment should be considered in pile foundation construction, and corresponding environmental protection measures should be taken.
6 conclusion
If the geotechnical investigation in the early stage of high-rise building is not carried out in place, it will affect the subsequent engineering treatment, thus affecting the overall engineering quality, eventually increasing the engineering cost and even causing safety accidents. The proposed buildings in this project are four 8-story super high-rise buildings (48 stories high), with the first-class construction importance, which are generally in a relatively stable geological structure area, with poor geotechnical engineering geological conditions, but belong to a relatively stable foundation. It is suitable for the construction of the proposed building on the premise of fully considering the unfavorable geological factors and taking corresponding measures.
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