If there are 7-8m cantilevers in areas with good soil quality, it can be used in soft soil areas with a pit depth of no more than 14m, and the economic benefit is good
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The combined type is used only when the depth of the foundation pit is very large（ 2) Reinforced concrete sheet pile This is a traditional support structure
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The cross-section with groove and groove has a certain water retaining effect
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Generally, the following data shall be provided for geotechnical investigation: (1) types, characteristics and properties of soil layers on the site（ 2) The distribution range and depth of unfavorable geological phenomena such as site fill, hidden Creek, ancient river channel and underground obstacles near the boundary of foundation pit and retaining wall show their impact on foundation pit engineering（ 3) The burial of shallow phreatic water and deep confined water at the bottom of the pit, the seepage characteristics of the soil layer and the possibility of quicksand and piping（ 4) Soil and water indexes required for support structure design and construction; 2、 The hydrogeological survey shall provide the following information and data: (1) apparent water level and static water level of underground aquifers（ 2) Recharge and dynamic change of water in underground aquifers and connection with nearby water bodies（ 3) The head height of confined water below the foundation pit bottom and the interface of aquifer（ 4) The influence of water level change on supporting structure and surrounding environment of foundation pit during construction is analyzed, and the measures that should be taken are put forward
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The depth of exploration points around the foundation pit shall not be less than 1 time of the excavation depth, and the soft soil layer shall be crossed in the soft soil area
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Regional 4
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The former is anti bending, and the latter is made into a waterproof curtain for water retaining.
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Exploration points should be arranged within 1 ~ 2 times the excavation depth outside the excavation boundary, and the scope of soft soil investigation should be expanded
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The cast-in-place pile retaining wall has large stiffness, strong bending resistance and relatively small deformation
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This is the most frightening building collapse in the construction industry since the founding of the people’s Republic of China
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Overview of foundation pit engineering concept and current situation foundation pit engineering is the general name of support, foundation pit soil reinforcement, groundwater control, excavation and other projects taken to protect foundation pit construction, underground structure safety and surrounding environment from damage, including survey, design, construction, monitoring, test, etc
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The spacing of exploration points can be 15 ~ 30m
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Strong comprehensiveness 2
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In the area with dense buildings in the center of the city, the conditions for foundation pit slope excavation are often not available
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High temporary and risk 3
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The appalling building fall in Shanghai shames civil workers
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In addition, before the support structure design, the following underground structure design data should be collected and understood: (1) the plane layout of the basement of the main project and the relative position with the building red line are related to the selection of support structure type and support layout（ 2) The pile position layout of the foundation of the main project is related to the column layout in the support system
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( 3) Bored cast-in-place pile row pile retaining wall is usually ∮ 600 ~ 1000mm, which is made into row pile retaining wall, reinforced concrete ring beam is poured at the top, and internal support system is set
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In the past, it was mostly used in sections where it was difficult to pull out steel sheet piles
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For soft soil slopes and soft rock slopes that are easily weathered, protective measures such as shotcreting, plastering, inlaying and masonry shall be taken for the slope toe and slope surface after excavation, and drainage at the slope top and toe shall be done
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① Channel steel sheet pile ② hot rolled locking steel sheet pile, which has the forms of U-type, Z-type, I-type, H-type and combined type
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Characteristics of foundation pit engineering: 1
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The scope of investigation depends on the excavation depth and the geotechnical conditions of the site
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Limit state of bearing capacity 2
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3、 The environmental investigation around the foundation pit shall include the following contents: (1) find out the type of buildings (structures), number of floors, foundation type and buried depth, foundation load and current situation of superstructure within the influence scope（ 2) Find out the distribution and characteristics of various underground facilities around the foundation pit, including water supply, drainage, cable, gas, sewage, rainwater, heat and other pipelines（ 3) Find out the distance of roads around the foundation pit and vehicle load（ 4) Find out the confluence and drainage of surface water around the site and adjacent areas, the leakage of underground water pipes and its impact on foundation pit excavation
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When there are signs of instability such as dangerous rock, boulder and collapse body on the slope, it shall be properly handled first
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geotechnical investigation in the detailed investigation stage of building foundation, it is appropriate to investigate the contents required by foundation pit engineering at the same time
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Therefore, it should be selected preferentially in open areas or surrounding environment where grading is allowed and slope stability can be guaranteed
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At about 6:00 on June 27, a 13 storey residential building under construction in “Lianhua riverside community” in Minhang District, Shanghai collapsed
At present, it is difficult to be tangent during construction, and there is a gap of 100 ~ 150mm between the piles, so the water retaining effect is poor
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It is widely used in all parts of China, which is one of the most widely used supporting structures
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A ring beam is set at the top, which will not be pulled out after use and will be permanently retained in the foundation soil
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Safety level of foundation pit supporting structure according to technical code for building foundation pit support, the safety level of foundation pit side wall is divided into three levels (table L-1), and corresponding importance coefficients are adopted for different levels in design γ 0 2、 Foundation pit engineering investigation I
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At this time, the excavation can only be carried out vertically or basically vertically under the protection of the supporting structure
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3、 Scheme selection of support system (1) steel sheet pile Steel sheet piles commonly used are simple channel steel sheet piles and hot-rolled locking steel sheet piles
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Limit state of normal use 3
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The engineering pile shall be used as the column pile as far as possible to reduce the cost（ 3) The number of basement floors of the main structure, the layout and elevation of floor slabs and bottom plates of each floor and the ground elevation are related to the determination of excavation depth, the selection of enclosure wall and support type and layout, and support replacement
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Practice shows that engineering experience plays an important role in determining the design scheme of foundation pit support and ensuring construction safety
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However, it is permanently retained in the foundation soil, which may cause obstacles to the construction of underground works in the future
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2、 Design principles and methods of foundation pit support structure The design principles of foundation pit support structure are: (1) safe and reliable; (2) economical and reasonable; (3) convenient construction: according to the current national industry standard technical specification for building foundation pit support, the foundation pit support structure shall be designed by the limit state design method expressed by partial coefficient
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When the stratum changes greatly, more exploration points should be added to find out the distribution law
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Strict environmental conditions 1、 The content of foundation pit engineering there are generally two construction processes of foundation pit earthwork excavation: sloping excavation (excavation without support) and external excavation under the protection of support system (excavation with support)
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The limit state of foundation pit supporting structure is divided into the following two categories: 1
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In the case of supported excavation, the foundation pit engineering generally includes the following contents: (1) foundation pit engineering survey（ 2) Design and construction of foundation pit support structure（ 3) Control the groundwater level of foundation pit（ 4) Excavation and transportation of foundation pit earthwork（ 5) Engineering monitoring during foundation pit earthwork excavation（ 6) Environmental protection around foundation pit
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Developers, contractors, supervision engineers and government officials with regulatory responsibilities will be nailed to the stigma column in the history of civil construction! The main causes of foundation pit accidents are as follows: ( 1) The design theory is not perfect
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Many calculation methods are still in the semi empirical stage, and the theoretical calculation results can not well reflect the actual situation of the project（ 2) The designer has unclear concept, improper scheme, missing items or errors in calculation（ 3) The design and construction personnel are inexperienced
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The former is simple and economical, but it needs to have the conditions for slope excavation, that is, the foundation pit is not too deep, and there is enough space outside the plane of the foundation pit for slope
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Sometimes it is combined with the retaining wall of deep mixing cement soil pile
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In China, the U-shape is commonly used, that is, the sheet pile wall is formed by biting each other
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