In recent years, with the rapid development of economic construction and urban construction in China, there are more and more underground projects
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The construction of multi-storey basements, subway stations, underground garages, underground shopping malls, underground warehouses and underground civil air defense projects of high-rise buildings all need to excavate deep foundation pits
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The plane area of some multi-storey basements of high-rise buildings is tens of thousands of square meters, and the depth of some basements is 26.88m, which makes the construction difficult
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The correct design and construction of foundation pit engineering can bring huge economic and social benefits, and play an important role in accelerating the progress of the project and protecting the surrounding environment
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1、 The content of foundation pit engineering is slope excavation (excavation without support): simple and economic, which should be preferred when the open area or surrounding environment allows to ensure the stability of the slope
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Excavation under the protection of support system (excavation with support): the soil quality and geological conditions are relatively poor; in the urban center and densely built areas, the conditions for slope excavation are often not available
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2、 According to jgj120, the safety level of foundation pit side wall is divided into three levels, and the corresponding importance coefficient γ is used for different levels
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3、 Monitoring of foundation pit deformation code for acceptance of construction quality of building foundation engineering (GB 50202) stipulates the classification and deformation monitoring value of foundation pit as follows: Note: 1
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Class I foundation pit is one of the following situations: (1) important engineering or supporting structure is part of the main structure; (2) excavation depth is more than 10m; (3) the distance from adjacent buildings and important facilities is less than 10m (4) within the scope of the foundation pit, there are historical relics, modern excellent buildings, important pipelines and other foundation pits that need to be strictly protected
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2
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The third level foundation pit is a foundation pit with excavation depth less than 7m and no special requirements for surrounding environment
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3
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In addition to the first and third class foundation pit belongs to the second class foundation pit
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4
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When there are special requirements with the existing facilities around, these requirements should be met
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4、 Foundation pit engineering investigation 1
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Investigation of surrounding environment investigation of adjacent buildings (structures) around the foundation pit
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For foundation pit construction in densely built areas of large and medium-sized cities, the following contents should be investigated: (1) the distribution of surrounding buildings (structures) and their distance from the sideline of foundation pit; (2) the superstructure form, foundation structure and buried depth, pile foundation and sensitivity to settlement difference of surrounding buildings (structures)
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Relevant design drawings should be collected and referred to when necessary (3) whether the surrounding buildings belong to historical relics or modern excellent buildings, or have special and strict requirements for use; (4) if the surrounding buildings (structures) have been inclined, cracked, and used abnormally before the excavation of foundation pit, the relevant data should be collected through photography, drawing and other means
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If necessary, a qualified unit shall be invited to conduct analysis and appraisal in advance
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2
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Investigation of underground pipelines around the foundation pit construction in large and medium-sized cities, the main pipelines around the foundation pit are gas, water, water and cable
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(1) Gas pipe
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We should investigate and master the following contents: relative position with foundation pit, buried depth, pipe diameter, pipe pressure, joint structure, pipe material, length of each pipe section, buried age, etc
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The gas pipe is generally made of steel pipe and cast iron pipe, the length of pipe section is about 4 ~ 6m, and the diameter is usually 100, 150, 200, 250, 300, 400 and 500mm
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The structure of cast iron pipe joint is socket connection, flange connection and mechanical connection, and the steel pipe is mostly welded or flange connection
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(2) Water supply pipeline
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The following contents should be investigated and mastered: relative position with foundation pit, buried depth, pipe diameter, pipe material, length of pipe joint, joint structure, water pressure in pipe, buried age, etc
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Cast iron pipe, reinforced concrete pipe and steel pipe are commonly used for water supply pipe
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The length of pipe section is about 3 ~ 5M and the diameter is 100 ~ 2000mm
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Cast iron pipe joints are mostly socket joints and flange joints, reinforced concrete pipes are mostly socket joints, and steel pipes are mostly welded
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(3) Sewer
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The following contents should be investigated: relative position with foundation pit, pipe diameter, buried depth, pipe material, water pressure in pipe, pipe joint length, foundation form, joint structure, inspection well spacing, etc
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Prefabricated reinforced concrete pipes are commonly used for sewer pipes, and their joints are socket type, tongue and groove type, flat type, etc., with pipe diameter of 300 ~ 2400mm
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(4) Cable
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There are many kinds of cables, including high voltage cables, communication cables, lighting cables, defense equipment cables, etc
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Some are placed in the cable trench, some are overhead
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Some use a common trench, and a variety of cables are put together
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Cable can be divided into ordinary cable and optical cable, and the requirement of optical cable is higher
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For cables, the following contents should be mastered through investigation: relative position with foundation pit, buried depth (or overhead height), specifications and models, use requirements, protection devices, etc
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3
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The investigation of underground structures and facilities around the foundation pit, such as subway tunnel, subway station, underground garage, underground shopping mall, underground passage, civil air defense, pipeline common ditch, etc., should also investigate the relative position with the foundation pit, buried depth, foundation form and structural form, sensitivity to deformation and settlement, etc
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These underground structures and facilities often have higher requirements, so effective measures should be taken in the construction of adjacent deep foundation pit
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4
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The investigation of surrounding road conditions is carried out in the downtown area of the city, which is adjacent to the common roads
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The importance of these roads is different, some are secondary roads, and some are urban trunk roads
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Once they are damaged due to excessive deformation, they will have serious consequences
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Road conditions are also related to construction and transportation
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Therefore, before the construction of deep foundation pit, the following contents should be investigated: (1) the nature and type of surrounding roads, and the relative position with the foundation pit; (2) traffic condition and importance; (3) traffic rules (one-way road, two-way road, no parking, etc.); (4) road subgrade and pavement structure
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5
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The construction conditions around the foundation pit site have a direct impact on the design and construction of the foundation pit project, and must be investigated in advance
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(1) It is necessary to know whether there are restrictions on the transportation of earthwork, materials and concrete during the construction of foundation pit, and whether phased closed construction is allowed when necessary, which has an impact on the selection of construction scheme; (2) understand the restrictions on the noise and vibration generated by construction near the construction site
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If there are strict restrictions on construction noise and vibration, the selection of pile type and blasting demolition of concrete support of supporting structure will be affected; (3) understand the conditions of construction site, whether there are enough sites for transportation vehicles, stacking materials, parking construction machinery, processing steel bars, etc., so as to determine whether to carry out comprehensive construction, partition construction or reverse construction
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