1. 1 Shortcomings of top-down method
Because it involves underground construction, half of the top-down construction will be restricted, and it is impossible to adjust the height, mode and process as needed at any time as in aerial work. If it is a basement project with a large floor, it is necessary to set up another retaining wall and floor to support the whole interface operation. In this case, extra workload will be generated. On the other hand, due to the existence of semi-closed operation, it is necessary to improve the flexibility and efficiency of internal construction and the performance of related equipment and tools to adapt to closed and underground construction. Generally speaking, the top-down method has higher requirements for equipment and technical skills in construction than ordinary ground operations, and its limitations are relatively large.
1.2 Difficulties in top-down construction
Because it is constructed from the middle down, the biggest difficulty in the implementation of the top-down method is the waterproofing of the downward construction part and the treatment of construction joints. How to do all the preventive measures in the process of underground construction: the high quality of floor slab, the structural arrangement of piles, the structural selection of construction and the necessary water-stopping measures are all the keys to the waterproof problem, because once the water flow affects the construction process, it will bring irreversible damage to the whole bottom project from top to bottom, so all the preventive measures should be done before and at the beginning of the project.
The grid structure and vertical typesetting in the construction process, as well as the joint treatment of every detail, are the key points of construction joint treatment. In a word, the top-down method has greatly promoted the progress of the whole project, but it also has its own difficulties in operation. Only by taking various preventive measures before construction, strictly observing relevant regulations during construction, and doing a good job of acceptance after construction, can we perfectly present the advantages of the reverse construction method and provide convenience for the whole project.
2 the specific application of the reverse method
2. 1 Project Introduction
A shopping mall is planned to build an exhibition center with an area of 7000m2, a building area of 4,900m2, an aboveground building area of 9000m2 and an underground building area of 6,900m2 (the proposed two-storey underground parking lot), with a total area of 15900m2. It is required to give priority to manual hollowing, and mechanical equipment should cooperate to complete related operations, and the whole project should be completed by reverse method. Due to the wide coverage and large amount of work, in order to ensure that the project is completed on schedule, the mall agrees to the following reverse construction method and the following construction scheme.
2.2 Construction process
(1) Monitoring system installation and precipitation. Before the whole project starts, entrust relevant units to test the land stability and soil quality of the whole area. If there is any problem, the construction plan should be stopped immediately and adjusted appropriately until the quality and indicators of the whole related factors reach the national regulations.
(2) Concrete pouring of beam and slab. According to the required area, length, width, height, spatial structure, typesetting, etc. , theoretically calculate the data meeting the space requirements, and make relevant sketches. Then, according to the theoretical results, the qualified steel plates are used to cast beam-slab concrete in the early stage.
(3) Earthwork excavation. After the foundation beam slab completes the foundation working face, carry out the first earthwork excavation. In this process, attention should be paid to the ventilation at the entrance of the exit and the reservation of space. Mechanical transportation and manual cooperation are adopted to ensure the accuracy and safety of earthwork development to the maximum extent. Corresponding crane trucks can be installed at the exit of earthwork excavation to directly handle the earthwork excavated in the basement, so as to avoid accumulation affecting the progress of the project or causing unnecessary engineering hazards.
(4) Install the steel pipe column. After accurately locating the position of the steel pipe column, clean up other sundries around, use manual digging piles, install the steel pipe column as required, and pour concrete. In the whole process, the error of locator should be reduced as much as possible to ensure that the bottom pouring of steel pipe column meets the relevant requirements (to ensure sufficient bearing capacity). In a word, the installation of steel pipe column is an important part of the whole foundation, so as to minimize the errors in all aspects and ensure safety.
(5) Concrete pouring of beam and slab. After the steel pipe column is installed, the beam and slab concrete shall be poured again, and the beam and slab tension shall meet the requirements, and the outlet shall be reserved for later operation.
(6) Earthwork excavation. After the above work is completed, another round of earthwork excavation will be carried out. Just like the third step, ensure the construction safety and reserve the exit, and cooperate with manual machines to achieve maximum efficiency and reliability. (7) Inspection and acceptance. In fact, the acceptance work runs through the whole project, and each link has to go through relevant acceptance after completion. After the completion of the above seven tasks, a comprehensive acceptance should be carried out, and the unqualified parts should be recreated in time. All inspection results shall be reported and recorded.
3 abstract
More and more high-rise buildings bring diversity to urban life, and the basement structure also follows. The application of top-down method in construction has really brought convenience and cost savings to the construction process. However, it is extremely important for the construction unit to pay attention to the key points and difficulties related to the top-down method before and during the construction. Only by making preparations and preventive measures in every detail can the top-down method be applied to high-rise building construction more maturely and perfectly, bringing benefits to urban life.
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