Design of diaphragm wall of the headquarters build

The researchers found that the Bank of China headquarters building is located at the intersection of Xidan North Street and Chang'an Street in Beijing. The architectural design is presided over by the famous American architect I.M. Pei. The geographical location of the project is very important. In order to make full and effective use of the site, the outline of the building is arranged along the planned red line, and the basement of the building occupies all the planned land (see Figure 1). The total area of the building is 174000m2, covering an area of 13299.6m2. Fifteen floors above the ground and four floors underground. The East and south sides are 55m span main entrance. In the middle of the building is a four seasons sharing hall with an area of about 4200m2 and a height of about 50m and natural lighting. The surface layer of the site is an artificial accumulation layer of 2 ~ 8m, and below it is the Quaternary sedimentary clay and sand pebble interaction layer. The first layer of groundwater is perched water in the upper layer, with a depth of about 5 ~ 6m; The second layer is confined water, with a buried depth of about 20.5 ~ 22m. The architectural design of this project is unique and novel, which brings great difficulties and challenges to the structural design, and also provides a rare opportunity for the innovation and development of structural design. Among them, the design and construction of the underground support structure of deep foundation pit has solved many technical difficulties in building a global super large optical communication whole industry chain base integrating optical fiber preform, optical fiber and optical cable, optical devices and raw and auxiliary supporting materials, obtained very valuable experience, and made a useful exploration for the development of construction technology. Figure 1 site plan 2 selection and determination of basement structure exterior wall scheme

in order to maximize the use of planned land, architects adopted the method of developing effective building space underground when the height of buildings is limited by planning. There are four floors in the basement, and the maximum depth of foundation pit excavation is -22m. At the same time, the outer contour of the building coincides with the red line of the building. In the scheme selection of structural design, first of all, the scheme of large-scale opening and power excavation of the basement slope is denied. The site is located in the downtown area, surrounded by important buildings and main streets of the city, so the slope excavation is impossible. In the second scheme, a slope protection pile support structure is built around the building. The advantages of this scheme are mature experience, reliable technology and convenient basement construction after excavation; The problem is that the supporting structure needs to occupy the municipal planning land outside the red line. This plan was also rejected. After analysis and demonstration, the design adopts the structural scheme that the outer wall of the basement is the diaphragm wall. The outer wall of the basement coincides with the red line of the building, and does not occupy the land outside the red line, which can not only ensure the effective transmission of the vertical load on the upper part of the building, but also ensure that the internal use space of the building is not affected, so as to meet the requirements of building use. With this scheme, its function and stress working state are different in different stages. During the construction and excavation stage, the diaphragm wall is a construction support structure for retaining soil and intercepting water; The use stage is the underground outdoor retaining wall. This scheme fully meets the requirements of planning and architectural design, but brings certain difficulties to structural design and construction

for the foundation pit with a depth of more than 20 meters, the construction support and use of the integrated diaphragm wall structure is currently inexperienced. The problems to be solved are: ① strict control of the displacement of the diaphragm wall. As a general support structure, the displacement of the diaphragm wall is not strictly required, as long as the safety and reliability of the excavation process are ensured; However, as the basement outer wall in the use stage, in order to ensure the reliable transmission of vertical load, the displacement of the wall top should be strictly controlled. Therefore, in order to avoid the influence of eccentricity and the reduction of building use space, large displacement of the wall is not allowed during the excavation process. ② The calculation method of internal force redistribution should be considered for the diaphragm wall in the construction support stage and use stage. ③ Reliable connection method between floor slab and diaphragm wall. ④ Under the action of vertical load, the settlement difference between the diaphragm wall settlement (caused by foundation sediment and other reasons) and the foundation slab shall be controlled, and waterproof measures shall be taken. The above problems are technically difficult, and there is no mature experience to learn from. Therefore, in the design and construction of the diaphragm wall of the Bank of China building, we have explored and adopted new design concepts and many new technologies and methods. Now the construction of the main structure has been basically completed. The test results and on-site observation show that the design is correct, the structure is reliable, and the effect is good, which provides useful experience for the application of the expanded earth wall technology. 3 Calculation Analysis and design of diaphragm wall 3.1 calculation analysis of diaphragm wall

because the side length of foundation pit is very long, the calculation is considered as a plane problem. Take the wall section of unit length for calculation, and analyze it with elastic fulcrum method and bar finite element method. The continuous wall is discretized into beam elements along the depth direction, and the anchor bolt and foundation pit subsoil are simplified into springs acting on corresponding nodes. The linear stiffness of the anchor bolt spring is determined according to the anchor bolt tension test curve (see Table 1); The linear stiffness of soil spring is calculated by m method according to different soil layers; The load on the continuous wall is the lateral earth pressure, water pressure at the back of the wall and the thrust of the bolt preload on the wall. The earth pressure is calculated in layers according to Rankine's earth pressure theory. The calculation diagram is shown in Figure 2. Linear stiffness of bolt spring table 1 linear stiffness of bolt elevation ki (kn/m) -4.001.10 ×. one point one eight ×. one point two five ×. 00 -14.001.45 ×. 00 -18.501.74 × 104 Figure 2 calculation diagram of underground diaphragm wall calculation and analysis simulates the actual construction process of foundation pit layered excavation and anchor rod layered driving and tensioning, which is divided into several continuous related working conditions. The calculation and analysis of each working condition should use the calculation results of the previous working condition. The actual situation that the wall has produced a certain displacement before the bolt is applied is considered in the calculation, and the situation that the wall sometimes displaces outside the foundation pit during the excavation of the foundation pit is also considered, making the calculation and analysis closer to the actual situation. See Figure 3 for the calculation results of continuous wall with typical section