The design and construction technology of building foundation pit engineering are varied, and there are many influencing factors in practical engineering. Like geotechnical engineering, foundation pit engineering has the characteristics of "practice first, theory later". So far, there have been a lot of successful practical experiences in deep foundation pit engineering in China, but there are also some failed lessons. In order to fully understand the design and construction characteristics of building foundation pit and make it convenient for designers to refer to engineering experience in calculation, this chapter selects some successful foundation pit engineering examples. The selected example mainly considers the following points: (1) Large typical deep foundation pit; (2) It has outstanding characteristics in a certain aspect; (3) It has guiding significance for future foundation pit engineering. In addition, the design and calculation of several typical cantilever pile-wall supporting structures are introduced in detail through examples. Example 1 structural design of pile wall 1. The design of cantilever pile wall is known: the retaining height of cantilever pile wall structure = 3m sand y =19kn/m2; P-30, no groundwater, allowable stress of steel sheet pile = 240mpa, as shown in Figure 8- 1. Determine the required length L of sheet pile wall and the required section bending moment Ⅳ. You can choose 300×300 I-beams (W-365 cm3/m) with a unit weight of 845 N/m.. 2. Known single-support pile wall design: retaining height H=6m, sandy soil 7 = 19KN/m3, no groundwater, lateral support, spacing 2m. The action point is below the ground behind the wall1m; The allowable bending stress of steel sheet pile is 240MPa, and it is designed as "free bearing". Find the required length l, supporting force f and required section bending moment w of sheet pile (see Figure 8-2). Solution 3. Anchor sheet pile is used to calculate the excavation depth of 6m project. Anchor sheet piles are used to retain the soil, and a trench with a depth of 1m and a width of 1 ~ 2m is dug behind the sheet piles. The ground load is 30kn/m2, with a width of 6m and a distance of 2m from the sheet pile. See Figure 8-3 for geological conditions. Reinforced concrete piles are dense in the foundation pit, and well point dewatering is arranged outside the sheet piles, and the well point pipe is 7m long. Solution (1) Adjust the P and C values of the selected layers of soil within the dewatering range of the well point, and the active lateral pressure behind the sheet pile (2) Ground load: Because the trench with the depth of Im is dug in advance after the sheet pile, the earth pressure is calculated from the depth of Im, and the soil with the thickness of Im is taken as the ground load, and its value is 4. Multi-layer support sheet pile wall calculates the basement of a project, with excavation depth of 9m and pile foundation. ESP V is selected for steel sheet piles, with section modulus Ⅳ of 3.82× 106 mm3/ linear meter, moment of inertia of 9.55× 108mm4 and elastic modulus e = 2.06x105n/mm2. Solution: Because the well point dewatering is set in sheet pile and is a dense pile foundation, the internal friction angle P and cohesion F below the first 9m of sheet pile wall are adjusted and multiplied by 1.4 and 1.3 coefficients respectively. The procedure of excavation and support is as follows: the first stage of excavation-the first layer of support-the second stage of excavation-the second layer of support-the third stage of excavation-the third layer of support-the fourth stage of excavation-adding a cushion-removing the third layer of support. At present, the stress of sheet pile in each stage is analyzed and calculated respectively. (1) The first-stage excavation is completed, and the sheet pile is cantilever-shaped, with an excavation depth of 3.2m See Figure 8-5 for the calculation diagram of sheet pile in the first-stage excavation. The largest and deepest foundation pit project-Shanghai Jinmao Mansion Jinmao Mansion is located on the south side of the Lujiazui Tunnel exit in Pudong. The project covers an area of 23,000m2, with a total construction area of 290,000m2, three floors underground, 88 floors above ground, and the spire elevation of 420m (see Figure 8- 10). The three underground floors cover an area of about 60,000m2, and the excavation area of the foundation pit is nearly 20,000m2 (see Figure 8- 1 1). The excavation depth of the main building is 19.65m, and the excavation depth of the podium is 15.6438+0m .. There are 429 steel pipe piles with a diameter of 9 14 under the main building, with a length of 65m and a delivery length of17.5m; There are 632 steel pipe piles with a diameter of 609 under the podium, the pile length is 33m, and the pile is delivered13.5m. This project is invested by China Shanghai Foreign Trade Center Co., Ltd., designed by SOM Design Office of the United States and contracted by Shanghai Construction Engineering Group Corporation. 1. characteristics of foundation pit engineering this project is the largest and deepest project in Shanghai's basic engineering construction. Its main features are as follows: (1) The underground continuous wall is used as the foundation external wall enclosure project, and has the function of a load-bearing wall, with a thickness of 1m and a depth of 36m. Because there is no lining on the inner wall of the underground wall, it requires the construction unit to ensure the construction quality in the construction of the underground wall, especially in the joint treatment of the groove section, the cleaning of the sediment at the bottom of the groove, and the seepage prevention of the whole wall. (2) The temporary support of foundation pit adopts cast-in-place reinforced concrete support. (3) The foundation earthwork is large, reaching 300,000 rn3. (4) Because the foundation construction adopts a two-stage excavation scheme, when hoisting the core tube of the main building and the steel structure of the basement, the concrete support should not touch these structures, so the support design should achieve four avoidances: avoiding the core tube of the tower, avoiding the steel structure of the basement, avoiding the floor beam of the podium and avoiding the foundation steel pipe pile. All these bring many difficulties to the layout of the supporting plane. 2. Comparison and selection of design schemes for foundation pit support design (1) In the foundation project of Jinmao Building, the original design of SOM design office adopted the cable-stayed anchor scheme. The main building part, cable-stayed anchor * * * six roads; There are four cable-stayed anchors in the podium. The cable-stayed anchor is anchored in 7 ~ 2 layers of sandy soil at an angle of 45, and the cement slurry is poured at the root 10 ~ 15m. The cable-stayed anchor is composed of steel beams, and the anchorage design strength of the cable-stayed anchor is 150t (service load). The reinforced concrete internal support scheme was put forward by Shanghai Construction Engineering (Group). There are four internal supports in the main building, and the elevation of the first support is 3.4m；; The elevation of the second bearing is ~ 8.3m, and the elevation of the third bearing is13.1m; The elevation of the fourth bearing is17.1m. The podium part has three internal supports with the same elevation as the main building part. Because this construction scheme has mature construction experience in Shanghai, strong construction reliability, and does not increase the project cost compared with the cable-stayed anchor construction scheme, it is considered that the construction method of reinforced concrete internal support is more suitable for the foundation pit support of Jinmao Building. (2) See Table 8- 1 for geotechnical parameters and earth pressure values. Except for the calculation of active earth pressure, the rest are obtained from geological data. For the active earth pressure of retaining wall in foundation pit, the calculation result of Rankine theory is more suitable for the objective situation of soft soil foundation in Shanghai, so the distribution of earth pressure can be obtained according to Rankine formula. (3) According to the active earth pressure distribution diagram, the design reaction envelope diagram of foundation pit support is synthesized, and the support reaction envelope diagrams of four internal support action points are obtained (see Figure 8- 12). According to Rankine theory, the reaction force of the fourth support should be greater than that of the third support. However, according to various data and documents, when the excavation reaches a certain depth, the reaction force of the support can be appropriately adjusted due to the lagging deformation of the deep soil, so the fourth support is reduced to 79 1kn/m. (4) The design working condition of the foundation pit support is 1: the main building and podium are excavated for the first time; Working condition 2: the first support of the main building is completed, and the second excavation of the main building is completed; Working condition 3: the main building is supported for the second time, and the main building is excavated for the third time; Working condition 4: the third support of the main building and the fourth excavation of the main building are completed; Working condition 5: the fourth support of the main building and the fourth excavation of the main building are completed; Working condition 6: the second excavation of the podium is finished; Working condition 7: the podium is supported for the second time, and the third excavation of the podium is over; Condition 8: The podium is supported for the third time, and the podium is excavated for the fourth time; Condition 9: Remove all internal supports and complete the third floor of the basement. According to the above boundary conditions and various working conditions, the bending moment envelope, shear force envelope and displacement envelope of underground continuous wall and bored pile can be obtained by using computer SAP90 program. (5) Reinforcement design of diaphragm wall and bored pile; According to the envelope diagram of underground continuous wall under various working conditions, the reinforcement envelope diagram of underground continuous wall can be obtained, and then reinforcement can be carried out according to the reinforcement envelope diagram. Figure 8- 13 is the reinforcement diagram of a standard trough section in the main building. According to the envelope diagram of bored pile arrangement under various working conditions, the pile arrangement and reinforcement are obtained (Figure 8- 14). The diameter of bored piles is 56 1200, the spacing is 1400, the elevation of pile top is 8.7m, the length of pile is 24m, and the elevation of pile bottom is 32.7m According to the principle of "Four Avoidances" of reinforced concrete internal supports in this project, the first, second and third internal supports are arranged in a plane. The displacement value, horizontal bending moment value, vertical bending moment value, axial force value and reaction force value of each node can be obtained by computer SAP90 program. The cross section of the waist beam of the first horizontal support is 1000×800(6×), the cross section of the walking support of the tower crane is 800× 1000, and the other cross sections are 800×800, 700×800 and 600×600 respectively. The second horizontal support waist beam is 1200×800, the side support section of the large bay is 900X 800, and the other support sections are 800X 800 and 600X 600. The waist beam of the third horizontal support is 1200X800, most bays are 1000×800, some members are ii00×800, and other support sections are 900×800 and 700×700 respectively. The fourth bracket is the same as the third bracket. According to the previous analysis, the reinforcement diagram of each section can be obtained, and Figure 8- 15 and Figure 8- 16 are the reinforcement diagrams of typical sections. The column support consists of bored piles buried under the pit bottom and lattice steel structure columns above the pit bottom. The column is inserted into the bored pile 5m, the diameter of the bored pile in the tower area is 1000, the pile length is 20m, the section size of the lattice column is 600×600, and the limb length is 1 1600× 65438. The steel of lattice column is A3 steel. According to the stress diagram of each bearing, the reinforcement of bored pile can be obtained (see Figure 8- 17). 3. Foundation pit support construction The designer SOM of this project requires rigid joints, which brings difficulties to the construction. As a supporting project of foundation, the underground wall has the function of bearing, and the inner side of the basement outer wall is lined, which requires high waterproof and quality. C40 high-strength underwater concrete is used for the first time in this project, which brings new problems to the project. Because the engineering piles are constructed before the diaphragm wall, and some pile feeding holes are close to the diaphragm wall, it has brought adverse effects to the diaphragm wall construction. The underground continuous wall is 36m deep and supported on 7-2 soil layer, while 7- 1 soil layer and 7-2 soil layer are hard, so it is extremely difficult to form trenches. The diaphragm wall adopts a new type of flexible joint (see Figure 8- 18), with a standard female trench length of 5.4m and a standard male trench length of 6. Om, and adopt the construction method of interval jump. Two imported hydraulic trenching machine distributary water systems are used for construction. Grouting should be carried out at the pile delivery hole near the diaphragm wall to ensure the quality of the diaphragm wall. Split grouting construction is carried out in the joint area outside the completed diaphragm wall to ensure the seepage capacity of the diaphragm wall. At the elevation of 7- 1 floor and 7-2 floor, it is difficult to hydraulic trenching machine with guide rods. That is, the guide bar trencher is used to groove the part above 7- 1 floor. Trenches of 7- 1 layer and 7-2 soil layer are made with a rope trencher. Use two hydraulic jacks and a shoulder pole to jack up the pre-tensioned joint box in sections. The soil layer at the bottom of the guide wall must be undisturbed soil to prevent the upper mouth from collapsing when the channel is formed. When using the guide bar trencher, use theodolite to control the verticality of the groove; In order to ensure the stability of the tank wall, the height of mud level in the tank should be controlled at about 200mm below the top surface of the guide wall. During the construction of bus duct, it is required to brush the bus duct joint, and wash the joint brush with clear water at any time to ensure the quality of joint connection meets the requirements. Air suction method is used to clean the foundation, so that the sediment is controlled within 200 mm Because the original pile sinking hole is close to the groove wall, the pore water pressure is high, which is easy to cause the groove wall to collapse. Therefore, before the groove is formed, the foundation around the original pile sinking hole should be strengthened. Bored cast-in-place pile is a kind of supporting structure, which is divided into two categories: the first category is to support the reinforced concrete internal support, and the second category is the retaining soil and retaining piles in the main building. See Table 8-2 for the diameters and hole bottom elevations of various bored piles. Bore-forming construction of daily crawler hydraulic drilling rig (dry drilling rig) cast-in-place pile. Due to the simultaneous construction with the diaphragm wall, it is necessary to carry out flow operation with the diaphragm wall construction at the use site. Acoustics 850 piles 1 100 protection pipe; 6 1300 mouth liners are acoustic 1000 piles; Acoustic 1200 pile adopts acoustic 1400 mouth guard pipe, and the length of mouth guard pipe is 6 ~ 7m. The reinforcing cage is hoisted in two sections, the steel column is assembled on the ground and hoisted once, and the reinforcing cage and the steel column are connected by electric welding at the mouth of the cave. Artificial mud is used to keep the hole wall stable. The specific gravity of mud is 1.06 ~ 1. 15, and the viscosity is controlled between 20s and 30s. Secondary hole cleaning adopts positive circulation mode. When the hole cleaning effect is not ideal, combined with the reverse circulation method, the column pile sediment is controlled within 100 omm, the row pile sediment is controlled within 300 mm, and the bored pile is marked C30 underwater and mixed on site. When pouring concrete, it is required that the conduit be buried in the concrete for at least 3m to ensure the compactness and frost resistance of the concrete. For retaining piles with large reaming phenomenon, nylon cloth is used for construction. The label of each reinforced concrete internal support is C30. Every time the earthwork is excavated to the bottom of each support, the internal support construction is started. The connection between the inner support waist beam and the underground wall is anchored by sound 28 and connected by cone thread. The connection between the internal support waist beam and the bored pile adopts 5628 steel bar anchorage and welding connection. Temporary construction joints are set at the junction of the main building and podium support, and steel dowel and embedded parts are reserved. During the supporting construction of the podium, the connecting steel bars of the podium are welded on the supporting prefabricated parts of the main building, so that the main building and the podium are supported as a whole. 4. In the foundation pit dewatering engineering construction, according to the formula of confined water, k- the bulk density of layered soil; Ti-layered soil thickness; Ratio-bulk density of water: f- distance from the bottom of foundation pit to the top of impervious layer; In order to meet the requirements of dewatering in the excavation stage of the main building foundation pit, the method of combining shallow dewatering with deep dewatering is adopted. In the first stage, the main building was excavated to the elevation of 19.65 m, and the podium was excavated to the elevation of 4.0 m. Therefore, in the first stage, the deep dewatering method was adopted for the main building and the shallow dewatering method was adopted for the podium. Sl light well point is used for shallow dewatering, with a well point pipe length of 7m, and SB- 1 deep well pump is used for deep dewatering, with a well pipe length of 22m. The arrangement of deep well pump in the foundation pit can be divided into two types, one is fixed on the support of the main building, and the other is in the large space of support, which will be removed with the excavation process. There are 28 deep well pumps in the foundation pit, 9 of which will be removed with the excavation. Before the first excavation, six groups of light well points were set in the whole foundation pit in the diaphragm wall. The spacing between light well points is controlled at about 2.4m, and the length of well points is 6m. This part of the light well point will be demolished after the first excavation. During the construction of the main building, in order to ensure the stability of earthwork slope and driveway slope, eight groups of light well points will be set up in the main building slope and driveway in the podium area, which will be set up after the first excavation.