The evolution of super large deep foundation pit support design?

The article focuses on the design process of the foundation pit support of a large-scale subway superstructure complex project in the Shenzhen Bay Reclamation Area. Facing the complex environment adjacent to three subway lines, the article combines the unstable construction plan, high development quality, and sales Due to the characteristics of tight planning, short construction period and high engineering difficulty, how to carry out selection analysis and evolution optimization of foundation pit support design step by step, so as to effectively protect the surrounding subway facilities and achieve good results. It can provide design and management reference for similar projects.

Pile foundation pit support design and overall project development planning play a very important role in every project. It is subject to many aspects and is related to the development and sales plan of the entire project and the smoothness of the project. implementation. The article takes the foundation pit project of the Metro Mangrove Bay Property Development Project (i.e. Shenwan Huiyun Center Project) in the Shenzhen Bay Reclamation Area as an example, focusing on the design of the supporting structure of the ultra-large deep foundation pit adjacent to the subway station and tunnel. When selecting a model, in addition to ensuring its own stability and safety, it is important to consider the precautions and proactively adapt to the complex large-scale subway superstructure complex. In the early stage, it faces the overall development needs of building plan adjustment, phased excavation, and phased sales. , and can control the displacement and deformation of the subway so that it does not exceed relevant regulations.

1 Project Overview

The Mangrove Bay Property Development Project is located on the south side of Shenzhen Bay, bounded by Shenwan 2nd Road to the east, Baishi 4th Road to the south, Shenwan 1st Road to the west, and Shenwan 1st Road to the north. White stone three ways. The proposed project covers an area of ??about 68,000 m2. It is a large-scale subway-topped complex with offices, hotels, apartments and large commercial functions, with a total construction area of ??nearly 600,000 m2. The project is divided into east and west areas by supporting municipal roads in the middle. The east and west districts are connected underground; the east district is a 4-story basement, of which the first and second underground floors and the upper four-floor podium form a large shopping mall of nearly 100,000 m2, with the Line 9/11 station on the south side and the Line 2 station on the north side. Shen Plaza realizes transfer outside the station through diagonal connectivity on the second underground floor of the project. The surrounding environment is relatively complex. The basement side line on the southwest side is only 4.4 to 6.8 meters away from the tunnel side line of Metro Line 11. Most of the basement side lines on the south side (except the southwest side) are close to the transfer station between Line 9 and Line 11; the basement side line on the northeast side is It is about 7 to 28 meters away from the tunnel edge of Metro Line 2, and is adjacent to the Metro Line 2 station on the northwest side (some locations are adjacent to the sunken square). The foundation pit of this project is about 320m long from east to west and 190m/130m from east to west respectively. The plane shape is roughly an irregular quadrilateral. The supporting perimeter is about 980m. The excavation depth of the foundation pit is about 12.34~20.08m. The area of ??the foundation pit It covers an area of ??about 54,000 m2, and most of the remaining land is used for subway stations. It is a typical ultra-large deep foundation pit near the subway (see Figure 1).

2 The evolution of the project’s architectural plan

The winning bidder for the Mangrove Bay project was determined in early 2014. Its architectural plan is for the subway on the north and south sides to pass through an above-ground and underground open diagonal interchange. Divide the plot into two pieces (see Figure 2). After the cooperative developer Vanke was introduced through public bidding at the end of 2014, the design unit and the program deepening unit combined Vanke's development concept. The commercial area of ??the original plan was scattered and did not meet the needs of setting up a larger commercial mall, as well as commercial and apartment buildings. Due to the reality that various entrances and exits cannot be set up on the surrounding main roads, entrances and exits are set up on the municipal supporting roads built in the middle of the project land, and functional isolation of the east and west sides is achieved. The layout of four apartment buildings plus one office building on the west side, two shorter office buildings and one 350m super high-rise office building on the east side has been initially determined, but the above-ground and underground transfer channels are retained (see Figure 3). Later, it was abandoned due to the excessive impact of the above-ground and underground transfer passages on underground commerce and the general entrance and exit (see Figure 4). Later, due to the landscape and line of sight requirements of the West District apartments and the implementation of new fire regulations.

3 Impact and Evolution on Support Design Selection

3.1 Selection Process of Envelope Structure

Mangrove Bay Project Foundation Pit Support Design and In the early stages of the project management work, conventional design was carried out based on the proximity of the existing subway lines, surrounding roads, site geological conditions, etc. on the north and south sides of the project. Based on the preliminary optimization plan, the boundary conditions of the basement were stabilized. The north side of the foundation pit is adjacent to the operating Metro Line 2, and the south side of the foundation pit is adjacent to the soon-to-be-operated Metro Lines 9 and 11. Preliminary analysis does not meet the conditions for conventional grading or rowing of piles and anchor cables. Within the sea area, the overall foundation pit area exceeds 50,000 m2.

3.2 Proposal of split-pit piles and ideas for setting up internal support structures

While ensuring the safety of the foundation pit, it is necessary to strictly control the deformation and displacement of the subway so that it does not exceed the relevant protection regulations It is the top priority in the design of the internal support in the foundation pit of this project. Therefore, the support system of this foundation pit is determined in principle by the designer, owner, and construction party to be a reinforced concrete internal support with greater stiffness. In the early stage, the relative positions of each tower in the scheme in Figure 3 are combined to minimize the interference of the designed internal supports to the underground structure of the tower; and to achieve the purpose of starting construction as early as possible, after the basement outline of the scheme is stabilized, the owner cooperates with the design unit to actively carry out support Design and construction preparation of underground diaphragm wall of protective structure. Since the foundation pit is about 320m long from east to west and about 130 to 190m from north to south, the deformation of the foundation pit has a long-side effect, that is, during the excavation process of the foundation pit, the deformation displacement near the middle of the long side is often the largest.

The middle of the long side of the north side of the foundation pit is right next to the junction of the Mangrove Bay Station of Metro Line 2 and the shield tunnel. The stiffness of the station is obviously much greater than that of the tunnel structure assembled from prefabricated segments. , after judgment and analysis, it was believed that this area was the weakest and most sensitive part around the foundation pit. During the initial communication with the Metro Technical Committee, it was recognized by relevant experts of the metro and requested to set up a pit pile here. At the same time, the support depth and number of layers on the west and east sides of the foundation pit are also different, and the construction progress requirements are inconsistent. The development and construction progress on the west side of the site is more urgent, and the completion time is scheduled to be about one year earlier than the east side. Furthermore, there is a gap between the Line 2 station on the west side of the project and the sunken plaza about 100m long and 10m deep, which cannot effectively provide reaction force for the support system in the foundation pit, so that the water and earth pressure on the north side cannot be directly effective. The pass and balance to the south side.

Therefore, in view of the needs and characteristics of this foundation pit, the design unit set up a dividing pile at the junction of the third and fourth floors of the basement at the location of the municipal road built in the middle of the project, that is, in the east and west areas of the foundation pit. , forming two relatively small foundation pits on the east and west sides, which can be constructed independently without affecting each other, and can effectively control and reduce the deformation of the connection between the subway station and the tunnel. The layout of internal supports needs to be combined with the characteristics of this project, and its specific types should be analyzed in depth and detail. On the basis of divided pit piles, three types of internal supports are proposed. Since the architectural scheme and project planning are constantly evolving and refining, and also require the multi-angle and multi-dimensional in-depth participation of various functional departments of the developer, the internal support scheme was initially a conceptual design selection in the conventional sense, using annular concrete internal supports. (See Figure 7), single ring plus counter brace and corner brace (See Figure 8), counter brace and corner brace (See Figure 9) and other forms. Although the ring braces in the East District were used to accommodate the ultra-high towers, the diameter of the braces was too large, reaching nearly 150m. The small ring braces in the West District did not consider the adverse effects of the sunken square, and the long braces were unstable. The calculation was later reviewed by the Design Institute. The above three solutions all have many technical problems and restrictions, so the design ideas have been adjusted.

3.3 Optimization of the zoning support design of the internal support structure

On the premise of two foundation pits in the east and west, taking into account the unfavorable factors of the sunken square, the foundation pit in the west area will be supported internally. The form is stabilized at the four corners with four large corner braces. Initially, in its central area, two large symmetries in the shape of a cross were set up to balance the earth pressure on both sides. In the area with existing underground diaphragm wall in the south, due to the structural rigidity close to the transfer station of Line 9 and Line 11, the earth pressure is not large, so only side trusses are provided for support. In the area of ??the sunken square of Line 2 in the north, since there is no solid soil outside the foundation pit to provide corresponding support reaction force, no internal supports are provided in this area.

In order to ensure sufficient support stiffness and meet the smooth construction of the high-rise tower in the middle, combined with the shape of the site, the support structure in the east area adopts a double-ring ring bracing type (see Figure 10). The problem caused by the support type is that the middle support in the foundation pit in the west area plays the main role of east-west support. The length exceeds 180m. The stiffness of the support is greatly weakened. The ability to control deformation, especially the balance ring support, is very low and cannot be effective. It resisted the huge thrust from the east ring brace and affected the construction of some tower basement structures. In view of the above shortcomings, as well as the consideration of excavation construction and convenient support removal, corresponding optimization was gradually carried out (see Figure 11).

① The large cross-shaped bracing is eliminated in the foundation pit on the west side, and only four large bracings are installed at the four corners, which strengthens the support rigidity and is conducive to the construction of the tower basement structure; ② According to The latest architectural adjustment plan for the sunken square directly connects it to the basement of this project. Only high-pressure jet grouting piles are set up to stop water. The disconnection of the ground wall can be made up of a "pier structure" composed of several cast-in-place piles. to strengthen. However, the support type still has certain shortcomings at the technical level: whether it is an angle support or a ring support, there are too many connecting beams, resulting in too many redundant structures and making the force transmission system too complicated and unclear; At the same time, it also increases the difficulty of construction and dismantling of bracing, reduces construction efficiency, and places more tower B in the west area under the southeast corner bracing. The ring bracing in the east area also limits the super high-rise giant columns.

For more information about engineering/service/purchasing tender document writing and production to improve the bid winning rate, you can click on the official website customer service at the bottom for free consultation: /#/?source=bdzd