Evolution of supporting design for super-large deep foundation pit?

In the process of foundation pit support design of a large subway superstructure complex project in Shenzhen Bay reclamation area, in the face of the complex environment adjacent to three subway lines, combined with the characteristics of unstable architectural scheme, high development quality and tight sales plan, under the circumstances of short construction period and great engineering difficulty, how to analyze and optimize the foundation pit support design step by step, so as to effectively protect the surrounding subway facilities and achieve good results. It can provide reference for the design and management of similar projects.

The design of pile foundation pit support and the overall development planning of the project play a very important role in each project, which is restricted by many aspects and is related to the development and sales plan of the whole project and the smooth implementation of the project. Taking the foundation pit engineering of Hongshuwan property development project in Shenzhen Bay reclamation area as an example, this paper mainly introduces the design and selection of super-large deep foundation pits near subway stations and tunnels, which not only ensures its own stability and safety, but also pays attention to consideration, and actively adapts to the large-scale subway superstructure complex with complex formats. In the early stage, it can meet the overall development needs of architectural scheme adjustment, phased excavation and phased sales, and control the displacement and deformation of the subway.

1 project overview

Hongshuwan property development project is located in the south of Shenzhen Bay, with Shenwan No.2 Road in the east, Baishi No.4 Road in the south, Shenwan No.1 Road in the west and Baishi No.3 Road in the north. The land area of the proposed project is about 68,000m2, which is a large-scale subway superstructure complex project integrating office, hotel, apartment and large-scale commercial functions, with a total construction area of nearly 600,000m2. The municipal road in the middle of the project is divided into east and west areas, which are connected underground; The east area is a four-story basement, in which the first and second floors underground and the upper four-story podium form a large shopping center with an area of nearly 65,438+10,000 m2. The station on the south side of Line 9/1 1 and the sinking square on the north side of Line 2 are obliquely connected outside the station through the underground second floor of the project. The surrounding environment is relatively complicated. The basement sideline on the southwest side is only 4.4~6.8m away from the subway 1 1 line tunnel sideline, and most of the basement sideline on the south side (except the southwest side) is close to the transfer stations on Line 9 and 1 1 line. The basement sideline on the northeast side is about 7 ~ 28m away from the subway line 2 tunnel sideline, and the northwest side is close to the subway line 2 station (local location is close to the sinking square). The foundation pit of this project is about 320m long from east to west, and the length from east to west is about190m/130m respectively. The plane shape is roughly irregular quadrangle, the supporting perimeter is about 980m, the excavation depth is about 12.34 ~ 20.08 m, the foundation pit area is about 54,000 m2, and most of the rest covers the subway station.

2 The evolution process of the project architectural scheme

At the beginning of 20 14, the winning bidder was determined for the mangrove bay project. Its architectural scheme is that the North-South subway divides the plot into two parts by means of an open interchange between the ground and the underground (see Figure 2). At the end of 20 14, Vanke, a cooperative developer, was introduced through public bidding. The design unit and the scheme deepening unit combine Vanke's development concept. In view of the fact that the original commercial areas of the project are scattered, which can not meet the needs of setting up large-scale commercial malls, and various entrances and exits of businesses and apartments can not 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 plot to achieve dynamic and static isolation on the east and west sides. There are four bedrooms and one living room in the west, two shorter offices in the east, and a 350m super high-rise office. The plane layout has been preliminarily determined, but the transfer passage between the ground and underground is reserved (see Figure 3). Later, it was forced to give up because of the influence of the underground interchange on underground commerce and the great influence of the entrance and exit of the general floor (see Figure 4). In the later period, due to the requirements of the landscape and sight of apartments in the west area and the implementation of the new fire code.

Influence and evolution of 3 on stent design and selection

3. 1 envelope selection process

In the initial stage of foundation pit support design and engineering management, the Hongshuwan project was designed in a conventional way, combining the geological conditions of the north and south sides of the project near the operated subway lines, surrounding roads and sites, and the boundary conditions of the basement were stabilized on the basis of the preliminary optimization scheme. The north side of the foundation pit is adjacent to the operated Metro Line 2, and the south side of the foundation pit is adjacent to the upcoming Metro Line 9 and Line 1 1. According to preliminary analysis, it is impossible to use traditional slope laying or pile row anchor cable. The foundation pit is located in the reclamation area, with a total area of over 50,000 m2.

3.2 The proposal of sub-pit pile and the idea of setting internal supporting structure

While ensuring the safety of foundation pit, it is the most important thing to strictly control the deformation and displacement of subway so that it does not exceed the relevant protection regulations. Therefore, in principle, the supporting system of foundation pit is determined by the designer, the owner and the constructor as the reinforced concrete internal support with greater rigidity. In the early stage, combined with the relative position of each tower in the scheme of Figure 3, the designed internal support will minimize the interference to the underground structure of the tower; As well as the goal of starting construction as soon as possible. After the outline of the basement of the scheme is stable, the owner will actively cooperate with the design unit to design and prepare for the construction of the underground continuous wall of the supporting structure. Because the length of foundation pit is about 320m from east to west and about 130 ~ 190 m from north to south, the deformation of foundation pit has long side effect, that is, the deformation and displacement near the middle of its long side are often the largest during excavation.

The middle part of the long side on the north side of this foundation pit is just near the junction of Hongshuwan Station and shield tunnel on Metro Line 2, and the stiffness of the station is obviously much greater than that of the tunnel structure assembled by prefabricated segments. After judgment and analysis, it is considered to be the weakest and most sensitive part around the foundation pit. It was recognized by subway experts in the preliminary communication with the subway technical Committee, and it is required to set up sub-pit piles here. At the same time, the supporting depth and layers on the east and west sides of the foundation pit are also different, and the construction progress requirements are also inconsistent. The development and construction progress on the west side of the site is urgent, and the planned completion time is about one year ahead of the east side. In addition, the station of Line 2 on the west side of the project and the sinking square with a length of about 100m and a depth of about 100m are vacant, which cannot effectively provide the reaction force for the supporting system in the foundation pit, so that the water and soil pressure on the north side cannot be directly and effectively transferred and balanced to the south side.

Therefore, according to the requirements and characteristics of this foundation pit, the design unit set up sub-pit piles at the junction of the third and fourth floors of the basement in the middle of the project, that is, the east and west areas of the foundation pit, forming two relatively small foundation pits on the east and west sides, which can be independently constructed without affecting each other, and can effectively control and reduce the deformation at the junction of the subway station and the tunnel. The layout of internal support should be combined with the characteristics of this project, and the specific types should be analyzed in depth. On the basis of split-pit piles, three forms of internal support are put forward. Because the architectural scheme and engineering planning have been evolving and refining, it also needs the deep participation of various functional departments of developers from multiple angles and dimensions. At first, the internal support scheme was a conceptual design choice in the conventional sense, which took the form of circular concrete internal support (see Figure 7), single ring plus diagonal brace and corner brace (see Figure 8), diagonal brace and corner brace (see Figure 9). Although the ring braces in the central and eastern regions have given up making super towers, the diameter of the ring braces is too large, reaching nearly 150m, and the small ring braces in the western region have not considered the adverse effects of sinking blocks and instability of long braces. After review and calculation by the design institute, there are many technical problems and limitations in the above three schemes, and the design ideas have been adjusted.

3.3 Design optimization of partition support for internal support structure

On the premise of east-west foundation pit, considering the unfavorable factors of sinking square, four large angle braces are set up in the form of stable internal support at the four corners of foundation pit in the west area. At first, in its middle area, two large cross-shaped symmetrical objects were set to balance the earth pressure on both sides. In the area with underground continuous wall in the south, because the transfer stations near the ground on Line 9 and Line 1 1 are rigid structures, the earth pressure is not great, and only side trusses are provided for support. In the sinking square area of Northern Line 2, because there is no solid soil outside the foundation pit to provide corresponding supporting reaction, there is no internal support in this area.

In order to ensure sufficient supporting rigidity and satisfy the smooth construction of the middle high-rise tower, combined with the shape of the site, the supporting structure in the east area adopts double-ring bracing (see figure 10). The problem caused by the support type is that the length of the middle support, which plays the main supporting role in the east-west foundation pit in the west, exceeds 180m, which greatly weakens the stiffness of the support, and the ability to control deformation, especially to balance the annular support, is low, which can not effectively resist the huge thrust from the eastern annular support and affect the construction of some tower basement structures. In view of the above shortcomings, and considering the convenience of excavation construction and dismantling and replacing the support, the corresponding optimization was carried out step by step (see figure 1 1).

① The foundation pit on the west side cancels the cross-shaped big scissors, and only four big scissors are set at the four corners, which strengthens the support stiffness and is beneficial to the construction of the tower basement structure; (2) According to the latest architectural adjustment scheme of sinking square, it is directly connected with the basement of this project, and only high-pressure jet grouting pile is set to stop water. The disconnection of diaphragm wall can be strengthened by setting a "pier structure" composed of several cast-in-place piles. However, there are still some shortcomings in technology: there are too many connecting beams in both corner braces and round braces, which leads to too many redundant structures and makes the force transmission system too complicated and unclear; At the same time, it also increases the difficulty of construction and support removal, and reduces the construction efficiency. More towers of Building B in the west area are pressed under the support of the southeast corner, and the circular support in the east area also restricts the super-high-rise giant columns.

For more information about project/service/procurement bidding, and to improve the winning rate, please click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd