Key words: pile positioning net control survey in top-down cover-and-dig method
1 overview
Xinjiekou Station of Nanjing Metro is located in nanjing xinjiekou Commercial Center, with a total length of 362.703 m and a buried depth of 24.7m. It is designed as a 3-span (partially multi-span) box structure. The whole station consists of the station main body, 16 entrance and exit and three ventilation pipes. The main body of the station is divided into four construction sections: large disc, neck section, straight section and south extension section. The station runs north and south, with high-rise buildings on both sides. The station site is composed of cohesive soil, soft soil and silt. The beam-slab structure of the station is supported by the continuous wall and the central column of the enclosure structure, and the connection between the continuous wall and the slab is realized by the connectors buried in the continuous wall. The station is built from top to bottom. Due to the long span, deep burial depth and poor geological conditions of the station, bored piles are used for the foundation of the intermediate steel pipe column, and the pile length is about 24.7 m. The construction quality of the intermediate steel pipe column is very important, which directly affects the overall construction quality of the station project.
2 intermediate steel pipe column measurement and positioning technology
There are four factors that affect the construction quality of intermediate steel pipe column: first, the quality of bored pile foundation of steel pipe column; Second, the production quality of steel pipe column itself; Thirdly, the installation and positioning accuracy of steel pipe columns; The fourth is the pouring quality of concrete in the column. Among them, the installation accuracy of the column is the most difficult to control, and it is difficult to remedy once there is a leak in the work; The position of steel pipe column (plane position, elevation and verticality), the stress of casing, waterproof performance (strength, stiffness and impermeability) and the pouring quality of concrete directly affect the bearing capacity of steel pipe column and the stability of the whole station.
The core problem of accurate positioning of steel pipe column is to control the position of pile core and automatic locator through measuring network. It is necessary to design a well-designed locator, establish a survey control network and control it from the construction technology. In the process of steel pipe column construction, measurement control is one of the most critical quality control factors.
2. 1 Design of automatic positioner
2. 1. 1 Design requirements of automatic positioner
The automatic locator is a cross-shaped cone welded by steel plates, which should be matched with the bottom of the steel pipe column during processing, so that the steel pipe column can be directly nested in place during hoisting. According to the plane position, elevation and verticality of each steel pipe column, the upper end of the locator is fixed at the bottom of the steel sleeve, and the lower end is anchored at the top of the foundation pile with C50 concrete.
2. 1.2 Structure and function of automatic positioner
The automatic locator is a cross-shaped cone, which plays the role of extradition and accurate positioning of steel pipe columns. There is a gap of 8 mm between the width of cone bottom and the inner diameter of steel pipe column, which includes cone plate, positioning transverse plate, annular anchoring steel bar and positioning iron piece. Among them, the cone plate and the positioning transverse plate can extradite the steel pipe column and limit the horizontal displacement of the steel pipe column; Annular anchoring steel bars support steel pipe columns, and control the horizontal position and elevation (Figure 1). The production quality of the positioner must be strictly controlled to ensure that it has sufficient strength, rigidity and accuracy, and that the positioner will not be damaged, deformed or displaced when installing the steel pipe column.
Figure 1 positioner component drawing
2.2 Precise control of intermediate steel pipe column with measuring net.
According to the survey of Xinjiekou traverse points and elevation leveling points in Nanjing urban survey control network, the survey control network of Xinjiekou Station (Figure 2) is formed to accurately control the steel pipe column in the middle of the station.
Fig. 2 Schematic diagram of measurement control network
2.2. 1 Preliminary survey lofting
Using survey control network to determine the core of the pile, mark the axis of the pile position, and set out according to the survey of the extended pile position of the steel pipe column to accurately determine the axis.
2.2.2 Precise positioning of steel casing center
In order to make the initial positioning of the locator not exceed the error range of 2 mm as much as possible, firstly, according to the center of the designed steel pipe column set out by the total station of the control network, two hanging lines are set out in the east-west direction and the north-south direction of the steel casing to determine the position of the pile center.
Preliminary positioning of automatic locator
Measure and set the horizontal axis at the top of the steel sleeve. Measure with total station, put the center of the locator (that is, the center of the corresponding steel pipe column) on the upper end of the steel casing, and mark it on the steel casing. Accurately determine the center of steel pipe column (that is, the center of automatic locator) with channel steel with holes. First, install the automatic positioner with steel wire plumb line, as shown in Figure 3.
Fig. 3 Preliminary positioning of locator
2.2.4 Precise positioning of automatic positioner
Using the traverse points in the station control network, the center of the steel pipe column is determined by the total station and the matching laser hammer calibrator (both laser hammer calibrator and prism table), and the locator is accurately positioned with an error of less than 3 mm, as shown in Figure 4.
2.2.5 Installation of automatic positioner
The center point of the steel pipe is transferred from the orifice to the bottom of the hole by hammering the calibrator. According to the center point, determine the center of the locator, measure the elevation of the automatic locator at the same time, and then fix the locator. The positioner is fixed with embedded connecting bolts. During installation, repeatedly measure and adjust the elevation and levelness of the anchor foot, and then fix it, as shown in Figure 5. The installation of the positioner must be retested to meet the accuracy requirements.
2.2.6 Precise positioning and fixing of steel pipe columns
The steel pipe column is positioned by simultaneous positioning of the upper and lower ends. The positioning of the lower end of the steel pipe column mainly depends on the automatic positioner, and the upper end is fixed by installing three-way basket screws (Figure 6). The precise positioning of the upper center is the same as that of the automatic positioner. Hoist the steel pipe string, embed the locator into the bottom of the steel pipe string, and then accurately locate the upper end of the steel pipe string. There are three flower basket screw fasteners between the upper steel pipe columns, which are used to fine-tune the position of the steel pipe columns. The center position and verticality of the steel tube column meet the design accuracy requirements after the screw of the steel tube column basket is accurately corrected, and the stability and strength of the steel tube column are guaranteed by firm welding. We think that once the positioner is installed in place, the position parameters of the middle steel pipe column are determined, because the plane position, elevation and verticality of the lower end of the steel pipe column are determined by the positioner.
Figure 4 Precise positioning of positioner
Fig. 5 Calibration and Fixation of Locator
Fig. 6 Positioning and fixing of the upper part of steel pipe column
2.3 Organization and implementation of construction survey
The south extension section of Xinjiekou Station is 30 m underground, and the straight section and the large disc section are three-story underground structures. The position control of steel pipe column is very important, so the following points must be done in construction survey: ① use precision total station and hammer leveling instrument to measure. (2) The survey control network must be adjusted, and its closure difference should meet the specification requirements. (3) Due to the tight construction period of the whole project and the arduous measurement work, the preparatory work must be arranged reasonably in advance to achieve a successful measurement. (4) The installation of the steel pipe column locator must be retested by the precision survey team of the project department after the initial measurement by the construction team.
3 Conclusion
The steel pipe columns of Xinjiekou Station of Nanjing Metro have all been installed, and the verticality of 98 steel pipe columns is 2 15 ‰, which meets the design requirements.
Seek the supervision and praise of the owner. At the same time, due to the self-designed locator, the survey progress has been greatly improved, and the construction period of steel pipe column has been advanced by 36 d, which has achieved good social and economic benefits, accumulated valuable experience for the construction of similar projects and is worth popularizing and applying.