The combined top-down and bottom-up (semi-top-down) construction technology is an operation mode for foundation pit engineering. By combining the peripheral layer-by-layer top-down/bottom-up reinforced support combination with the top-down floor slabs, it provides an effective support system for the foundation pit to meet the corresponding requirements of foundation pit construction operations. Based on this, this paper studies the combined top-down and bottom-up construction technology for foundation pit engineering. Taking the Foshan International Financial Center project as an example, it introduces the process principles of combined top-down and bottom-up construction, points out the technical key points of this construction method, and hopes to provide some reference for relevant practitioners.

Foshan International Financial Center Project

The Foshan International Financial Center project, constructed by China Railway Construction Group Co., Ltd., is located in the core area of the Central Business District of Foshan New Town. It is currently the tallest building under construction in Foshan, consisting of a 61-story, 318-meter-high main tower, a 7-story commercial podium, a commercial pedestrian street, and 4 high-end apartment towers. It is expected to be completed and put into use at the end of 2023, when it will become a landmark building in Foshan integrating shopping, leisure and entertainment, commercial activities, and cultural appreciation. The combined top-down and bottom-up (semi-top-down) construction technology has been actively applied and summarized in this project, achieving good economic, technical, and social benefits.

I.Preface

For special projects with deep foundations, complex geology, complex surrounding environments, and short construction periods, unconventional top-down construction technology is selectively adopted, which can effectively solve technical problems that conventional similar construction technologies cannot handle. The combined top-down and bottom-up method usually adopts the top-down method for local areas of the basement. Through this area, the top-down structure and the foundation pit support form effective support, and the foundation pit is excavated to the bottom. Adjacent areas adopt the bottom-up construction method. The structure in the top-down area adopts a combination of bottom-up exterior walls and top-down beams, slabs, and columns, with the beams acting as lateral support (the slab is capped at 1500mm inside the basement exterior wall, and completed after the exterior wall construction is finished) pressing against the retaining piles on the side wall while a shear wall is constructed bottom-up on the indoor side of the steel sheet piles. This not only ensures the safety of the peripheral side walls of the foundation pit but also provides a vertical working space through the floors for the bottom-up construction of the shear wall, accelerating the construction progress.

II. Project Overview

The Foshan International Financial Center has a total construction area of 486,900 m² and covers an area of 65,500 m². Above ground, it is divided into four major areas: shopping center, apartment buildings, office buildings, and landmark tower (H=317.2m). The single-floor construction area of the basement is 53,000 m², covering a large area. The basement is divided into 28 construction zones (as shown in Figure 1). The construction conditions are complex and technically difficult, and it is surrounded by subways on all four sides. According to different conditions and requirements in different areas, the shopping center area adopts the combined top-down and bottom-up construction technology.

The central core island area of the project adopts the bottom-up method, while the peripheral areas (the remaining zones) adopt the top-down method, and this area uses multiple types of top-down methods, including the bottom-up method, top-down method, semi-top-down method, and layered top-down method. The basement of the shopping center adopts the semi-top-down method.

Bottom-up method: Zones 2-4, 7-10, 13-16, 19-22; Top-down method: Zones 1, 5, 6, 11, 12, 18, 23, 24; Semi-top-down method: Zones 25, 26, 27, 28; Bottom-up method: Zone 17 (Phase IV landmark tower area).

Figure 1  Schematic diagram of basement structure construction flow

III. Technical Key Points of Combined Top-Down and Bottom-Up Construction

3.1 Construction Process

Top-down area beam and slab construction → Bottom-up area support replacement beam construction → Layer-by-layer earthwork excavation → Top-down area raft foundation construction → Bottom-up area raft foundation construction → Bottom-up construction of exterior walls in top-down area → Structure construction in bottom-up area

Figure 2 Schematic plan of the semi-top-down method for the shopping center

Figure 3 On-site rendering of the semi-top-down method for the shopping center

3.2 Counter-pressure Earth Technology

The foundation pit support of this project adopts a support system of rotary drilled retaining piles + 3 levels of internal bracing + central island structure, and the semi-top-down construction method is used. According to the design and construction process, it is divided into the bottom-up area and the top-down area. The top-down area is distributed within 40m to 70m along the edges of the foundation pit on the east, west, and north sides. The earthwork in the basement of the top-down area is excavated by stepped slope excavation. The basement roof structure is constructed first, then the earthwork within the remaining basement range of the top-down area is excavated layer by layer, and the three-level basement structure is constructed top-down, with support replacement by the beam-slab structure, and finally the basement exterior wall is constructed bottom-up. The third basement level of the project is a civil air defense area. Due to the influence of the basement floor height, only small excavators and small vehicles can be used for operations, resulting in serious efficiency reduction. Therefore, it is considered that the main earthwork transport operations are completed on the second basement level, which also maximizes the use of favorable construction conditions such as the height of the second basement level (clear height 4.6m, clear height of the lane 4m), the high bearing capacity of the floor slab (roof slab of the civil air defense area), and the main soil exit lane in Zone 24. Part of the third basement level is excavated at one time along with the excavation of the second basement level, only retaining the original mixing pile reinforcement area within the 1:3 range of the third basement level.

Figure 4 Schematic diagram of counter-pressure earth

3.3 Connection Method of Reinforced Ring Beam

The reinforced ring beam connection method is one of the connection methods between horizontal members and vertical members of underground structures. The ring beam reinforcement is installed by piece-by-piece assembly, and the beam-slab reinforcement is installed together with the vertical structure by penetrating into and anchoring within the ring beam. Except for the ring beam, the construction of beam-slab reinforcement adopts conventional methods. Since the reinforced ring beam cannot be assembled and installed as a whole, on-site piece-by-piece assembly can only be used. This paper focuses on the piece-by-piece assembly and installation technology of ring beam reinforcement.

Figure 5 Schematic plan of the reinforced ring beam

(1) Reinforcement setting-out and processing

Set out the ring beam reinforcement according to the dimensions of the design and construction drawings, cut the main reinforcement and waist reinforcement according to the drawing dimensions, optimize the stirrups into right-angle stirrups, and split some stirrups into two "U"-shaped open stirrups.

Calculate the circumference according to the different diameters of the circular shapes formed by the main reinforcement and waist reinforcement at different positions of the ring beam, and process them into rings using a bending machine. The total length L of the main reinforcement and waist reinforcement of each ring is: L = circumference + lap arc length, where the lap arc length is 10d, and d is the reinforcement diameter.

(2) Binding and installation

Lower the bottom main reinforcement into the bottom layer by layer and in batches (first lower the main reinforcement close to the steel pipe column), and bind and connect it with the bottom of the two U-shaped stirrups. Next, lower the waist reinforcement one by one according to the required spacing (first lower the waist reinforcement close to the steel pipe column), adjust the spacing, and bind and fix it with the stirrups. Then install the upper main reinforcement to the top position, adjust the spacing, and bind and fix it with the upper part of the stirrups. Finally, weld the lap joints of the main reinforcement and waist reinforcement to form a closed body. This new method makes the installation of complex ring beam reinforcement flexible and solves the difficulty of binding due to dense reinforcement under traditional construction methods.

Figure 6 Rendering of ring beam binding

3.4 Construction Technology of Annular Steel Corbel

The annular steel corbel connection method is another connection method between horizontal members and vertical members of underground structures. The annular steel corbel is installed by splitting, combining, and piece-by-piece assembly, and the beam-slab reinforcement is installed by welding and anchoring on the annular steel corbel. The steel structure annular steel corbel cannot be installed as a whole due to the closure of the top of the circular pipe column. To solve the installation problem of the annular steel corbel, the project adopts the installation construction technology of segmented processing and segmented installation.

Figure 7 Schematic plan of the corbel

(1) Setting-out and processing

Position the circular pipe column and annular plate according to the drawings, determine the corresponding positions of the connection plates and stiffener plates, and carry out the split detailed design and setting-out of the annular corbel. Use CNC cutting equipment to make special-shaped annular plates, mainly splitting and processing the cylindrical (or conical-like with trapezoidal vertical section) corbel into two half-cylinders.

Figure 8 Schematic diagram of corbel processing and welding part treatment

(2) Treatment of welding parts

Even a tiny burr at the welding part will affect the welding quality. To ensure welding quality, the groove and welding parts of the part plates must be ground before part assembly, and slag, oil stains, dust, floating rust, etc. on the groove and welding parts must be cleaned up to ensure welding quality.

(3) Installation

Use temporary lifting lugs welded above the steel pipe column as lifting points, and use manual hoists to lift the two half-cylinders to the support plate position respectively and adjust the position points. Assemble through the control lines on the top annular plate of the column and the column body, align the outer annular plate, stiffener plate, and connection plate of the column body to achieve precise positioning. According to the influence of factors such as the cross-sectional dimensions of the column top and the shrinkage of full penetration welding, control the cross-sectional dimensions of the combined corbel assembly of the column top annular plate and the distance between the holes on the corbel connection plate and the column body, ensure the precise positioning of the holes, and reserve welding shrinkage gaps to ensure that the assembly dimensions meet the requirements.

(4) Welding

Due to the concentrated nodes and large number of welds, preheating is carried out before welding to prevent welding deformation caused by concentrated welds. Symmetrical welding is adopted for the welds. When welding the second layer, the welding direction should be opposite to that of the first layer, and the welding joints of each layer should be staggered by 15-20mm. Layered multi-pass welding is adopted, and the thickness of each weld layer is not greater than 5mm.

Welding

(5) Weld inspection

Non-destructive testing is carried out 24 hours after welding is completed to ensure welding quality. If it is found that the welding does not meet the design and specification requirements, rework shall be carried out immediately. After the overall assembly is completed, assembly acceptance is carried out, and temporary anti-deformation measures are added after passing the acceptance.

Figure 9 Schematic diagram of weld inspection

3.5 Connection Technology between Exterior Wall and Beam-Slab

(1) The beam-slab is constructed top-down, and the exterior wall is constructed bottom-up. The beam-slab and exterior wall adopt a separated secondary construction method. During the construction of the beam-slab structure of the -1F basement level, the slab structure is not capped. The beams are connected to the concealed beams of the basement side wall and braced to the support waist beam. The slab structure is terminated at a distance of 1500mm from the concealed beam. The 1500mm is the construction space for the later basement side wall. Wall reinforcement dowel bars for the basement side wall are embedded on the concealed beam for reinforcement lapping during the later construction of the basement side wall. After the horizontal structures of the -2F and -3F levels are constructed according to this construction method, the basement side wall is constructed bottom-up. During the construction of the -3F side wall, it is poured together with the reserved 1500mm slab horizontal structure. According to this construction method, construction is carried out layer by layer upward until the side wall of the -1F level is closed and completed.

(2) When the side wall is constructed bottom-up, the earthwork at the bottom of the side wall is over-excavated by 500mm to facilitate the corresponding structural treatment agent and waterproof treatment of the piles at the bottom of the side wall. After the brick胎模 (brick formwork) of the pile heads at the bottom of the side wall is completed, small earthwork backfilling is carried out. During the construction of binding the preliminary reinforcement, attention should be paid to ensuring that the anchoring and lapping meet the design requirements, and when constructing to the upper roof slab, the anchoring between the slab reinforcement and the wall body should be ensured.

Figure 10 Schematic diagram of bottom-up construction of exterior wall

IV. Conclusion

According to the basement construction characteristics of the Foshan International Financial Center project, the semi-top-down construction technology effectively solves the respective shortcomings of the top-down method and the bottom-up method, fully leverages the advantages of both methods, and is an effective combination of the two methods.

The Foshan International Financial Center selects the combined top-down and bottom-up construction technology according to the actual project conditions. During the construction process, the top-down range and reinforced bracing of the overall structure are reasonably adjusted, which can meet the structural design requirements of the project and avoid gaps in the overall structure, effectively improving construction safety, quality, efficiency, and benefits. Therefore, relevant construction personnel should reasonably select and apply construction technology, continuously improve the overall construction progress and quality, thereby creating greater economic benefits and providing some reference for the construction of underground top-down construction projects.