Zhuojin Pipe Fitting Co., LtdConstruction Technology for Foundation Pit Reinforcement Using Steel Tube Diagonal Bracing on Existing Structures
Literature Overview
The paper by Wang Shidong, Yang Guangde, Xu Qinglu, Zhuo Shihua, and Yang Yongfu, published in Construction Technology (Chinese and English) (Vol. 54, No. 8, 2025, pp. 103–107), presents a novel construction technology for reinforcing existing foundation pit support systems while protecting the basement external walls and floor slabs. The technology employs steel tube diagonal bracing combined with internal concrete waist beams on the inner face of the basement external wall, along with concrete shear keys on the existing floor slab to transfer horizontal forces. The research was supported by the CSCEC3BSH-2023-04 project on industrial heritage renovation and expansion technology.
Core Technical Solution
Design Principle
The fundamental challenge addressed by this technology is the need to strengthen an existing foundation pit support system without damaging the already-constructed basement external walls and floor slabs. Traditional reinforcement methods often involve excavating and reconstructing portions of the existing structure, which is impractical or impossible when the structure is already in service or when the surrounding environment does not permit major excavation.
The proposed solution works on the following principle:
- Internal concrete waist beams: Constructed on the inner face of the basement external wall, matching the width of the force transmission bands, these beams provide a continuous load path for the horizontal forces generated by the soil pressure. By distributing the loads from the diagonal bracing along the wall, the waist beams prevent localized cracking or deformation of the external wall.
- Steel tube diagonal bracing: Installed between the waist beams and the floor slab shear keys, the diagonal bracing provides additional lateral support to the foundation pit wall system. The bracing acts as a truss member, transferring horizontal forces from the waist beams to the floor slab.
- Concrete shear keys: Constructed on the existing floor slab, these keys provide the anchor points for the diagonal bracing and transfer the horizontal forces into the floor slab mass, which acts as a reaction block.
Force Transfer Mechanism
The force transfer path can be described as follows:
- Soil lateral pressure acts on the foundation pit support system (e.g., sheet piles, diaphragm walls).
- The support system transfers horizontal forces to the internal waist beams through the basement external wall.
- The waist beams distribute the forces to the steel tube diagonal bracing.
- The diagonal bracing transfers the forces to the concrete shear keys on the floor slab.
- The floor slab, acting as a massive reaction block, absorbs and dissipates the horizontal forces.
This mechanism effectively converts the horizontal soil pressure into a system of internal forces that are carried by the existing structure, thereby reducing the demand on the original foundation pit support system and preventing damage to the basement walls.
Technical Implementation Details
Steel Tube Diagonal Bracing
| Parameter | Specification | Rationale |
|---|---|---|
| Steel tube diameter | Typically 219–325 mm | Adequate buckling resistance for expected loads |
| Wall thickness | 8–14 mm | Provides sufficient moment of inertia |
| Steel grade | Q235 or Q355 | Good weldability and ductility |
| Connection type | Welded gusset plates or bolted end plates | Secure load transfer to waist beams and shear keys |
| Bracing angle | Typically 45°–60° from horizontal | Optimizes force distribution between axial and shear components |
| Spacing | Determined by structural analysis, typically 3–6 m | Ensures adequate lateral support along the wall |
Concrete Waist Beams
The waist beams are cast in place on the inner face of the basement external wall, using the existing wall surface as one side of the formwork. The beam width matches the force transmission band width to ensure effective load transfer. The beam is reinforced with high-strength rebar and is bonded to the existing wall surface using high-strength epoxy adhesive or mechanical anchors.
Concrete Shear Keys
The shear keys are constructed on the existing floor slab by cutting grooves and casting new concrete into them. The keys are designed to resist the horizontal shear forces transmitted by the diagonal bracing. The connection between the new concrete and the existing slab is ensured through surface roughening, bonding agent application, and the mechanical interlock provided by the groove geometry.
Engineering Practice Implications
Advantages Over Traditional Methods
The proposed technology offers several significant advantages compared to conventional foundation pit reinforcement methods:
- Speed: The construction can be completed relatively quickly, as it does not require major excavation or demolition of existing structures.
- Simplicity: The construction sequence is straightforward, involving primarily formwork, reinforcement, and concrete placement operations that are well within the capabilities of standard construction crews.
- Cost-effectiveness: By utilizing existing structural elements as part of the reinforcement system, the technology reduces material and labor costs.
- Non-disruptive: The method does not interfere with subsequent basement structure construction, allowing the project to proceed without significant delays.
- Wall protection: By distributing the horizontal forces through the waist beams, the technology prevents cracking and deformation of the external walls, thereby avoiding water leakage risks.
Quality Control Considerations
- Welding quality: The connections between the steel tubes and the gusset plates or end plates must be inspected for full penetration, absence of cracks, and adequate weld profile. Visual testing and magnetic particle testing (MT) are recommended.
- Concrete quality: The concrete used for the waist beams and shear keys should meet the specified strength grade, with appropriate workability for placement in confined spaces. Cube testing and non-destructive testing (NDT) such as ultrasonic pulse velocity (UPV) should be performed.
- Bond strength: The bond between the new concrete and the existing structure is critical. Pull-off tests on bond coupons should be conducted to verify adequate bond strength.
- Alignment and geometry: The steel tube bracing must be installed with precise alignment to ensure proper force transfer. Deviations in alignment can introduce unwanted bending moments and reduce the effective capacity of the bracing system.
FMEA Analysis of Potential Failure Modes
| Failure Mode | Cause | Consequence | Mitigation |
|---|---|---|---|
| Diagonal brace buckling | Insufficient stiffness or unbraced length | Loss of lateral support | Adequate tube section, intermediate bracing |
| Shear key pull-out | Insufficient bond strength | Loss of force transfer path | Surface preparation, bonding agent, mechanical anchors |
| Waist beam cracking | Overloading or inadequate reinforcement | Loss of load distribution | Proper reinforcement design, load monitoring |
| External wall damage | Excessive localized pressure | Water leakage, structural damage | Waist beam width matching force band, gradual loading |
| Weld failure | Poor welding quality | Connection failure | Welder qualification, NDT inspection |
Key Questions and Reflections
One important consideration is the long-term durability of the concrete shear keys under cyclic loading conditions, such as those induced by traffic loads or seismic events. The repeated loading may cause fatigue cracking at the bond interface between the new and existing concrete. Additionally, the study does not address the seismic performance of the reinforced system, which is a critical concern in earthquake-prone regions.
Another question is the applicability of this technology to different soil conditions and groundwater levels. The effectiveness of the reinforcement system depends on the magnitude of the lateral soil pressure, which varies with soil type, water table elevation, and surcharge loads. A parametric study covering a range of geotechnical conditions would strengthen the general applicability of the proposed technology.
Summary
This paper presents a practical and innovative construction technology for reinforcing existing foundation pit support systems while protecting the basement external walls and floor slabs. The combination of steel tube diagonal bracing, internal concrete waist beams, and floor slab shear keys creates an effective force transfer system that enhances the lateral support capacity without disrupting existing structures. The technology offers significant advantages in terms of speed, cost, and minimal disruption, making it suitable for urban environments where space and time constraints are critical. For construction engineers working on foundation pit reinforcement projects in existing structures, this work provides a valuable reference, though further investigation into seismic performance, long-term durability, and geotechnical parametric effects is recommended to broaden the technology's applicability.