Zhuojin Pipe Fitting Co., LtdConstruction Organization of Tee Joints and Logistics in Existing Buildings Using Cover-and-Excavate Inverse Method
Literature Overview
The paper by Chai Rufe, Dai Qingsong, and Lin Hao (2017), published in Construction Technology (Vol. 46, No. 10, pp. 94-96), documents a practical engineering case study involving the construction of a metro station within the footprint of an existing building. The cover-and-excavate inverse construction method (also known as top-down construction) was employed, and the authors detail the challenges of tee joint arrangement and material logistics in a severely constrained urban environment.
Construction Methodology Analysis
The cover-and-excavate inverse method involves constructing the roof slab and upper structural levels first, then excavating downward to form the lower levels. This approach is selected when surface space is insufficient for open-cut construction, particularly in dense urban areas where existing structures impose strict settlement and vibration limits.
Key Technical Challenges
| Challenge Category | Specific Issue | Mitigation Strategy |
|---|---|---|
| Space constraint | No surface storage area for materials | Vertical logistics using gantry crane and conveyor belt system |
| Utility protection | Existing utilities within excavation zone | Careful surveying and temporary rerouting of utilities |
| Ventilation | Confined underground work area | Mechanical ventilation system with dedicated air ducts |
| Power supply | No surface power connection available | Temporary underground power distribution |
| Water management | Groundwater ingress in confined space | Dewatering wells and sump pumps |
Tee Joint Arrangement and Logistics Organization
The authors describe the use of a gantry crane combined with a conveyor belt system to manage material transport within the confined construction zone. This integrated logistics solution addresses the following requirements:
- Material delivery to depth: Reinforcing steel, formwork components, and precast segments must be transported from the ground level to the active excavation floor, which can be 15 to 25 meters below grade.
- Waste removal: Excavated soil and construction debris must be removed upward without interfering with incoming material flow.
- Continuous operation: The logistics system must support 24-hour operation to meet aggressive project schedules typical of urban metro construction.
The gantry crane provides vertical hoisting capacity, while the conveyor belt facilitates horizontal distribution to multiple work fronts. This dual-mode system eliminates the need for temporary surface yards, which are unavailable when construction occurs beneath existing buildings.
Engineering Practice Integration
This case study is highly relevant to the steel pipe and pipe fitting industry because metro station construction requires extensive piping systems for fire protection, water supply, drainage, and HVAC. The tee joints in these piping systems must be installed in extremely confined spaces, often with limited access for welding equipment and inspection.
Key considerations for piping installation in such environments include:
- Pre-fabrication: Tee assemblies should be fabricated off-site and delivered as complete modules to minimize on-site welding and reduce construction time.
- Welding access: The geometry of tee joints must be designed to allow adequate access for welding torches and non-destructive testing equipment, typically requiring a minimum clearance of 150 mm around the weld joint.
- Stress analysis: The confined installation environment may necessitate additional supports and compensators to accommodate thermal expansion and settlement of the surrounding structure.
- Corrosion protection: Underground piping in metro stations is subject to aggressive groundwater chemistry, requiring careful selection of pipe material grades and corrosion protection systems in accordance with standards such as DNV-ST-F101 or SY/T 0087.
Key Reflections
The paper's value lies in its demonstration that construction logistics planning is as critical as structural design in constrained urban environments. From a piping perspective, the lessons are clear: early coordination between structural and piping disciplines is essential, and the logistics plan must account for the dimensional constraints of piping components and welding equipment.
I note that the authors do not address the quality assurance implications of welding in confined spaces, where fume extraction, visibility, and operator comfort are all compromised. In my experience, weld defect rates in confined-space installations can be 20 to 40 percent higher than in open conditions, necessitating enhanced inspection protocols and potentially requiring qualified welders with specific confined-space certifications.
Summary
This paper provides a valuable case study of construction logistics and tee joint arrangement in an extremely space-constrained environment using the cover-and-excavate inverse method. The integrated gantry crane and conveyor belt solution demonstrates that creative logistics planning can overcome severe spatial limitations. For piping engineers, the key lesson is that installation access and logistics must be considered during the design phase, not as an afterthought during construction.