Underground continuous wall construction is a common and essential technique in municipal engineering, widely used in deep foundation pit projects in various cities. Upon completion, this method forms a continuous reinforced concrete wall underground, serving as a water barrier, a waterproofing solution, load-bearing structure, and flood prevention mechanism. In this article, we will explore the construction process of underground continuous walls and the necessary structural measures.
1. Overview of Underground Continuous Wall Construction
Construction Preparation
Road Construction
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Load-Bearing Capacity: The foundation should have a load-bearing capacity of over 100 kN/m² to accommodate large equipment like trenching machines, cranes, and concrete transport machinery. Consideration should be given to crane load and equipment movement, with a typical 300T crawler crane exerting about 0.08 MPa pressure on the ground.
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Road Specifications: The construction road should be designed as a circular path around the foundation pit, ideally 9 to 12 meters wide, based on the equipment dimensions, including the width of the rebar cage hoisting equipment. Roads should be reinforced with 250 mm thick reinforced concrete to support the heavy equipment.
Guide Wall
The guide wall serves as a benchmark for measurement and a support structure to ensure the stability of the slurry and trench walls during the construction. It should be built in a stable soil layer, with a thickness no less than 200 mm and a depth of at least 1.5 meters.
2. Construction Process of the Guide Wall
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Foundation of the Guide Wall: Ensure the base is set on stable soil to prevent slurry leakage.
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Common Structural Forms: Guide walls often take “L” or “[” shapes, depending on soil conditions. The “L” shape is used for stable soils, while the “[” shape extends the base to support weaker soils.
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Reinforcements and Improvements: In areas with soft soils or mixed fill materials, additional reinforcements like cement-based grouting or deep guide walls should be considered.
3. Slurry Management in Underground Continuous Wall Construction
Slurry is essential for stabilizing the trench walls and preventing collapse. It also facilitates the removal of debris and cools the machinery. The slurry’s composition should be tailored to the site’s geological conditions and groundwater levels.
Slurry Properties:
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Must be designed based on site conditions.
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Freshly mixed slurry and recycled slurry must meet specific performance standards.
Slurry Pool:
A slurry pool is used to store and circulate slurry, ensuring the continuous wall’s trench quality. The slurry pool should be able to handle at least twice the amount of slurry required for a day’s construction.
4. Trenching Equipment and Techniques
Common Trenching Machines
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Clamshell Bucket Trenchers: Most commonly used for trenching in soft soils.
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Impact Trenching Machines: Suitable for soft soils and rock layers, providing efficient digging.
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Double-Wheel Milling Machines: Effective in hard soils and rocks, offering high precision and speed, especially for depths up to 60 meters.
Trenching Precision
Trenching accuracy is vital to maintaining the integrity of the continuous wall. Depending on the machine type, the precision can reach 1‰ to 2‰ verticality, ensuring proper alignment and structure.
5. Jointing and Reinforcement Measures
The joints of underground continuous walls are designed to maintain structural integrity while withstanding varying loads.
Common Joint Types
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Flexible Joints: Circular locking pipe joints and hinged joints are often used for ease of construction.
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Rigid Joints: Steel plate joints (H-type or cross-type) provide enhanced strength, water resistance, and load distribution.
Installation of Joints:
Proper joint installation techniques are essential to prevent leakage and ensure the wall’s stability.
6. Rebar Cages for Continuous Walls
Rebar cages are integral to providing structural strength and stability. These cages are fabricated onsite using mechanical connections, and the length of the overlap should meet the design requirements. The cages are lifted into place using cranes, ensuring that they maintain the necessary alignment and verticality throughout the installation process.
Rebar Cage Specifications:
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The cages are typically made of HRB400 steel for optimal strength.
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The overall cage should be handled with care to prevent deformation and ensure safety during installation.
7. Concrete Pouring and Grouting Techniques
Underwater Concrete:
Waterproof concrete must be used for continuous walls. The concrete mix should be tested and adjusted to maintain the necessary slump (180-220mm). The placement of concrete should be done with a double-pipe system to ensure uniform distribution and prevent segregation.
Grouting at the Bottom:
Grouting ensures the structural foundation is solid and free from sediment. The grout used should be a P.O.42.5 cement slurry, with a water-cement ratio between 0.5 and 0.6.
8. Safety Measures
Equipment Handling:
All machinery must be operated by qualified personnel, and safety protocols must be strictly followed, especially in hazardous areas like the trenches.
Protection for Workers:
Effective protective measures should be in place to ensure workers’ safety, including barriers around crane operations and precautions against falls and equipment malfunctions.
9. Environmental Protection Measures
Proper waste disposal and slurry management are crucial to minimize environmental impact. The disposal of mud, wastewater, and waste soil should follow local regulations. Construction sites should be equipped with vehicle washing stations to prevent dirt and debris from spreading.
Conclusion
The construction of underground continuous walls is a sophisticated process requiring attention to detail at every stage—from initial trenching to final concrete pouring. By understanding the process, selecting the right equipment, and implementing the correct safety and environmental measures, successful completion of these projects is achievable.
By ensuring proper slurry management, reinforcement techniques, and high-quality material use, underground continuous walls can provide durable, long-lasting protection for urban infrastructure.
