1. Classification of Excavation Pits
Level 1 Excavation Pit:
These are important engineering projects where the support structure and foundation are integrated. Excavation depth is greater than 10 meters, and they are located near important buildings or facilities. Protection is required for historic or modern buildings, as well as crucial pipelines within the excavation range.
Level 2 Excavation Pit:
These pits are neither level 1 nor level 3 but may include general excavation sites with specific requirements.
Level 3 Excavation Pit:
These pits are relatively shallow, with excavation depths less than 7 meters and no special requirements.
2. General Support Methods for Shallow Excavation Pits
For Level 3 Excavation Pits with limited depth, where slope excavation may be difficult, various support methods can be applied, including short-column cross walls, temporary retaining walls, inclined column supports, and anchor support systems.
2.1 Trench Support (for Pit Excavations)
For excavation in trenches or pits, horizontal bracing support is typically used. This can be divided into two major categories:
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Horizontal retaining walls: These can be intermittent or continuous, depending on the conditions.
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Vertical retaining walls: These are used for loose, moist soils, where excavation depth exceeds 3 meters and can reach up to 5 meters.
Types of Retaining Walls:
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Continuous Horizontal Retaining Wall
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Intermittent Horizontal Retaining Wall
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Vertical Retaining Wall
2.2 Simple Support for Limited Excavation
When slope excavation is not possible in certain sections of the pit, simpler support systems can be used for foundation construction, such as:
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Short-column cross-wall support, which is suitable for sections with larger excavation widths.
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Temporary retaining wall support which is ideal when slope gradients are insufficient in parts of the pit.
2.3 Inclined Column Support
Inclined columns are used to provide additional stability in large pits. These columns are drilled along the edge of the pit and support the retaining walls with diagonal braces. Suitable for deep but less complex excavation.
2.4 Anchor Support
Anchor support involves setting up column piles along the pit’s perimeter. A retaining wall is placed inside the column pile, and tie rods are installed at the top of the piles to stabilize the structure. Suitable for large excavations with limited space for horizontal or diagonal bracing.
3. Support Methods for Deep Excavation Pits
For deep excavation pits, several fundamental requirements must be met for the safety and stability of both the pit and the surrounding infrastructure:
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Ensure the support structure effectively retains soil and maintains the stability of the pit’s sidewalls.
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Protect adjacent structures such as buildings, roads, and underground pipelines.
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Prevent soil deformation, settlement, or collapse from affecting the construction site.
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Manage groundwater levels to ensure construction occurs above the water table.
3.1 Pile Support System
Before excavation begins, concrete piles are set around the pit. The pile layout can vary, including:
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Intermittent piles: Used for shallower excavation pits.
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Double-row piles: Suitable for deeper or more complex projects.
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Continuous piles: Provide the most stable and secure support for deep pits.
Common Pile Types:
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Drilled concrete piles (0.6-1.1m in diameter) for depths between 7-13 meters.
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Sinking concrete piles (0.5-0.8m in diameter) are typically used for pits under 10 meters deep.
3.2 Soil Nail Wall Support
This method stabilizes the soil through the insertion of reinforcing rods (soil nails) into the soil layers. The nails are anchored and secured using grout injection, and combined with sprayed concrete panels, they form a retaining wall. This system is used for slopes or excavations with unstable soil.
Construction Process for Soil Nail Support:
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Excavation: Excavation is performed in layers, starting from the top and progressing downward. The working face should not be less than 6 meters wide.
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Spraying First Layer of Concrete: A sprayed concrete layer is applied immediately to prevent soil from loosening.
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Drilling for Soil Nails: Drill holes are created at specific angles, and nails are inserted into the soil to reinforce the structure.
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Grouting and Concrete Spraying: After installing the soil nails, grout is injected to bond the nails to the surrounding soil, followed by a final layer of sprayed concrete.
3.3 Anchor Bolt Support
Anchor bolts are drilled into undisturbed soil layers, then reinforced with a cement grout mixture to stabilize the pit’s sidewalls. This method is ideal for deeper excavations in hard soils or fractured rock, and it ensures the stability of the surrounding environment.
Anchor Bolt Support Process:
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Drilling: Drilling is done using rotary or impact drilling techniques, and the hole is cleaned thoroughly before insertion of the anchor bolt.
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Anchor Bolt Installation: The anchor bolts are installed, and grout is injected to bond them securely with the soil.
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Pressure Grouting: Pressure is applied during grouting to ensure a strong bond between the anchor and the surrounding soil.
4. Advanced Support Systems for Large or Complex Excavations
For larger or deeper excavation projects, advanced methods may be required to ensure stability and safety:
4.1 Continuous Underground Walls
These reinforced concrete walls are constructed in segments and provide significant strength, making them ideal for large excavation sites with high water levels. They are capable of blocking water, bearing loads, and preventing soil erosion.
4.2 Steel Sheet Pile Retaining Walls
In cases where the excavation is deep and groundwater levels are high, steel sheet piles can serve as a structural barrier. These piles help to retain the soil, prevent water seepage, and reduce the risk of sand flow.
Two Types of Steel Sheet Piles:
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Without Anchor Piles (Cantilever Method): Steel piles are driven directly into the soil.
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With Anchor Piles (Anchored Sheet Piles): Steel piles are supported with additional anchors to ensure stability.
4.3 Internal Bracing for Excavations
When the excavation depth is large and external wall systems fail to meet stability needs, internal bracing can be used to provide additional support. Common forms of internal bracing include steel pipe supports and reinforced concrete frames.
Types of Internal Bracing:
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Steel Pipe Support: Typically uses pipes of various diameters and thicknesses to provide stability.
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Reinforced Concrete Frame Support: Offers greater rigidity and minimizes deformation, ideal for sensitive environments.
Conclusion
Choosing the right support system for an excavation pit depends on various factors, including depth, soil conditions, proximity to nearby structures, and the overall complexity of the project. A well-planned support system is crucial for maintaining the stability of the excavation and ensuring the safety of both the construction site and the surrounding environment.
By using appropriate excavation techniques and support methods, engineers can ensure the successful completion of excavation projects while mitigating risks and reducing costs.
