In piling and geotechnical engineering, CBP stands for Contiguous Bored Pile. It refers to a type of retaining wall constructed from a series of closely spaced, bored concrete piles. This method is widely utilized for creating robust earth retention systems, particularly in challenging ground conditions or for deep excavation works.
Understanding Contiguous Bored Piles (CBP)
As highlighted in the provided reference, a Contiguous Bored Pile (CBP) wall is a common type of retaining wall. Its construction sequence involves drilling individual piles in close proximity to each other, forming a continuous barrier. This type of wall has been specifically noted for its use in various deep excavation projects, providing essential support and stability to the surrounding soil.
While the piles are bored side-by-side, they do not interlock or overlap. Instead, a small gap (typically 100-150mm) is left between adjacent piles. This distinguishes them from secant pile walls, where piles are designed to cut into each other to create a watertight seal.
Key Characteristics of CBP Walls
CBP walls offer a practical solution for earth retention due to their specific design and construction advantages:
- Contiguous Nature: The term "contiguous" means adjacent or touching. In CBP walls, the piles are installed in a linear arrangement with minimal spacing, forming a nearly continuous barrier.
- Flexibility: CBP walls can be designed to various depths and diameters, making them adaptable to different project requirements and soil conditions.
- Construction Method: They are constructed using bored piling techniques, which generally involve less vibration and noise compared to driven piles, making them suitable for urban environments.
- Water Permeability: Due to the small gaps between piles, CBP walls are not inherently watertight. For projects requiring a dry excavation, supplementary measures like grouting or shotcrete are often applied to the wall face.
Applications in Deep Excavation
The primary application of CBP walls, as per the reference, is for deep excavation works. This includes a variety of civil engineering projects:
- Basement Construction: Providing support for multi-story basements in commercial buildings, residential complexes, and car parks.
- Underground Infrastructure: Supporting excavations for tunnels, metro stations, and utility shafts.
- Bridge Abutments: Creating stable retaining structures for bridge approaches.
- Waterfront Structures: Occasionally used in port and harbor developments, though often requiring additional sealing for marine environments.
Examples of Deep Excavation Use Cases:
- Urban Redevelopment: Minimizing ground movement in densely populated areas during the construction of new high-rise foundations.
- Infrastructure Upgrades: Facilitating the expansion or creation of new underground transport links without disrupting existing surface structures.
Advantages and Considerations of CBP Walls
CBP walls offer several benefits but also come with specific considerations for project planners.
| Aspect | Advantages of CBP Walls | Considerations for CBP Walls |
|---|---|---|
| Speed | Generally faster to construct than diaphragm walls. | Installation can be slower than sheet piling in certain ground conditions. |
| Cost | Often more cost-effective than secant pile walls or diaphragm walls for retention. | May incur additional costs for waterproofing if required. |
| Ground | Suitable for a wide range of soil types, including challenging ground. | Not ideal for highly permeable ground unless supplemented with sealing. |
| Vibration | Low vibration and noise levels, making them suitable for urban areas. | Requires significant site area for piling rigs and spoil management. |
| Flexibility | Adaptable to various geometries, including curved walls. | Limited stiffness compared to a solid concrete wall, requiring careful design. |
| Usage | Excellent for temporary or permanent retention for deep excavations. | Potential for water ingress through gaps if not adequately sealed. |
Construction Process Overview
The construction of a CBP wall typically involves the following general steps:
- Survey and Layout: Marking the exact locations for each pile.
- Guide Wall Construction: A low concrete guide wall is often built to ensure accurate positioning and verticality of the piles.
- Boring: A piling rig drills a bore (hole) to the required depth using an auger or other drilling tools.
- Reinforcement Cage Installation: A prefabricated steel reinforcement cage is lowered into the bored hole.
- Concreting: Concrete is poured into the bore, usually using a tremie pipe, to displace any water or drilling fluid and fill the hole from the bottom up.
- Sequence of Piles: Piles are typically constructed in an alternating pattern (e.g., primary piles first, then secondary piles) to allow concrete to gain sufficient strength before adjacent bores are drilled.
Contiguous Bored Piles (CBP) provide a robust and versatile solution for retaining earth, particularly valuable in deep excavation scenarios where stability and control over ground movement are paramount.
