Deep foundations are used to transfer structural loads from a structure to deeper, stronger layers of soil or rock. They are typically used when shallow foundations are not suitable due to weak soils, insufficient bearing capacity, or the need to construct basements. Unlike shallow foundations, which rest directly on the surface of the soil, deep foundations extend below the ground to reach more stable strata.
Deep foundation types
There are several types of deep foundations, each with its own characteristics and applications:
Pile Foundations
- Description: Pile foundations consist of long, slender elements driven or drilled into the ground. They can be made of reinforced concrete, steel, or timber.
- Types:
- Friction piles: Transfer loads to the soil through skin friction along their lateral surface.
- End-bearing piles: Transfer loads to a bearing stratum at their tip.
- Composite piles: Combine the characteristics of friction and end-bearing piles.
- Applications: Widely used in various construction projects, including buildings, bridges, and retaining walls.
Drilled Shaft Foundations
- Description: Drilled shaft foundations are formed by drilling a hole into the ground and filling it with reinforced concrete. They can be constructed with or without a steel casing.
- Advantages: Can be used in a variety of soil conditions and can accommodate large loads.
- Applications: Commonly used for high-rise buildings, bridges, and industrial facilities.
Caissons
- Description: Caissons are hollow box foundations that are constructed by excavating a hole in the ground and then sinking the caisson into place. They can be open-ended or closed-ended.
- Advantages: Can be used in challenging soil conditions, such as those with high water tables or underground utilities.
- Applications: Often used for bridges, piers, and offshore structures.
Buoyancy Rafts
- Description: Buoyancy rafts are large, prefabricated concrete elements that are floated into place and then filled with concrete to create a foundation.
- Advantages: Can be used in deep water or where the ground is unstable.
- Applications: Primarily used for offshore structures, such as oil and gas platforms.
What is the strongest type of foundation?
The strongest type of foundation depends on the specific soil conditions and loading requirements of the structure. However, drilled shaft foundations and caissons are generally considered to be among the strongest types of deep foundations due to their ability to transfer large loads to deep, stable strata.
How deep can a deep foundation be?
The required depth of a deep foundation depends on several factors, including:
- Soil conditions: The depth of weak or unstable soils.
- Structural loads: The weight and type of structure.
- Groundwater conditions: The presence of groundwater can affect the depth of the foundation.
- Seismic considerations: In areas prone to earthquakes, foundations may need to be deeper to resist seismic forces.
Deep foundations can extend to depths of several hundred feet or more, depending on the specific project requirements.
Pile foundations vs. drilled shafts
- Installation: Pile foundations are typically driven into the ground, while drilled shafts are formed by excavating a hole and filling it with concrete.
- Soil Conditions: Piles are suitable for a wider range of soil conditions, including cohesive and granular soils. Drilled shafts are often preferred in challenging conditions such as soft soils or those with underground utilities.
- Capacity: Both pile foundations and drilled shafts can support large loads, but the specific capacity depends on factors such as soil properties, pile length, and diameter.
- Cost: The cost of deep foundations can vary depending on the type of foundation, soil conditions, and project location. Drilled shafts may be more expensive to install due to the excavation process.
Caissons vs. buoyancy rafts
- Installation: Caissons are sunk into place, while buoyancy rafts are floated into position and then filled with concrete.
- Applications: Caissons are commonly used for bridges, piers, and offshore structures, while buoyancy rafts are primarily used for offshore platforms.
- Cost: Buoyancy rafts can be more expensive due to the specialized equipment required for their installation.
Design onsiderations for deep foundations
When designing deep foundations, several factors must be considered:
- Soil properties: The bearing capacity, compressibility, and permeability of the soil.
- Structural loads: The weight and type of structure.
- Groundwater conditions: The presence of groundwater can affect the stability of the foundation.
- Seismic considerations: In areas prone to earthquakes, foundations must be designed to resist seismic forces.
- Construction constraints: Site conditions, such as access and utilities, may limit the type of foundation that can be used.
Case studies of deep foundation projects
- Burj Khalifa: The world’s tallest building, Burj Khalifa, is supported by a combination of bored piles and raft foundations.
- Golden Gate Bridge: The iconic Golden Gate Bridge is supported by caissons, which were sunk into the San Francis co Bay.
- Channel Tunnel: The Channel Tunnel, connecting England and France, was constructed using bored tunnels and caissons.
Advances in deep foundation technology
Recent advancements in deep foundation technology include:
- High-capacity drilling equipment: Allows for the construction of deeper and larger diameter drilled shafts.
- Innovative pile driving techniques: Improved methods for driving piles, such as impact hammers and vibratory hammers.
- Use of geosynthetics: Geosynthetics can be used to improve the performance of deep foundations by reinforcing the soil or providing lateral support.
- Monitoring technologies: Advanced monitoring systems can be used to track the performance of deep foundations and detect potential problems.
These advancements have enabled the construction of even larger and more complex structures on challenging foundations.
Deep foundations are essential for supporting structures in challenging soil conditions or when shallow foundations are not suitable. The choice of deep foundation type depends on various factors, including soil properties, structural loads, and project requirements. By carefully considering these factors, engineers can select the most appropriate deep foundation for a given project.
Recommendations
- Geotechnical investigation: Conduct a thorough geotechnical investigation to determine the soil conditions at the site.
- Foundation design: Consult with a qualified geotechnical engineer to design the appropriate type of deep foundation.
- Construction supervision: Ensure proper construction techniques are used to ensure the quality and integrity of the deep foundation.
- Regular inspection: Conduct regular inspections of the foundation after construction to monitor its performance.
