In this lesson, we will discuss the design and installation of drilled shafts and caissons, both of which are types of deep foundations used for large structures where shallow foundations are insufficient. These foundations are typically used in bridges, towers, and high-rise buildings. By the end of the lesson, you'll understand how these foundation systems are designed, their applications, and how they are installed.
Learning Objectives
By the end of this lesson, you will:
Understand the differences between drilled shafts and caissons.
Learn the design principles for drilled shafts and caissons.
Understand the installation process for these deep foundations.
Introduction to Drilled Shafts and Caissons
Drilled shafts and caissons are deep foundation systems designed to support heavy loads by reaching deeper, more stable soil layers or bedrock. Both types of foundations transfer structural loads deep into the ground, providing stability for structures built in areas with poor surface soil conditions.
1. Drilled Shafts
Also known as bored piles, drilled shafts are cylindrical foundations created by drilling deep into the ground and filling the hole with reinforced concrete. Drilled shafts are typically used for large-scale construction projects, including bridges, towers, and heavy structures.
2. Caissons
Caissons are watertight retaining structures used as deep foundations. They are constructed in such a way that workers can enter the caisson to excavate and then fill it with concrete. Caissons are often used for marine projects, bridges, or areas with soft or water-logged soil.
Design of Drilled Shafts
Drilled shafts are designed to transfer loads to deeper, more stable layers of soil or rock. The design depends on several factors, including the soil type, the weight of the structure, and the groundwater conditions.
1. Load-Bearing Capacity
The load-bearing capacity of a drilled shaft depends on the end-bearing resistance (support from the material at the bottom of the shaft) and the side friction resistance (support from the friction between the shaft's surface and the surrounding soil).
Key Considerations:
Soil Conditions: Soil testing is critical to determine the bearing capacity and frictional resistance of the soil.
Shaft Diameter: The larger the diameter of the drilled shaft, the more load it can support.
Shaft Length: Shafts must be long enough to reach a stable layer of soil or rock.
2. Reinforcement and Concrete Placement
Drilled shafts are typically reinforced with steel rebar cages to increase their load-bearing capacity. Once the rebar is in place, the shaft is filled with concrete. The quality of concrete placement is crucial to ensure that no air pockets or weak points are formed.
Design of Caissons
Caissons are designed similarly to drilled shafts but have additional complexities due to their use in underwater or soft ground conditions. The key design considerations include structural stability, water-tightness, and load distribution.
1. Structural Stability
Caissons must be designed to withstand the pressures exerted by the surrounding soil and water. The walls of the caisson are typically reinforced with steel or concrete to provide additional strength.
2. Load Distribution
Like drilled shafts, caissons transfer loads through end-bearing and side friction. In the case of marine applications, caissons often rest on the seabed or are drilled into rock layers to ensure stability.
Installation of Drilled Shafts
The installation process for drilled shafts involves several steps to ensure that the foundation is strong and stable.
1. Drilling the Shaft
The first step is drilling the hole to the required depth using a specialized drilling rig. The depth of the hole depends on the soil conditions and the required load-bearing capacity.
2. Soil Stabilization
Once the hole is drilled, it may be necessary to stabilize the soil to prevent it from collapsing. This can be done using temporary casing or by adding bentonite slurry to the hole. The casing or slurry prevents the surrounding soil from falling into the drilled shaft during the excavation and concrete placement process.
3. Rebar Placement and Concrete Pouring
A steel rebar cage is then placed inside the shaft to provide reinforcement. Concrete is poured into the shaft to fill the hole completely, creating a solid, load-bearing foundation.
Installation of Caissons
Caisson installation is more complex due to the challenges of working in water or soft ground.
1. Excavation
Caissons are typically constructed by sinking a large, watertight structure into the ground or water and then excavating the material from inside. The caisson is gradually lowered as the excavation proceeds.
2. Concrete Placement
Once the caisson reaches the desired depth, the inside of the structure is filled with concrete, providing a solid foundation for the structure above.
3. Seal and Waterproofing
In marine applications, the bottom of the caisson is sealed to prevent water ingress. The walls must be waterproofed to ensure that the caisson remains stable in the long term.
Load Distribution in Drilled Shafts and Caissons
Both drilled shafts and caissons rely on a combination of end-bearing and side friction to transfer the load of the structure to deeper soil layers or rock.
End-Bearing: The bottom of the shaft or caisson rests on a stable layer of soil or rock, providing a strong base for the structure.
Side Friction: The sides of the shaft or caisson provide additional support by frictional resistance between the foundation and the surrounding soil.
Conclusion
Drilled shafts and caissons are critical components of deep foundation systems, offering stability and support for heavy structures in challenging soil conditions. By understanding their design principles and installation processes, construction professionals can ensure that structures are built on solid, stable foundations that can withstand the forces they are subjected to over time.
Both foundation systems play an essential role in modern construction, particularly for large structures that require deep support due to weak or unstable surface soils. Understanding how to design and install these foundations is a key skill for engineers and construction professionals alike.
