Deep Foundation Systems Using Subsurface Boxes or Cylinders
This entry details a construction technique employing a hollow structural element, sunk into the ground to reach a competent load-bearing stratum. These structures, often constructed of reinforced concrete, act as deep foundation elements transferring structural loads from above to stronger soil or rock at depth.
Types and Construction Methods
- Box: Characterized by a rectangular or square cross-section, often used for large structures requiring substantial load-bearing capacity. Construction typically involves excavating within the box, allowing it to sink under its own weight or with added ballast, while removing soil.
- Open: A shaft-like structure open at both the top and bottom during sinking. Soil is excavated from within, and the unit descends as material is removed.
- Pneumatic: A watertight structure with a working chamber at the bottom pressurized with compressed air to keep water and soil from entering. Workers enter the chamber to excavate, and the structure is sunk. This method is particularly useful in areas with high water tables and unstable soil.
- Drilled: Constructed by drilling a large-diameter hole and then filling it with concrete. These are often used when the bearing stratum is rock. Sometimes referred to as bored piles or drilled shafts.
Advantages
- Ability to transfer loads through weak or unstable soils to a stronger bearing stratum at depth.
- Minimized vibrations and noise compared to driven piles.
- Suitable for sites with high water tables or challenging soil conditions.
- Provides a large bearing area, distributing loads effectively.
Disadvantages
- Construction can be more complex and time-consuming than other foundation methods.
- Requires specialized equipment and skilled labor.
- Potential for soil disturbance during excavation.
- Higher initial cost compared to some other foundation options.
Applications
Frequently employed in the construction of bridge piers, retaining walls, dams, and high-rise buildings where substantial load-bearing capacity is required and subsurface conditions are challenging.
Design Considerations
Design includes assessing soil properties, determining the required bearing capacity, calculating the settlement, and designing the structural elements of the box or cylinder to withstand the applied loads and earth pressures. Stability during sinking is also a critical aspect of the design process.