Deep foundations are crucial for supporting structures in challenging soil conditions. They transfer loads to stable layers deep underground, ensuring stability for buildings and infrastructure.

Types of deep foundations include piles, drilled shafts, and caissons. Each has unique advantages and is chosen based on soil conditions, loads, and site constraints. Understanding these options is key for effective foundation design.

Deep Foundation Types and Applications

Types of Deep Foundations

• Deep foundations transfer loads from superstructure to deeper, more competent soil or rock strata
• Piles serve as slender, column-like elements driven or installed into the ground to support structures
• Drilled shafts (bored piles) function as large-diameter cylindrical foundations constructed by drilling a hole and filling it with reinforced concrete
• Caissons act as hollow structures sunk into the ground and filled with concrete, used for deep foundations in water or unstable soils
• Micropiles operate as small-diameter (typically less than 300 mm) reinforced piles drilled into various soil and rock conditions

Applications and Selection Factors

• Support high-rise buildings, bridges, offshore structures, and structures on weak or compressible soils
• Choice of deep foundation type depends on soil conditions, structural loads, site constraints, and economic considerations
• Piles excel in supporting structures in areas with poor surface soil conditions (coastal areas)
• Drilled shafts prove effective for heavy loads and when minimal ground disturbance required (urban environments)
• Caissons perform well in marine environments and for structures requiring large lateral load resistance (bridge piers)
• Micropiles offer solutions for limited access areas or retrofitting existing structures (historic building foundations)

Construction Methods for Deep Foundations

Pile Installation Techniques

• Driving methods include impact (diesel hammers, hydraulic hammers) or vibratory techniques
• Jacking involves pushing piles into the ground using hydraulic jacks
• Screwing utilizes helical piles rotated into the soil
• In-situ techniques encompass cast-in-place concrete piles (auger cast piles, continuous flight auger piles)
• Driven piles made of various materials timber (for light structures in marine environments), steel (H-piles for deep penetration), and precast concrete (for heavy loads and corrosive environments)

Drilled Shaft and Caisson Construction

• Drilled shaft construction involves excavating a cylindrical hole using specialized drilling equipment (rotary drilling rigs, bucket augers)
• Reinforcement cage placement follows excavation, ensuring proper alignment and cover
• Concrete filling completes the shaft, often using tremie methods for underwater concrete placement
• Caisson construction typically involves sinking a prefabricated hollow structure through soil or water
• Sinking achieved using caisson's weight, additional ballast, or hydraulic jacks
• Slurry techniques employed in unstable soil conditions for both drilled shafts and caissons
  • Bentonite slurry maintains borehole stability during excavation
  • Polymer slurries used for temporary support in certain soil types

Quality Control and Environmental Considerations

• Integrity testing methods include sonic logging, thermal integrity profiling, and pile dynamics analysis
• Load testing verifies design assumptions and confirms foundation capacity (static load tests, dynamic load tests)
• Environmental factors considered noise restrictions (urban areas), vibration limits (near sensitive structures)
• Potential damage to adjacent structures mitigated through pre-construction surveys and vibration monitoring
• Construction methods adapted to minimize environmental impact (low-noise pile installation techniques, containment systems for drilling fluids)

Load Transfer Mechanisms in Deep Foundations

End-Bearing and Skin Friction

• End-bearing capacity transfers load through the tip of the deep foundation element to underlying competent soil or rock layer
• Skin friction (shaft resistance) transfers load along the sides of the deep foundation element through friction or adhesion with surrounding soil
• Relative contribution of end-bearing and skin friction varies depending on foundation type, soil conditions, and installation method
• Piles in cohesionless soils (sand, gravel) typically develop significant skin friction
• Piles in cohesive soils (clay) may rely more on end-bearing capacity
• Drilled shafts often utilize both end-bearing and skin friction, distribution depends on shaft geometry and soil profile
• Caissons primarily rely on end-bearing capacity, especially when founded on bedrock, but may also develop skin friction along their sides

Soil-Structure Interaction and Load Distribution

• Load transfer affected by soil properties (strength, stiffness) and foundation characteristics (surface roughness, shape)
• Stress distribution in soil around deep foundation influences load-carrying capacity
• Negative skin friction can develop in settling soils, adding load to the foundation element
• Group effects, such as overlapping stress zones and reduced efficiency, considered for closely spaced deep foundations
• Load transfer in layered soils complex, requiring careful analysis of each stratum's contribution
• Dynamic load transfer (seismic events, machine vibrations) differs from static load transfer, necessitating specialized analysis

Deep Foundation Systems: Advantages vs Limitations

Driven Pile Systems

• Advantages
  • High load-carrying capacity achieved through densification of surrounding soil
  • Quick installation process increases construction efficiency
  • Quality control facilitated by uniform manufacturing processes (precast concrete, steel piles)
• Limitations
  • Noise and vibration issues during installation may restrict use in urban areas
  • Potential for damage to adjacent structures if not properly managed
  • Limited ability to penetrate very dense soils or bedrock without pre-drilling

Drilled Shaft Systems

• Advantages
  • Large diameters and depths achievable, allowing for high load-carrying capacity
  • Minimal vibration and noise during installation suitable for urban environments
  • Ability to visually inspect soil conditions during drilling process
• Limitations
  • Require specialized equipment and expertise for installation
  • Construction process can be slowed by unexpected soil conditions or groundwater issues
  • Quality control challenges in ensuring proper concrete placement and shaft integrity

Caisson Systems

• Advantages
  • Well-suited for deep water applications (offshore structures, bridge foundations)
  • Can resist large lateral loads due to their size and mass
  • Ability to be prefabricated off-site, potentially reducing on-site construction time
• Limitations
  • Costly and time-consuming to construct, especially for deep installations
  • Challenges in achieving proper sealing against water ingress during sinking process
  • Limited to specific soil conditions and project types due to size and installation requirements

Micropile Systems

• Advantages
  • Low vibration installation ideal for work near existing structures
  • Can be used in limited access areas or for retrofitting existing foundations
  • Versatile in various soil and rock conditions due to drilling installation method
• Limitations
  • Generally lower load-carrying capacity compared to larger deep foundation types
  • Higher cost per unit of load-carrying capacity compared to conventional piles
  • May require a large number of micropiles to support heavy loads, increasing complexity