Pile Foundations and Their Classifications

Pile Foundations

• Deep foundation elements used to transfer loads to deeper, more competent soil layers or bedrock
• Used in situations where surface soils are weak or compressible
• Provide support for heavy structures like buildings, bridges, and towers

Classification of Piles

• Material:
  • Timber: Used for low-load applications and in environments with low corrosion risk
  • Concrete: Suitable for high-load applications, can be precast or cast-in-situ
  • Steel: Used for heavy loads and in aggressive environments, provide high strength and durability
• Installation Method:
  • Driven: Installed by driving them into the ground using a hammer, commonly made of concrete or steel
  • Bored: Created by drilling a hole into the ground and filling it with concrete, suitable for weak soils
• Function:
  • End-Bearing: Transfer load through the tip into a strong layer
  • Friction: Rely on skin friction along the sides to support loads

Static Analysis: Determining Pile Capacity

• Driven Piles:
  • In Sandy Soil:
    • ( Q_p = Q_s + Q_b ) where ( Q_s ) is skin friction, ( Q_b ) is end bearing
    • ( Q_s = \alpha \cdot A_s \cdot \sigma'{v} ) where ( \alpha ) is adhesion factor, ( A_s ) is surface area, ( \sigma'{v} ) is vertical effective stress
  • In Clayey Soil:
    • ( Q_s = \tau \cdot A_s ) where ( \tau ) is average shear strength of the clay
• Bored Piles:
  • Similar calculations apply, with adjustments based on the drilling method and soil type

Dynamic Pile Formula

• Hiley formula: ( Q = \frac{W \cdot h}{D} ) where ( W ) is hammer weight, ( h ) is drop height, ( D ) is pile displacement

Negative Skin Friction

• Downward forces, like soil settlement, act against the pile reducing load-carrying capacity
• Factors affecting negative skin friction:
  • Soil Settlement: Differential settlement of surrounding soil
  • Pore Water Pressure: Changes in pore pressure can influence friction
  • Soil Type and Density: Coarse soils have less negative friction compared to fine-grained soils

Piles in Groups

• Group Efficiency: Load-carrying capacity of a group compared to individual piles
  • Factors affecting group efficiency:
    • Spacing of Piles: Closer spacing can lead to negative interactions between piles
    • Soil Type: Group efficiency varies between cohesive and granular soils
    • Load Distribution: Uniform loading can improve efficiency
• Behavior of Pile Groups:
  • In Sandy Soil: Piles can behave cohesively under load due to shear transfer
  • In Clayey Soil: Behavior is influenced by adhesion and soil strength, reduced group efficiency may occur due to negative skin friction

Pile Load Test

• Static Load Test: A vertical load is applied incrementally to measure settlement
• Dynamic Load Test: Measures pile behavior during installation

Pile Cap

• Distributes loads from the superstructure to the piles
• Reduces differential settlement
• Improves load distribution
• Enhances stability and rigidity of the pile group

Criteria for Spacing and Depth of Piles

• Spacing: Piles should generally be spaced at least 2.5 to 3 times their diameter to minimize interaction effects
• Depth: Piles must extend to a depth where they can effectively mobilize sufficient capacity in the bearing layer

BIS Design Criterion

• IS 2911: Outlines methods for determining the load-carrying capacity of piles in both clay and sand
• Underrammed Piles: Specific design criteria are given for under-reamed piles in clay, focusing on depth, load capacity, and safety factors

Conclusion

• Understanding the concepts of pile foundations is essential for civil engineers involved in foundation design
• Ensures safety, stability, and durability in construction projects