Pile Foundation Basics

Need for Pile Foundation

• Necessary for structures with very large design loads.
• Essential when soil conditions are poor at shallow depths.
• Utilized in sites with constraints, e.g., property lines.

Timber Piles

• Made from tree trunks, installed with the small end as a point.
• Maximum length is 35 m; optimum length ranges from 9 to 20 m.
• Typical maximum load: 450 kN; optimum load range: 80 - 240 kN.
• Advantages: High availability, easy to work with.
• Disadvantages: Susceptible to decay and low durability.

Steel Piles

• Practically unlimited maximum length, with an optimum range of 12 to 50 m.
• Load capacity depends on maximum allowable stress and cross-sectional area.
• Common shapes include rolled HP shapes and pipe piles.
• Closed-end piles can be filled with concrete post-installation.
• Advantages: High load capacity and efficient under hard driving conditions.
• Disadvantages: Vulnerable to corrosion; can be damaged by obstructions.

Concrete Piles

• Types include precast, prestressed, cast in place, and composite constructions.
• Precast piles designed to resist bending during transport and lateral loads.
• Prestressed piles use tensioned high-strength steel cables.
• Max lengths: 10 - 15 m for precast and 20 - 30 m for prestressed.
• Load capacity varies: 900 kN for precast, 8500 kN for prestressed; optimum: 350 - 3500 kN.
• Advantages: High load capacities and corrosion resistance.
• Disadvantages: High initial costs, handling difficulties, susceptible to soil-induced damage.

Composite Piles

• Combination of materials, taking advantage of both timber and concrete properties.

Installation Techniques

• Displacement Piling: Earth is displaced as piles are driven in.
• Replacement Piling: Soil is removed to make space for piles.

Classification by Load Transfer

• End Bearing Piles: Transfer loads directly to the soil.
• Friction Piles: Load is supported through friction along the pile's surface.
• Tension Piles: Designed to resist upward loads.
• Compaction Piles: Used for ground improvement.
• Anchor Piles, Fender Piles, and Dolphins: Specific functions related to stabilization and marine applications.

Factors Affecting Pile Selection

• Load carrying capacity assessed through methods like static analysis, dynamic analysis, and pile load testing.
• Established through correlation with field tests (SPT, CPT, etc.).

Pile Load Testing

• Determines settlement and ultimate bearing capacity.
• Initial or routine tests involve applying incremental loads.
• Failure load reached when settlement exceeds a specific percentage of pile diameter, followed by recovery measurements.

Efficiency of Pile Group

• Closely spaced piles may experience overlapping soil resistance.
• Total bearing capacity of a group may differ from the sum of individual piles.
• Efficiency of group depends on factors like characteristics, spacing, and number of piles, calculated using efficiency factor (ηg).

Pile Spacing Considerations

• Affects overlapping stress distribution and foundation costs.
• Influences settlements from various causes, such as soil consolidation and groundwater level changes.

Example Calculation

• A pile group of 9 piles in soft clay with specific parameters illustrates load capacity estimation based on bearing and adhesion factors, underscoring practical applications of theoretical concepts in pile foundation design.

Explore the essential concepts of pile foundations, including their need for large design loads and suitability in poor soil conditions. This quiz covers various types of piles, particularly timber piles, detailing their specifications and limitations. Test your understanding of pile foundations and their applications in construction.