Pile Foundations: Necessity, Diverse Types, and Key Classifications

Foundations are the critical interface between a structure and the ground, transferring the loads safely to the supporting soil or rock. While shallow foundations like spread footings and mat foundations are common, there are many situations where they are simply not adequate. This is where pile foundations come into play, serving as a vital component of deep foundation systems. Understanding the necessity and diverse types of piles is fundamental for any civil and structural engineer.

This detailed guide will delve into why pile foundations are indispensable for various construction projects, their different classifications based on material, load transfer mechanism, and installation methods, providing a comprehensive overview for students and professionals alike.

Necessity of Pile Foundations

Pile foundations are chosen over shallow foundations when the upper soil strata are weak or compressible, and cannot adequately support the structural loads. Here are the primary reasons why pile foundations become necessary:

• Weak or Soft Soil Conditions: When the bearing capacity of the soil near the ground surface is insufficient to support the structure, piles transfer the loads to deeper, stronger soil layers or rock.
• High Structural Loads: For heavy structures like high-rise buildings, bridges, and industrial plants, the loads are too immense for shallow foundations, necessitating deep foundations to distribute the weight effectively.
• Presence of Expansive or Collapsible Soils: Soils that undergo significant volume changes (swelling or shrinking) due to moisture variations, or those that collapse upon wetting, pose risks to shallow foundations. Piles can extend below these problematic layers.
• Liquefiable Soils: In seismic regions, saturated loose granular soils can lose their strength and stiffness during an earthquake, a phenomenon known as liquefaction. Piles can extend through liquefiable layers to stable ground.
• Scour Protection: For structures subjected to water currents (e.g., bridge piers, offshore platforms), piles provide stability against erosion and scour of the surrounding soil.
• Uplift Forces: Structures like transmission towers, offshore platforms, or basement rafts below the water table can experience significant uplift forces. Piles can be designed to resist these forces.
• Reduced Settlement: Piles help in significantly reducing and controlling differential settlement, which can otherwise cause structural damage.
• Adjacent Structure Considerations: When constructing new foundations close to existing structures, piles can minimize disturbance and potential settlement of the neighboring buildings.
• Below Water Table Construction: Constructing shallow foundations below the water table can be challenging and costly due to dewatering requirements. Piles often provide a more practical solution.

For more general information on foundations, you can refer to the Wikipedia article on Foundations.

Types of Pile Foundations

Pile foundations are classified based on various criteria, each influencing their application and behavior.

1. Based on Material:

• Timber Piles:
  • Made from tree trunks, commonly pine, fir, or oak.
  • Economical in areas where timber is readily available and costs are low.
  • Can be susceptible to decay (unless treated) and insect attacks, especially above the water table.
  • Used for lighter structures or in marine environments below permanent water level.
• Concrete Piles:
  • Most common type due to versatility and durability.
  • Can be precast (manufactured off-site and driven) or cast-in-situ (bored and concreted on-site).
  • Offer high compressive strength and good resistance to decay and corrosion (if properly designed).
  • Reinforced concrete piles are widely used for various structures.
• Steel Piles:
  • Typically H-piles (rolled H-sections) or Pipe Piles (circular sections).
  • Have high bearing capacity and good resistance to driving stresses.
  • Can penetrate dense soil layers or soft rock.
  • Susceptible to corrosion, especially in aggressive soil or marine environments, often requiring protective coatings.
  • Useful for deep foundations and structures subjected to large lateral loads.
• Composite Piles:
  • Composed of two or more different materials (e.g., timber below water table and concrete above).
  • Used to combine the advantages and mitigate the disadvantages of individual materials.
  • An example is a concrete upper part and a timber lower part, where the timber is continuously submerged.

2. Based on Load Transfer Mechanism:

• End-Bearing Piles (Point Bearing Piles):
  • These piles derive their primary support from a strong, incompressible soil or rock layer at their tip.
  • The load is transferred through the pile tip to the strong stratum.
  • Suitable when a hard stratum is available at a reasonable depth.
• Friction Piles (Floating Piles):
  • These piles primarily rely on the frictional resistance developed along their shaft length with the surrounding soil.
  • The load is transferred to the soil through skin friction or adhesion between the pile surface and the soil.
  • Used when the firm stratum is very deep or non-existent, and the upper soil layers provide sufficient frictional resistance.
• Combined End-Bearing and Friction Piles:
  • Most piles derive resistance from both end-bearing and skin friction.
  • The contribution of each mechanism depends on the soil type, pile geometry, and installation method.

For more information on the mechanics of load transfer in piles, you can refer to the Wikipedia article on Pile Foundations.

3. Based on Installation Method:

• Driven Piles (Displacement Piles):
  • Piles are driven into the ground using hammers (drop hammers, diesel hammers, vibratory hammers).
  • This method displaces soil laterally and densifies granular soils, increasing their bearing capacity.
  • Can be precast concrete, steel H-piles, or pipe piles.
  • Advantages include speed of installation (for precast), known quality of materials, and relatively high load capacity.
  • Disadvantages include noise, vibration, and potential for heave in surrounding soil.
• Bored Piles (Replacement Piles / Drilled Shafts / Caissons):
  • A hole is bored or drilled into the ground, and then reinforced and filled with concrete.
  • This method removes soil during drilling, causing minimal ground displacement.
  • Can be constructed to very large diameters and depths.
  • Advantages include less noise and vibration, suitability for urban areas, and ability to inspect the bore.
  • Disadvantages include slower installation, reliance on concrete quality control on-site, and potential for caving in unstable soil.
• Screw Piles (Helical Piles):
  • Piles with helical blades that are screwed into the ground.
  • Minimal disturbance to the surrounding soil.
  • Can be quickly installed and removed.
  • Ideal for lighter structures, tension piles, or in environmentally sensitive areas.

4. Based on Function or Use:

• Load Bearing Piles: Designed to carry vertical compressive loads from the structure.
• Compaction Piles: Used to densify loose granular soils to increase their bearing capacity. These piles are typically driven and not designed to carry direct structural loads themselves.
• Tension Piles (Uplift Piles): Designed to resist uplift forces from structures, such as those caused by wind, buoyancy, or overturning moments.
• Batter Piles (Raker Piles): Driven at an inclination to resist horizontal or inclined forces, commonly used for bridge abutments, retaining walls, and offshore structures.
• Fender Piles: Used to protect marine structures (jetties, wharves) from impact by ships.
• Sheet Piles: Interlocking sections driven to form a continuous wall for earth retention (e.g., coffer dams, retaining walls) or cut-off walls to prevent seepage.

Conclusion

Pile foundations are indispensable in modern construction, providing robust support for structures in challenging ground conditions. Their diverse types, materials, and installation methods offer engineers a wide array of solutions to suit specific project requirements and site constraints. A thorough understanding of their necessity, behavior, and classification is paramount for designing safe, economical, and durable deep foundations that form the invisible yet critical backbone of our built environment.

Frequently Asked Questions

When are pile foundations necessary?

Pile foundations are necessary when surface soil is weak, structural loads are very high, expansive/collapsible soils are present, there's a risk of liquefaction, or uplift forces need to be resisted.

What are the main types of piles based on material?

The main types based on material are timber piles, concrete piles, steel piles, and composite piles.

What is the difference between end-bearing piles and friction piles?

End-bearing piles transfer load to a strong stratum at their tip, while friction piles primarily rely on frictional resistance along their shaft with the surrounding soil.

What are bored piles and driven piles, and what are their key differences?

A bored pile (or drilled shaft) is installed by drilling a hole, inserting reinforcement, and filling it with concrete, removing soil and causing minimal disturbance. Driven piles are forced into the ground using hammers, displacing soil laterally and causing more noise/vibration but often faster installation and increased soil density.

When would composite piles be a cost-effective solution?

Composite piles are cost-effective when combining materials to leverage their best properties, such as using timber below the permanent water table (where it resists decay) and concrete above, to save on more expensive materials.

What are tension piles used for?

Tension piles, also known as uplift piles, are designed to resist upward (uplift) forces on structures, common in situations with high wind loads, buoyancy, or overturning moments.

What is liquefaction, and how do piles help?

Liquefaction is when saturated granular soils lose strength during an earthquake. Piles help by extending through these liquefiable layers to deeper, stable, non-liquefiable ground, ensuring structural support.

What are batter piles?

Batter piles (or raker piles) are piles driven at an inclination to effectively resist horizontal or inclined forces, often used in structures like bridge abutments or retaining walls.

Why are steel piles commonly used for very deep foundations?

Steel piles are commonly used for very deep foundations because they have high bearing capacity, can withstand significant driving stresses, and can effectively penetrate dense soil layers or even soft rock.