Pile Foundations Overview

Pile Foundations

• Piles are deep foundations, significantly wider than their width.
• They are used when shallow foundations are inadequate for large loads or poor soil conditions.
• Piles can be made of steel, concrete, or timber.

Pile Types and Their Characteristics

• Pile types depend on the load, soil conditions, water table position, and installation method.
• Categories include steel, concrete, timber, and composite piles.
• Steel piles are often pipe or rolled steel H-section.
• Concrete piles are precast or cast-in-place.
• Precast concrete piles can be prestressed using high-strength steel cables.
• Cast-in-place piles are installed by making a hole and filling with fresh concrete.

Pile Installation

• Different installation methods exist, including impact hammers and vibratory hammers.
• Traditional impact hammers use repeated blows to drive piles.
• Vibratory hammers create a more controlled method, often using two counter rotating weights.
• Cast-in-place (bored) piles are installed by placing reinforcement in a hole and pouring fresh concrete.

Pile Load Transfer

• Load transfer occurs through skin friction along the pile shaft and the bearing capacity at the pile point.
• Ultimate load (Qu) is a combination of shaft resistance (Qs) and point resistance (Qp)
• Point bearing occurs when soil near the surface is weak and a stiff or rock stratum is located at relatively shallow depth.
• Friction piles occur when a stiff stratum is located at a reasonable depth.

Load Transfer in Sand

• The nature of variation in f(skin friction) with depth in sand is approximately linear.
• The maximum frictional resistance is fully achieved once relative displacement between the pile and soil is 5-10 mm.
• Frictional resistance increases with depth.
• Point resistance in sand fully mobilized once the pile tip is displaced about 10 - 25% of the width.

Load Transfer in Clay

• In saturated clays, the net ultimate load can be expressed Qp = A₄c₄ where c is the undrained cohesion of the soil below the tip of the pile.

Pile Capacity Estimation

• Several methods exist to estimate the ultimate pile capacity, including the use of standard penetration test results.
• Equations for estimating ultimate load bearing capacities (Qp, and Qs).

Negative Skin Friction

• Negative skin friction is a downward drag force exerted on a pile by surrounding soil.
• Negative skin friction can occur when fill is placed above pile if the ground is consolidation.
• Negative skin friction can also occur if fill is placed over soft clay.

Pile Load Tests

• Pile load tests are performed to verify the load-carrying capacity of a pile.
• The test procedure involves applying step loads and measuring pile settlement over time
• Methods for applying load include kentledge systems and reaction piles.

Group Piles and Efficiency

• Piles in groups are used to transmit structural load to the soil.
• The efficiency of group piles is affected by pile spacing, which determines how much load transfer is blocked by pile interaction.

Pile Capacity in Group Piles

• To estimate the pile's ultimate capacity in a group, one can consider the ultimate bearing capacity of each pile in the group (Q₂₁ ) + ultimate skin friction for each pile in the group (Q₂).