Basic Principles and Classifications of Pile Foundations

Pile foundations are structural elements, made of timber, concrete, or steel, that transfer loads from large structures to strong soil or rock layers deep underground. By reducing settlement, increasing stability, and supporting structures in challenging soil conditions, pile foundations are essential in constructing high-rises, bridges, offshore structures, and industrial facilities. Let's dive deep into the fascinating world of pile foundations and learn about their intriguing principles and classifications.

TABLE OF CONTENTS

• Basic Principles of Foundations
  
• Types of Foundations
  
• Classification of Pile Foundations
  
• Design Considerations for Pile Foundations
  
• Construction and Installation of Pile Foundations
  
• Pile Foundations on the FE Exam

What is a Foundation in Construction?

In construction, a foundation is the lowermost part of a building or structure that serves as a stable base to support the weight above it. Its primary function is to distribute the structural loads to the underlying soil or rock. Foundations are crucial for transferring loads safely, preventing excessive settlement, and ensuring the stability and integrity of the structure.

Foundations are essential elements that provide a solid and stable base for construction, ensuring structural integrity, load distribution, and overall safety.

Purposes of a Foundation in Construction

The purpose of a foundation in construction is to provide a stable and solid base for a building or structure. Foundations play a crucial role in transferring the loads from the structure to the underlying soil or rock in a safe and efficient manner.

A foundation provides level structure when building.

Foundations create a level surface for construction, allowing for the accurate and stable placement of walls, floors, and other structural components. They provide a supportive framework upon which the entire building or structure is erected.

A foundation distributes the weight of the structure.

Foundations distribute the weight and loads of the structure evenly to the ground, preventing excessive stress on any specific area. By spreading the loads over a larger area, foundations minimize the risk of settlement or structural failure.

A foundation supports a structure against natural disasters.

Foundations ensure the stability and integrity of the structure by anchoring it to the ground. They resist horizontal forces, such as wind or seismic loads, and prevent excessive movement or deformation. Foundations provide a solid base that keeps the structure in place, ensuring its safety and durability.

The Two Different Types of Foundations in Buildings

There are two main types of foundations: shallow and deep. Shallow foundations are used when the load-bearing soil is near the surface, while deep foundations are employed when the load-bearing soil or rock is at a greater depth.

Deep Foundation

A deep foundation is a type of foundation used when the load-bearing soil or rock is located at a significant depth below the ground surface. It is employed to transfer structural loads from weak upper layers to stronger soil or rock layers below.

Deep foundations, such as pile foundations, are essential when shallow foundations are insufficient or unsuitable due to soil conditions or heavy loads. They distribute the loads over a larger area and prevent excessive settlement or movement that could compromise the structure's stability and integrity.

Shallow Foundation

A shallow foundation is a type of foundation that is constructed near or at the ground surface to distribute structural loads to the stronger soil layers. It is commonly used for smaller or lighter structures where the upper soil layers have adequate load-bearing capacity.

Shallow foundations, such as strip footings, isolated footings, and raft foundations, are designed to prevent excessive settlement or movement by spreading the loads over a larger area of soil. They are less expensive and easier to construct compared to deep foundations but are limited to areas with suitable soil conditions near the surface.

Shallow foundations play a crucial role in residential buildings, small commercial structures, and low-rise constructions. They rely on the strength of the upper soil layers and do not require extensive excavation or reinforcement.

Site investigations and soil tests are conducted to assess the soil's bearing capacity and suitability for shallow foundations. Essentially, shallow foundations offer a cost-effective and practical solution for structures where the load-bearing capacity of the upper soil layers is sufficient.

What are Pile Foundations?

Pile foundations are the most common type of deep foundation. They consist of long, slender columns (piles) made of materials like timber, concrete, or steel. Piles are driven or installed into the ground, reaching the load-bearing strata.

They transfer the loads vertically through the weak upper layers to provide stability and support. Deep foundations, including pile foundations, are widely used in the construction of tall buildings, bridges, and other projects where soil conditions require a deeper level of support to ensure the structure's safety and longevity.

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Pile Foundation Design

Pile foundation design refers to the process of determining the appropriate configuration, dimensions, and specifications of piles for construction projects. The design process involves analyzing the soil conditions, structural loads, and other relevant factors to make sure the piles can effectively transfer loads from the structure to the underlying soil or rock layers.

Pile foundation design requires the expertise of geotechnical and structural engineers who utilize specialized software, engineering principles, and codes and standards to secure the stability and safety of the foundation system.

When to Use Pile Foundations

Pile foundations should be used when shallow foundations are inadequate to support the loads or when soil conditions pose challenges for stability and settlement control. They provide a practical solution for transferring loads to stronger soil or rock layers, assuring the safety and durability of the structure.

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Types of Pile Foundations

There are several types of pile foundations commonly used in construction. Which type of pile foundation ultimately gets used and how they are installed depends on

• soil conditions
• load requirements
• site constraints
• construction methods
  

Each type of pile foundation has its advantages and limitations, and the selection depends on the specific project requirements and site conditions.

Bored Pile Foundations

Bored piles, also known as drilled piles or augered piles, are created by drilling holes into the ground and then filling them with concrete or placing reinforcing steel. Bored piles are commonly used in cohesive soils or rock formations and can provide high load-bearing capacity.

They are suitable for deep foundation applications and can be installed with or without casing, depending on the soil conditions.

Cast In Place Pile Foundations

Cast-in-place piles are formed by drilling a hole into the ground and then pouring concrete into the hole while withdrawing the drilling tool. The concrete is usually reinforced with steel rebar.

Cast-in-place piles offer flexibility in terms of diameter and depth, making them suitable for a variety of soil conditions. They are often used in cohesive soils and can provide high load-bearing capacity.

Composite Pile Foundations

A composite pile foundation is a type of pile foundation that combines different pile materials, such as concrete, steel, or timber, to optimize load-bearing capacity and performance. Composite pile foundations can provide advantages such as increased load capacity, flexibility in design, improved structural performance, and potential environmental benefits.

Concrete Pile Foundations

A concrete pile foundation is a deep foundation system that utilizes cylindrical or square columns made of reinforced concrete to support and transfer structural loads to the underlying soil or rock layers. Concrete piles are driven or installed deep into the ground to reach load-bearing strata.

Concrete pile foundations can be advantageous because they provide high load-bearing capacity, resistance to vertical and horizontal forces, durability, and are suitable for various soil conditions.

Concrete pile foundations are commonly used in the construction of structures with very heavy loads, such as bridges and high-rise buildings.

Driven Pile Foundations

Driven piles are installed by driving them into the ground using impact hammers. The most common materials for driven piles are concrete, steel, and timber. Use this method for pile installation works for a wide range of soil conditions and are relatively easy to install. Driven piles can be precast or cast-in-place and are available in various shapes, such as H-piles, pipe piles, and timber piles.

Micropile Foundations

Micropiles, also called mini piles or pin piles, are small-diameter piles typically ranging from 100mm to 300mm. They are often used in limited-access sites or in areas with weak soils or unstable rock formations.

Screw Pile Foundations

Screw piles, also known as helical piles or screw anchors, are piles with helical plates that are screwed into the ground. They are often used in areas with soft soils or where there are limitations on noise, vibrations, or access. Screw piles can be easier to install than other types of pile foundations because they don’t disturb the site as much as other methods.

Sheet Pile Foundations

Sheet piles are thin, interlocking sections of steel or concrete driven into the ground to create retaining walls or excavation support systems. They are commonly used in waterfront or marine projects and can withstand lateral loads and provide soil stability.

Steel Pile Foundations

A steel pile foundation is a deep foundation system that utilizes high-strength steel piles to support and transfer structural loads to the underlying soil or rock layers. Steel piles, which can be cylindrical or H-shaped, are either driven into the ground or drilled and filled with concrete.

Steel pile foundations are preferred when challenging soil conditions or high load-bearing capacity is required. As you can imagine, they provide lots of strength, are very versatile, and commonly used in the construction of bridges, marine structures, high-rise buildings, and industrial facilities.

Timber Pile Foundations

A timber pile foundation is a deep foundation system that uses cylindrical columns made of hardwood or treated softwood to support and transfer structural loads to the underlying soil or rock layers. Timber piles are cost-effective, easy to install, and environmentally sustainable.

They are often used in construction projects with relatively light loads, such as residential buildings and piers. However, proper treatment, protection, and maintenance are necessary to prevent decay or insect damage. Regular inspections and specific design considerations are crucial for maintaining these types of pile foundations.

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How to Install a Pile Foundation

Pile foundations are installed using various methods depending on the type of piles being chosen to use and the conditions of the construction project site.

• Driven piles are installed by repeatedly striking them with an impact hammer, while bored piles are created by drilling a hole and then filling it with concrete. 
• Cast-in-place piles are formed by drilling a hole and pouring concrete while gradually withdrawing the drilling tool. 
• Screw piles are twisted into the ground using specialized equipment.
  

The installation process of pile foundations requires lots of planning and preparation in order to successfully install them and have them last.

Mastering the Basics of Pile Foundations for the Fundamentals of Engineering (FE) Exam

Understanding the basics of pile foundations is an important topic when preparing for the FE exam. The FE exam will assess your understanding and proficiency in various engineering principles and practices, including the design and analysis of foundation systems.

Therefore, having a solid grasp of pile foundation principles and classifications will equip you with the necessary skills to address questions related to foundation design, load transfer mechanisms, and soil-structure interaction on FE exam day.