Well Foundation Types – Features and Construction

Well foundation types are essential in civil engineering for structures over water bodies or weak soils. Engineers widely use deep foundations, known as caissons, to support bridge substructures, piers, abutments, and other heavy-load applications. Their ability to transfer loads deep into the ground makes them a preferred choice in deep foundation engineering. The main types of well foundations include circular wells, double-D wells, twin circular wells, and rectangular wells. Each type offers unique advantages in terms of load distribution and construction feasibility. Understanding these types is crucial for selecting the appropriate foundation system for specific project requirements.

This article provides an in-depth understanding of well foundations, including their types, construction process, design considerations, advantages, and comparisons with other deep foundations.

What is a Well Foundation?

Well foundation types are crucial in civil engineering, especially for supporting heavy structures like bridges, piers, and abutments over water or loose soils. Engineers design caissons as hollow structures to be sunk into the ground to the desired depth. They design these foundations to withstand substantial vertical and horizontal loads, making them ideal for challenging environments.

Key Features of Well Foundations types

• Suitable for heavy axial and lateral loads.
• Effective in soft soils and waterlogged areas.
• Resistant to scouring in underwater conditions.
• Commonly used in bridge substructures, jetties, and transmission towers.

Understanding the various well foundation types and their applications is essential for selecting the appropriate foundation system for specific project requirements.

Types of Well Foundations

Well foundations come in various shapes and designs, each suited to specific conditions and
requirements. The primary well foundation types include

Open Caisson (Open Well foundation)

Engineers design open caissons, also known as open wells, as box-like structures with open tops and bottoms. They sink these structures into the ground by excavating soil from within. This method allows the caisson to reach the desired depth, providing a stable foundation for various construction projects. Once it reaches the desired depth, workers seal the bottom with a concrete plug and fill the structure with concrete to form the foundation. Open caissons are particularly suitable for penetrating through water and soft soils to reach a stable stratum. They are economical for deep penetrations. However, during sinking, there is a potential for tilting, which requires careful monitoring and control. Understanding the various well foundation types, including open caissons, is essential for selecting the appropriate foundation system for specific project requirements.

Box Caisson

A box caisson is a type of well foundation with a closed bottom. Engineers prefabricate it and then float it to the desired location. Once in place, they sink it and fill it with concrete to provide the necessary weight and stability. Box caissons are often used in situations where dewatering is challenging. One advantage of box caissons is good quality control due to prefabrication. However, they are limited to relatively shallow depths and require a level bearing surface.

Pneumatic Caisson

Engineers design pneumatic caissons as sealed structures with open bottoms and closed tops. They pump compressed air into the chamber to maintain a higher pressure than the surrounding water, keeping the working area dry and facilitating safe construction activities. This keeps the workspace dry. It allows workers to excavate safely. This method is particularly useful for deep foundations in challenging underwater environments. However, it poses health risks to workers due to the high-pressure conditions, including decompression sickness, commonly known as caisson disease. Symptoms can range from joint and muscle pain to dizziness and, in severe cases, paralysis or death. Therefore, strict safety protocols and monitoring are essential to protect workers during pneumatic caisson operations.

Construction of Well Foundation types

The construction of a well foundation types involves several key steps

Site Survey and Setting Out of well foundations

Accurate layout of the bridge centerline and pier locations is marked using surveying
instruments. Reference points are established away from potential settlement zones to ensure
precision during construction.

Fabrication and Placement of Cutting Edge

The cutting edge, typically made of steel, is fabricated and assembled on-site. It is placed on a
levelled and compacted surface, ensuring proper alignment and level.

Construction of Well Curb

The well curb, a wedge-shaped reinforced concrete ring beam, is constructed above the cutting
edge. This involves setting up inner formwork, fixing reinforcement bars, and pouring concrete
to form the curb.

Sinking of the Well foundation types

Constructing a well foundation involves several key steps to ensure stability and load-bearing capacity.

• Sinking the Well: The process begins by excavating soil from within the shaft, allowing the well to sink under its own weight. To maintain vertical alignment and prevent tilting, engineers monitor and adjust the sinking process carefully.
• Seating the Well: Upon reaching the desired depth, engineers seat the well foundation into a hard stratum to ensure stability. This may involve additional excavation or dredging to achieve proper seating. In cases where the hard stratum has steep slopes or consists of rocky material, they may employ pneumatic sinking methods to achieve uniform seating.
• Bottom Plugging: Engineers place a concrete plug at the bottom of the well to seal it and provide a solid base. They achieve this by pouring concrete into the well and allowing it to set. This process ensures the foundation’s stability and prevents water ingress.
• Sand Filling and Top Plugging: Engineers fill the well with sand up to a certain level to reduce buoyancy and provide additional stability. They then place a top plug of concrete to seal the well. This process ensures the foundation’s stability and prevents water ingress.
• Construction of Well Cap: Engineers construct a reinforced concrete slab, known as the well cap, on top of the well to distribute the load from the superstructure to the foundation. This cap ensures the stability and integrity of the structure by effectively transferring the superstructure’s weight to the well foundation.
• Understanding the various well foundation types and their construction processes is essential for selecting the appropriate foundation system for specific project requirements.

Design Considerations for Structural Stability of Well Foundations

Designing a well foundation requires careful consideration of various factors to ensure structural stability:

• Soil Conditions: Engineers conduct thorough geotechnical investigations to assess soil properties, bearing capacity, and the presence of any obstructions. This information guides the design and construction process.
• Load Analysis: Engineers design the foundation to withstand vertical loads from the superstructure, as well as horizontal forces due to wind, water currents, and seismic activity.
• Scour Depth: In riverine environments, the potential for scour around the foundation is evaluated, and appropriate measures are taken to prevent undermining.
• Tilting and Shifting Control: During sinking, continuous monitoring is essential to detect and correct any tilting or shifting of the well to maintain vertical alignment.

Understanding the various well foundation types and their construction processes is essential for selecting the appropriate foundation system for specific project requirements.

Well Foundation types in Bridge Construction

In bridge construction, well foundations are particularly advantageous due to their ability to
support heavy loads and resist lateral forces. They are commonly used for:

The choice of well foundation types in bridge construction depends on several factors. These include soil conditions, water depth, and load requirements. Their robust design ensures durability and
stability, even in challenging environments.

Advantages of Well Foundation types

Understanding the benefits and limitations of well foundations is important for their effective
application: Here are its advantages:

● Uncontrolled tilting: Wells may tilt or shift laterally due to non-uniform soil conditions,
improper excavation, or external forces such as water currents.
● Corrective measures required: Engineers may need to add kentledge (extra weight),
apply water jets, or remove excess soil to maintain vertical alignment, further
complicating construction.

Well Foundation types vs. Other Deep Foundations

Well Foundation types vs. Pile Foundations

Aspect	Well Foundations	Pile Foundations

Definition

Deep foundations with large hollow structures, typically used in bridge piers and abutments.

Deep foundations consisting of slender columns driven or drilled into the ground to transfer loads to deeper, more stable soil layers.

Load Capacity

Suitable for heavy axial and lateral loads; provides a massive foundation.

Can support substantial vertical loads; often used in groups to achieve desired capacity.

Soil Conditions

Ideal for underwater constructions and locations with boulders or obstructions; provides resistance to scouring.

Suitable for a variety of soil conditions, including expansive or collapsible soils; advantageous in areas with high water tables.

Construction Cost

Higher initial cost due to complex construction processes.

More cost-effective with faster installation times.

Durability

High resistance to scour; well-suited for environments with strong water currents.

May require additional protection against scour and corrosion, especially in aggressive environments.

Well Foundations types vs. Raft Foundations

Feature	Well Foundations	Raft Foundations
Depth	Deep foundation	Shallow foundation
Load Distribution	Suitable for heavy loads	Used for light-to-moderate loads
Application	Best for bridges and marine structures	Commonly used in buildings and basements