Deep Foundation Design

A deep foundation is a type of foundation used in construction to provide support for structures by transferring the load to deeper, more stable soil or rock layers.

This is in contrast to shallow foundations, which typically transfer loads to the soil layers near the surface. Deep foundations are employed when the soil near the surface has insufficient bearing capacity, the structure's loads are very large, or when the structure is subjected to lateral forces such as wind or seismic loads.

When To Use a Deep Foundation

For the integrity of a structure, there are a number of reasons why you would want to employ a deep foundation. However, the main causes would be weakened or damaged soils, undocumented fills, and liquefaction.

Weak Soils

The term “weak soils” refers to soils that could potentially fail if a shallow foundation were to be employed.

Compressible Soils

Compressible soils are those that can become denser through the process of volume reduction. Fundamentally, the soil will compress and pull down over time when a structure is built on compressible soil without deep foundation

Undocumented Soils

When the soil’s stability is unknown, it is referred to as undocumented soil.

Liquefaction

Liquefaction occurs when waterlogged, loosely packed sediments at or near the ground’s surface start to lose strength as a result of violent ground shaking. During earthquakes, liquefaction beneath buildings and other structures can result in significant damage.

Types of Deep Foundations

The following are the several kinds of deep foundations now in use:

• Basement foundations

• Buoyancy rafts (hollow box foundations)

• Caisson foundations

• Drilled shaft foundations

• Pile foundations

Basement Foundations

These are hollow underground buildings intended to give workspace or storage. Instead of taking into account the most effective way to withstand external earth and hydrostatic pressures, the structural design is dictated by their functional needs. They are built on-site in open excavations.

Buoyancy Rafts( Hollow Box Foundations)

Buoyancy rafts, also known as floating foundations or float-out installations, are a type of foundation system used in the construction of structures on poor or saturated soil conditions where traditional foundations may not be suitable. These foundations work by using the principle of buoyancy, where the weight of the structure is counteracted by the buoyant force exerted by the displaced soil or water. This reduces the overall bearing pressure on the soil, helping to prevent settlement and instability.

Buoyancy rafts are typically constructed using lightweight materials such as reinforced concrete, and their design may incorporate hollow or cellular sections to further reduce the weight of the foundation. The structure's weight is evenly distributed across the buoyancy raft, which in turn displaces an equivalent volume of soil or water to generate buoyant uplift.

These types of foundations are commonly used in areas with high water tables, marshy lands, or very soft soils, where the bearing capacity of the soil is low or where settlement is a significant concern. They can also be used in areas with environmental constraints or contamination, where minimizing ground disturbance is essential.

It is important to note that the design and construction of buoyancy rafts require careful engineering and analysis to ensure the proper balance between the structure's weight and the buoyant force. This helps maintain stability and prevent excessive uplift or settlement.

Caisson Foundations

A caisson foundation, also referred to as a pier foundation, is a watertight retaining structure used as a bridge pier, in the construction of a concrete dam, or for ship repairs. It consists of a prefabricated hollow box or cylinder that is used as a foundation and is lowered into the ground to a particular depth before being filled with concrete.

When constructing bridge piers and other structures that must be anchored beneath rivers and other bodies of water, the caisson foundation is most usually used. This is made possible by the ability to float caissons to the project site and then bury them there.

If a geotechnical engineer determines that the soil is suitable to carry the building load, caissons are either drilled deep into the underlying soil strata or to bedrock (referred to as “rock caissons”). Caissons are typically “belled” at the bottom when they rest on soil to disperse the weight over more ground. For these “belled caissons,” specialized drilling bits are used to remove the soil.

Types of Caisson Foundations

There are various types of caisson foundations, they include;

• Box Caissons

• Excavated Caissons

• Floating Caissons

• Open Caissons

• Pneumatic Caissons

• Sheeted Caissons

Box Caissons

Box caissons are heavy-timbered, watertight boxes with an opening at the top. They are often floated to the proper location, where they are then buried into the ground with a masonry pier inside.

Excavated Caissons 

Excavated caissons are exactly what their name implies—caissons that are erected inside an area that has been dug out. They are typically cylindrical in shape, with a concrete backfill.

Floating Caissons 

Floating caissons are prefabricated boxes with cylindrical cavities, commonly referred to as floating docks.

Pneumatic Caissons

Little cofferdams called open caissons are positioned, pumped dry, and then filled with concrete. They are typically utilized while building piers.

Sheeted Caissons 

Large waterproof cylinders or boxes called pneumatic caissons are typically utilized for underwater construction.

Cylinders

Cylinders are small single-cell caissons.

Drilled Shaft Foundations

Drilled shaft foundations, also known as drilled piers or bored piles, are a type of deep foundation used to support structures by transferring loads to deeper, stable soil layers or bedrock. Constructed by drilling a large diameter hole, inserting a steel reinforcement cage, and filling it with concrete, they are suitable for a wide range of soil conditions and provide high load-bearing capacity and resistance to lateral loads. Commonly used for bridges, tall buildings, and other structures, drilled shaft foundations are a reliable alternative when shallow foundations are insufficient.

Pile Foundations

The most typical deep foundation, which is frequently utilized for big projects. Pile foundation is appropriate when the soil is clayey or has a low bearing capability. Concrete, steel, and timber are all utilized to make piles, but reinforced concrete is most frequently used for pile foundations. Through a vertical pile, the load will be transferred from the superstructure to the deep-seated soil.

Types of Pile Foundation

There are several types of pile foundations in use. They are;

• End bearing pile

• Friction pile

• Anchor pile

End Bearing Pile

The end bearing piles are driven into the hard, soft soil at a depth of no more than 40 meters. Over the hard rock, at a great depth, the pile’s end is set. Through the vertical elements of the foundation, the structure’s load will be transmitted to the ground soil.

Friction Pile

The end-bearing pile is driven using the same methods as the friction pile. When the pile depth is greater than 40 meters, the friction pile is used. The weight Is transferred by the skin friction that occurs when the friction piles happen by the surrounding dirt.

Anchor Pile

A particular kind of pile foundation called an anchor pile is utilized to withstand uplift forces that may otherwise cause the pile to be pulled out of the earth.

Advantages of Deep Foundations

• They can withstand heavy loading conditions

• High-rise buildings can be built in areas with poor soil holding ability.

• Utilized for large-scale building structures.

• It is impact-resistant against seismic loads.

Disadvantages of Deep Foundations

• Building a deep foundation is expensive.

• A highly skilled workforce is needed.

• While executing, many additional safety precautions are needed.

Deep Foundations Design Steps

Deep foundations are structural components that divert stresses away from weak or unstable surface soils and onto deeper, more stable soil layers or rock formations. When high bearing capacity and resistance to lateral stresses are required for buildings, bridges, dams, and other structures, they are frequently used. Yet, there are a number of difficulties involved in planning and constructing deep foundations on slopes that are prone to landslides, including slope stability, soil-structure interaction, seismic stress, and construction viability. Listed below are some of the steps required to carry out an effective deep foundation design.

• Slope Stability Risk Assessment

Assessing the danger of slope failure and its potential effects on the effectiveness of the foundation is the first stage in any foundation design. The evaluation of the geotechnical qualities of the soil, the geometry and orientation of the slope, the groundwater conditions, the external loads and stresses, and the probable collapse causes and modes all come under the complicated and site-specific process known as slope stability analysis. Several techniques and tools, such as limit equilibrium analysis, finite element analysis, probabilistic analysis, or physical and numerical modeling, might be employed, depending on the level of specificity and precision required.

A factor of safety (FOS), which represents the margin of safety against slope failure, is the result of the slope stability analysis. If the FOS is less than 1, the slope is unstable; if it is more than 1, the slope is stable. The project needs, the level of uncertainty, and the design codes all affect the minimum allowable FOS.

• Selection of Suitable Deep Foundation

There are several options to take into account when choosing the best deep foundation type for the slope conditions and the structural requirements, including piles, drilled shafts, caissons, micropiles, helical piles, and ground anchors. Depending on the soil type, installation technique, load capacity, stiffness, durability, and cost, each type has pros and cons. Certain general standards should be considered while making a choice.

The selected foundation must have the following properties: it must be capable of withstanding vertical, horizontal, and moment loads from the structure and slope movements; it must be able to pierce through weak or unstable surface soils and reach a competent bearing layer or rock formation; it should have a minimal impact on the slope stability and soil-structure interaction; and it should be practical to install with the equipment and resources currently available without causing excessive disturbance or damage to the slope or environment.

• Design And Analysis of Deep Foundation

The axial and lateral load capacities of the foundation, as well as its settlement and deflection, must all be taken into account when building and analyzing deep foundations on slopes that are prone to landslides. The foundation’s ability to support a seismic load should also be considered, as well as interactions between nearby foundations that could have an impact on the system’s load distribution and collective efficiency. These elements must all adhere to design guidelines and standards.

• Installation And Monitoring of Deep Foundation

The deep foundations on the slope must be installed and maintained as the final phase, in accordance with the contract requirements and design criteria. Quality control, instrumentation, and observation should all be taken into account during installation and monitoring, as well as the manner and order of installation. The foundation materials may need to be tested and inspected, and measurements and records of displacement, load, strain, stress, and vibration may need to be made both during and after installation in order to confirm the design assumptions and find any anomalies or issues.

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