Friction piles play an important role in the construction when it comes to providing support in the areas that have weak and unstable soil. These piles can be a part of deep foundations and mainly relies on the frictional force between the surface of the pile and the surrounding soil for bearing a load. This mainly depends on the resistance generated along the pile shaft length. Frictional piles can provide cost-effective and reliable solutions for stability in areas where the soil is too weak to support the structural load with shallow foundations.
This blog will provide you with various details regarding friction piles and how to apply them practically in the construction.
What Are Friction Piles?
Frictional pile is a kind of foundational pile which mainly uses the frictional resistance force between the surface of the pile and adjacent soil for the transfer of superstructure load. Depending upon the subsoil condition, resistance force caused by friction can develop in a definite length of the pile. For a stable foundation condition, the frictional force must be adequate to support the superstructure.
Working Principle of Friction Piles
Friction piles are also called floating piles. It works on the principle of using frictional resistance of pile surface and soil for supporting the structural loads. The frictional piles will distribute the load to its full pile length by soil interaction.
- Load Transfer Mechanism
The friction will be created when friction piles are moved into soil in a downward direction. This force helps in supporting the load and prevents settlement according to shear strength of soil.
- Load Distribution Along the Pile
Friction piles will carry loads along the shafts into the soil. The load amount carried by each section will depend on the friction produced between the soil and the pile.
- Factors Affecting Efficiency
- Soil Type
- Pile Material
- Pile Length
- Installation Method
- Role of shear stress
The shear stress generated between the pile surface and soil is the key component in friction pile design.
- Friction Piles as Wedges
Friction piles function like wedges driven into the soil. As the pile penetrates the soil, it induces shear forces along its shaft. This behavior allows the pile to develop significant load-bearing capacity even in soils that do not provide a firm base for end-bearing.
What Is Friction Pile Made of?
Friction piles are long, slender columns that help in transferring the load of a structure into the ground. Depending on the project needs and soil conditions, these piles are made from,
- Reinforced concrete: Used in large projects for strength and durability
- Steel: It is used when high tensile strength and resistance to bending is needed.
- Timber: An economical option for lighter structures or in less aggressive environments, though less commonly used today.
These piles are driven deep into the ground, often to a depth at least three times the width of the structure, ensuring they engage with enough soil to support the building through friction along their length. The deeper they go, the more they rely on the friction between the pile surface and the surrounding soil to carry the load.
Types of Friction Piles
- Precast concrete piles
- Cast-in-Situ Piles
- H-Piles and Wide Flange Beams
- Composite Piles
Applications of Friction Piles in Construction
- Friction piles can be used in the areas where the soil is weak or unstable, but needs to support a high amount of loads. One such building is a skyscraper.
- There is a need for deep foundations near soil that has a weak surface for the construction of bridges. Here the friction pile can give the necessary load-bearing capacity.
- Large industrial structures like factories and warehouses must rely on friction piles for support. These piles can help in making sure that the structure can support any heavy machinery and equipment.
- In constructing coastal and offshore structures, frictional piles can be used to support marine infrastructure.
- Frictional piles can be used with retaining walls for lateral support in earth retention systems.
Frictional piles are commonly used for larger structures requiring high load-bearing capacity in places with high-strength deep soil. Also, if there is properly compacted and consolidated sand, it can provide additional benefits. One of the most well-known examples of frictional pile application is a tall and super heavy skyscraper that is located in Dubai, which is the Burj Khalifa.
How To Calculate Skin Friction On Piles
Skin friction on pile surfaces can be calculated using the formula:
Qs=∑f*AsQs
Where:
- Qs = skin friction
- As = surface area of the pile
- f = unit shaft resistance
To find f, use the following equation:
f=Kσotan(δ)+ca
where:
- K = earth pressure coefficient,
- σo = vertical stress at the pile depth,
- tan(δ) = friction angle between soil and pile,
- ca = adhesion coefficient.
The earth pressure coefficient reflects how soil produces lateral pressure on the pile. The value of K depends on the pile installation method:
- Driven piles: The earth pressure coefficient is higher due to soil compaction around the pile.
- Bored piles: The coefficient is lower because the soil is less compacted around the pile.
- Jetted piles: The coefficient can vary significantly, as the surrounding soil is often disturbed.
Factors Affecting Skin Friction
Several factors can influence skin friction, some are:
- Pile material: The material used (steel, concrete, or wood) impacts the skin friction characteristics.
- Soil type and installation method: The interaction between the pile and soil, along with the method of pile installation, can significantly affect skin friction.
What is Positive Skin Friction in Piles?
Positive skin friction is the force that resists the penetration of friction piles into the soil and transfers the building loads into the soil. It provides the needed stability to support the superstructure and is the main force behind the concept of friction piles. More than 80% of the piles’ bearing capacity comes from this force and 95% in the case of piles longer than 30m.
What Is Negative Skin Friction in Piles?
Negative skin friction happens when the surrounding soil around a pile or pile group moves downward relative to the pile. This occurs if the pile is located in a compressible soil layer. It leads to changes in the consolidation and settlement of the soil. Negative skin friction is a downward drag force that is produced by the settling soil on the pile’s surface.
Conclusion
Frictional piles can be used in deep foundation systems and can help provide stability and support in challenging soil conditions. They help transfer the structure’s loads to the deepest and more stable layers of soil. They can also ensure the stability and longevity of large structures. Thus, frictional piles are an important element of modern construction.
FAQs
The friction pile capacity can be calculated by multiplying the surface area of the pile by the friction force per unit area that the soil can safely provide. More contact with the soil will lead to high frictional resistance that helps piles to support loads.
To increase the capacity of a friction pile, several factors can be adjusted:
Pile Diameter: A larger diameter increases the surface area in contact with the soil, boosting friction.
Pile Depth: Deeper piles engage more soil, improving overall resistance.
Pile Number: Increasing the number of piles distributes the load more effectively.
Surface Roughness: Rougher pile surfaces create more friction, increasing load capacity.
Frictional piles are normally made of H-piles and wide flange beams. H piles are normally as wide as they are deep but have the same thickness in the flange and web. When compared to the standard beams, piles will provide better distribution of weight over a wider area. Due to this, the H-Piles are an excellent choice for the friction piling, as they can support the large and heavy structures, and each pile can withstand loads up to 1000 tons.