Foundations are often overlooked but are a critical element of any structural engineering project. The commitment civil engineers make to design and implement robust, reliable foundations are of paramount importance. Pile foundations are one of the most commonly used types of foundations and have been employed in many engineering applications. This article will provide a comprehensive guide to pile foundation design and construction, to help civil engineers ensure their foundations are safe and reliable.
Pile foundations are commonly used in a variety of engineering projects, such as bridges, docks, and buildings. Due to their load-bearing capacity and resistance to lateral forces, they are an ideal choice for many applications. Pile foundations are also relatively easy to construct, which makes them a popular choice for many civil engineers.
Despite their popularity, pile foundations are not without their challenges. In order to ensure a safe and reliable foundation, it is important to understand the challenges associated with pile foundation design and how to address them. Some of the challenges associated with pile foundations include:
- Designing for serviceability: Pile foundations must be designed to withstand the loads imposed by the structure they are supporting.
- Construction: Due to the nature of pile foundations, construction can be challenging. In order to ensure a safe and reliable foundation, it is important to understand the challenges associated with pile foundations and how to address them.
- Corrosion: Pile foundations are susceptible to corrosion due to their exposure to the elements. In order to ensure a safe and reliable foundation, it is important to understand the challenges associated with pile foundations and how to address them.
This article will provide a comprehensive guide to pile foundation design and construction, to help civil engineers ensure their foundations are safe and reliable.
Pile design capacity is based on following factors.
- End bearing
- Skin friction
Pile design capacity
Pile capacity = allowable end bearing + allowable skin friction.
The allowable values are after the relevant safety factors are applied. If you used the ultimate values, factors safety shall be applied in this equation.
Pile End Bearing Capacity
Piles are commonly used in the construction industry to support building foundations, platforms and retaining walls. Pile end bearing capacity is the maximum load a pile can carry at its tip and is one of many types of pile load test. The pile end bearing capacity test evaluates the load-bearing capability of a pile by measuring the largest vertical load that it can bear at its tip and it is very important information in the pile foundation design.
Depending on the type of piles, the method of specifying the end bearing is varying. For the piles socketed into the rock, it is the capacity of the rock. When the pile is rested on the soil, it is the soil bearing capacity. The acceptable method to obtain the allowable end bearing is to carry out soil investigation.
For rock, the tests like uniaxial compressive strength (UCS) test are done to evaluate the allowable end bearing of rock. If we know the ultimate end bearing, allowable ending can be calculated by applying suitable safety factor. The following equation is used to calculate the end bearing capacity of rock of soil.
End Bearing Capacity = [allowable end bearing] x [ area of the pile toe ]
or
End Bearing Capacity = {[ Ultimate end bearing] / [Factor of Safety]} x [ area of the pile toe]
Worked example
Ultimate end bearing capacity 5000 kN/m2, Diameter of pile 600mm, Factor of safety to convert ultimate end bearing to allowable end bearing 2.5; this value may vary by the standard.
End bearing Capacity = [ 5000 / 2.5 ] [π x 0.62 /4] = 565 kN
Pile Skin Friction
Pile skin friction is a type of dry friction that occurs between two surfaces in contact. The amount of friction between the two surfaces is proportional to the force pressing them together and the area of contact. Pile skin friction plays an important role in many applications, including: The pile theory provides a mathematical description of the frictional behavior of piles and is used extensively in geotechnical engineering. The theory was first proposed by Robert Hooke.
Pile skin friction plays an important role in soil-structure interaction and foundation engineering. It forms a critical part of both static and dynamic analyses, especially those involving pile foundation design. This includes analyses of pile loads, settlements caused by piledriven compression, pile-soil interaction (such as pile stability or pile bearing capacity) and vibration generated by nearby piles being driven into the ground or movement of water through groundwater.
The pile skin friction can be calculated from the following equation.
Allowable Skin Friction = [ Allowable soil skin friction ] x [ Surface area of pile ]
Skin friction in the soil and the rock is considered for the design when pile is socked into the rock. For friction piles, the soil skin friction will be considered. There are situation where the development in the negative skin friction, designer shall be alert in those situation and calculation shall be done accordingly.
Worked Example
This example only indicates the calculation of soil skin friction of a layer of soil. Soil layer thickness 1.5m, allowable skin friction 75 kN/m2, diameter of pile 600mm
Skin Friction = 75 x π x 0.6 x 1.5 = 212 kN
Pile Negative Skin Friction
The friction force applied opposite to the skin friction is called the negative skin friction. It is applied downwards and reduce the pile bearing capacity. Negative skin friction occurs when there are compressible soils such as organic soils. Having the clay what would settle with the time due to the overburden pressure will also generate the negative skin friction on the pile.
Pile skin friction is balance between the positive and negative skin frictions.
Pile Skin Friction = Positive Skin Friction – Negative Skin Friction
Structural Capacity in Pile Foundation Design
In addition to the geotechnical capacity of pile, structural capacity shall be checked. There are some simple check such as consideration of the 25% of the pile compressive strength and calculate the pile structural capacity by multiplying the cross-sectional area of pile.
However, it is more correct and best practice is to analyze the pile for buckling behaviors also considering the soil pile interaction. When there are soft soils in the area piles are constructed, lateral restrain would be lesser and could cause buckling failures.
The pile capacity will be take as follows.
Pile Design Capacity = Minimum of [ allowable geotechnical capacity or structural capacity ]
