Foundations can come in all different shapes, sizes and construction materials. A very diverse foundation system is the pile foundation. Pile foundations are used throughout the globe to support structures of all different scales. This article will take a deep dive down the rabbit hole and explore all the different pile foundation types and the benefits of each…
As a point of reference, the following table provides the full extent of pile foundation types. Click on each pile type to jump to the relevant section of this article for more details. Or alternatively grab a coffee and read from start to finish…
|Different Pile Foundation Types
|Suitable In Rock
|Suitable in Water Table
|Bored Pile (Rotary Bored Pile)
|Bored Pile (CFA Pile)
I’d like to thank Keller Piling for the use of their pile installation animations which are imbedded throughout this article. I’ve had the pleasure of working with Keller on some of my past projects throughout Melbourne. Keller Group are a multi national organisation who have experience in the installation of many of the pile foundation types explored in this article. You can take a look at their website HERE.
What is a Pile Foundation
A pile foundation is a deep foundation type with structural form resembling that of a column. Pile foundations penetrate the support soil and use friction forces between the side of the pile and the soil and/or end bearing between the soil and its toe to support the required design load. The quantity of piles, plan dimension and the embedment depth into the support soil are parameters that Structural Engineers can modify in order to meet the required load demands on the pile foundation.
Often, multiple piles may be required to support a given load under the one column or wall. This is called a “pile group”. Generally multiple piles are tied together with a stiff concrete element called a Pile cap. In some cases, a single pile foundation doesn’t require a pile cap, for a detailed look at this arrangement take a look at THIS article.
Piles can also form part of basement walls and shoring systems for large commercial building projects. For an in-depth article on basement types and in particular how piles form elements of basement walls, take a look at THIS article.
How do Pile Foundations Work
Although pile foundations come in many different types. The way they work from a Structural Engineering perspective is very similar.
Pile foundation types can broadly fit within three separate categories, regardless of construction methodology or material…
- Friction Piles
- End Bearing Piles
- Combination Friction Bearing Piles
Friction piles are those that gain the majority of their load bearing capacity from the friction interface between the sides of the pile and the support soil.
End Bearing Piles
End bearing piles are those that gain the majority of their load bearing capacity from the toe (base) of the pile bearing against the support soil.
Combination Friction Bearing Piles
Predictably, a combination friction bearing pile type are those that gain their load bearing capacity through a combination of friction and end bearing.
When Should Pile Foundation Types Be Used
The decision on which foundation system to use on a given building is influenced by:
- Soil conditions
- Potential for undermining (and allowing for future flexibility)
- Proximity to neighbouring buildings and below ground structures.
- Presence and depth of ground water table
- The forces applied to the structure (down, up and horizontal)
- Project cost
- Ability and specialty of local contractors
The following sections identify the scenarios where pile foundation types are generally a better alternative to shallow foundations…
The soil condition, type and depth of competent founding material influence whether pile foundations should are used and what pile foundation types to choose from.
If the top soil layers are not strong enough to support the required load, Structural Engineers need to design footings which go deeper into the soil profile. Deep pile foundation types unlock the potential to found upon stronger soil materials.
In Melbourne’s south-bank region in Australia, the upper soil layers are extremely prone to settlement and have minimal load bearing capability. This soil layer is known as Coode Island Silt. Any major buildings constructed in this region are required to be founded upon the underlying bedrock beneath. The bedrock in this region is in the order of 35 to 40m below ground surface level!
Prevent Foundation Undermining
Most pile foundation types can allow shallow excavation to occur in close proximity to the footing without becoming undermined.
Consideration needs to be made in design to allow this to occur. However if it is anticipated that future excavations may occur adjacent to a footing, a piled foundation is an ideal solution to facilitate this.
The Structural Engineer simply needs to discount the friction capacity provided by the the length of pile which corresponds to the adjacent excavation.
It is much costlier and in some cases impossible to allow this to occur adjacent to shallow foundations such as isolated pads. For shallow footings large volumes of blinding concrete need to be introduced to achieve a similar result. However there is a limit to what is feasible in terms of providing additional founding depth with blinding. To learn more about blinding concrete and its application, take a look at THIS article.
Prevent Applying Additional Surcharge Load on Sensitive Neighbouring Structures
In some instances, Structural Engineers may want to prevent additional surcharge load being applied to existing structures. A deep pile foundation type allows a new building to be constructed next to an existing building without applying additional unwanted surcharge load.
Take an example scenario where it is proposed to construct a new multi storey office tower adjacent to an existing basement car park. If the new office tower is founded upon shallow foundations, the load transmitted by the foundations to the soil will result in a large horizontal thrust load resulting on the existing basement walls.
However if a deep pile foundation is used, which supports the majority of the load through end bearing between the foot of the pile and the soil, the majority of this surcharge load is prevented from effecting the basement wall as illustrated below..
Sites with Shallow Water Table
Pile foundations are ideal for sites which have shallow water table. Excavating trenches and pits for large isolated shallow pad footings can be messy when the base of excavation is below the water table.
The dig needs to be constantly de-watered during construction. If the flow rate of the water table is significant enough, this can be a non-feasible solution.
Many pile foundation types allow easy construction where water table may be an issue, these types are discussed later in this article and include:
- Driven piles
- Bored Piles (when constructed with a Continuous Flight Auger)
- Screw Piles
Footings Requiring Large Uplift Capacity
In buildings which have large upward forces at the base of their columns or walls, piles can often be an efficient foundation solution.
Uplift in columns and walls can be generated within the structure due to :
- Large overturning wind loads on tall slender buildings
- Wind uplift on very light structures
- Rocking effect on shear walls in buildings during a seismic events
Shallow footings solve the uplift issue by harnessing the self weight of the building as well as using an adequate plan area for the footing itself to provide stabilisation. However if the uplift is sufficiently large, the friction capacity of some pile foundation types can quickly solve this dilemma in a more efficient manner.
Pile Foundation Types by Material
Structural Engineers and builders have a number of choices when it comes to the construction materials of different pile foundation types. Some pile construction types are suitable for multiple different types of construction materials, while others are only suitable for one:
Pile foundation types can be constructed from either:
- Cast in-place concrete
- Precast Concrete
- Structural Steel
- Structural Timber (lumber)
- Composite Concrete and Steel
Pile Foundation Types by Construction Methodology
The following sections outline the most common pile foundation types, provides a brief explanation of how they are constructed and lists advantages/disadvantages of each.
Bored piles have the highest load rating potential of all the pile foundation types. They can also reach extremely large depths.
Bored piles come in two main types:
- Rotary Bored Piles (often referred to as simply Bored Piles)
- Continuous Flight Auger Piles (often referred to as CFA Piles, or simply CFAs)
Both of these pile types utilise a circular reinforced concrete construction type. The main difference between the two is how they are constructed and more specifically, which piling rig is used…
A bored piling rig uses a drill bit which comprises a short steel helical cutting blade attached to a rotating shaft. The piled hole is constructed by progressively screwing the cutting blade into the soil then removing the blade from the excavated hole along with the cut soil where it is placed in a truck as spoil and taken from site. This process is repeated until the excavated hole reaches the required design depth.
Once the excavation is completed, the reinforcing cage is placed inside and the concrete is then placed using the tremie method.
The tremie method of placing concrete involves lowering a pipe to the base of the excavation, the pipe end at the top of the excavation is connected to a concrete hopper. The hoper and the pipe are then charged with the wet concrete mix. The pipe is then slowly lifted out of the excavated hole, placing concrete as it goes. During the lifting process, concrete is continuously replenished to the hopper in order to keep the system fully charged.
A CFA piling rig uses a drill bit which comprises a long steel helical cutting blade which runs the full length of its drill shaft. The drill shaft is a hollow tube through which concrete can be pumped.
The pile is constructed by first using the cutting blade to penetrate the soil to the required design depth (in this case, the spoil is not removed progressively but left in the hole between the cutting blades). As the full length helical blade is retracted from the hole, it takes the soil along with it, while at the same time placing concrete as it rises out of the excavation.
Once the drill bit is removed from the hole, the reinforcement cage is plunged into the still wet concrete.
What’s the Difference Between CFA Piles and Bored Pile Foundation Types?
In summary, the differences for both a CFA Pile and a Bored Pile are best illustrated with a comparison table…
|Drill Bit Used
|Short steel helical bit
|Full length helical bit
|Up to 29m (95 foot)
|Up to 80m+ (262 foot)
|Up to 1.2m (4 foot)
|Up to 2.4m (7.5 foot)
|Suitability for Rock
|Suitability for Deep Gravel/Sand
|Suitability below Water table
|Concrete Placement Method
|Through drill shaft
|Suitability to support large vertical loads
Advantages of Bored Pile Foundation Types
- Founding depth can be extremely deep.
- Depending on the piling rig, socketing in to rock is possible.
- If a CFA piling technique is used, this can be adopted in shallow water table zones.
- Extremely high load capacity can be achieved
Disadvantages of Bored Pile Foundation Types
- Significant spoil material can be generated which needs to be disposed off-site. This can be problematic for soil contaminated sites.
- Dust generated during construction can be higher than other pile foundation types
- Some specialist equipment is required for the installation and construction compared to some other pile foundation types
- Moderate vibration is generated during the boring process, especially if rock is encountered
- For rotary bore method, large excavated holes are present during construction which can pose a safety risk
Driven piles can be constructed from pre-cast concrete, steel or timber.
The pre-formed piles are usually fabricated off-site then delivered at the required dimensions and lengths.
The piles are then installed using a rig with an impact hammer attachment (or sometimes a vibration attachment). Like hammering a nail into a piece of timber, the rig rams the pile into the support soil until the desired capacity is reached.
How Driven Piles Work
The process of hammering a driven pile into the support soil has multiple benefits. From a geotechnical perspective, the hammering process displaces the soil around the pile shaft, this compacts the soil during installation and increases its support capacity.
Driven piles work best in sandy soil conditions. If embedment into support rock is required, driven piles are not a desired solution.
To achieve the desired capacity, the hammer rig continuously impacts the top of the pile. With each blow a reference mark is scribed on the side of the pile. With each subsequent blow, further reference marks are scribed so the displacement of the pile with each blow can be measured. Site Geotechnical Engineers can then calculate the piles load bearing capacity at any given time because the applied force (hammer blow) is a controlled constant force and the resulting displacement is proportional to the stiffness of the system and therefore its load bearing capacity.
There is usually a point at which no further impact can be applied to the top of the pile as the soil can become so compacted that subsequent blows do not result in further displacement of the pile. For this reason, it can often be difficult to achieve the desired up-lift capacity from a driven pile as the installation method can prevent further embedment from being achieved (and therefore higher uplift capacity).
Advantages of Driven Pile Foundation Types
- Can support moderate loading
- Does not require the disposal of soil. This is particularly advantageous for contaminated soil sites.
- Can be used in aggressive soil conditions
- Very quick installation
- As piles are fabricated off-site, quality is higher due to being manufactured in a controlled environment
- Wide range of pile sizes can be used from 350mm to 2400mm diameter (14 to 95 inch)
- No exposed holes during construction which improves site safety
- Can be used where water table is present
Disadvantages of Driven Pile Foundation Types
- Noisy installation process
- Installation can cause excessive vibration which may effect neighbouring properties (both its residents and structures)
- Less flexibility to achieve high uplift capacity
- Generally not suitable for rocky type soils
Screw piles are generally constructed from structural steel.
If you’ve ever enjoyed opening a bottle of wine (the cork variety, not a screw top), you already know how the principles of a screw pile works…
How Screw Piles Work
Screw piles are pre-fabricated structural steel elements comprising a circular steel shaft with a circular cutting blade at their base.
A screw pile can be installed using non-specialised equipment. A simple excavator with a rotary head attachment is all that is required.
Screw piles come in sections, the first section which is installed has the cutting blade. If the desired depth and therefore load bearing capacity has not been reached, subsequent lengths are then used. each subsequent length interlocks with the previous and the screw installation continues until the desired depth is reached.
Advantages of Screw Pile Foundation Types
- Very quiet installation
- Minimal dust produced during construction
- Very minimal vibration is produced during construction
- Light-weight, most pieces can be lifted into place using manual labour
- Minimal spoil is produced, this is especially advantageous for soil contaminated sites.
- Non-specialised equipment required, making this pile type ideal for residential construction
- Can be used in settings where water table is present and close to ground surface.
Disadvantages of Screw Pile Foundation Types
- Not suitable for rock soil types
- Detailed understanding of soil conditions are required prior to construction.
- Loadbearing capacity is limited, especially compared to some other different pile foundation types.
Micro piles (also referred to as minipiles) are very similar in nature to bored piles however on a much smaller scale in terms of pile diameter and depth.
Micropiles are generally in the order of 150 to 300mm (6 to 12 inch) diameter whereas bored piles are 300mm and above…
How Micropiles Work
Due to the small diameter and short depth of Micropiles, their installation differs slightly to bored piles.
A steel casing is first pressed into the soil to the required depth. Soil is then removed using a flushing method. This method involves introducing pressurised water to the soil then the water-soil mixture is literally sucked out of the casing.
Concrete is then placed in the hole using the tremie method explained earlier in this article. As the concrete placement is occurring from bottom to top, the steel casing is removed at the same time.
Reinforcement is then plunged into the still wet concrete. Due to the small diameter of the pile, instead of a full reinforcement cage being installed, it is usually a single centralised bar.
Advantages of Micropile Foundation Types
- Can be installed in confined spaces, making it ideal for strengthening existing foundations in car parks etc.
- Relatively quiet installation
- Relatively low vibration during construction
- Spoil material is minimal due to small diameter and pile length
Disadvantages of Micropile Foundation Types
- Not suitable for large load (ideal for providing additional strength to existing footings)
- Length is restricted. If the foundation is required to reach large depths, micropiles are not a feasible solution.
- Limited application in shallow water table settings
The Franki pile is perhaps one of my favourite pile foundation types in the bunch. Franki piles harness the benefits of driven piles with bored piles.
Franki piles are often called pressure injected piles. This pile type was patented by Edgard Frankignoul in 1909
How do Franki Piles Work
Franki pile installation starts with insertion of a steel casing into the ground. The steel casing is driven through the soil layers with the assistance of a drop hammer.
In the lower section of the steel casing, a dry cement mix is provided. The drop hammer drives the dry cement mix and the steel casing lower and lower until the desired depth is reached. The dry cement mix acts as a hammer head (driving plug) for the system to penetrate deep below ground.
Once the desired depth has been reached, the steel casings position is locked, preventing it from continuing further. Final hammer blows are applied to the dry concrete mix which further compacts the soil and causes the mix to exit the steel casing. In doing so, the dry cement mix forms a bulb like shape at the base of the pile. This bulb form will later give the Franki pile exceptional uplift capacity when its completed.
A reinforcement cage is then installed and the remainder of the excavated hole and is filled with concrete.
Advantages of the Franki Pile Foundation Type
- Due to the steel casing, the Franki pile can be installed in the presence of shallow water table.
- The bulb form at the base of the Franki pile gives it very high uplift capacity compared to its length
- The compaction process during construction also gives the Franki pile very good compression capacity compared to its relative length
Disadvantages of the Franki Pile Foundation Type
- The depth of installation for Franki piles is limited
- Construction can be loud due to the drop hammer
- Relatively high vibration is produced during construction
- Not suitable for rocky soils
Sheet piles are an outlier in the pile foundation types for this article. This is because sheet piling is predominantly used for temporary shoring and retaining of soil. They can only support nominal vertical loading.
Sheet piling is also used for temporary coffer dams to assist with the construction of bridge piers and foundations which are in the middle of a water bodies.
How Sheet Piles Work
Sheet piles are constructed of structural steel sheets. The steel sheets are usually profiled in a trapezoidal shape to give the sheet pile system its stiffness in bending.
The profiled steel sheets are pressed into the soil side-by-side to form an interlocked continuous wall. The installation of the segments can be performed either by a specialised rig or in shallower applications they can simply be pressed into the ground with an excavator using the bucket attachment to press them into the soil.
Sheet piling is usually used in temporary retaining and shoring applications to assist in the construction of other building elements such as deep foundations and basement walls.
Advantages of Sheet Piles
- Quiet installation
- Low dust is produced during construction
- Relatively low vibration generated during construction.
- Can be used in locations where water table is present and shallow.
Disadvantages of Sheet Piles
- Can only support small vertical loads (it is not designed for this application)
- Design life of sheet piles is low due to steel being imbedded into soil. Application of galvanizing can prolong the life-span however only for so long.
- Not suitable for rock conditions
Have you ever wondered how Structural Engineers verify that the final constructed pile is strong enough to support the design loading? Take a look at THIS article which covers all the different types of pile testing.