FOUNDATION ENGINEERING

What is Sectional Flight Auger (SFA) Piling and when is it used?

Sectional Flight Auger piling, commonly shortened to SFA piling, is a bored piling method particularly suited to projects with restricted access, limited headroom or other site constraints that prevent the use of a larger Continuous Flight Auger rig.

Unlike CFA piling, which uses one continuous auger to reach the full pile depth, SFA piles are installed using shorter auger sections that are progressively connected as drilling advances.

This allows compact piling rigs to install deep bored piles within existing buildings, beneath overhead structures and on sites with narrow or difficult access.

At Dawson Foundation Engineering, we routinely design and install SFA piles for residential, commercial, restricted-access and retaining-wall projects throughout the UK.

Further technical information is available on our Sectional Flight Auger Piling service page.

How Does Sectional Flight Auger Piling Work?

SFA piles are formed by boring into the ground using a lead auger, also known as the boring head.

Once the first auger section has entered the ground, additional short auger sections are progressively connected until the required design depth has been reached.

The augers are then disconnected and removed one section at a time as the pile is formed.

Depending on the ground conditions and installation method, concrete or grout may be pumped through the hollow stem of the auger as it is withdrawn. This leaves a wet concrete column within the bore.

The required reinforcement is then lowered or plunged into the completed pile.

The installation process generally involves:

Positioning the compact piling rig over the proposed pile location
Drilling into the ground using the lead auger
Adding further auger sections as the bore advances
Continuing until the required depth is reached
Removing the auger sections during concreting or before concrete placement
Installing the required reinforcement
Preparing the completed pile for connection to the wider foundation system

Why Is the Auger Installed in Sections?

A CFA rig must carry an auger long enough to install the complete pile in one continuous drilling stroke. This generally requires a tall piling rig and significant vertical clearance.

SFA piling overcomes this restriction by using shorter auger sections. The individual sections can be added and removed within a much smaller working height.

This makes the method particularly useful where piling must be completed:

Inside an existing building
Within a basement
Beneath a roof or mezzanine floor
Under bridges or other structures
Beneath overhead power lines
Close to sensitive infrastructure
On sites with narrow or difficult access routes

Read more about the practical challenges involved in restricted access piling.

Restricted-Headroom SFA Piling in Wilmslow

Dawson Foundation Engineering completed an SFA piling project in Wilmslow, Greater Manchester, installing 300mm diameter piles to depths of 10 metres.

The available headroom was as low as 3 metres, preventing the use of a conventional full-height CFA piling rig.

By progressively adding and removing shorter auger sections, the SFA system enabled the required pile depth to be achieved within the restricted working height.

This project demonstrates how sectional piling equipment can provide substantial pile depth and capacity even where the available headroom is severely limited.

View the project in more detail: Restricted Headroom SFA Piling Works in Wilmslow.

What Are the Main Advantages of SFA Piling?

Advantage	Why It Matters
Compact equipment	Smaller SFA rigs can operate where access or headroom prevents larger piling equipment from being used.
Low noise	The bored installation process creates less noise than many impact-driven piling methods.
Almost vibrationless	SFA piling can be suitable close to existing buildings, utilities and sensitive infrastructure.
Wide range of diameters	The method can support both compact residential schemes and higher-capacity commercial foundations.
Versatile installation	Hollow-stem and solid-stem techniques allow the method to be adapted to different ground conditions.
Substantial load capacity	SFA piles can be designed to support compressive, tensile and shear loads.
Retaining-wall applications	Closely spaced SFA piles can be used to form contiguous bored pile walls.

What SFA Pile Diameters Are Available?

Dawson Foundation Engineering installs the following standard SFA pile diameters:

250mm
300mm
350mm
400mm
450mm
500mm
600mm

The most appropriate diameter depends on the ground conditions, structural loading, pile depth, access and equipment required.

Smaller 250mm and 300mm diameter piles can be installed using compact plant to depths of around 12 metres, subject to the site and ground conditions.

Larger-diameter or deeper piles normally require a more powerful and therefore larger piling rig.

What Loads Can SFA Piles Support?

SFA pile capacity varies according to the pile diameter, depth, reinforcement and ground conditions.

As an indication:

300mm diameter piles are often specified for compressive safe working loads of approximately 300–400kN
Loads of around 600kN may be achievable for a 300mm pile in suitable ground conditions
Shear loads of around 30kN are not uncommon for a 300mm diameter pile
600mm diameter piles may support compressive safe working loads of 2,000kN or more
Shear forces exceeding 100kN may be achievable with 600mm piles

These figures are not standard capacities that can be applied to every site. The pile design must be based on the structural loads and the available ground investigation information.

Hollow-Stem vs Solid-Stem SFA Piling

Sectional Flight Auger piles can be installed using either a hollow-stem or solid-stem method.

Consideration	Hollow-Stem SFA	Solid-Stem SFA
Concrete placement	Concrete or grout is pumped through the hollow auger stem as the augers are withdrawn.	The augers are removed before concrete is placed into the open bore.
Ground suitability	More versatile where the bore may not remain open after the auger is removed.	Requires unsaturated and self-supporting ground capable of maintaining an open bore.
Groundwater	Concrete is placed from the base upwards as the auger is withdrawn.	Generally unsuitable where water or unstable ground would enter the open bore.
Reinforcement	Reinforcement is plunged into the wet concrete after the auger has been removed.	Reinforcement may be installed before or after concrete placement, depending on the design and sequence.

When Is the Hollow-Stem Method Used?

The hollow-stem method is generally the more versatile SFA installation technique.

Concrete or grout is pumped through the auger stem as the augers are progressively withdrawn. This fills the pile bore from the base upwards and helps maintain the integrity of the pile during concreting.

The hollow-stem method can offer:

Suitability for a wider range of ground conditions
Concrete placement from the base of the pile upwards
No requirement to leave the completed bore unsupported before concreting
Reduced risk of groundwater or unstable ground entering the pile bore
Greater control over pile formation

Once the augers have been removed, the reinforcement is plunged into the wet concrete or grout.

When Is the Solid-Stem Method Used?

The solid-stem method can only be used where the ground is sufficiently stable to remain open after the augers are removed.

This normally means unsaturated, self-supporting soils that will not collapse or fill with groundwater before concrete is placed.

Clay soils can provide suitable conditions for solid-stem SFA piling because the bore may remain stable after drilling.

Potential advantages of the solid-stem method include:

Rapid installation where the ground is suitable
Reduced reliance on continuous concrete pumping during drilling
Less dependence on individual concrete delivery timings
The ability to prepare several pile bores before a concrete delivery
The option to install reinforcement before concrete placement
A clean and controlled working process in suitable ground

The method must not be selected where the ground or groundwater conditions would prevent the bore from remaining stable.

SFA Piling vs CFA Piling

SFA and CFA piles are both bored, cast-in-place foundation systems. In both methods, an auger is used to excavate the soil before the pile is filled with concrete or grout.

The main difference is the auger arrangement and the equipment required.

Consideration	SFA Piling	CFA Piling
Auger arrangement	Short auger sections are progressively added and removed.	One continuous auger reaches the full pile depth.
Rig size	Can be installed using compact, lightweight piling rigs.	Normally requires a taller and larger piling rig.
Headroom	Suitable for low-headroom and internal works.	Generally suited to open sites with unrestricted headroom.
Access	Suitable for narrow, restricted or complex access routes.	Requires access suitable for a larger rig and associated equipment.
Installation speed	Auger sections must be connected and removed individually.	Typically faster because drilling and withdrawal take place in one continuous operation.
Typical project	Confined, restricted or lower-volume schemes.	Open, higher-output and larger-volume projects.

Where a site has sufficient access and headroom, CFA may provide a faster installation programme. Where the site is confined or a large CFA rig cannot practically operate, SFA may provide the more suitable solution.

Read the full comparison: CFA vs SFA Piling: What’s the Difference?

When Is SFA Piling Used?

Sectional Flight Auger piling may be considered for:

Internal piling works
Low-headroom sites
Basements and existing buildings
Restricted or narrow access routes
Sites beneath bridges or overhead services
Projects close to railways, substations or sensitive infrastructure
House extensions and residential developments
Commercial and industrial foundations
Underpinning and structural strengthening
Contiguous bored pile retaining walls
Smaller projects where mobilising a large CFA rig may not be practical

Can SFA Piles Be Used for Retaining Walls?

SFA piles can be installed in a closely spaced row to form a contiguous bored pile wall.

These walls are commonly used to support the ground around:

Basements
Underground car parks
Deep excavations
Highway and railway cuttings
Excavations close to existing structures

The piles are installed with small gaps between them and are normally connected at the top using a reinforced concrete capping beam.

The piles may require substantial reinforcement to resist the bending forces generated as excavation proceeds.

Learn more in our guide: What Is a Contiguous Bored Pile Wall?

What Ground Conditions Are Suitable?

SFA piling can be used in a wide range of ground conditions, but the specific installation method must be selected around the site investigation findings.

Relevant factors include:

Soil type, strength and consistency
Whether the ground is self-supporting
The presence and level of groundwater
Made ground or variable fill
The risk of bore collapse
Buried obstructions
The required pile depth and capacity

The hollow-stem method may be required where the open bore cannot remain stable, while solid-stem installation may be suitable within stable, unsaturated and self-supporting soils.

Sites containing significant obstructions, dense overburden or rock may require a different piling method, such as overburden drill and case piling.

What Access and Working Area Are Required?

Although SFA rigs can be substantially smaller than CFA piling rigs, the site must still provide safe access and a suitable operating area.

The assessment should consider:

The dimensions and weight of the selected piling rig
The available doorway or access width
Headroom throughout the access route and working area
The delivery and storage of auger sections
Concrete or grout delivery arrangements
Reinforcement storage and installation
Spoil handling and removal
Nearby excavations, structures and services
The suitability of the working platform or existing surface

Ground Beams, Pile Caps and Reinforced Concrete Works

SFA piles normally need to be connected to the supported structure through an appropriate engineered foundation detail.

Depending on the project, this may include:

Reinforced concrete ground beams
Pile caps
Floor slabs
Capping beams for retaining walls
Internal piled rafts
Underpinning beams or structural connections

Dawson Foundation Engineering can undertake associated reinforced concrete works, as well as setting out, excavation, piling attendances and spoil removal.

Is SFA Piling Suitable for Your Project?

Sectional Flight Auger piling can provide a versatile bored foundation solution where access, headroom or site logistics prevent a larger CFA rig from operating.

However, the correct piling method must be selected using the ground conditions, structural loads, required pile capacity and practical site constraints.

At Dawson Foundation Engineering, we routinely install SFA piles from 250mm to 600mm in diameter for residential, commercial, restricted-access and retaining-wall projects.

To discuss your project, call 0333 224 5196 or contact our team to arrange a free site visit and no-obligation quotation.