How to Calculate Screw Pile Load Capacity

What Is a Screw Pile Torque Chart?​​​

​​​

A screw pile torque chart provides a reference between the pile being installed, the installation torque and the corresponding capacity or design information.

​

It is useful because torque is not simply a machine setting.​

​

During installation, torque provides real-time information about the resistance being encountered by the pile. That information can then be assessed against the requirements established for the foundation.​

​

The required torque should therefore come from the pile design or specification rather than an installer choosing an arbitrary figure on site.​

​

ABC Anchors' pile range has different dimensions and torque limits, so the pile and installation equipment must also be matched correctly.

​

Why Does Installation Torque Tell Us About Load Capacity?​​​

 

Imagine screwing something into very soft material compared with dense material.

 

Less resistance requires less rotational force. Greater resistance requires more.​

 

A screw pile behaves on the same basic principle.​

 

As its helices advance through the ground, the installation equipment records the rotational resistance. Increasing resistance produces increasing installation torque.​

 

That measurement gives the installer information about the ground the pile is encountering.​

 

The established relationship between installation torque and ultimate pile capacity allows that measured torque to become useful engineering information rather than simply an installation reading.​

 

This is why installation torque should be measured and recorded for the individual piles.

​

What Ground Conditions Are Suitable for Screw Piles?​

 

There is no single ground condition that determines whether every screw pile will or will not work.​

 

Clay, sand, gravel, made ground and other soil conditions behave differently during installation.​

 

Ground conditions influence:​

​

• resistance during installation

• torque achieved

• required pile depth

• helix and pile selection

• whether extensions are required and

• the capacity that can ultimately be achieved

 

Denser or stronger ground will generally create greater resistance, while softer ground can require the pile to continue deeper or require a different pile configuration before the necessary installation criteria are achieved.

 

Obstructions, buried material and significant changes in strata can also affect installation.

 

This is why a calculation based on an assumed torque value cannot replace information from the actual site.

​

Why Is Soil Investigation Important for Screw Pile Design?​

 

A soil investigation provides information about what is beneath the proposed structure before the full screw pile installation begins.​

 

One method is the Standard Penetration Test (SPT), which measures soil resistance at different depths.

 

At ABC Anchors, our preferred SPT range for screw piles is approximately 17 to 38 N, with the higher end being more favourable. Lower values may require larger pile diameters or larger flights to provide sufficient surface area to achieve the required torque and capacity.

 

A soil report can therefore help the structural engineer and screw pile supplier understand the likely ground conditions and select an appropriate starting specification.

 

However, conditions can vary across a site.

 

Installation torque remains valuable because it provides information from the position of the individual pile as it is installed.​

​

What Is a Test Pile and Why Use One?

 

Where there is uncertainty about the ground, installing a test pile before committing to the full foundation can provide valuable site-specific information.​

 

We offer a Test Pile Kit for hire.

 

Our kit typically includes a selection of screw piles, usually 60R or 76R piles, extension shafts and torque-monitoring installation equipment.​

 

A test installation can help assess:​

​

• ground resistance

• achievable installation torque

• likely pile depth

• soil behaviour during installation

• pile selection and

• torque-to-load assumptions

 

The results can then contribute to the information used for the final pile design.

 

Testing one location does not mean that the ground will be identical across the entire site, so test results still need to be considered alongside the wider site and engineering information.​

​

How Does Screw Pile Design Work?​

 

Good screw pile design starts with the load the foundation needs to support and the ground into which the pile will be installed.

 

The process may involve:​

​

1. Establishing the structural loads

2. Reviewing available ground information

3. Carrying out a soil investigation where required

4. Installing a test pile where appropriate

5. Selecting the pile diameter and helix arrangement

6. Establishing the required installation torque and depth

7. Selecting suitable installation equipment

8. Installing and monitoring each pile

9. Recording the installation torque

10. Confirming the foundation against the project specification

 

The pile with the highest available torque capacity is not automatically the correct pile.

 

The objective is to specify a pile capable of meeting the structural requirements in the actual ground conditions while remaining within the pile and installation equipment limits.

​

How Is Torque Measured During Screw Pile Installation?​

​

Torque is measured during installation using a gauge associated with the torque head.

 

This applies whether the installation uses handheld equipment or an excavator-mounted torque head.

 

The installer can monitor resistance as the pile advances and compare the readings with the project requirements.

 

The torque reading should therefore not be treated as something checked only when the pile reaches an assumed depth.

 

It provides information throughout the installation.

 

At ABC Anchors, we also specify a minimum embedment requirement with the first flight of the screw pile should be at least 2 metres below ground level.

 

Depth and torque must both be considered. Reaching a particular depth alone does not demonstrate that the required torque has been achieved.

​

What If the Screw Pile Does Not Reach the Required Torque?​

 

If the pile reaches the intended depth without achieving the required installation torque, it may need to continue deeper where the pile specification and site conditions permit.

​

Extensions can allow the pile to continue until suitable conditions are encountered.

 

A persistently low reading may indicate softer ground than anticipated.

 

If the pile stops advancing or behaves unexpectedly, possible causes can include an obstruction, buried debris or other local ground conditions.

 

The correct response depends on the project and what is being encountered. Unexpected readings should not simply be ignored to complete the installation.

 

The installer and your project's engineer can determine the appropriate next step where site conditions differ from the design assumptions.

​

What If the Required Torque Is Reached Early?

 

High torque can indicate that the pile has encountered stronger or denser ground earlier than expected.

 

However, reaching the target torque does not mean installers should automatically stop without considering the complete specification.

 

Minimum embedment, pile limits, structural requirements and the installation design still apply.

 

If the required torque is reached unexpectedly early, stop and check the project requirements before proceeding.

​

Can You Calculate Screw Pile Capacity Before Installation?

 

You can design for a required load and determine a target installation torque before work starts, but the actual installation provides important information about the ground encountered by each pile.

 

That distinction matters.

 

A structural engineer may establish that a foundation requires a particular Safe Working Load. From that requirement, the design can work backwards through the selected safety factor and appropriate torque relationship to establish the required pile performance.

 

For example, if a pile requires a 60 kN Safe Working Load and a safety factor of 2 is specified:

​

Required ultimate capacity = 60 × 2 = 120 kN

​

If the relevant Kt factor is 30:

​

Required torque = 120 ÷ 30

​

Required torque = 4 kNm

 

The installation then needs to demonstrate the required conditions in the actual ground.

​

Why Should Torque Readings Be Recorded?

 

Torque records provide evidence of what happened during installation.

 

Rather than stating that a pile was installed "deep enough", the record shows the resistance measured during the installation.

 

Records may be important for:

​

• the structural engineer

• Building Control

• warranty providers

• contractors

• clients and

• future project documentation

 

The required documentation should be agreed for the project, and readings should be recorded consistently for the installed piles.

​

Screw Pile Installation Training

 

Understanding the formula is only one part of installing screw piles correctly.

 

Installers also need to understand how to read torque during installation, recognise unexpected ground behaviour, use the equipment correctly and document the installation.

 

At ABC Anchors, we provide screw pile installation training covering both the theory behind its screw piling system and practical installation using Autoguide equipment.

​

Training covers areas including torque and load capacity, ground conditions, pile selection, installation equipment, record keeping and safe working practices.

 

One-day and two-day training options are also available, with the practical programme designed for contractors, groundworkers, site teams and other members involved with screw pile installation.

​

Calculate the Load, Then Verify It in the Ground

 

A screw pile calculator can make the mathematics straightforward, but the calculation is only one part of screw pile design.​

 

The basic relationship is:

​

Qu = Kt × T

 

Installation torque gives the calculated ultimate pile capacity. The appropriate safety factor is then applied to establish the Safe Working Load.​

 

But the calculation still needs the correct pile, torque factor, structural loads and information about the ground.

 

For projects where ground conditions are uncertain, consider a soil investigation or hire an ABC Anchors Test Pile Kit before the full installation.​

 

For help with screw pile selection, torque requirements, installation equipment or project-specific technical information, contact us with your structural loads and available ground information.​

​

Frequently Asked Questions About Screw Pile Load Capacity​

 

1. How do you calculate screw pile load capacity?​

Screw pile load capacity can be estimated using Qu = Kt × T, where Qu is the ultimate pile capacity, Kt is the empirical torque factor and T is the average installation torque. A suitable safety factor is then applied to the ultimate capacity to determine the Safe Working Load for the pile.

 

2. How does installation torque relate to screw pile capacity?

Installation torque measures the rotational resistance encountered as a screw pile advances through the ground. There is an established relationship between installation torque and ultimate pile capacity, which allows measured torque to be used as part of assessing pile performance. The appropriate torque factor and project requirements must also be considered.

 

3. What is the difference between ultimate capacity and Safe Working Load?

Ultimate capacity is the calculated maximum capacity derived from the torque-to-capacity relationship. The Safe Working Load (SWL) is determined after an appropriate safety factor has been applied. For example, an ultimate capacity of 120 kN with a safety factor of 2 would give a Safe Working Load of 60 kN.​

 

4. What safety factor should be used for a screw pile load calculation?

The appropriate safety factor depends on the project and should be confirmed by the structural engineer. For example, applying a safety factor of 2 to an ultimate capacity of 120 kN gives a 60 kN Safe Working Load, while a safety factor of 3 gives a 40 kN Safe Working Load.

 

5. Can you calculate screw pile load capacity before installation?

A screw pile foundation can be designed around the required structural load, and a target installation torque can be established before installation. However, actual installation provides important information about the ground resistance encountered by each pile. Measured installation torque should therefore be checked against the project requirements.

 

6. How do ground conditions affect screw pile load capacity?​

Ground conditions affect the resistance encountered by the screw pile, the installation torque achieved and potentially the depth and pile configuration required. Soil conditions can vary across the same site, which is why a soil investigation or test pile may be recommended before the full installation.

 

7. What happens if a screw pile does not reach the required torque?​

If the required torque has not been achieved, the pile may need to continue deeper where its specification and the site conditions allow. Extensions may be used where appropriate. Unexpectedly low torque can indicate different ground conditions from those anticipated, so the installation should be assessed against the project specification rather than stopping at an assumed depth.

 

8. What is a screw pile torque chart?​

A screw pile torque chart provides reference information relating pile specifications and installation torque to capacity or other design information. The relevant pile limits, torque factor, ground conditions and required structural load still need to be considered rather than selecting a pile from a torque figure alone.

 

9. Is there a screw pile calculator?​

Screw pile capacity can be calculated manually using Qu = Kt × T, followed by the appropriate safety factor to determine the Safe Working Load. An interactive screw pile calculator can automate this calculation, but its result should not replace project-specific structural engineering, ground information or installation verification.

 

10. Do I need a test pile before calculating screw pile capacity?​

Not for every project, but a test pile can provide useful site-specific information where ground conditions or achievable installation torque are uncertain. At ABC Anchors, we offer a Test Pile Kit for hire, allowing ground resistance and installation behaviour to be assessed before committing to the full installation.

 

11. Does screw pile depth determine load capacity?​

Depth alone does not determine screw pile load capacity. The pile needs to satisfy the project's installation and design requirements, including the required torque. A pile reaching an assumed depth without achieving the specified installation conditions should not automatically be considered capable of carrying the required load.

 

12. Who should confirm the final screw pile design and working load?​

ABC Anchors can provide technical information about its screw pile systems, installation equipment and torque requirements, while our installation records provide evidence of the conditions achieved on your site. Your project's structural engineer should therefore confirm the required loads, appropriate safety factors and final design requirements.