Screw piers, Pile caps and Houses

What Are Screw Piers?

Screw piers, also called screw piles or helical piers, are steel foundation elements installed by rotating a shaft with one or more helical plates into the ground.

They are commonly used where near-surface soils cannot provide adequate foundation support, or where the design requires loads to be transferred to deeper ground.

In residential construction, screw piers may support house footings, slabs, posts, extensions and other structural elements.

Their main attraction is that they can often be installed relatively quickly with limited excavation and spoil removal compared with conventional bored concrete piers.

However, a screw pier is not a complete foundation system on its own. Its suitability depends on the ground conditions, the loads it must resist, its connection to the structure and the behaviour of the surrounding soil.

Watch the explanation

Screw Pier Installation and Foundation Problems — Video

In this video, Matthew Cornell discusses screw pier installation, the loads applied to house foundations, the role of pile caps and problems that can arise in reactive soils.

The video includes an installation demonstration credited to Ideal Foundations, followed by Matt’s engineering commentary.

Installation

How Are Screw Piers Installed?

A screw pier generally consists of a steel shaft with one or more helical plates near its lower end.

Installation equipment rotates the pile into the ground. Additional shaft sections can be connected as the pile advances to the required depth.

Depending on the system and ground conditions, the installation torque may be used as part of the process for verifying the pile’s geotechnical capacity. The required torque, depth and acceptance criteria should be established by the responsible designer and pile supplier.

The helix transfers loads into the surrounding ground. Screw piles can be designed for downward compression loads and upward tension loads, subject to the relevant ground conditions and structural design.

Installation records are important. An engineer investigating an existing screw-pier foundation may need to know the pile locations, shaft sizes, helix configurations, installation depths and recorded torques.

Structural behaviour

What Loads Act on Screw Piers?

House foundations do not carry downward loads alone.

A screw pier designed primarily for axial loading should not automatically be assumed capable of resisting significant lateral loads or bending moments.

Those actions may require additional structural elements, an appropriate pile-head connection, greater shaft stiffness or a different foundation arrangement.

Lateral restraint

Why Sideways Support Matters

In the original video, Matt highlights the importance of considering the steel shaft as a structural member, rather than simply assuming that the surrounding ground will prevent it from moving sideways.

Where the upper soils are soft or disturbed, their ability to provide lateral restraint may be limited.

A long, slender steel shaft subjected to compression must be checked for stability and buckling, with realistic assumptions about the restraint provided by the soil.

The lateral resistance of an embedded pile is a geotechnical and structural design question. It depends on factors including soil stiffness, shaft properties, embedment, loading and pile-head restraint.

Matt’s concern is particularly relevant when a screw pier is installed directly beneath a house post without an adequately designed means of transferring lateral forces into the ground.

Connections

What Is a Screw Pile Cap?

A pile cap is a structural element that connects a pile or group of piles to the supported building. It transfers loads between the superstructure and the piles and, where designed to do so, helps distribute forces and resist moments.

For some residential screw-pier arrangements, Matt favours combining the screw pier with a bored concrete footing or pile cap near ground level.

The screw pier can transfer the required axial loads to deeper ground, while the concrete element may be designed to provide additional support, connection capacity and lateral resistance.

This is not a universal detail. The dimensions, reinforcement, embedment, connection and interaction between the concrete footing and the steel pile must be designed for the actual loads and ground conditions.

A shallow concrete cap will not necessarily provide sufficient lateral restraint simply because concrete has been placed around the top of the pile.

Reactive clay

Screw Piers in Reactive Clay Soils

Reactive clay soils expand and contract as their moisture content changes.

In Queensland, this can cause substantial seasonal or longer-term ground movement beneath residential foundations.

Screw piers may be used to transfer foundation loads to deeper, more stable material, but this does not automatically remove the effects of reactive ground movement on the slab or footing system above.

What Happens When the Ground Shrinks?

During drying, reactive soil may shrink away from the underside of a foundation. Compressible fill can also settle, although the causes and implications differ.

If the house is properly supported on deeper piles, a gap may develop between the ground and parts of the suspended or partially supported foundation without the building necessarily settling by the same amount.

However, a visible gap is not enough to establish that the foundation is performing adequately. The load path, pile locations, slab behaviour and reason for the ground movement need to be understood.

What Happens When the Ground Swells?

When reactive soil becomes wetter, it may swell and exert upward pressure on the underside of a slab or footing system.

Deep screw piers may restrain downward settlement at their support points, but they do not necessarily prevent the ground between those points from heaving.

This can create a difficult combination of restrained pile-supported areas and soil-supported areas that move vertically.

The slab and its connections must be designed for the expected behaviour of the entire foundation system.

Waffle slabs

Problems With Screw Piers Beneath Waffle Pod Slabs

Matt’s original discussion raises an important concern about waffle pod slabs supported by screw piers around their perimeter.

If perimeter footings are supported on deeper, relatively stable ground while internal areas of the slab remain influenced by reactive near-surface soils, differential movement can occur.

The internal slab may be subjected to upward heave or loss of support while the perimeter remains restrained.

Adding screw piers to a waffle slab therefore requires consideration of more than the perimeter wall loads.

The designer needs to consider:

• the location and spacing of pile supports;
• the behaviour of internal and perimeter slab beams;
• potential ground shrinkage and swelling;
• the capacity of the slab to span between supports;
• the possibility of uplift acting on the slab;
• the required lateral stability of the foundation system; and
• the intended behaviour of any void formers or movement-isolation details.

Matt’s preference in the video is to consider internal pile support as well as perimeter support where the foundation arrangement warrants it. Whether that is appropriate depends on the slab design, ground conditions and intended structural behaviour.

Movement details

Can a Pile Cap Allow a Slab to Move?

Some proprietary foundation systems incorporate details intended to accommodate relative movement between a slab and its supporting piles.

Such arrangements may be used where the designer expects reactive soil movement and needs to control how forces are transferred into the pile.

However, movement-isolation details introduce their own design considerations.

The engineer must understand how the connection behaves during upward movement, whether support is maintained or temporarily disengaged, and what happens when the slab returns toward its original position.

Connections, alignment, lateral restraint and the capacity of the slab to bridge between supports all remain important.

These are system-specific questions and cannot be resolved by assuming that an isolation cap automatically protects the building from heave.

Visible foundation movement

Gaps Beneath Houses Supported on Screw Piers

A gap beneath a house slab can develop when the surrounding ground settles or shrinks while the structure remains supported on deeper piles.

This can be alarming to a homeowner, particularly where the gap is visible at the edge of a waffle pod slab.

The appropriate response depends on why the gap formed.

If the soil has consolidated, the movement may differ significantly from seasonal shrinkage of reactive clay, which could reverse when moisture conditions change.

Simply filling a gap beneath a slab without understanding its cause can be problematic. New fill may interfere with an intended movement allowance or transfer unwanted loads into the slab during subsequent soil swelling.

Investigation should establish whether the slab is intended to span between piles, whether the pile supports remain effective and whether the observed movement is consistent with the foundation design.

Construction defects

What Happens When a House Post Misses the Centre of a Screw Pier?

Another issue Matt discusses is a house post positioned away from the intended centreline of its supporting screw pier.

Where a vertical post load is applied eccentrically, it creates a moment in addition to the vertical force.

The magnitude of that moment is related to the load and the offset distance.

If the original design assumed a centrally applied load, the pile cap, pile-head connection and supporting pile may not have been designed for the additional bending effects.

Possible engineering responses depend on the actual geometry and may include:

• checking whether the existing cap and pile can resist the eccentric loading;
• modifying or enlarging the supporting concrete element;
• introducing appropriately designed ground beams to transfer loads to adjacent supports; or
• providing additional support where required.

Matt describes using ground beams to redistribute loads between pile caps where the original post location cannot be achieved.

That approach requires structural design. The beams, adjacent pile reactions, connections and foundations must all be checked for the resulting forces.

Engineering investigations

Common Screw Pier Foundation Problems

When investigating a house supported on screw piers, the relevant question is not simply whether the piles were installed.

It is whether the complete foundation system has been designed, constructed and maintained in a way that adequately supports the building.

Issues that may require investigation include:

• unexplained slab or footing movement;
• cracking associated with differential foundation movement;
• gaps beneath waffle pod slabs;
• uncertainty about pile depths, locations or installation records;
• insufficient or undocumented lateral restraint;
• offset posts and eccentric pile-cap loading;
• damage or movement affecting pile-head connections; and
• foundation behaviour inconsistent with the original design assumptions.

Assessment may involve reviewing the original engineering drawings, geotechnical report, installation records, site levels, photographs and the observed condition of the building.

Where information is missing, further investigation or testing may be necessary before reliable conclusions can be reached.

Independent engineering

Are Screw Piers a Good Foundation System?

They can be. Screw piers offer a practical means of transferring loads to deeper ground and can be particularly useful where conventional shallow footings are unsuitable.

But their effectiveness depends on appropriate engineering and installation.

The pile’s axial capacity, shaft stability, lateral resistance, connections, supporting concrete elements and interaction with the building must be considered together.

For reactive clay sites, the design must also address how the slab or footing system behaves when the surrounding soil shrinks or swells.

Matt’s central point is that specifying screw piers should not become a shortcut around properly designing the rest of the foundation.

Cornell Engineers

Investigating Screw Pier and Foundation Failures

Cornell Engineers specialises in investigating structural problems, foundation movement and building defects.

We provide independent assessments of suitable complex matters involving screw-pier-supported structures, slab movement, cracking, suspected construction defects and disputed causes of damage.

Our role is to investigate the available evidence, explain the likely causes and provide clear recommendations for further investigation or rectification.

Related reading: What Is Underpinning? and our Industrial Structures services.