Understanding AS 2870:2011 – Residential Slabs and Footings Standard

The Standard is essential in the construction industry to ensure the stability of residential foundations and is aimed at engineers, builders, building designers, concreters, carpenters, building certifiers, surveyors, manufacturers, suppliers and building inspectors.

This blog will cover the Standard’s purpose, key requirements, and why it is critical in residential projects.

What is AS 2870:2011?

The AS 2870—residential slabs and footings Standards set out:

• the methods for classifying a site, and
• guidelines for designing and constructing

-         slab-on-ground and footing systems,

-          stiffened rafts,

-          waffle slabs,

-          strip footings,

-          pad footings, and

-          piled footings

for a single-dwelling house, townhouse, or the like, which may be detached or separated by a party wall or common wall but not situated vertically above or below another dwelling.

It consists of six sections:

1. Scope of the Standard, application, performance requirements for footings and defined terms.
2. Required methods for undertaking site classification and site investigation.
3. Requirements for footing system designs: slab-on-ground, stiffened rafts, waffle slab, strip footings, pad footings, and piled footings.
4. Designs of footing systems by engineering principles.
5. Specific requirements for footing systems such as drainage, reinforcement placement, concrete grade and cover, vapour barriers and dealing with aggressive soils.
6. Construction of footing systems on filled sites and additional requirements for reactive soils.

The Standards address problem areas identified since the last revision, including:

• footing performance,
• normal and abnormal sites (such as zero lots),
• site classification and impacts of construction,
• guidance for concrete strengths,
• review of reinforcement detailing requirements,
• review of plumbing requirements, and
• soil reactivity and managing soil movement,

to ensure the structural systems of residential buildings remain stable and prevent damage to homes. 

Soil Classification in AS 2870:2011

The soil classification under AS 2870 is based on their ability to change volume in soil moisture. Extreme weather in Australia, such as drought and floods, can leave soils prone to cracking or rapid expansion, which means an adequately designed foundation is critical.

Seven site classifications in AS 2870 indicate how reactive soil may be to moisture changes. These are:

1. Class A – typically sites with sand or rocks with little to no movement due to changes in moisture.
2. Class S – Slightly reactive – slight ground movement from fluctuations in moisture.
3. Class M – Moderately reactive – may experience moderate ground movements from fluctuations in moisture.
4. H1 – Highly reactive – may experience high levels of ground movement.
5. H2 – Highly reactive – may experience very high levels of ground movement.
6. E – Extremely reactive – may experience extreme ground movement from changes in moisture.
7. P – Problematic – are sites with filled materials that can vary widely across the site. Class P sites are typically composed of clay, sand, and silt and may have abnormal water conditions.

A lab technician tests a soil sample from the site to determine its texture and how it will react to various environmental conditions. Then, as part of foundation design, a site classification report is prepared, which will guide the type of footing or foundation that can be constructed based on the site's soil classification. Foundations must be designed based on the site classification report to avoid failure and damage to the foundation. 

Key Design Requirements of AS 2870:2011

The key changes in the AS 2870 Australian Standards regarding slabs and footings include:

• Revised slab footing designs, including mesh upgrades for slab lengths exceeding 20m external beam continuity at re-entrant corners and additional top reinforcement.
• Extensive recommendations on design and construction to minimise the effects of aggressive soil, such as high-saline soils and acidic soils on slabs.
• Extensive footing designs for the effects of trees on the site.

A qualified engineer will determine the type of foundation or slab used based on the site classification and the recommendations in the Standards.

Types of foundations include:

• Slabs,
• Stiffened rafts,
• Waffle rafts,
• Strip footings,
• Pad footings,
• Pier or pile footings, and
• Raised sub-floors.

Types of slabs include:

• Slab-on-grade – the simplest slab designed to be cast directly on soil or rock.
• Waffle slab – contains reinforced concrete edge beams and strip footings that form a grid on the slab for support, hence the name. This slab type doesn’t have good drainage and is unsuitable for highly reactive and expansive soils.
• Raft slabs are cast–in–place slabs with large internal footings to support columns. They are most suited to commercial buildings and large homes with a large area and low bearing capacity.

A typical residential building will have a slab 85-385mm thick. The engineer will analyse the site, existing conditions, and the loads applied on the slab to determine the slab type and thickness. For example, if the site contains dry, sandy soil that is not subject to movement, it will require a thinner, simpler slab. A site with reactive clay may need a thicker slab and, if the load it will carry is significant, may need heavier steel reinforcements.

The Standards contain detailed slab and footing design recommendations to cater to different soil conditions.

Performance Criteria for Residential Footing Systems

The performance criteria in AS 2870 ensure that residential foundations are designed to handle soil movement and resist damage. According to the Standards, footing systems that comply with the Standards are:

1. Not subject to abnormal moisture conditions, and
2. Maintained so that abnormal moisture conditions do not develop.

Examples of events that may cause abnormal moisture conditions include:

• Removal of trees prior to construction or having trees too close to a footing.
• Unusual moisture conditions caused by drains, channels, ponds, and dams.
• Failure to maintain site drainage.

Examples of maintenance to prevent abnormal moisture conditions from developing include:

• Providing proper drainage.
• Restriction of trees and shrubs near the foundation.
• Prompt repair of any leaks, such as leaks in plumbing or stormwater drainage.

While the Standards acknowledge that shifting will occur over time, it is important to repair any damage promptly. Following the recommendations will help mitigate building cracking, shifting, and long-term damage.

Who Needs to Follow AS 2870:2011?

The AS2870 applies to everyone involved in the building and construction industry, particularly in residential construction:

• Engineers
• Builders
• Building designers and architects
• Concreters
• Carpenters
• Building certifiers and surveyors
• Manufacturers and suppliers
• Building inspectors

It is critical for residential projects to follow the Standards for several reasons:

• To ensure the design and construction of a building is stable and safe.
• To ensure consistent performance of a building.
• To prevent structural failure of a building.
• To avoid litigation due to cracking houses.

If the Standards are not followed, there is a high risk of the site developing “abnormal” moisture conditions, which can lead to structural failure of the building. This damage is catastrophic and very costly to rectify.

Conclusion

Footing design and construction are critical to ensuring a building's performance and structural integrity. However, the movement of soil and the varied weather patterns in Australia present challenges when building a strong foundation for residential buildings.

The AS2870 Australian Standards ensure the investigation, design, construction, and performance of footings in residential construction for the health and safety of people in the buildings and guarantee the quality and performance of buildings to protect the public.

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