AS 2870-2011: A Practical Guide to Residential Slabs and Footings in Australia

Introduction to AS 2870-2011

AS 2870-2011 is one of the key Australian Standards used in the design and construction of residential slabs and footings. The standard provides requirements and recommendations for the design of residential footing systems, particularly where soil movement can affect the performance of a building.

For builders, engineers, designers and homeowners, understanding AS 2870-2011 Residential Slabs and Footings is important when planning a new residential development or assessing foundation requirements.

The standard focuses on how residential footing systems should be designed to accommodate expected ground movement and reduce the risk of excessive cracking, distortion and structural damage.

What Is AS 2870-2011?

AS 2870-2011 is the Australian Standard titled Residential Slabs and Footings. It establishes principles for the design and construction of footing systems for residential buildings.

One of the most important concepts within the standard is the classification of residential sites according to their expected soil movement.

Australian residential sites can experience significant changes in ground conditions because of factors such as:

• Reactive clay soils
• Changes in soil moisture
• Seasonal weather conditions
• Tree root activity
• Poor drainage
• Site filling
• Groundwater conditions
• Changes to surrounding landscaping

Because soil behaviour varies significantly from one location to another, foundation design needs to consider the specific characteristics of the site.

Why Soil Classification Matters

Soil classification is a fundamental part of residential footing design under AS 2870-2011.

Reactive soils can expand when they absorb water and shrink when they dry. This repeated movement can place stress on residential slabs, footings, walls and other building elements.

AS 2870-2011 uses site classifications to describe the expected ground movement characteristics of a residential site.

Common classifications include:

• Class A – Generally stable sites with little or no ground movement
• Class S – Slightly reactive sites
• Class M – Moderately reactive sites
• Class H – Highly reactive sites
• Class E – Extremely reactive sites
• Class P – Problem sites requiring special consideration

The appropriate site classification can influence the type of footing system required and the engineering approach used for the project.

Reactive Soil and Residential Foundations

Reactive soil is one of the major considerations when designing residential foundations in Australia.

Clay soils can change volume as their moisture content changes. During wet periods, clay may swell. During dry periods, it may shrink.

This movement can result in differential movement beneath a building.

If different parts of a residential slab move by different amounts, cracking may occur in:

• Internal walls
• External brickwork
• Plasterboard
• Floor finishes
• Windows and doors
• Tiled areas
• Concrete slabs

The purpose of appropriate footing design is not necessarily to eliminate all movement. Instead, the design should account for reasonably anticipated soil movement and provide a footing system suitable for the site conditions.

Common Residential Footing Systems

Residential foundation systems may include different forms of slabs and footings depending on the site and building requirements.

Common systems include:

Slab-on-Ground Foundations

A slab-on-ground system uses a reinforced concrete slab constructed at or near ground level. The slab and supporting ground work together as part of the foundation system.

The design needs to consider factors such as soil reactivity, slab geometry, reinforcement, edge beams and internal beams.

Stiffened Raft Slabs

A stiffened raft slab is commonly used for residential construction and incorporates beams and reinforcement to improve the structural performance of the slab.

The configuration of the slab depends on the site classification, building loads and other design considerations.

Strip Footings

Strip footings can be used to support walls and other structural elements. Their suitability depends on the site conditions and the overall structural design.

Pier and Beam Systems

On some sites, particularly where ground conditions present challenges, a footing system incorporating piers may be considered.

The appropriate solution should be determined by a qualified engineer based on site-specific conditions.

Site Investigation Before Foundation Design

A reliable foundation design