If you are developing land or building in coastal areas of South East Queensland—particularly the Gold Coast, Sunshine Coast, or low-lying parts of Brisbane and Moreton Bay—you may be required to complete an Acid Sulfate Soil (ASS) investigation.

What are Acid Sulfate Soils?

Acid sulfate soils are naturally occurring soils containing iron sulfides. When left undisturbed beneath the water table, they are harmless. However, when exposed to air through excavation or drainage, they react with oxygen to form sulfuric acid. This acid can severely damage concrete and steel structures, harm the environment, and kill aquatic life.

When Do You Need an ASS Test?

You typically need an ASS investigation if your project involves:

• Excavating more than 100 cubic metres of soil in a mapped ASS risk area.
• Filling land with more than 500 cubic metres of material with an average depth of 0.5 metres or greater.
• Extracting groundwater or lowering the water table.
• Developing land below 5 metres Australian Height Datum (AHD).

Your local council or private certifier will advise if an ASS investigation is required as part of your Development Application (DA). Express Soil Testing provides preliminary ASS assessments to determine if a detailed investigation and management plan are necessary.

If you are planning to build a new home, add an extension, or construct a granny flat in Brisbane, one of the first steps you will need to take is securing a soil test and site classification report.

Why is it Required?

Under the National Construction Code and Australian Standard AS 2870, a site classification report is mandatory. It tells your engineer and building designer how reactive the soil is—meaning how much it will shrink and swell with moisture changes. This information is critical for designing a safe, compliant foundation system that won’t crack or fail over time.

What Does the Process Involve?

A trained field technician will visit your site to drill boreholes and collect soil samples. We assess the soil profile, bearing capacity, and reactivity. The entire site visit usually takes about an hour.

What Do You Get?

You receive a certified AS 2870 Site Classification Report. This document assigns a classification (such as Class M for moderately reactive, or Class H for highly reactive) and provides the geotechnical data your engineer needs to finalise the slab and footing design. Your certifier will require this report before issuing building approval.

Acid sulphate soils (ASS) are a significant environmental and construction concern across coastal South East Queensland. When ASS is disturbed — through excavation, drainage changes, or development — it can release sulfuric acid, causing severe structural and environmental damage.

Key Triggers for Investigation

Local councils across SEQ (including Brisbane, Gold Coast, and Sunshine Coast) have strict planning schemes regarding ASS. An investigation is generally triggered when development occurs in mapped risk areas (typically below 5m AHD) and involves:

1. Excavation: Removing soil, such as for basement car parks, swimming pools, or deep trenching for services.
2. Filling: Placing significant amounts of fill on the site, which can displace groundwater and expose underlying ASS.
3. Dewatering: Lowering the groundwater table temporarily or permanently during construction.

The Investigation Process

If triggered, a preliminary ASS assessment is required. This involves drilling boreholes, collecting samples at specific depth intervals, and conducting field screening tests. If field tests indicate the presence of ASS, samples are sent to a NATA-accredited laboratory for detailed analysis. Based on these results, an ASS Management Plan may be required to safely neutralise the soil during construction.

Since 1 July 2019, the Queensland Plumbing and Wastewater Code has required a site classification and soil test for new plumbing work on both new and existing projects. The change ensures that plumbing and drainage systems are designed to withstand the specific soil conditions of the site.

Why the Change?

Highly reactive soils (which shrink and swell significantly) can cause underground pipes to move, bend, or break. By requiring a soil test, plumbers and engineers can specify appropriate flexible joints, lagging, and bedding materials to prevent pipe failure and costly repairs.

When is it Required?

A soil test is now required for:

• All new dwellings.
• Extensions or additions that involve new underground drainage.
• Installing new on-site wastewater treatment systems.

How We Help

Express Soil Testing provides the necessary AS 2870 site classification reports required for plumbing approvals. If your project is in an unsewered area, we can also complete the required soil percolation testing and wastewater design in the same site visit, saving you time and money.

A soil test and site classification identifies a range of conditions that must be addressed before construction. One of the most common issues in Brisbane and surrounds is reduced bearing capacity.

What is Bearing Capacity?

Bearing capacity is the maximum load or pressure that the soil can support without failing or settling excessively. If a building’s load exceeds the soil’s bearing capacity, the foundation can sink, leading to severe structural damage.

Common Causes in SEQ

Reduced bearing capacity is frequently encountered due to:

• Uncontrolled Fill: Sites that have been levelled with fill material that was not properly compacted or certified.
• Soft Alluvial Soils: Common in low-lying areas near rivers and creeks.
• High Moisture Content: Poor drainage or high groundwater tables can soften the soil profile.

The Solution

When a soil test identifies low bearing capacity, the site is typically classified as Class P (Problem site). This doesn’t mean you can’t build — it simply means standard footing designs cannot be used. Your structural engineer will use the data from our report to design a site-specific foundation — such as deeper piers, screw piles, or a stiffened raft slab — to safely transfer the building’s load to stronger soil layers below.

Every new building, extension, or development project in Australia requires a soil test and site classification before construction begins. This requirement was introduced in the early 1990s in response to widespread foundation failures and cracked homes.

The Core Purpose

The primary purpose of a soil test is to determine the soil’s reactivity — how much it will expand when wet and shrink when dry. This movement exerts significant force on concrete slabs and footings, and without knowing the soil type, a safe foundation cannot be designed.

The AS 2870 Standard

Our investigation results in a site classification under Australian Standard AS 2870. The site is assigned a class:

• Class A, S, or M: Non-reactive to moderately reactive soils.
• Class H1 or H2: Highly reactive soils requiring stronger foundations.
• Class E: Extremely reactive soils.
• Class P: Problem sites (due to fill, soft soil, trees, or poor drainage) requiring custom engineering.

Protecting Your Investment

Without a soil test, an engineer would have to guess the foundation requirements. Guessing low leads to cracked walls and structural failure. Guessing high leads to massively over-engineered slabs that cost thousands of dollars more than necessary. A soil test provides the exact data needed for a safe, cost-effective design — and is a mandatory requirement for your building approval.