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.