Acid Sulfate Soils On Sydney Development Sites

Acid sulfate soils are natural sediments holding iron sulfides that stay harmless while they sit under water, and turn into sulfuric acid once they are exposed to air. On a Sydney development, that exposure comes from digging them up or from pumping down the water table, so the risk lands on basements, deep trenches and dewatering.

You can't see the problem in a fresh excavation. Most of the time, potential acid sulfate soil looks like any other dark, wet, low-lying mud or sand until the lab results come back.

Where they turn up

These soils form in low-lying coastal land, along floodplains, estuaries, rivers and creeks. They usually sit below 5 metres above sea level, measured as Australian Height Datum, and beneath the water table.

In Sydney that points to the low ground around Botany Bay, the Georges River, the Cooks River and the Parramatta River, plus the flats along creeks that feed them. A site on a sandstone ridge won't have them. A site on a former wetland, a few hundred metres from that ridge, can.

How the five classes work

Councils across coastal Sydney map this risk in their Local Environmental Plan, with land sorted into five classes. The class tells you which works need development consent because of the soil.

Class 1: any works.

Class 2: works below the natural ground surface, or works that are likely to lower the water table.

Class 3: works more than 1 metre below the natural ground surface, or that lower the water table by more than 1 metre.

Class 4: the same test at 2 metres.

Class 5: works within 500 metres of Class 1 to 4 land that is below 5 metres AHD, where the water table on that adjoining land is likely to drop below 1 metre AHD.

Class 5 catches people out. A basement on higher ground, sitting on land that looks safe on the map, can still trigger the clause if its dewatering pulls down the water table next door. The wording differs slightly between councils, so we read the clause in the LEP that applies to the site.

The map only tells you the soils are possible. It says nothing about what is really under your site, and that is the job of the assessment.

The assessment

The first step is a preliminary assessment, prepared in the way the 1998 Acid Sulfate Soils Manual sets out. It looks at how deep the works go, how the groundwater will be affected, and whether soils on the site test as acid sulfate.

Samples come from the same boreholes as the [geotechnical investigation](/blog/understanding-geotechnical-investigation-reports), taken down through the depth of the proposed works and sent to a laboratory. If the soils are there and the works will disturb them, a management plan follows. We carry out the assessment as part of our geotechnical scope, and a specialist environmental consultant writes the management plan, which we then coordinate with the shoring and excavation design.

What it changes in the engineering

Shoring and water. A dewatering pump lowers the water table beyond the site boundary, and that can expose acid sulfate soils on land you don't own. On these sites, a cut-off wall that keeps the water table where it is often makes more sense than pumping. The options are compared in [dewatering and groundwater control in excavations](/blog/dewatering-and-groundwater-control-in-excavations).

Concrete and steel. Acid attacks concrete and corrodes reinforcement, buried pipes and steel piles. We check the soil and groundwater results against the exposure tables in AS 2159 when we design the piles, then carry the same results into the cover and concrete grade for the basement walls and slab. The reasoning is in [concrete cover and why it decides durability](/blog/concrete-cover-and-why-it-decides-durability).

Permanent basements. A drained basement that pumps forever keeps the water table down forever. A tanked basement, sealed against the water, avoids that, and it needs designing for the uplift described in [basement tanking and hydrostatic uplift](/blog/basement-tanking-and-hydrostatic-uplift).

Spoil. Excavated acid sulfate soil is treated, usually by blending it with lime, and classified before it leaves site. That takes space on the site and time on the programme, and a tight CBD-style site doesn't have much of either.

Water discharge. Water pumped from the excavation gets tested, and treated if needed, before it goes anywhere near the stormwater system.

Timing on the programme

An assessment is a small job at the start of a project and a very large one in the middle of it. If the LEP clause applies to your works, the council will want it with the DA.

We raise the map class in the first review of any low-lying site, along with the depth of the basement, because those two facts together tell us whether the question needs answering. Finding acid sulfate soils during bulk excavation stops the trucks, and the soil keeps reacting with the air while everybody waits for a plan.

How we approach it

We treat the map, the boreholes and the groundwater as one question, and answer it before the shoring type is locked in. Where the water table has to stay up, we design the walls and the basement around that from the start.

Soil like this has sat quietly under water for thousands of years, and it only becomes a problem on the day somebody lets the air in.

ACSES provides geotechnical engineering, including acid sulfate soil assessment, across Sydney. Talk to us about a project.

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