Why this is a contract question before it is a landscaping question
Most pages about soil tell you what to build. This one is about what to sign, because the report itself makes the point better than we could. The government's guidance for developing on this ground is that "a survey at a suitable scale as well as geotechnical analysis should be undertaken because of variability of materials throughout the sites", and it advises seeking advice from local councils about localised areas of disturbed terrain. Nobody is suggesting you commission a geotechnical report for a lawn. The relevant part is the reason given: variability of materials throughout the sites.
If the material varies within a site, then the cost of excavating and replacing it varies too, and that cost is roughly 30 per cent of a residential turf job before you count disposal of whatever comes out. So the practical consequence for a homeowner is not "get a soil test". It is this: on Unanderra ground, be suspicious of a firm total price given before anyone has opened the surface, and be equally suspicious of a fixed price that is conspicuously higher than everyone else's, because that is the same risk priced the other way.
What a good Unanderra quote does instead is separate the earthworks from the supply, state what it assumes about the subgrade, and say what happens if that assumption is wrong. That is a normal way to price uncertain groundwork and it protects both sides.
What the fill classification does and does not tell you
It does tell you the ground was reworked to at least a metre. It tells you the landfill component can include "soil, rock, building and waste material" and, in the Wollongong sheet's wording, dredged sand or mud, demolition rubble and industrial or household waste. It tells you the limitations include a subsidence hazard and "soil impermeability leading to poor drainage", and low fertility.
It does not tell you how reactive the ground is, because neither Illawarra sheet's Disturbed Terrain report carries a Surface Movement Potential section. That is a genuine gap in the source data and we leave it as one. It also does not tell you anything about a specific address: the mapping is at 1:100,000 and describes a suburb polygon including roads, parks and rail corridor, not a backyard.
The other 52.7 per cent of Unanderra is not an easy alternative either. Fairy Meadow alluvium at 23.6 per cent brings "high seasonal watertables" and "Low wet bearing strength". Gwynneville at 23.2 per cent brings shallow soils with a low-permeability subsoil. Berkeley at 5.9 per cent is the unit whose reactivity section reads "Highly reactive soils", and it dominates the ground under Figtree lawns a few kilometres north. The full mechanical picture is in our guide to fill, reactive clay and impeded drainage in Unanderra and the western suburbs.
Questions worth asking on the day
- How deep are you excavating, in millimetres, and what are you assuming you will find?
- Is spoil removal in the price, and what happens to the price if the volume doubles?
- What base material are you using, how many compaction passes, and how is the level checked?
- Where does water leave this base, specifically, and what happens to that path in a heavy February?
- What is restraining the perimeter?
Those five questions are worth more on this suburb's ground than any amount of product comparison. Our stage-by-stage account of an Unanderra turf installation explains what a good answer to each sounds like, and the Illawarra square metre pricing guide shows how the earthworks share moves the total.
Unanderra: fill, quotes and the questions that follow
Should I be worried that half my suburb is mapped as fill?
Worried is the wrong word. Prepared is the right one. Disturbed Terrain covers 47.3 per cent of Unanderra, and all the mapping tells you is that the ground there was reworked to at least a metre deep at some point. Plenty of perfectly good lawns sit on fill. What the classification changes is the amount of confidence anyone can have about what is underneath before they open it, and therefore how a price should be structured.
How should an Unanderra quote be worded, then?
Ideally with the excavation and disposal stated separately from the turf supply, and with a stated assumption about what is expected below the surface. If an installer prices a fixed lump sum for the whole job sight unseen, they have either priced in a large contingency or they are carrying a risk they will want to renegotiate later. Neither is dishonest, but you should know which one you are looking at. Ask what happens to the price if the excavation hits rubble, concrete or unexpected material.
How reactive is fill?
We do not know, and neither does the mapping. Neither Illawarra sheet's Disturbed Terrain report includes a Surface Movement Potential section at all, which is the section that would state reactivity. We are not going to borrow a figure from a different map sheet to fill that hole, and you should be sceptical of anyone who quotes you one. What the report does state is that limitations 'are dependent on nature of fill material', which is precisely the point.
What is the other half of Unanderra made of?
Fairy Meadow alluvium at 23.6 per cent, Gwynneville footslope at 23.2 per cent, and Berkeley at 5.9 per cent, with 100 per cent mapping coverage. So a block that is not on fill is most likely on either alluvium with 'high seasonal watertables' or on shallow footslope soils with a low-permeability subsoil. There is no part of this suburb where the ground is simply unremarkable.
Does fill mean the base has to be deeper?
Not automatically, and be careful with anyone who says so as a rule. There is no Australian Standard or government source that sets excavation depth or base build-up for artificial turf as a function of soil type, so any depth figure is trade practice rather than a requirement. Typical practice is around 75mm for a lawn and about 100mm under vehicle traffic. On variable ground an installer may reasonably go deeper, and the right response is to ask why for your site rather than to accept a number as a standard.
Can I lay turf on my Unanderra nature strip?
Wollongong City Council has no published position on synthetic turf out there, and we will not invent one. Its verge guidelines prohibit structures and hard surfaces in a verge garden, require a 1500mm level pedestrian strip and 600mm of kerb clearance, and note that neither Council nor a utility has any obligation to reinstate a verge garden after excavating it. The verge is public land, so speak to Council before you start. Inside your own boundary this does not apply.
Is there anything good about building on fill?
Sometimes, yes. Fill areas have generally been levelled to slopes under 5 per cent, which means access is often easier and the finished levels are simpler to set than on a terraced escarpment block. Where the material happens to be granular and well compacted, it can make a perfectly serviceable subgrade. The problem is never that fill is bad, it is that fill is unpredictable, and unpredictable is expensive to price from a satellite image.
What water rules apply while I still have a natural lawn?
Sydney Water's Water Wise Guidelines. Lawns may be watered before 10am or after 4pm using a hand-held hose with an instant on-off mechanism, a sprinkler or a watering system, while smart and drip systems may run between 10am and 4pm. Run-off onto paths is not permitted. The penalty notice amount is $220 for an individual and $550 for a corporation, and Sydney Water notes it cannot issue a fine unless its authorised staff personally witness the misuse.
On this ground, a measured quote is the only honest one
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