Reactive clay soils pool removal is a more involved job than the same pool on sand or gravel: the clay swells when wet and shrinks when dry, and the backfill choices you make today will affect how well the ground behaves under a future lawn, slab or structure for years to come. The Hunter has extensive Class M and Class H clay deposits, particularly across the Maitland floodplain, the Lake Macquarie hinterland and parts of western Newcastle. Our guide on sandy coastal soils covers the opposite end of the soil spectrum; this page focuses on the reactive clay challenge.
Quick answer (BLUF)
On reactive clay sites (Class M, H1 or H2 under Standards Australia AS 2870 residential slabs and footings), pool fill-in requires careful backfill selection, controlled compaction and moisture management during the work. Getting it wrong leads to ongoing differential settlement, surface cracking and, if you plan to build over the fill, problems with any slab or structure. This guide is general information only, not engineering advice. Talk to a licensed geotechnical engineer before committing to a backfill specification on a reactive site.
Soil classes M, H1 and H2 in plain English
Standards Australia AS 1289 methods of testing soils underpins the classification system used in Australia. AS 2870, which governs residential slabs and footings, defines site classes based on the potential ground surface movement (ys) that a soil can produce:
- Class M (moderately reactive): ys between 20 mm and 40 mm. Common across many Hunter residential areas. Manageable with standard engineered fill and controlled compaction.
- Class H1 (highly reactive): ys between 40 mm and 60 mm. Found in heavier clay zones, including parts of the Maitland floodplain and some western Newcastle areas. Requires more careful backfill selection and moisture control.
- Class H2 (very highly reactive): ys above 60 mm. Found in the heaviest black clay zones. Demands engineered design for any future structure above the filled pool.
These classifications matter for pool removal because the on-site clay excavated from the pool void is the most reactive material available. Simply pushing it back in as backfill is rarely the right answer, particularly if you intend to build over the area or want minimal long-term surface movement.
Where reactive clays sit in the Hunter
Reactive clay soils are not evenly distributed across the Hunter. The heaviest concentrations tend to be:
- The Maitland and East Maitland floodplain, where heavy alluvial clay has built up over centuries of river flooding. For pool owners in this area, see our Maitland City Council pool removal rules for the additional approval considerations on the floodplain.
- Parts of Cessnock, Kurri Kurri and the western Hunter Valley, where older geological clay horizons are shallow.
- Pockets across western Lake Macquarie and parts of Newcastle, particularly where natural drainage channels have deposited fine-grained material.
Sandy coastal soils (described in our sandy coastal soils guide) dominate the lake foreshore and coastal areas, but a short distance inland the soil type can shift markedly to heavier clay.
A soil classification report from a geotechnical engineer confirms the site class for your specific block. Our guide on geotechnical reports after pool fill-in explains when a geotech report is worth commissioning and what it covers.
Backfill choices on reactive ground
The key decision on a reactive clay site is what material to fill the pool void with:
Recycled concrete crushed aggregate (RCA) or granular fill: Granular fill does not swell or shrink with moisture changes, which makes it a more predictable base for the surface above. It compacts well and is available from demolition and recycling operations in the Hunter. The pool’s broken concrete can often be crushed on-site or at the tip and reused as part of the fill profile, placed at depth beneath a clean granular layer.
Imported engineered clay: Some specifications use engineered clay placed at optimal moisture content to reduce the reactivity contrast between the fill and the surrounding ground. This approach is more common when the adjacent natural clay must remain undisturbed and you want the fill to behave similarly.
On-site excavated clay returned as fill: Using the excavated material directly as fill is lower cost but higher risk on reactive sites. Placement and compaction must be carefully managed, and moisture content during compaction significantly affects the outcome. In an uncontrolled scenario, returned clay can result in visible surface humping or cracking within one to two seasons as moisture equilibrium is re-established.
The right choice depends on your intended use of the surface after removal. Our pool excavation and backfill service page explains what is included in a standard fill specification.
Compaction lifts and moisture control
Regardless of fill material, compaction on reactive clay sites follows a lift-by-lift approach: fill is placed in controlled layers (typically 150-300 mm per lift, depending on the material and compaction equipment used), and each lift is compacted to the specified density before the next is placed.
Moisture content during compaction is critical on clay sites. Clay placed too dry will compact initially but absorb moisture later, causing swelling and surface heave. Clay placed too wet will be soft during compaction, achieving low density and later consolidating as it dries. The target moisture content is typically near the material’s optimum moisture content (OMC), determined by Proctor compaction testing under Standards Australia AS 1289.
On reactive sites, our post-removal site handover survey guide explains what compaction test results should look like in the paperwork you receive at project completion.
Building over a filled pool on reactive soil
If you intend to build a structure over the filled pool area, reactive clay significantly complicates the footing design. See our guides on pouring a concrete slab over a filled pool, building over a filled pool and building a deck over a filled pool for the options available.
For a slab on reactive clay, AS 2870 governs the footing system design. A structural engineer will typically specify:
- A waffle raft or stiffened raft to accommodate differential movement
- Perimeter and internal beams sized to the site class
- Control joints to accommodate expected shrink/swell movement
- Edge moisture barriers to slow the moisture change at the slab perimeter
The fill placed in the pool void is only part of the system: the transition zone between the fill and the surrounding natural clay is where differential movement is most likely to occur, and the footing design must account for it.
When a geotech report is worth the money
A geotechnical report is not always required for a pool removal, but it is worth the investment when:
- You plan to build a slab, deck or shed over the filled area
- The pool is on a known clay site and you are unsure of the class
- The surrounding house is showing signs of foundation movement (cracking in walls, sticking doors), suggesting the site may be more reactive than average
- The groundwater table is elevated, which can complicate compaction on clay. Our guide on groundwater and pool removal in the Hunter covers this interaction.
- The certifier or council specifies one as a condition of approval
A geotech report typically costs a few hundred to a few thousand dollars depending on scope. The cost of getting the backfill wrong, particularly if you later want to build over the area, is substantially more.
FAQs
What is the most common sign that a pool fill-in on clay has gone wrong?
Uneven surface settlement over the first one to two years is the most common indicator: you may see a slight dip or hump in the lawn where the pool used to be, or surface cracking in paving or paths near the edge. This typically indicates that the fill was placed without adequate compaction control, or that the moisture content during placement was outside the target range.
Can I use the concrete from the demolished pool as backfill on a clay site?
Crushed concrete (RCA) is often a good backfill component on reactive clay sites because it is granular and non-reactive. It is typically used in the lower lift of the fill profile, with a specified clean granular material above it. Do not mix uncrushed large concrete chunks with clay fill, as this creates voids that can cause irregular settlement.
Does reactive clay affect the cost of pool removal?
It can. On sites where imported granular fill is required rather than using on-site material, the fill purchase and delivery adds cost. Tighter compaction specifications may also require more passes with the compaction plant. The typical cost difference is modest compared with the overall project cost, but it is worth including in the quote scope.
How do I find out what soil class my block is?
A geotechnical engineer can conduct a site classification report using bore logs, field testing and laboratory analysis. For a quick indication before commissioning a full report, the local council’s soil type maps or a neighbouring property’s building approval can give an indication, though these are not a substitute for site-specific testing.
Will a reactive clay site affect how quickly I can use the space after removal?
Yes, somewhat. Allow the fill to undergo at least one wet season and one dry season before placing heavy loads (a slab or shed foundation) on the filled area. Light use such as lawn and garden is fine earlier. Discuss the timing with your contractor and, if you have a geotech report, with the engineer.
Related reading
- Geotechnical report after pool fill-in
- Pouring a concrete slab over a filled pool
- Building over a filled pool
- Building a deck over a filled pool
- Post-removal site handover survey
- Sandy coastal soils and pool removal
- Maitland City Council pool removal rules
- Groundwater and pool removal in the Hunter
- Pool excavation and backfill service
- Full pool removal service