Pool removal on clay soil in the Southern Highlands requires a different backfill specification than a standard sandy suburban lot, because reactive clay shrinks in dry summers and swells in wet winters in a cycle that undermines compacted fill if the work is not done correctly. Getting this right from the start is what separates a pool site that settles neatly over one season from one that subsides, humps or cracks for years afterwards. Our pool excavation and backfill service accounts for the actual soil beneath your property, not a one-size-fits-all spec, and the Highlands soil map is varied enough that the right approach for Moss Vale looks different from what suits Bundanoon.
The Highlands clay belt, where it dominates
Wingecarribee Shire sits on several distinct geological units, and soil reactivity varies considerably across the region. The broad pattern is:
- Moss Vale and the western Highlands: Heavy red-brown clay derived from Permian shales. Typically Class H to Class E reactivity under AS 2870.
- Bowral and Mittagong: A mix of clay and sandy loam over Hawkesbury sandstone. Class M to H depending on the block and its micro-topography.
- Robertson and Barrengarry plateau: Basalt-derived soils, dark, rich and fertile but very plastic when wet.
- Bundanoon and the eastern escarpment fringe: Lighter sandy loam over sandstone. Class S to M, lower reactivity.
Two properties 20 kilometres apart can need quite different backfill specifications. A site inspection that includes a soil assessment is essential before any pool removal pool removal clay soil Southern Highlands quote is finalised.
How reactive clay damages pools
Clay soils expand as moisture content rises and contract as they dry. This cyclical movement is the reason many Highlands pools develop cracks, stepped joints and shell distortion well before they reach the end of their expected life. A pool shell is a relatively rigid structure, it cannot flex with significant ground movement without cracking.
Pools installed into Class H or E clay without adequate bedding and perimeter drainage will often show structural damage within 5 to 15 years. By the time an owner decides on removal, the shell is usually already cracked and displaced by the same ground movement that was causing slow leakage long before any visible sign appeared.
Why clay backfill alone is a bad idea
When a pool is demolished, the void left behind is typically 20 to 80 cubic metres. The obvious instinct is to refill with the excavated material, it came from there, after all. On a reactive clay site, this approach leads to problems.
Recompacted clay fill will:
- Develop shrinkage cracks through summer, creating small internal voids
- Swell and heave during winter as moisture reactivates the clay minerals
- Never compact to the same density as undisturbed native soil
- Create a zone of differential settlement that damages adjacent paving, fence lines and structures
The practical result is a pool site that settles unevenly over 2 to 5 years, forming a depression in the wet season and humping slightly in a dry summer. This is a common complaint on Highlands properties where cheap pool removals used excavated clay as the sole backfill.
Granular fill and geotextile layering
The correct approach on reactive sites uses imported granular fill, typically crushed rock or road base, placed in compacted layers of no more than 200mm. A geotextile separation fabric is placed between the granular fill and any remaining clay subgrade to prevent clay migration up into the fill column over time.
AS 2870, residential slabs and footings provides the site classification framework that should inform backfill design on reactive soils.
| AS 2870 soil class | Reactivity | Recommended backfill |
|---|---|---|
| Class S | Slightly reactive, sandy | Native fill with layer compaction acceptable |
| Class M | Moderately reactive clay | Blend of granular and conditioned native fill |
| Class H1 | Highly reactive clay | Full granular fill, geotextile liner |
| Class H2 | Very highly reactive | Full granular fill, drainage layer, geotextile |
| Class E | Extremely reactive | Engineered fill, geotechnical review required |
For pools on Class H and E sites, which are common through Moss Vale and Robertson, a full granular fill is the appropriate specification. It costs more in materials than using excavated clay, but the long-term result is a stable site.
Drainage that actually works on clay
Granular fill placed in a clay-lined void will still retain water unless drainage is addressed at the base. Without an outlet, the granular fill column acts as a collection sump: it fills with water in winter and slowly dries in summer. This moisture cycling can still cause differential movement even in granular fill if there is no path for water to escape.
The standard approach is a perforated ag pipe at the base of the excavation, wrapped in geotextile filter fabric, connected to a drainage outlet at the property boundary or a soakage pit in a non-reactive zone. Our pool excavation and backfill service includes drainage design for reactive sites as part of the standard scope.
Future build-over considerations
If there is any chance you may want to build over the filled pool site later, whether a shed, patio, extension or granny flat, the backfill specification now directly determines what is structurally possible. See our dedicated building over a filled-in pool guide for what geotechnical requirements apply to any future structure.
On Class H and E sites, a geotechnical engineer’s report will typically be required before any slab is designed over filled ground. The fill type and compaction records must be documented. Specifying granular fill from the outset keeps your options open; using compacted clay may require re-excavation before any future building work can begin.
A partial pool removal fill-in on a reactive clay site still requires the same backfill specification for the crushed shell section, the clay soil does not behave differently just because the concrete shell is partially retained.
Signs your site has reactive clay
Not sure what you have? Some field indicators worth looking for:
- Cracks in paths, driveways or brick walls that open through summer and close in winter
- Pool shell that has moved laterally, shows stepped joints, or has wide surface cracks
- Ground that becomes sticky and adhesive when wet, then hard, cracked and powdery in dry conditions
- Previous garden landscaping that has heaved or settled unevenly over time
If your property shows two or more of these signs, assume a Class H soil profile and specify accordingly. The cost difference between a Class M and Class H backfill specification is modest relative to the cost of digging the site out again in five years.
Winter removal adds another layer of complication on clay, see our pool removal in winter guide for how season timing interacts with clay behaviour. And if your plan after removal is a productive garden, our guide to converting a pool site to a garden or orchard covers the soil depth and drainage requirements for growing above compacted fill.
FAQs
Can I backfill the pool void with the clay I dug out?
Not recommended on reactive clay sites, which are common through Moss Vale and Robertson. Recompacted excavated clay will crack in dry conditions, swell in wet, and never consolidate to the density of undisturbed soil. Imported granular fill with a geotextile liner is the correct specification for Class H and E profiles.
Does the fill type affect a future slab or structure over the pool site?
Yes, significantly. Granular fill that has been compacted in layers and documented is far better as a base for a future slab than recompacted clay. A geotechnical engineer will review the fill specification and compaction records before designing any footing over old pool ground. See our building over a filled-in pool guide for details.
How does clay soil affect the pool removal process timeline?
Reactive clay can extend the pool removal process timeline by requiring extra compaction passes and longer curing between fill layers. A site that would take 2 days on sandy loam may take 2.5 to 3 days on heavy clay, particularly in winter when compaction takes longer to achieve specified density.
What’s the typical extra cost for granular fill vs clay backfill?
A typical uplift is $800 to $2,500 for a standard pool, depending on haulage distance and local aggregate prices. This is often worth spending, particularly on Class H sites, to avoid ongoing settlement issues and keep future build-over options open. Ask your contractor to specify fill type in the written quote.
Does frost make clay backfill worse?
Yes. Frost can cause heave in saturated fill, particularly on reactive clay sites. Our frost heave and backfill guide explains how to specify fill and schedule works to minimise freeze-thaw damage on Highlands properties.