When a pool is removed from a Southern Highlands property, the visible result, a level, restored garden surface, can look complete within a day of the final site tidy. But what happens below the surface over the weeks, months and years following removal matters enormously for the long-term integrity of the area.
Understanding soil compaction and settlement after pool removal is important for homeowners who plan to use the space for landscaping, construction, or any purpose where ground stability matters. This guide explains the mechanics of settlement, what good compaction practice looks like, and what you should expect (and watch for) in the months after removal.
Quick Answer (BLUF)
Some degree of settlement after pool removal is normal, expect minor surface undulation within the first year as the backfill consolidates. Settlement should be minor if the backfill was properly compacted in layers. If settlement is significant (more than 50-100mm of surface drop) or uneven, it suggests inadequate compaction methodology. In the Southern Highlands’ clay and basalt soils, proper layered compaction and drainage aggregate at the base are critical. Any structure, paving, shed, pergola, should not be installed on the filled area until at least six months of settlement has occurred.
Why Settlement Occurs
When a pool is excavated, the surrounding soil is in its natural, undisturbed state, it’s been compacting under its own weight for thousands of years and is stable. The pool shell holds the excavation shape while it’s in place.
When the shell is removed and the excavation is filled with backfill material, that material is in a loose, disturbed state. No matter how well it’s compacted during installation:
- The individual particles within the fill re-arrange under load and moisture cycling
- Air voids gradually compress under gravity and the weight of any surface loading
- Moisture cycling (wet-dry seasons) causes fine particles to migrate and resettle
- In the Southern Highlands specifically, frost action (freeze-thaw cycling) causes additional micro-movement in the near-surface zone
All of this adds up to measurable settlement over time. The question is not whether settlement will occur, but how much and over what timeframe.
What Good Compaction Practice Delivers
A backfill that’s been compacted properly to engineering standards will typically achieve:
- Less than 25mm of surface settlement in the first year
- Stable surface suitable for lawn and garden use within weeks of completion
- Sufficient bearing capacity for standard garden structures after 6-12 months
Poor compaction (filling the excavation in a single lift without compaction) can produce:
- 100-300mm of surface settlement in the first year
- Uneven settling creating low spots and tripping hazards
- Ongoing settlement continuing for several years
- Insufficient bearing capacity for any structures
The difference is entirely in the methodology: layered placement and compaction vs bulk dumping.
Layered Compaction: What It Means in Practice
Proper backfill compaction for a pool removal cavity involves:
Layer thickness: Fill material is placed in 200-300mm lifts (layers). Each lift is compacted before the next is added. This means a 1.5m deep excavation requires five to seven separate lift-compact cycles, not a two-hour job.
Compaction equipment: A vibrating plate compactor (walk-behind) is suitable for smaller areas. A vibrating roller or drum compactor achieves better compaction in the centre of larger excavations. The equipment must reach all areas of the excavation.
Fill material type matters: Uniform, well-graded material (sandy loam, crushed road base) compacts better than variable material. Angular crushed concrete rubble compacts differently from fine soil. Good fill is a mix of fine and coarse particles.
Moisture content: Compaction is most effective when the fill is at optimum moisture content, not bone dry (particles don’t bind) and not saturated (fills with water and compacts poorly). In the Southern Highlands’ variable climate, managing moisture during backfill can require judgement and sometimes pausing during wet weather.
Southern Highlands Soil-Specific Issues
Clay Soils (Bowral, Mittagong, Moss Vale)
Clay soils in the lower Wingecarribee are highly plastic, they shrink when dry and swell when wet. This shrink-swell behaviour means that:
- The surrounding undisturbed soil is moving seasonally, and the fill material must accommodate this movement
- Backfill that’s placed when the surrounding clay is in a wet, swollen state may develop surface cracks during the subsequent dry season as the clay shrinks
- Drainage is critical, backfill must not become a sump that traps water, as saturated clay around and beneath the fill causes uplift and instability
The recommended approach in clay soil areas is to use select fill with good drainage characteristics rather than excavated clay as the backfill material, and to install a drainage aggregate base layer at the bottom of the excavation.
Our clay soil pool removal guide covers these issues in more detail.
Basalt Soils (Robertson, Burrawang)
Basalt rock fragments create large air voids in the fill profile. These voids are resistant to initial compaction but collapse over time as the rock fragments shift. The solution is blending, mixing basalt rubble with finer material to fill the inter-particle voids before compaction.
See our basalt soil guide for specific advice on Robertson and Burrawang properties.
Frost Action (All Highlands Elevations)
At higher elevations (Robertson, Burrawang, Bundanoon), frost penetrates the near-surface soil to depths of 150-300mm in severe winters. This freeze-thaw cycling moves near-surface soil particles annually. The effect on a newly filled pool area is most pronounced in the first two to three winters, after which the material has reached a more stable state. See the frost heave guide for detailed coverage.
Compaction Testing
Your certifier may require formal compaction testing as a condition of the CDC or DA approval. The two most common methods are:
Nuclear Density Gauge (NDG) Test: Measures the density and moisture content of the compacted fill at a specific point. Quick, accurate, non-destructive.
Dynamic Cone Penetrometer (DCP) Test: A cone is driven into the fill by a standard hammer. The rate of penetration indicates the bearing capacity of the material.
If compaction testing is required, the results must meet specified benchmarks (usually expressed as a percentage of maximum dry density under Australian Standard AS 1289). Your certifier will specify the standard required for the anticipated future use of the area.
What to Watch for After Removal
In the first 12 months after removal, monitor the filled area periodically:
- Surface undulation: Minor undulation (under 25mm) is normal and self-corrects as the fill consolidates. More than 50mm of undulation suggests under-compaction.
- Cracking: Fine surface cracks in clay-based topsoil are normal in dry summers. Significant cracks (more than 5-10mm wide, or extending more than 100mm deep) warrant investigation.
- Waterlogging: The filled area should drain normally after rain. Persistent waterlogging suggests the drainage aggregate layer or topsoil drainage isn’t functioning, may need investigation.
- Surface level change: If you mark the corners of the filled area at completion and check again at 6 and 12 months, you can track total settlement.
FAQs
How long before I can lay paving over the old pool area?
A minimum of six months is advisable before installing any paving. Twelve months is better. Paving locks in the surface level, if you pave over a fill area that hasn’t finished settling, you’ll have cracked or uneven paving. For any substantial paving project, consider a compaction test before proceeding.
Can I build a garden shed over the old pool area?
Yes, but not immediately and not on a simple surface slab. After six to twelve months of settlement, engage a structural engineer to assess the compaction and design appropriate footings. A shed with standard steel frame footings (point loads) concentrates load on small areas, the engineer will determine whether the fill’s bearing capacity is sufficient. See our building over a filled pool guide for the full process.
My pool was filled several years ago, should I be worried about settlement?
If the fill was done correctly and you haven’t noticed significant ongoing settlement, the fill has probably reached a stable state. Minor undulation that hasn’t changed in two to three years is typically fine. If you’re planning to build over the area, get a compaction test to confirm the current bearing capacity regardless of how it looks visually.
How is settlement different for a partial fill-in vs a full removal?
In a partial fill-in, the pool shell remains as a structural container. Settlement within the shell is contained by the shell walls and typically appears as a depression within the shell footprint rather than spread across the whole area. However, the shell itself may shift or crack over time in the Southern Highlands’ cold, wet conditions, see our partial fill-in guide for the implications.