The Southern Highlands sits on one of the most geologically varied landscapes in NSW. Drive from Bowral up through Moss Vale and Robertson toward the escarpment and you move through distinctly different soil profiles, from the clay-heavy lowlands of the Wingecarribee valley to the rich, dark basalt-derived soils of the Robertson plateau and Burrawang area, some of the most fertile agricultural land in the state.
That fertility comes with a catch for pool owners. Basalt soil means basalt rock, and rock creates significant challenges when it comes time to remove or fill a pool. This guide explains what makes basalt-country pool removal different, why it can cost more, and what a well-executed backfill looks like when rock is involved.
Quick Answer (BLUF)
Basalt soils on the Robertson plateau and around Burrawang often contain boulders, rock shelves and fragmentary rock at varying depths. When removing a pool, excavation into basalt may require rock breaking equipment, hydraulic rock hammers or even controlled blasting. Backfilling a pool surrounded by basalt requires careful material selection, the angular, free-draining nature of basalt rubble must be accounted for to prevent long-term settlement. Expect 20-40% higher excavation costs on basalt sites compared to standard clay soil sites.
The Geology of the Robertson Plateau
Robertson sits on the southern edge of the Illawarra escarpment at around 700 metres elevation. The area is underlain by Tertiary basalt flows, the same volcanic geology that created the fertile “black soil” dairy country across the plateau. This basalt weathers into a dark, humus-rich clay-loam at the surface (the famous black Robertson soil), but the underlying basalt rock can be encountered at depths as shallow as 300mm in some areas or as deep as several metres in others.
The variability is the challenge. A pool might be installed without hitting rock at all, or the excavation might have skirted around a rock shelf with the pool sitting partly on rock and partly on soil. When you come to remove the pool decades later, what’s beneath it is often a surprise.
How Basalt Rock Complicates Pool Removal
The Excavation Stage
For a full pool removal, the shell must be broken up and excavated. In clay or sandy soils, this is straightforward, excavators can dig down, break the shell and remove material efficiently. Where basalt rock is present:
- The machine hits rock at unexpected depths while excavating around the shell edges. Progress slows significantly.
- Rock hammers are needed. A standard excavator bucket can’t break rock, a hydraulic breaker attachment (rock hammer) must be used. This requires either swapping attachments on a tracked excavator or bringing in a dedicated breaker machine. Day rates for hydraulic rock hammers are 30-50% higher than standard buckets.
- Disposal costs rise. Basalt rock is heavy. Even if it’s being used as fill material on-site, the additional volume of rock to be managed increases handling time.
- Unexpected delays occur. A job quoted for two days can extend to three or four when substantial rock is encountered.
A good contractor will inspect the site thoroughly before quoting and ask about the pool’s original installation, whether rock was encountered during the original dig. Old photos of the pool construction, if you have them, can give useful clues. See our full pool removal service page for what the process involves.
The Backfill Stage
Backfilling a pool cavity in basalt terrain requires particular care. The pool excavation sits in a mix of:
- Loose soil and debris from demolition
- Angular basalt rock fragments
- The crushed pool shell material itself (for concrete pools)
The challenge with basalt rock as backfill material is that angular rock creates large voids between pieces that don’t self-compact like soil does. Over time, as the rock settles and those voids compress (a process that can take several wet-dry cycles), you get surface subsidence, the ground above the old pool sinking or cracking.
A properly managed backfill for a basalt-country pool removal typically involves:
- Layered compaction. Backfill is added in 200-300mm layers and compacted with a vibrating plate compactor before the next layer is added. This is time-consuming but essential.
- Material selection. Imported sandy loam or select fill is often brought in to blend with the basalt rubble, reducing void space.
- Drainage aggregate. A layer of drainage aggregate (typically 20mm crushed rock) is placed at the bottom of the excavation to manage groundwater, which can be substantial in the wet Robertson and Burrawang area.
- Geotextile fabric. A geotextile layer between the drainage aggregate and the soil backfill prevents fine soil particles from migrating down into the drainage layer over time.
Our soil compaction and settlement guide covers what good backfill practice looks like in more detail.
Clay Soils in the Lower Highlands: A Different Problem
While Robertson and Burrawang sit on basalt, the lower-lying areas of Wingecarribee, Bowral, Mittagong, Berrima and the valley floor toward Moss Vale, are predominantly clay-based soils. Clay presents different challenges: it’s plastic when wet (making excavation sides unstable), it retains moisture and contributes to frost heave, and it shrinks and swells seasonally. The clay soil pool removal guide covers these challenges specifically.
Many Highlands properties sit on a transition between clay and basalt, a clay surface layer over basalt-derived subsoil and rock. These mixed-profile sites require the most careful assessment.
Can Basalt Rock Be Reused On-Site?
Yes, and it often should be. Basalt is a durable, free-draining material that can be used as:
- Sub-base material beneath driveways or hardstand areas
- Fill material on sloping sections of the property
- Drainage fill around French drains or soakage pits
- Garden edging or feature stone if the pieces are appropriate in size
Using rock on-site reduces disposal costs and transport. For rural-residential properties in Robertson and Burrawang where tipping fees and truck access can add significantly to costs, smart on-site reuse of basalt material makes real economic sense. Discuss this with your contractor when getting your pool removal quote.
Cost Implications
On a standard clay or sandy soil site, pool excavation and backfill costs for a full removal might run $2,000-$4,000. On a basalt-rock site in Robertson or Burrawang, allow 20-40% more, sometimes higher if a significant rock shelf is encountered and must be broken out. The variability makes accurate quoting difficult without a site inspection; responsible contractors price basalt sites with a provisional rock-breaking allowance and provide a clear explanation of what triggers additional charges.
FAQs
How do I know if my Robertson or Burrawang property has rock at pool depth?
Ask about the original pool installation, contractors typically hit rock during the original dig and may have documentation. If the pool was built on footings, there’s a good chance rock was encountered. Alternatively, a site inspection by an experienced pool removal contractor who works in the Robertson/Burrawang area will give you a reasonable assessment based on soil profile at the surface and the local geology.
Can I use the basalt rock from the pool area as fill elsewhere on my property?
Absolutely. Basalt rock is excellent fill and sub-base material. If you’re planning any future earthworks, sheds, driveways, retaining walls, retaining the excavated basalt on-site rather than carting it away can save significant money. Discuss storage and placement with your contractor as part of the project planning.
Does basalt rock affect the risk of subsidence after pool removal?
Yes, if the backfill isn’t managed correctly. Angular basalt rubble contains more void space than compacted soil and will settle more over time. The solution is proper layered compaction during backfill and appropriate use of finer material to fill voids. Ask your contractor specifically about their compaction methodology on rocky sites. See our soil settlement guide for what to look for.
Is pool removal in Robertson or Burrawang harder to access than in Bowral?
Generally, yes. The Robertson plateau and Burrawang area are more rural and the road access to many properties involves steep, narrow roads. Machine access can be limited and transport distances from the closest equipment depot are greater. These factors add to project costs compared to Bowral or Mittagong properties. Our rural property access guide covers access considerations in detail.