A geotechnical report (also called a soil investigation report or geotech assessment) is a formal document produced by a geotechnical engineer that characterises the soil at a site and makes recommendations for construction. In the context of pool removal, it becomes relevant when the filled-in pool site is going to be used for future construction, or when site conditions create uncertainty about how well the fill will perform over time.
Quick Answer (BLUF)
A geotechnical report is not always required after pool fill-in, but is strongly advisable, and in some cases mandatory, when: you plan to build over or near the filled site in future; your certifier or council conditions the approval on one; the pool is on a steep or unstable site; or the Southern Highlands clay soils on your property create unusual settlement risk. Expect to pay $1,500 to $4,500 for a residential geotechnical report in the Southern Highlands, depending on depth and complexity.
What a Geotechnical Report Is (and Isn’t)
A geotechnical report is:
- A professional assessment by a registered geotechnical engineer of the soil and subsoil conditions at your site
- Based on physical investigation (test bores or test pits, plus laboratory analysis of soil samples)
- A document that makes specific, testable recommendations for foundation design and fill specifications
- Typically required (or at minimum, recommended) as input for any building design on filled ground
A geotechnical report is not:
- The same as a structural engineer’s report (though the two are often used together)
- An automatic part of standard pool removal (it’s an additional engagement)
- Required for all pool removals, only when specific triggers are met
When a Geotechnical Report Is Required After Pool Fill-In
1. You Plan to Build on the Site
If any structure is to be built over or adjacent to the filled-in pool excavation, a geotechnical report is essential. The engineer needs to assess:
- The quality of the backfill material and its compaction level
- The native soil profile around and below the fill
- The load-bearing capacity of the filled zone
- Any differential settlement risk between the fill and native soil
- The appropriate foundation design for the proposed structure
This applies to any building: granny flat, garage, shed, carport, outdoor entertaining area with a slab, or even a decked area over the filled zone.
2. Your Certifier or Council Requests One
For some pool removal approvals in Wingecarribee Shire, the certifier may include as a condition of the CDC or DA that a geotechnical assessment be obtained before or after backfill. This is more common when:
- The site is on a slope
- The soil is highly reactive (classified as H, E, or P under AS 2870)
- There’s evidence of poor soil stability
- The pool is large relative to the lot size
Ask your certifier at the outset whether a geotech report will be required as a condition of approval.
3. The Site Is on a Slope or Near an Unstable Area
For pool fill-in on gradient sites in the Southern Highlands, particularly near the Robertson or Bundanoon escarpments, or on hillside properties in Bowral, the geotechnical engineer assesses:
- Whether the filled excavation could slide on the slope
- What retaining measures are needed
- What drainage specification is needed within the fill to prevent water build-up
See pool removal on steep gradient sites and retaining wall requirements after pool fill-in for related guidance.
4. The Southern Highlands Clay Soil Creates Settlement Risk
Wingecarribee Shire is largely underlain by reactive clay soils derived from basalt. These soils are classified under AS 2870 as highly reactive (H or E class in some locations). A geotechnical engineer can:
- Confirm the actual reactivity classification of your specific site
- Specify appropriate fill material to minimise differential movement
- Recommend foundation types for any future buildings
- Confirm whether the backfill meets the standard required
The guides on pool removal in clay soil and pool removal in basalt soil cover the soil science dimension.
What a Geotechnical Investigation for Pool Fill-In Involves
The physical investigation typically involves:
| Method | Description | When Used |
|---|---|---|
| Hand auger borings | Shallow soil samples to 3-4m depth by hand | Small sites, minimal access constraints |
| Machine boring (rotary or cable tool) | Deep soil samples to 10m+ using a drilling rig | Larger structures planned, deeper investigation required |
| Test pits | Excavation to inspect soil profile directly | Where lateral variation needs to be assessed |
| In-situ testing (SPT, CPT) | Strength testing within the bore hole | For foundation design load calculations |
| Laboratory analysis | Swell, Atterberg limits, moisture content testing | Confirms reactivity classification |
For a standard residential pool removal in the Southern Highlands, the typical investigation is 2-3 hand auger borings plus laboratory analysis, sufficient to confirm the existing backfill condition and native soil profile.
What the Report Recommends
A geotechnical report for pool fill-in typically provides:
- Existing fill assessment: Was the backfill placed correctly? What is its current compaction state?
- Native soil classification: What AS 2870 site classification applies? (A, S, M, H1, H2, E, P)
- Future development recommendations: What foundation type is appropriate for any future structures? What footing depths and bearing capacities apply?
- Drainage recommendations: What subsoil drainage is needed to manage groundwater and moisture?
- Any special provisions: Specific requirements for the site due to unusual conditions
This report then feeds directly into the design of any future structure (a granny flat design by the building designer, for example, would reference the geotech report for foundation specifications).
Timing of the Geotechnical Report
The geotechnical report can be:
- Commissioned before pool removal works: To inform the backfill specification before works start. This ensures the contractor uses material and methods that the engineer has specified.
- Commissioned after works: To certify that the works were completed correctly. This is the certification-stage approach, sometimes required as a condition of the compliance certificate.
For future construction planning, the pre-works report gives the best outcome, it lets you specify the fill correctly from the start, rather than discovering a problem in certification.
Cost of a Geotechnical Report in the Southern Highlands
| Report Type | Indicative Cost |
|---|---|
| Basic residential geotech (2-3 borings + lab) | $1,500 to $2,500 |
| Intermediate (4-5 borings, complex site) | $2,500 to $4,000 |
| Full investigation (machine boring, slope stability) | $4,000 to $8,000+ |
| Certificate only (reviewing existing fill without investigation) | $800 to $1,500 |
These are Southern Highlands market estimates. Geotechnical engineers servicing the Highlands include Bowral-based consultants and Wollongong and Canberra-based firms who regularly work in the region.
Frequently Asked Questions
Is a geotechnical report the same as a structural engineer’s report for pool fill-in? No, they are separate documents from different professionals. A geotechnical engineer assesses the soil; a structural engineer designs the structure based on (among other things) the geotech findings. Both may be needed for future construction on a filled pool site. See do I need an engineer’s report after pool fill-in for more on the structural engineer question.
If I’m not planning to build on the filled site, do I still need a geotech report? Not necessarily. If you’re just grassing the site over and have no plans to build, a geotech report is rarely needed. Check with your certifier whether any conditions of your pool removal approval require one.
What happens if the backfill from the pool removal doesn’t meet the geotech specification? The engineer will identify this in their report and recommend remediation, typically additional compaction or replacement of fill material. This can be an unexpected cost if a poor contractor placed inferior backfill. This is why specifying quality fill and compaction in the original contract matters.
Can I use the same geotechnical report for pool removal and a future granny flat application? Possibly. If the geotech investigation was comprehensive and relatively recent (within 5 years for most purposes), the report can often be referenced for a future development application. Discuss with the engineer and the DA certifier whether a new investigation is required.
Who can perform a geotechnical investigation in the Southern Highlands? Registered geotechnical engineers (typically with RPEQ or CPEng registration) with relevant soil investigation experience. Ask your pool removal contractor or certifier for a referral to a local or regional geotechnical consultant.
Need to understand geotechnical requirements for your pool removal project? Contact Southern Highlands Pool Removals for a free consultation and referral to local geotechnical professionals.