Frequently Asked Questions.

In most cases, yes. Whether you are navigating a Development Application (DA) or Complying Development Certificate (CDC), a geotechnical report or AS 2870 site classification is commonly required for new builds, knockdown rebuilds, extensions, granny flats, in-ground pools, and retaining works. We provide the parameters your engineer needs for foundation design on Sydney’s reactive clay, shale, and sandstone sites.

Unsure what your project needs? Send us your site address and we will advise.

A Site Classification (AS 2870) primarily assesses how reactive (prone to shrinking and swelling) the surface soils are, which is used for standard residential slabs and footings (Class M, H1, H2, E, etc.). A full Geotechnical Investigation goes deeper. It involves drilling boreholes to assess the underlying soil and rock profile, providing the specific bearing capacities required for heavier or more complex designs like retaining walls, rock excavations, or deep piers.

'Class P' stands for a 'Problem' site, common in areas like Liverpool and Campbelltown. It sits outside the standard reactivity scale used in an AS 2870 site classification and means site-specific geotechnical advice is required for your footing design.

Class M is a moderately reactive site under AS 2870: clay that shrinks and swells with seasonal moisture change, enough to move a footing but within the range a standard stiffened raft or strip footing is designed for. Many Sydney clay sites classify as M. Your structural engineer reads the class straight into the footing design. See the full class scale.

Class H1 and H2 are highly reactive clay sites under AS 2870, with larger seasonal ground movement than Class M, and H2 moves more than H1. Both are buildable: the standard sets stiffer rafts and deeper footings for each class, and your structural engineer designs to the class. Deep residual clays over shale are the usual H1 and H2 sites in Sydney. See the full class scale.

Our investigations are hands-on and site-specific. We extract soil and rock samples, log the geological profile, perform in-situ strength testing (like DCPs), and analyse this data. We then translate the raw site data into an engineering report written for your design team.

Bearing capacity measures how much weight the ground can safely support. We provide precise parameters so your engineer doesn’t have to rely on generic, worst-case assumptions.

Yes. The engineer inspects at bearing level and gives the call before leaving site: proceed, deepen or adjust. The written record follows within days. Single visits for builders from $600 + GST per inspection, standing arrangements for contractors. See Construction Phase Support.

Proof rolling is an on-site test where heavy machinery (like a loaded water cart) is driven over prepared earthworks to observe how the ground reacts. It is commonly used as part of earthworks verification under AS 3798 and helps identify unsuitable or soft areas before construction proceeds.

Yes. If your local council requires On-Site Stormwater Detention (OSD) or absorption trenches, we conduct in-situ permeability testing. This provides the specific infiltration rates your civil engineer needs for their stormwater design submission.

All SFGEO testing, investigations, and reporting are carried out in accordance with the relevant Australian Standards and project requirements, where applicable. Commonly referenced standards include AS 1726 (Geotechnical Site Investigations), AS 2870 (Residential Slabs and Footings), and AS 3798 (Guidelines on Earthworks).

Our reports are written to be immediately usable by your structural or civil engineer. We provide the specific bearing capacities, soil profiles, and design parameters in the format engineers need to get straight to work. If you don’t yet have a structural engineer, we’re happy to introduce you to Sydney firms we work with regularly.

For a standard house, extension, granny flat or pool on a level block, an AS 2870 site classification is typically what the certifier is asking for. It gives the structural engineer the class and the parameters for the slab and footing design. A full geotechnical investigation is typically requested where the land is steep or mapped for landslip, where a basement or excavation runs close to a boundary, where retaining walls are proposed, where the block carries fill or soft ground, or where the classification comes back Class P.

Send us the certifier’s or council’s exact wording with the address and we scope to it rather than guess. If the classification is enough, that is what we quote.

It depends on the council and on what is mapped over the block. The common triggers are sloping or landslip-mapped land, cut or fill beyond what the council’s DCP allows without a report, excavation near a boundary or an existing structure, retaining walls above the height the DCP sets, and land flagged for acid sulfate soils or contamination. Some councils name the trigger in the DA checklist; others raise it as a request for further information after lodgement.

Each council’s threshold is its own, so the DCP and the certifier’s checklist are the reference, not a rule of thumb. If you are not sure whether your block is caught, send us the address and any correspondence from council and we will tell you before you commit to anything.

There is no expiry date written into a geotechnical report, because the ground does not change on a schedule. What changes is the project and the site. Certifiers and councils typically want a report that reflects the current proposal and the current ground: if the design has moved (a basement added, the footprint shifted, a pool where there was not one), or the block has been cut, filled or built on since the fieldwork, they may ask for it to be reviewed or updated. A report written for a different building on the same block may not cover yours.

If you have an older report, send it through with the current plans. Often a review letter or an addendum is all that is needed; sometimes a fresh visit is, and we will say which.

Yes. Where the 4WD rig can’t get in, motorised hand augers carry through a doorway or down a side passage: terrace backyards, courtyards, under a carport with 2.1 m of clearance. Hand-auger holes are logged to the same standard and cross-checked with penetrometer tests, so the report is no thinner for the access. See tight access drilling.

Yes. On steep blocks the rig sets up where it can (the street, or the council verge as at Coogee) and motorised hand augers reach the rest of the block. The report addresses what the slope actually raises: differential settlement, slope stability and the retaining parameters your engineer needs, with the rock and soil data behind them.

Yes. We work across Greater Sydney from Marrickville: the reactive shale clays of the Inner West and Western Sydney, the Hawkesbury sandstone of the North Shore and Northern Beaches, and the sands of the Eastern Suburbs. For drilling programs the 4WD rig also travels into regional NSW, see subcontract drilling.

The engineer drills your site. The person who signs your report is the person who extracted the samples, cross-verified the data on site, and sat down to interpret it. No subcontracted drill crews on our investigations, no drive-by inspections.

Every SFGEO report is a comprehensive document, not a template with a few values dropped in. Where practical, we use the drill rig to reach greater depth, and we supplement boreholes with on-the-spot strength tests (penetrometers) so the data is cross-checked on site before it reaches you. The intent is simple: the right data, the first time, so your structural engineer designs accurately, your certifier’s requirements are met, and your builder prices against a design that reflects the ground.

All laboratory work is conducted through a third-party NATA-accredited facility. Speak to our team directly.

Absolutely. We are a boutique, independent geotechnical firm. This means our advice is conflict-free and strictly focused on providing objective geotechnical data. We don’t sell construction materials or construction services.

Yes. We carry comprehensive Professional Indemnity (PI) and Public Liability (PL) insurance covering geotechnical engineering and drilling operations. Certificates of currency can be provided on request for both residential and commercial projects.