Job ads for geotechnical roles often list "CPEng or CGeol" as if the two are interchangeable, and on a site investigation. The two roles are different professions with different training, different registrations, and different legal authority to sign off work, even though they spend a lot of time on the same project.
What a geotechnical engineer actually is
A geotechnical engineer is a civil engineer who specialises in soil and rock mechanics. In Australia, this is its own occupation code, ANZSCO 233212, and the standard path is a civil engineering degree with a geotechnical major or a postgraduate qualification like UNSW's combined Master's in Geotechnical Engineering and Engineering Geology. The job is fundamentally about design: taking data from a site investigation and using it to size a footing, check a slope's factor of safety, or design a retaining wall, then standing behind that design as an engineer.
That design authority is backed by registration. Most practising geotechnical engineers hold CPEng, NER, or RPEQ (or a state equivalent), covered in more detail in our guide to those three credentials and our walkthrough of getting Chartered.
What an engineering geologist actually is
An engineering geologist is an earth scientist, not an engineer. The base qualification is a geoscience degree, and the relevant Australian registration is RPGeo through the Australian Institute of Geoscientists (AIG), which has its own five-year experience requirement and a nominated field of practice covering engineering geology specifically. Where a geotechnical engineer starts from structural loads and works down, an engineering geologist starts from the ground itself: rock mass structure, weathering profiles, fault zones, landslide history, groundwater behaviour, and what all of that means for a site before any design work happens.
Where the two overlap
Site investigation is where the roles blur most. Both professions do borehole logging, both interpret ground conditions, and both can end up writing sections of the same report. A common way people describe the split: engineering geologists identify and characterise the problem, geotechnical engineers design the solution. On a landslide assessment, for example, the geologist might map the failure surface and describe the rock mass structure, while the engineer runs the slope stability analysis and specifies remediation.
The sign-off difference
This is the part that trips people up. RPEQ (and equivalents like Victoria's RPEng) is specifically a licence to provide "professional engineering services." An engineering geologist can sign off on geological characterisation and hazard assessment, but design work, such as certifying a foundation or a retaining structure, generally requires a geotechnical engineer who holds RPEQ or the relevant state registration. Some project specifications (Queensland Transport and Main Roads guidelines are a public example) explicitly name both an RPEQ geotechnical engineer and a suitably registered engineering geologist as acceptable signatories, but for different parts of the report. Check the specific project or client requirement rather than assuming either credential covers everything.
Which one should you actually become
If you're drawn to calculations, structural loads, and signing off on a design, geotechnical engineering is a more direct path, and it starts with a civil engineering degree. If you're drawn to fieldwork, rock and soil formation, mapping, and figuring out what's actually going on underground before anyone designs anything, engineering geology is the better fit, and it starts with an earth science degree. Plenty of people end up doing both: UNSW's combined program exists precisely because the two disciplines overlap enough that dual training has real value on complex projects.
If you're hiring instead of choosing a career
For most residential and straightforward commercial site investigations, a geotechnical engineer alone covers the job. For complex ground: active landslides, karst, faulted rock, contaminated sites with a geological dimension, bringing in an engineering geologist alongside the engineer gets you a level of ground characterisation that an engineering-only team often can't match.