What Does RL m AHD Mean in a Geotechnical Report?

A geotechnical report written in Australia carries sentences like these:

Example 1 — a building platform in cut
The underside of the proposed slab-on-grade is understood to be at RL 24.60 m AHD. All uncontrolled fill beneath the building footprint is to be removed to expose natural soils and replaced with engineered fill.

Example 2 — a hardstand platform in fill
The proposed hardstand subgrade level is RL 5.90 m AHD. Existing surface levels across the area range between RL 5.10 and RL 5.55 m AHD. Topsoil and any uncontrolled fill are to be stripped prior to placement of engineered fill to design level.

To a surveyor, both are unambiguous. You may ask: how deep do we dig, and how much material do we move?

This article guides you, works through both examples end to end, and covers the level-related mistakes.

RL: Reduced Level

RL stands for Reduced Level (a height above a nominated datum surface.)

"Reduced" is surveying language. Field measurements (staff readings, total station observations, GNSS positions) are reduced through a calculation to give a single height referenced to a common surface. That process is why a page of backsights and foresights collapses into one clean value per point.

The usefulness of a reduced level is that it is comparable. A borehole collar, an existing surface spot height, a footing level, a stormwater invert and a finished floor level can sit in the same schedule and be compared directly, because all of them are measured from the same starting surface.

Depths, by contrast, are only meaningful alongside the surface they were measured from. "1.8 m of fill" tells you nothing on its own. "Base of fill at RL 23.75" tells you exactly where the interface sits, on any drawing, from any direction.

AHD: Australian Height Datum

AHD is the Australian Height Datum, the national vertical reference surface for the mainland and Tasmania.

It was established in 1971 by fixing mean sea level (as observed at 30 tide gauges around the coast between 1966 and 1968) to zero, then adjusting the national levelling network to suit. Tasmania has its own realisation, adopted in 1983 from two tide gauges. Because of that history, older survey plans sometimes label it AHD71.

AHD is close to mean sea level, but it is not mean sea level. Ocean topography, the age of the original adjustment and accumulated distortion in the levelling network mean AHD departs from the true geoid by up to roughly half a metre across the continent.

AHD heights are not GNSS heights. A GNSS receiver natively produces an ellipsoidal height above the GDA2020 reference ellipsoid. Converting that to AHD requires a geoid model (AUSGeoid2020) and the two differ by tens of metres across most of Australia. If a rover not correctly configured, or has been localised to the wrong base, produces numbers that look entirely plausible and are wrong by an amount nobody notices until something is poured. Geoscience Australia now also publishes the Australian Vertical Working Surface (AVWS) as a more accurate option for GNSS-based work, though AHD remains the datum specified on the overwhelming majority of development approvals and design drawings.

Put together: RL 24.60 m AHD means the point sits 24.60 metres above the Australian Height Datum. Nothing more complicated than that.

Example 1: the building platform in cut

The first statement contains two separate instructions, and the second one is the expensive one.

The design level is fixed; the cut is not

RL 24.60 is a level, not a depth. To get a depth, subtract it from the existing surface at the point you care about.

• Existing ground at the north-east corner, RL 25.85 → 1.25 m of cut to underside of slab
• Existing ground at the south-west corner, RL 24.95 → 0.35 m of cut

One design level, very different excavation. That is precisely why levels are given as RLs — the design surface stays put while the ground does whatever it does.

Stripping uncontrolled fill is a separate obligation

Uncontrolled fill is material placed without documented compaction control, density testing or supervision: site clearing spoil, demolition rubble, whatever the previous occupier pushed around. Its density, composition and compressibility are unknown and variable across a site, which is the whole problem. It may perform perfectly well. It may also contain a buried stump, a slab of broken concrete, or a pocket of loose sand that produces differential settlement under a stiffened raft.

Critically, the requirement to remove it is not limited by the slab level. Where the fill extends to RL 23.75, you excavate to RL 23.75 — 850 mm below the underside of slab — and then reinstate back up to RL 24.60 in engineered (controlled) fill: placed in specified layer thicknesses, moisture conditioned, compacted to a nominated density ratio, and density tested at an agreed frequency.

So excavation depth and slab level are two different numbers, and only one of them appears on the architectural drawings.

Reading it off the borehole logs

The logs will normally record the collar RL and the depth to the fill/natural interface. Convert those depths to RLs and the required over-excavation reads straight off the page:

Interpolate across the footprint, add an allowance for battering or benching around the perimeter, and you have a defensible engineered fill volume for the tender rather than a guess. Note BH3: where natural soils already sit above the design level, you are simply cutting into competent material and no replacement fill is required there.

One caution on interpolation: three boreholes describe three points, not a surface. Uncontrolled fill is notoriously variable, and the base of an old fill body can step abruptly where a previous excavation edge or a filled watercourse runs through the site. Treat interpolated volumes as an estimate to be confirmed during bulk earthworks, and say so in the tender.

Example 2: the hardstand platform in fill

The second statement inverts the geometry, and it catches people out in a different way.

Here the design subgrade level (RL 5.90) sits above the existing surface (RL 5.10 to 5.55). There is no bulk cut. The platform has to be built up.

The trap is assuming the fill depth equals the difference between the two levels. It does not, because you strip first and fill afterwards.

Take a point where the existing surface is RL 5.35 with 250 mm of topsoil over it:

1. Strip topsoil → exposed subgrade at RL 5.10
2. Fill from RL 5.10 up to RL 5.90 → 0.80 m of engineered fill

The net raise at that point is 0.55 m. The fill actually placed is 0.80 m—a 45% difference, before anyone touches an uncontrolled fill pocket. Multiply across a hardstand area, and the gap between those two numbers is a substantial quantity of imported material, cartage and compaction. Estimators who price the net difference between surfaces, rather than the volume above the stripped subgrade, underprice the job every time.

Further points specific to filling:

• Placing fill loads the ground underneath. A raised platform over soft alluvial or estuarine soils — which RL 5.90 on a coastal site may well imply — induces settlement in the underlying material. The report should say whether that settlement is significant, how long it will take, and whether surcharging or a waiting period is needed before pavement construction.
• The design level may be subgrade, not finished surface. RL 5.90 as a subgrade level sits below the pavement layers. Finished surface will be higher by the full pavement thickness. Check which one the number refers to before you set out.

Remember

Don't mix datum with AHD. Contractors routinely set a local benchmark at an arbitrary value — RL 100.00 on a manhole lid is the classic — because it is fast and fine for internal setout. The moment you compare a level from that survey against an AHD design level, everything shifts by a constant offset. Any drawing carrying levels should state its datum explicitly. If a plan does not say AHD, do not assume it.

Ellipsoidal heights straight off the rover. Covered above, and worth repeating, because it is the failure mode most likely to survive undetected until something is built.

Ambiguity in "underside of slab". Does it mean the underside of the structural concrete, or the top of the prepared subgrade or blinding layer? Typically 50–100 mm apart. Over a large slab that is a meaningful volume, and it is a five-minute email to the designer to resolve.

Mixing FSL, USL and subgrade level. Finished surface, underside of slab and subgrade differ by slab thickness, bedding and pavement depth. A 200 mm slab at FSL 24.80 has an underside at 24.60. Numbers pulled from the wrong sheet look entirely reasonable.

Benchmarks that have moved. Site benchmarks get bumped by plant, established on fill that later settles, or destroyed and re-established from a different origin. Any job running longer than a few months should recheck its benchmark against an official Permanent Survey Mark. State authorities publish PSM coordinates and AHD heights — in Queensland through the DNRM survey control database, in NSW through SCIMS, with equivalents in every jurisdiction.

Transposed digits. RL 24.60 becomes RL 26.40 in a re-typed schedule. Check levels against the drawing, not against the last document someone else typed.

Before earthworks start

• Confirm the datum in writing — AHD, or a stated assumed datum with a documented relationship to AHD.
• Confirm the site benchmark against a published PSM, and record how and when it was checked.
• Confirm what the design RL refers to: underside of concrete, top of subgrade, or finished surface.
• Convert every borehole depth to an RL and map the base of the uncontrolled fill.
• In cut, identify where natural soils sit above design level — those areas need no replacement fill.
• In fill, price from the stripped surface up, not from the existing surface up.
• Agree the earthworks specification: layer thickness, target density ratio, testing frequency, and who signs off. AS 3798 is the reference point for commercial and residential earthworks and sets out levels of supervision and testing.
• Agree an inspection hold point at exposed subgrade, before any fill is placed. This is where proof rolling and geotechnical inspection confirm that the uncontrolled fill has actually gone.

Working through a geotechnical report and want more detail on how these levels are recorded in the first place? See our guides to borehole logs, geotechnical investigation for site classification, and proof rolling assessment. All free, all written for Australian practice.