Bearing strata
Bearing strata is the soil layer with adequate capacity to support footing loads. Identified by geotech; drives pier depth, pad sizing, and footing design under AS 2870.
Ask Chalkline about this →Bearing strata is the soil or rock layer with adequate bearing capacity to support the loads imposed by a structure’s footings. The geotech identifies the bearing strata through site investigation (boreholes, test pits, penetrometer probes); the engineer designs footings to transfer loads into the bearing strata at the required depth. Bearing strata is the central concept behind pier depth, pad sizing, and footing-down-to-bearing design under AS 2870 and AS 1726.
Why builders care:
Identifying the bearing strata depth is what drives the cost of the foundation system. Cost-relevant cases:
| Bearing strata depth | Typical foundation system | Cost band (3-bed residential, 2026 AUD) |
|---|---|---|
| Surface (0-300 mm) | Strip footing or waffle slab on standard fill | $25-$40k |
| 0.3-1.5 m | Stiffened raft slab with edge beams | $35-$55k |
| 1.5-3.0 m | Bored piers to bearing strata + ground beam | $45-$80k |
| 3.0-6.0 m | Deep bored piers (cased) + ground beam | $70-$130k |
| > 6.0 m | Driven piles or screw piers | $100k+ |
How bearing strata is identified:
- Geotech investigation under AS 1726:
- Boreholes with split-spoon sampling and SPT (Standard Penetration Test) blow counts.
- Test pits (excavator-dug pits to 2-3 m).
- DCP (Dynamic Cone Penetrometer) for shallow surface bearing.
- Bearing capacity criterion depends on the structure:
- Residential strip footing: allowable bearing typically 100 kPa minimum.
- Residential pad footing: 150-200 kPa allowable.
- Pier: socket the pier 1-2 diameters into competent rock, OR base of pier on stiff clay / dense sand at allowable 200+ kPa.
- Verification at construction: the engineer or geotech may inspect during pier-hole drilling to confirm the strata as drilled is the strata as designed.
Distinguishing related terms:
- Bearing strata: the layer that actually supports the load.
- Founding strata: the specific stratum the footing or pier base sits on; effectively a subset of bearing strata for the actual footing location.
- Competent material: a broader term for soil or rock with adequate strength for the application; not necessarily the bearing layer (e.g. fill can be competent for pavement but not residential footing).
- Active zone (AS 2870): the depth to which seasonal moisture cycling penetrates; usually above bearing strata.
Common builder issues:
- Bearing strata deeper than expected at construction: pier hole keeps drilling; cost blows out. Insist on per-pier sign-off by geotech.
- Bearing strata sloping or variable: one pier hits at 2 m, the adjacent pier needs 4 m. Common on rocky sites.
- Fill misidentified as bearing: an old uncontrolled fill can look firm but compress under load. Always verify with proper soil investigation, not just visual inspection.
- Water table at bearing strata depth: piers below water table need casing or different installation method; cost penalty.
For builders:
- Spec a proper geotech report for any non-trivial foundation system (any pier, any sloping site, any sandy or filled site). $1,500-$3,000 for the report saves orders of magnitude on the foundation.
- Brief the geotech on the proposed footing system so they investigate to the relevant depth. A surface DCP isn’t enough for piered design.
- Allow contingency in the foundation budget on sites where bearing depth is variable. 15-25% contingency is typical for piered sites.
- Sign off every pier with the engineer or geotech during installation. Don’t accept “looks ok” from the piling contractor alone.
Also known as: founding strata, competent material, load-bearing layer.
Category: Geotechnical / footings / foundation design.
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Last updated: 2026-05-15. Verified: 2026-05-15.