Describing rock
Strength, weathering grades, defect logging, core recovery and RQD.
Before you use this section
The conventions below follow general Australian practice for soil and rock description, broadly after AS 1726. Your own QA manual, and any project specification, take precedence. Where the two differ, follow your procedures — and if you are unsure which applies, ask the reviewing engineer before you log, not after.
Rock is described by different properties from soil, and the two most important are strength and weathering.
The word order
- Strength
- Weathering grade
- Colour
- ROCK NAME — in capitals
- Structure, bedding and defects
- Additional observations
For example:
low strength, moderately weathered, grey SILTSTONE, thinly bedded, with closely spaced sub-horizontal joints
Strength
Assessed by response to hammer and pick, or by point load index.
| Term | Abbreviation | Guide |
|---|---|---|
| Extremely low | EL | Crumbles under firm hand pressure |
| Very low | VL | Broken by light hammer blow |
| Low | L | Broken by hand-held hammer |
| Medium | M | Broken by several hammer blows |
| High | H | Difficult to break with hammer |
| Very high | VH | Chipped only by repeated blows |
| Extremely high | EH | Rings under hammer |
Weathering
| Term | Abbreviation | Meaning |
|---|---|---|
| Residual soil | RS | Original fabric destroyed |
| Extremely weathered | XW | Soil strength, rock fabric preserved |
| Highly weathered | HW | Substantially discoloured and weakened |
| Moderately weathered | MW | Discoloured, strength reduced |
| Slightly weathered | SW | Slight discolouration on defects |
| Fresh | FR | No visible weathering |
Extremely weathered rock is not soil. It has soil strength but retains rock fabric, and it behaves differently from a residual soil. Logging it as clay loses information that matters for excavation and foundation design.
Defects
Record for each defect:
- Depth
- Type — joint, bedding parting, seam, shear
- Orientation — dip and dip direction where measurable
- Spacing
- Surface condition — roughness, coating, infill
Defects usually govern rock mass behaviour more than intact strength does. A high-strength rock with closely spaced clay-infilled joints is a weaker mass than a moderate-strength rock with widely spaced tight joints.
Use the Defects tab. See Rock core, photos, construction, methods and defects.
Core recovery and RQD
For cored holes, record per run:
- Total core recovery — recovered length as a percentage of run length
- RQD — the sum of intact, sound core pieces 100 mm or longer, as a percentage of run length
Low recovery is itself a result: it usually means broken or weak ground, and should be described rather than left as a blank.
Record these in the Rock core tab.