Docs / Borehole Logging / Worked example: a cored rock hole

Worked example: a cored rock hole

Soil to rock transition, rock descriptions, core recovery, RQD and defects.

A hole augered through soil into rock, then cored. The additional demand here is describing the rock and recording core properly.

Location tab

Field	Value
Location ID	`BH01`
Type	Borehole
Final depth	8.50 m
Drilling method	Auger to 4.20 m, NMLC coring 4.20–8.50 m

Record the method change and the depth it happened. The Methods tab holds method intervals; the drilling method field on the Location tab carries the summary.

Lithology

Depth	Description
0.00–1.20 m	firm, moist, brown, medium plasticity silty CLAY, trace fine gravel
1.20–4.20 m	stiff to very stiff, moist, orange-brown mottled grey, high plasticity CLAY — residual
4.20–6.10 m	very low to low strength, highly weathered, grey-brown SILTSTONE, thinly bedded
6.10–8.50 m	medium strength, moderately weathered, grey SILTSTONE, thinly bedded, closely spaced sub-horizontal joints

What to notice

The soil–rock transition is a described boundary, not a gap. The auger stopped and coring started at the same depth the material changed — which is usually, but not always, the case. Where they differ, log the material change, not the equipment change.
Rock descriptions lead with strength and weathering. Soil descriptions lead with consistency and moisture. The order changes because the governing properties change.
Two rock intervals, not one. Strength and weathering both improve at 6.10 m. That boundary matters for excavatability and founding depth, so it is a separate interval.
Residual is flagged. The clay from 1.20 m is identified as residual — weathered in place from the underlying siltstone — which links it to the rock below.

Rock core

Run	Depth	Recovery	RQD
1	4.20–5.70 m	82%	25%
2	5.70–7.20 m	96%	61%
3	7.20–8.50 m	100%	78%

Recovery and RQD both improve with depth, consistent with the weathering description. When the numbers and the words tell the same story, both are more credible.

Defects

Depth	Type	Orientation	Condition
6.40 m	Joint	Sub-horizontal	Clean, rough, tight
6.85 m	Bedding parting	Sub-horizontal	Iron-stained
7.30 m	Joint	70°	Clay-infilled, 2 mm

The clay-infilled joint at 7.30 m matters more than the intact strength either side of it. Defects usually govern rock mass behaviour — see Describing rock.

Photographs

Photograph every core run in the tray, with the hole ID, run depths and a scale in frame. Attach via the Photos tab. Core photographs are frequently the only surviving physical record once the core is disposed of.

# Worked example: a cored rock hole

A hole augered through soil into rock, then cored. The additional demand here is describing the rock and recording core properly.

## Location tab

| Field | Value |
|---|---|
| Location ID | `BH01` |
| Type | Borehole |
| Final depth | 8.50 m |
| Drilling method | Auger to 4.20 m, NMLC coring 4.20–8.50 m |

Record the method change and the depth it happened. The **Methods** tab holds method intervals; the drilling method field on the Location tab carries the summary.

## Lithology

| Depth | Description |
|---|---|
| 0.00–1.20 m | *firm, moist, brown, medium plasticity silty CLAY, trace fine gravel* |
| 1.20–4.20 m | *stiff to very stiff, moist, orange-brown mottled grey, high plasticity CLAY — residual* |
| 4.20–6.10 m | *very low to low strength, highly weathered, grey-brown SILTSTONE, thinly bedded* |
| 6.10–8.50 m | *medium strength, moderately weathered, grey SILTSTONE, thinly bedded, closely spaced sub-horizontal joints* |

### What to notice

- **The soil–rock transition is a described boundary**, not a gap. The auger stopped and coring started at the same depth the material changed — which is usually, but not always, the case. Where they differ, log the material change, not the equipment change.
- **Rock descriptions lead with strength and weathering.** Soil descriptions lead with consistency and moisture. The order changes because the governing properties change.
- **Two rock intervals, not one.** Strength and weathering both improve at 6.10 m. That boundary matters for excavatability and founding depth, so it is a separate interval.
- **Residual is flagged.** The clay from 1.20 m is identified as residual — weathered in place from the underlying siltstone — which links it to the rock below.

## Rock core

| Run | Depth | Recovery | RQD |
|---|---|---|---|
| 1 | 4.20–5.70 m | 82% | 25% |
| 2 | 5.70–7.20 m | 96% | 61% |
| 3 | 7.20–8.50 m | 100% | 78% |

Recovery and RQD both improve with depth, consistent with the weathering description. When the numbers and the words tell the same story, both are more credible.

## Defects

| Depth | Type | Orientation | Condition |
|---|---|---|---|
| 6.40 m | Joint | Sub-horizontal | Clean, rough, tight |
| 6.85 m | Bedding parting | Sub-horizontal | Iron-stained |
| 7.30 m | Joint | 70° | Clay-infilled, 2 mm |

The clay-infilled joint at 7.30 m matters more than the intact strength either side of it. Defects usually govern rock mass behaviour — see **Describing rock**.

## Photographs

Photograph every core run in the tray, with the hole ID, run depths and a scale in frame. Attach via the **Photos** tab. Core photographs are frequently the only surviving physical record once the core is disposed of.