Borehole drilling is a geotechnical and geoenvironmental investigation technique used to create a narrow, vertically (or sometimes diagonally) drilled shaft into the ground.
The primary purpose of Borehole Drilling is to extract soil, rock, and groundwater samples from various depths to assess the subsurface conditions of a site. This process is essential for determining the engineering properties of the earth before constructing buildings, bridges, roads, or other infrastructure.
What Is Borehole Drilling and Logging?
Borehole drilling creates a vertical shaft into the ground to access subsurface soils and rock for examination and sampling.
Soil logging is the systematic description and recording of the materials encountered, including:
- Soil/rock type and classification
- Layer boundaries and thickness
- Colour, moisture content, and consistency
- Structure, fabric, and discontinuities
- Groundwater observations
- Field test results (SPT, DCP, shear vane)
Drilling Methods
| Method | Suitable For | Key Features |
|---|---|---|
| Auger drilling | Cohesive soils, clay, silt | Continuous recovery, disturbed samples |
| Wash boring | Sands, gravels, stiff clays | Fast, limited sample quality |
| Rotary coring | Rock, stiff clay | Intact core samples, rock quality designation (RQD) |
| Percussion drilling | All soil types, rock | Good for deep boreholes, groundwater monitoring |
| Hand auger / hand excavation | Shallow depths (< 3 m), limited access | Light equipment, low cost |
| Direct push | Sands, silts, soft clays | Continuous soil profiling, CPT compatibility |
Purposes
- Subsurface Investigation: Determining the sequence and thickness of soil and rock strata beneath a site.
- Sample Recovery: Retrieving undisturbed and disturbed samples for laboratory testing of physical and chemical properties.
- Groundwater Assessment: Measuring the groundwater table level and collecting water samples for quality analysis.
- In-Situ Testing: Facilitating downhole testing methods such as the Standard Penetration Test (SPT), Cone Penetration Test (CPT), or vane shear testing.
Soil Logging to AS 1726
AS 1726-2017 provides the Australian standard for soil and rock description and logging. A compliant log must include:
Required Information
| Field | Description |
|---|---|
| Project details | Site address, borehole number, drilling method |
| Coordinates | Easting, Northing, RL (reduced level) |
| Depth scale | Vertical scale (typically 1:25, 1:50, or 1:100) |
| Soil/rock type | Descriptive name per AS 1726 classification |
| Layer boundaries | Depth of each layer change |
| Colour | Soil colour using standard descriptors |
| Moisture condition | Dry, moist, wet, saturated |
| Consistency/density | Very loose to very dense (sands), very soft to hard (clays) |
| Structure | Bedding, laminations, fissures, root holes |
| Additional observations | Odour, staining, carbonate content, organic material |
| Field test results | SPT N-values, DCP, vane shear, pocket penetrometer |
| Sample information | Sample type (disturbed, undisturbed), depth, reference |
| Groundwater | Depth encountered, depth after stabilisation |
| Water level | Date and time of measurement |
Soil Description Order (AS 1726)
The standard order for describing soil in a log is:
- Particle size — gravel, sand, silt, clay (in order of increasing proportion)
- Plasticity — non-plastic, low, medium, high, very high
- Colour
- Moisture condition
- Consistency (cohesive) or density (granular)
- Structure and fabric
- Additional information
Sampling Techniques
| Sample Type | Method | Use For |
|---|---|---|
| Disturbed (DS) | Auger or bag sample | Classification, moisture content, compaction testing |
| Undisturbed (UDS) | Thin-walled tube sampler | Shear strength, consolidation, intact structure |
| SPT sample | Standard Penetration Test sampler | N-value, classification, disturbed sample |
| Core sample | Rotary coring | Rock quality, discontinuity logging, intact strength |
| Bulk sample | Large volume from test pit | Compaction testing, large-scale testing |
Groundwater Monitoring
During drilling, groundwater observations are critical:
| Observation | Significance |
|---|---|
| Depth first encountered | Initial water strike during drilling |
| Depth after stabilisation | Groundwater level equilibrium (recorded 24+ hours later) |
| Artesian flow | Pressurised groundwater — requires casing |
| Fluid loss | Highly permeable zone — gravel, fissured rock |
Rock Logging
For rock cores, additional parameters are recorded:
| Parameter | Description |
|---|---|
| Rock type | Igneous, sedimentary, metamorphic |
| Weathering grade | Fresh to completely weathered (W1–W5 per AS 1726) |
| Strength | Extremely low to extremely high (AS 1726 classification) |
| RQD (Rock Quality Designation) | % of intact core pieces > 100 mm |
| Fracture spacing | Very closely to very widely spaced |
| Joint condition | Roughness, coating, aperture, infilling |
Logging Software and Formats
| Software | Features |
|---|---|
| gINT | Industry-standard geotechnical data management |
| OpenGround (Bentley) | Cloud-based logging and data management |
| AGS 4.1.1 AU format | Standard electronic data transfer format (Australian Geomechanics Society) |
Australian Standards
| Standard | Title |
|---|---|
| AS 1726-2017 | Geotechnical site investigations — soil and rock description |
| AS 1289.6.3.1 | Standard Penetration Test |
| AS 1289.6.2.1 | Determination of the shear strength of a soil — Field test |
| AGS 4.1.1 AU | Electronic data transfer format for geotechnical data |
Terminology
- Refusal: The point at which the drilling equipment cannot reasonably penetrate further, typically due to striking solid bedrock or an obstruction.
- Standpipe / Piezometer: A pipe installed into a finished borehole, screened at specific depths, used to measure groundwater levels and pressures.
- Cuttings: The broken fragments of soil and rock flushed or lifted out of the borehole during drilling.
- SPT N-Value: A standardised measure of soil density derived from the Standard Penetration Test, indicating the number of blows required to drive a sampler 300 mm into the soil.