The Standard Penetration Test (SPT) is the most widely used in-situ geotechnical test worldwide.
In Australia, the SPT is conducted in accordance with AS 1289.6.3.1 and provides essential data for soil strength, density, and foundation design parameters.
What Is the SPT?
The SPT measures the resistance of soil to penetration by a standard split-barrel sampler driven by a 63.5 kg hammer falling 760 mm. The result is the N-value — the number of blows required to drive the sampler 300 mm after an initial 150 mm seating drive.
SPT Equipment
| Component | Specification (AS 1289.6.3.1) |
|---|---|
| Hammer | 63.5 kg ± 0.5 kg |
| Drop height | 760 mm ± 10 mm |
| Sampler | Split-barrel, 51 mm OD, 35 mm ID |
| Drill rods | AW or NW size |
| Anvil | Hammer strikes anvil on top of drill rods |
SPT Procedure
Step 1: Borehole Preparation
The borehole is advanced to the desired test depth. Any disturbance at the base of the borehole is cleaned out.
Step 2: Seating Drive
The SPT sampler is lowered to the bottom of the borehole. The hammer is dropped 150 mm to seat the sampler, recording the number of blows for the first 150 mm penetration (N₁ — not used in the N-value calculation).
Step 3: Test Drive
The sampler is driven a further 300 mm, recording the number of blows for each 75 mm interval. The N-value is the total blows for the final 300 mm (N₂ + N₃ + N₄).
Step 4: Recovery
The sampler is withdrawn and the soil sample is recovered for classification.
Step 5: Record
The blow counts, penetration depth, and refusal (if any) are recorded. If 50 blows are reached before 300 mm, the test is terminated (refusal) and the penetration is noted.
SPT N-Value Interpretation
| N-value (blows/300mm) | Relative Density | Consistency |
|---|---|---|
| 0–4 | Very loose | Very soft |
| 4–10 | Loose | Soft |
| 10–30 | Medium dense | Firm |
| 30–50 | Dense | Stiff |
| > 50 | Very dense | Hard |
Correlations
SPT N-values correlate with a wide range of geotechnical parameters:
| Parameter | Correlation |
|---|---|
| Friction angle (φ') | φ' = √(12N) + 15 (for sands) |
| Undrained shear strength (su) | su ≈ N × 6.25 kPa (for clays) |
| Relative density (Dr) | Dr = √(N / 60) × 100 |
| Modulus of elasticity (E) | E ≈ 500 × (N+15) kPa |
| Allowable bearing capacity | q_all ≈ N/10 MPa (for footings on sand) |
Corrections to SPT N-Values
1. Overburden Correction (N₆₀)
The measured N-value is corrected to a reference effective overburden stress of 100 kPa:
$$ N_{60} = N \times \left(\frac{ER}{60}\right) \times C_N $$Where:
- ER = hammer energy ratio (%)
- C_N = overburden correction factor
2. Hammer Energy Correction
Australian SPT hammers typically deliver 50–60% of theoretical energy. Most Australian practice uses N₆₀ directly unless a specific energy measurement is made.
3. Dilatancy Correction
For fine sands below the water table (N > 15):
$$ N_{corr} = 15 + 0.5(N - 15) $$When Is the SPT Used?
| Application | Purpose |
|---|---|
| Site investigation | Soil profiling and density assessment |
| Foundation design | Bearing capacity and settlement estimates |
| Liquefaction assessment | Cyclic resistance evaluation for seismic design |
| Ground improvement | Verify compaction or grouting effectiveness |
| Retaining walls | Earth pressure parameter estimation |
| Pile design | End bearing and skin friction estimates |
Limitations of the SPT
- Discontinuous sampling — data points at discrete depths only
- Disturbed sample — provides classification and moisture content only, not intact strength
- Energy variation — hammer energy varies between operators and equipment
- Soil type effects — gravel, cobbles, or cemented layers can give misleadingly high N-values
- Low sensitivity — not ideal for soft clays or loose sands
SPT in Australia
Key Documents
| Standard | Title |
|---|---|
| AS 1289.6.3.1 | Determination of the penetration resistance of a soil - Standard Penetration Test (SPT) |
| AS 1726-2017 | Geotechnical site investigations |
| AGS 4.1.1 AU | Geotechnical data format — SPT logging |
SPT Equipment Calibration
NATA-accredited laboratories in Australia are required to calibrate SPT hammers periodically. Energy measurements should be performed using an SPT calibration system to verify the hammer energy ratio.