The Cone Penetration Test (CPT) is an advanced in-situ geotechnical test that provides continuous soil profiling and direct measurement of soil strength parameters. In Australia, CPT testing is widely used for major infrastructure, mining, and coastal developments.
What Is the CPT?
The CPT involves pushing a cone-tipped probe (typically 15 cm² base area, 60° apex angle) into the ground at a constant rate of 20 mm/s while continuously measuring:
- Cone resistance (q_c) — end bearing resistance
- Sleeve friction (f_s) — side friction on the 150 cm² friction sleeve
- Pore pressure (u₂) — (for CPTu) measured at the cone shoulder
The resulting data is recorded at 10–20 mm intervals, providing a continuous profile of soil behaviour.
Measured Parameters
| Parameter | Symbol | Unit | What It Indicates |
|---|---|---|---|
| Cone resistance | q_c | MPa | Soil strength, density |
| Sleeve friction | f_s | kPa | Soil type, grain size |
| Friction ratio | R_f | % | f_s / q_c × 100 — soil type classification |
| Pore pressure | u₂ | kPa | Groundwater, drainage characteristics |
| Pore pressure ratio | B_q | — | (u₂ — u₀) / (q_t — σ_v₀) |
CPT vs SPT
| Aspect | CPT | SPT |
|---|---|---|
| Continuity | Continuous profile | Discrete points (every 1–1.5 m) |
| Speed | Faster (20–30 m/h) | Slower (requires rod handling) |
| Sample | No physical sample | Disturbed sample collected |
| Accuracy | Highly repeatable | Operator-dependent |
| Cost | Higher mobilisation | Lower mobilisation |
| Soil types | Limited in gravels | All soil types |
| Penetration | Typically to 30–40 m | Typically to 30–50 m |
CPT Equipment in Australia
Onshore
- 15–20 tonne rigs — standard for most projects
- Track-mounted CPT rigs — for soft or uneven terrain
- Lightweight CPT rigs — limited access, environmental sites
Offshore / Marine
- Seabed CPT systems — lowered to seafloor
- Downhole CPT — deployed through drill string
- Free-fall CPT — for very soft seabed sediments
Soil Behaviour Type (SBT) Charts
The standard Robertson (1990, 2010) SBT chart classifies soil into 9 zones using q_c and R_f:
| Zone | Soil Behaviour Type | Typical q_c (MPa) | Typical R_f (%) |
|---|---|---|---|
| 1 | Sensitive fine-grained | < 0.5 | < 0.5 |
| 2 | Organic soils / clay | 0.5–1.0 | 1–3 |
| 3 | Clays — silty clay | 1.0–2.5 | 2–4 |
| 4 | Silt mixtures | 2.5–5.0 | 1–3 |
| 5 | Sand mixtures | 5.0–10.0 | 0.5–2.0 |
| 6 | Sands — clean sand | 10.0–20.0 | 0.2–1.0 |
| 7 | Dense sand — gravelly | > 20.0 | < 0.5 |
| 8 | Very stiff sand / clay | N/A | N/A |
| 9 | Very stiff fine-grained | N/A | N/A |
Correlations
Undrained Shear Strength (Clays)
$$ s_u = \frac{q_t - \sigma_{v0}}{N_{kt}} $$Where N_kt = 14–16 typically (site-specific calibration recommended).
Friction Angle (Sands)
$$ \phi' = 17.6 + 11.0 \times \log_{10}\left(\frac{q_c}{\sqrt{\sigma_{v0}}}\right) $$Relative Density (Sands)
$$ D_r = \frac{1}{2.96} \times \ln\left(\frac{q_c}{0.064 \times \sigma_{v0}^{0.5}}\right) $$Constrained Modulus (Settlement)
| Soil Type | M (MPa) |
|---|---|
| Clays | M = 2–8 × q_c |
| Silts | M = 3–6 × q_c |
| Sands | M = 3–12 × q_c |
Applications
| Application | CPT Use |
|---|---|
| Foundation design | Stratigraphy, bearing capacity, settlement parameters |
| Pile design | End bearing, skin friction, continuity of bearing stratum |
| Liquefaction assessment | Cyclic resistance ratio (CRR) from CPT data |
| Ground improvement verification | Pre- and post-treatment CPT profiling |
| Land reclamation | Consolidation assessment, fill performance |
| Offshore investigations | Seabed strength, pipeline trenching |
| Contaminated sites | Stratigraphy for contaminant transport modelling |
CPTu (Piezocone)
The CPTu adds pore pressure measurement for enhanced capabilities:
| Measurement | Indication |
|---|---|
| u₂ at cone shoulder | Most reliable pore pressure reading |
| High u₂ | Compressible clay, silt |
| Low or negative u₂ | Dense sand, overconsolidated clay |
| Dissipation test | In-situ permeability, consolidation coefficient (c_h) |
Pore Pressure Dissipation Test
The penetration is stopped at a target depth and the pore pressure decay is recorded:
- Sands — dissipation in seconds
- Silts — dissipation in minutes
- Clays — dissipation in hours
The dissipation test gives:
- Equilibrium pore pressure (u₀)
- Coefficient of consolidation (c_h)
- Hydraulic conductivity (k)
Advantages and Limitations
Advantages
- Continuous, repeatable data
- Fast (20–30 m per hour)
- Direct measurement — no operator bias
- Detailed stratigraphy identification
- Pore pressure and dissipation data (CPTu)
- Correlated to many design parameters
- Good for soft soils
Limitations
- Cannot penetrate gravel, boulders, or hard rock
- No soil sample for visual classification or lab testing
- Higher mobilisation cost than SPT (typically)
- Requires reaction weight (20-tonne rig)
- Limited depth near bedrock or very dense layers
Australian Standards
| Standard | Title |
|---|---|
| AS 1726-2017 | Geotechnical site investigations |
| AS 1289.6.5.1 | Determination of the cone resistance of a soil — Static cone penetrometer test |
| ISSMGE TC16 | International reference test procedure for CPT |
Frequently Asked Questions
Can CPT replace SPT?
CPT provides more detailed data for most applications but does not collect a soil sample. For projects where physical samples are critical (classification, lab testing), CPT is best combined with limited boreholes for sample collection.
What is the maximum depth of CPT?
Onshore CPT typically reaches 30–40 m in ideal conditions. Greater depths (40–60 m) are possible with heavier rigs. Offshore CPT can reach 50–80 m depending on the system.
Can CPT detect gravels?
CPT can identify gravel layers through tip resistance fluctuations, but cannot penetrate through thick gravel beds. SPT or rotary drilling is needed where gravels are anticipated.
How is CPT used for liquefaction assessment?
CPT is the preferred method for liquefaction assessment. The cone resistance is used to calculate the Cyclic Resistance Ratio (CRR) and is less ambiguous than SPT-based methods for silty soils.