CPTu Piezocone Test — Pore Pressure Measurement and Interpretation

The CPTu — the piezocone penetration test — is the standard cone penetration test with one critical addition: a pore water pressure sensor built into the cone body at a precisely defined location. That single measurement transforms what the test can tell you about fine-grained soils, enabling a pore-pressure correction to the raw tip resistance that is essential for accurate interpretation in clays and silts, and opening the door to dissipation testing for consolidation parameters.

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Where pore pressure is measured — the u2 position #

Pore pressure can be measured at three positions on a cone: at the cone face (u1), immediately behind the cone shoulder (u2), or behind the friction sleeve (u3). The u2 position — behind the cone shoulder — is the universally adopted standard, specified in ASTM D5778 and ISO 22476-1, because it most directly influences the net force on the tip load cell and produces the most consistent, interpretable pore pressure response across different soil types.

At u2, the pore pressure transducer connects to a porous filter element (typically sintered HDPE, ~10–20 micron pore size) that allows pore water to reach the transducer while excluding soil particles. The filter element must be saturated before the test — if it contains air, the pore pressure response will be slow, attenuated, or entirely absent in low-permeability soils.

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The q_t pore pressure correction #

The cone tip load cell measures total force on the cone tip, but that force is shared between the soil skeleton and pore water pressure acting on the annular unequal area at the cone shoulder. The correction is:

q_t = q_c + u₂(1 − a)

where a is the cone area ratio — the ratio of the net cross-sectional area at the shoulder to the cone tip area (A_c = 10 cm²). The area ratio is cone-specific, typically 0.70–0.85, determined by laboratory calibration of each individual cone in a pressure vessel.

In clean sands, u₂ is approximately hydrostatic and the correction is negligible — often less than 2% of q_c. In soft clays, high excess pore pressures can make u₂ several times q_c. Using uncorrected q_c instead of q_t in soft clay can underestimate normalised cone resistance Q_tn by 20–50%, producing significant errors in SBT classification, undrained shear strength, and bearing capacity. All modern CPT interpretation must use q_t throughout — not raw q_c.

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The pore pressure ratio B_q #

The pore pressure ratio B_q normalises the excess pore pressure generated during penetration:

B_q = (u₂ − u₀) / (q_t − σ_v0)

where u₀ is in-situ hydrostatic pore pressure and σ_v0 is total vertical stress. B_q forms the third axis of the extended Robertson SBT classification charts, complementing Q_tn and F_r. Sensitive clays generate B_q values approaching 1.0 — nearly all penetration resistance is carried by pore pressure rather than the soil skeleton. Dense dilative sands generate slightly negative B_q due to dilation suction. For the complete normalised parameter framework, see Normalised CPT plots — Q_tn, F_r, and B_q explained.

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Dissipation testing — estimating c_h #

When the CPTu cone is held stationary in a fine-grained layer, the excess pore pressure dissipates as water drains away. Monitoring this decay gives the time for 50% dissipation (t₅₀), from which the horizontal coefficient of consolidation c_h is estimated:

c_h = T^*₅₀ × r² / t₅₀

where r is the cone radius and T^*₅₀ ≈ 0.245 (Houlsby & Teh, 1988) for a standard 10 cm² cone. Dissipation tests are typically run for 15–60 minutes at selected depths through a clay profile, building a c_h depth profile for consolidation settlement time-rate analysis at a fraction of the cost of multiple laboratory oedometer tests. The cone must remain completely stationary during the test — any rod movement resets the dissipation curve. For how c_h feeds into settlement calculations, see the foundation design guide.

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Filter saturation — the field-critical step #

A desaturated piezometer filter is the most common cause of poor CPTu data quality — it produces sluggish or absent u₂ response that cannot be corrected after the fact. Standard saturation protocol:

1. Place the filter in glycerin or silicone oil under vacuum for at least 30 minutes before the test.
2. Assemble the cone under the saturating fluid — keep the filter away from air throughout.
3. Store the assembled cone tip-down in a sealed container of glycerin until deployment.
4. Push immediately after removing from the container, minimising surface air exposure time.

Quality can be checked by observing u₂ as the cone passes through standing water in the borehole casing — a properly saturated filter responds immediately to hydrostatic pressure. Post-test zero drift greater than 5% of the maximum measured u₂ should be flagged in the sounding report.

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When CPTu is essential vs optional #

Soil condition	CPTu essential?	Reason
Soft to firm clay	Essential	qt correction significantly changes Qtn; without it, su and SBT are unreliable
Sensitive or structured clay	Essential	Bq identifies sensitivity; high excess u2 is the key diagnostic
Interbedded sand and clay	Essential	u2 drops to hydrostatic in sand layers — most reliable layer boundary indicator
Consolidation design needed	Essential	Dissipation tests give ch profile for time-rate settlement calculations
Clean dense sand only	Optional	u2 ≈ hydrostatic; qc ≈ qt; correction is negligible

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How DartiGeo uses CPTu data #

When CPTu data is imported into DartiGeo — depth, q_c, f_s, u₂ columns from Excel — the software applies the q_t correction automatically using the cone area ratio a entered in the sounding parameters. All subsequent calculations — Q_tn normalisation, SBT classification, undrained shear strength, and bearing capacity — use q_t throughout. B_q is computed at every depth and available for the extended SBT chart. Where no u₂ column is present (standard CPT without pore pressure), DartiGeo assumes u₂ = hydrostatic and applies a small default correction using an area ratio of 0.80.

Download a free 14-day trial of DartiGeo →

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Frequently asked questions #

What does CPTu stand for? #

CPTu stands for Cone Penetration Test with pore water pressure measurement — the lowercase “u” refers to the pore pressure parameter u₂, measured at the cone shoulder position. It is also called the piezocone penetration test, and the instrumented probe is the piezocone. The u2 position is the ASTM D5778 and ISO 22476-1 standard location for pore pressure measurement.

Why does negative u2 occur in CPTu data? #

Negative excess pore pressure below hydrostatic occurs in dense dilative sands and heavily overconsolidated clays. During penetration, these materials want to expand in volume (dilate) but are constrained — this creates a local suction, drawing pore water toward the cone and generating negative excess u₂. It is a valid physical response indicating dense or overconsolidated material. It should not be confused with filter desaturation, which produces erratic or zero u₂ from the start of the push rather than a physically consistent negative trend.

How long does a CPTu dissipation test take? #

Duration depends on soil permeability and the target dissipation level. In very soft marine clays (c_h ~0.1–1.0 m²/year), t₅₀ can exceed 30–60 minutes. In firm clays or silts (c_h ~1–10 m²/year), t₅₀ is typically 5–15 minutes. Many practitioners run tests to 50% dissipation only and use curve-fitting on early-time data to estimate t₅₀ without waiting for full dissipation, reducing total hold time in deep investigations.

Can I use standard CPT without u2 for clay interpretation? #

Technically yes, but with significantly reduced accuracy. Without u₂, the q_t correction cannot be applied. In soft clays where excess u₂ is large relative to q_c, this can underestimate Q_tn by 20–50%, leading to overestimation of undrained shear strength and potentially unconservative bearing capacity. The cost difference between standard CPT and CPTu is small; the interpretive improvement in fine-grained soils is substantial. CPTu should always be the default choice.

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Related articles #

• Cone Penetration Test (CPT) — complete guide
• CPT procedure — equipment, push rate, and data output
• Robertson SBT chart — how to classify soils from CPT data
• Normalised CPT plots — Q_tn, F_r, and B_q explained
• Estimating soil parameters from CPT
• Foundation design — bearing capacity and settlement guide
• Geotechnical laboratory testing — complete guide