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CPT/CPTU Interpretation of Stratigraphy: Soil Layering and Soil Classification

1. Stratigraphy Key signatures of soil layering from CPT/CPTU data

2. Soil Classification - development and application of soil classification charts

3. Examples of results in different soil types.

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Measured Data and Calculated Variables1. Measured Data

most common = qc, fs, and u2

2. Calculated Variables (for u2 measurement): Corrected tip resistance: qt = qc + u2(1-a) Excess pore pressure u = u2 u0 Friction Ratio: Rf = fs/qc Normalized net tip resistance: Qc = (qc vo)/'vo Normalized sleeve resistance: Fr = fs/(qc vo) Pore Pressure Parameter: Bq = (u2 uo)/(qt vo) Normalized Excess Pore Pressure: U = (u2 uo)/'vo Normalized Corrected Tip Resistance: Qt = (qt vo)/'vo

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Stratigraphic ProfilingExcellent application for the CPT and especially the CPTU

Approach:1.Reply on fundamentals of soil behavior, i.e., stiffness (e.g., dense sand vs. soft clay) and drainage (drained behavior during shear in sand vs. undrained behavior during shear in clay).

2.Use all information available qc or qt, fs, u, Qt, Rf, Bq(+ other sensors when available).

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Stratigraphic ProfilingKey Signatures to look for in measured data, e.g.:

1. Shape and magnitude of qt profile e.g., high in dense sand, low in soft clay

2. Shape of u profile and magnitude, especially relative to equilibrium pore pressure profile e.g., high in soft clay, u = 0 in medium density sand

3. Magnitude of Rf relative to that of qt e.g., if high and coupled with low qt = soft clay.

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Example CPT in Western MassachusettsInspect relative values of qc, fsand Rf

Med.

Dense

Sand

Clay(CVVC)

UNITS:1 ksc 100 kPa 0.1 MPa 2000 psf 1 tsf

Loose

Sand

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Example CPTU in Eastern Massachusetts

StiffClayCrust

UniformSoftClay

SPT N = WOR (i.e., = 0)

Linear increase in qt and u2 with depth

High u2 relative to u0

Boston Blue Clay

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Example CPTU in NE Massachusetts

Significant variations in qt, fs and u2 with depth

Boston Blue Clay- Newbury, MA

Stiff, high OCR CLAY Crust

Sensitive, soft CLAY

Dissipation Test

Interbedded Layers, Silt, Clay, Sand

Increasing silt content

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Example CPTU - Holland

Note:- jump in Rf in Peat

Layers

- low qc, fs but high u in Clay

- high qc, fs but low Rf in sand + u close to u0

- apparent significant stratification in middle sand layer

0

5

10

15

20

25

30

Dep

th (m

)

Sand

PeatClay

Sand

Peat

Clay

Sand

FrictionRatio

(%)

Pore water pressure(MPa)

Sleevefriction(MPa)

Cone resistance(MPa)

SoilProfile

1.0 0.5 0 0.2 0.1 0 4 8 12 16 20 8 6 4 2 0

uo

Pre-drilled

[Zuidberg et al. 1982]

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Example CPTU profiles in Venetian soils

Significant interbeddingof soils from sands to siltyclays

CPTU19x=0.0m

CPTU9x=66.3m

CPTU8x=134.2m

0 10 20 30qt, MPa

0 10 20 30qt, MPa

0 10 20qt, MPa

2 0u2, MPa

2 0u2, MPa

2 0u2, MPa

0.0 0.2 0.4 0.6fs, MPa

0.0 0.2 0.4 0.6fs, MPa

0.0 0.2 0.4fs, MPa

50

40

30

20

10

Dep

th b

elow

MW

L (m

)

qtfsu2

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Example CPTU Offshore Deep Water Site

Location of seabed anchors

199000 199500 200000 200500 201000 201500 202000 202500Easting (m)

648500

649000

649500

650000

650500

651000

651500N

orth

ing

(m)

FPSO

CPTU LocationsAnchor LocationsFPSO LocationAnchor Lines

c

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Deep water site

CPTUsconducted at one anchor location

199700 199800 199900 200000 200100Easting (m)

648500

648600

648700

648800

648900

Nor

thin

g (m

)

c

approx. 80m between CPTUs

CPTU A

CPTU BCPTU C

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0 0.1 0.2 0.3 0.4sleeve friction, fs (MPa)

0.0 1.0 2.0 3.0 4.0cone resistance, qt (MPa)

28.0

26.0

24.0

22.0

20.0

18.0

16.0

14.0

12.0

10.0

8.0

6.0

4.0

2.0

0.0

dept

h be

low

sea

bed

(m)

0 0.1 0.2 0.3 0.4sleeve friction, fs (MPa)

0.0 1.0 2.0 3.0 4.0cone resistance, qt (MPa)

28.0

26.0

24.0

22.0

20.0

18.0

16.0

14.0

12.0

10.0

8.0

6.0

4.0

2.0

0.0

dept

h be

low

sea

bed

(m)

0 0.1 0.2 0.3 0.4sleeve friction, fs (MPa)

0.0 1.0 2.0 3.0 4.0cone resistance, qt (MPa)

28.0

26.0

24.0

22.0

20.0

18.0

16.0

14.0

12.0

10.0

8.0

6.0

4.0

2.0

0.0

dept

h be

low

sea

bed

(m)

CPTU B CPTU CCPTU A

Deep water site CPTUs at one anchor location

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0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0corrected cone resistance, qt (MPa)

28.0

26.0

24.0

22.0

20.0

18.0

16.0

14.0

12.0

10.0

8.0

6.0

4.0

2.0

0.0

dept

h be

low

sea

bed

(m)

Deep water site CPTUs at one anchor location

Spatial variability in CPTU data required custom dimensions for each anchor (Note: for anchor design initial penetration and final resistance important)

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Example CPTU in Japanese volcanic soil

[Takesue et al. (1995)]

Note correlation between SPT N values and CPTU but SPT testing was continuous

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X-ray of fixed piston sample of Connecticut Valley Varved Clay (CVVC) Amherst, MA

Silt = "summer" depositClay = "winter" deposit

Example CPTU in Connecticut Valley Varved Clay (CVVC),

Western MA

= 10 inches

Clay

Silt

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Example CPTU in CVVC, Amherst, MA

Increasing silt content and thickness of silt layers

Stiff desiccated CVVC crust

Lightly overconsolidated CVVC = soft, moderately sensitive "clay"

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Piezoprobe picture

Projected tip area = 1.25 cm2

u1(face), u2, u1(tip)

u1 - Wissau2u1 - button

Miniature Piezoprobe for high resolution profiling of thin soil layers

18/33Pore Pressure (kPa)

300 400 500 600

Dep

th (m

)

7.50

7.55

7.60

7.65

7.70

7.75

7.80

Wissa u1u2u1 button

push at 2 cm/s sample at 64 Hz

Example Miniature Piezoprobe CVVC Amherst, MA

419/33Pore Pressure (kPa)300 350 400 450 500 550

Dep

th (m

)

10.30

10.32

10.34

10.36

10.38

10.40

Wissa u1

u1 (tip) at 0.5 cm/s

10 cm

Example Miniature Piezoprobe CVVC Amherst, MA

Clay-Silt Interface = spring thaw

Increasing clay content (going upwards) = deposit of finer grained particles in calm waters of ice covered lake

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Soil Classification from CPT/CPTU dataMethodology:

1. Quantify observations used to identify soil stratigraphy.

2. Empirically based, i.e., measured CPT/CPTU data are correlated with know soil profiles.

3. Early charts relied on direct use of reduced data, e.g., qc or qtand fs or Rf.

4. Later charts make use of normalized parameters to account for increasing overburden stress with depth, e.g., Qt, Bq.

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CPT Soil Classification/Behavior Chart

Based on qc and fsfrom CPT

[Figure 5.6Douglas and Olsen 1981]

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Measured CPTU pore pressure by location and soil type

[Robertson et al. 1986]

u1 > u2 > u3

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Pore Pressure (via Bq) for soil Classification

Note: measured u is function of location chart is for u2 position. Hence, negative pore pressures can occur.

[Janbu and Senneset 1984]

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Soil Behavior Type Classification ChartChart making use of qt

[Robertson et al. 1986]

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Soil Behavior Type Classification Chart

[Robertson 1990]

Based on normalized CPTU data

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Example CPTU Soil Classification Oslo Airport

[Sandven et al. 1998]

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Newbury BBC classification chart= "crust" = "Interbeddd silt, clay, sand= Soft, moderately sensitive Clay

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Example of "Automated" Soil Identification Chart

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Example of "Automated" Soil Identification Chart

CVVC Amherst, MA30/33

Additional Measurements for better definition of soil type/behavior

Options include:[Note: additional sensors covered in later topic] Short dissipation tests with CPTU

Dual or Triple element (pore pressure) CPTU

Seismic CPTU to get Shear Wave Velocity (Vs)

Electrical conductivity (or resistivity) = relate to soil porosity, degree of saturation, relative density, leaching of quick clays

Nuclear density/Gamma Cone = density of soil units

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[Campanella et al, 1984]

Example CPTU Mine Tailings with ice lenses

Ice lenses = sharp spikes in qc and u2

Use of dissipation tests to aid in classification

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[Robertson et al. 1995]

Soil Classification/Behavior Chart using Gmax

- G0 = Gmax- Vs direct measure from seismic CPTU- t must be estimated

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- Use all information available, e.g., qc or qt, fs, u, Fr, Bq- Shape and magnitude of qt profile gives indication on whether you are in uniform clay layer, sand layer, etc.

- Pore pressure profile readily indicates a drained condition (e.g., sand with u = 0) or undrained (e.g., clay with u > 0)- Use qt - Rf - Bq and/or Qt-Fr-Bq diagrams to identify soil type. Accumulate local experience to create/modify diagrams.

- Short dissipation tests can help in identifying soil type

- Measurements using other sensors (e.g., Vs) can enhance soil identification

Recommendations: CPT/CPTU based Soil Identification/Classification