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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
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