Interpretation of SoilParameters -

Coarse-grained soils

Peter K. Robertson

CPT in Geotechnical Practice

Santiago, Chile

July, 2014

Soil Parameters

Interpretation of the CPT in

coarse-grained soils

such as: sand, silty sand, sandy silt,gravelly sand, sandy gravel

Robertson, 2014

Perceived applicability of CPT forderiving soil parameters

Initial stateparameter

StrengthParameters

DeformationCharacteristics*

FlowCharact.

SoilType

γ Dr/ψ

Ko OCR St su Φ’ E,G M Go k ch

Fine-grained

2-3 2-3 1 2-3 1-2 4 2-3 2-3 2-3 2-3 2-3

Coarse-grained

2-3 2-3 4-5 4-5 2-3 2-3 2-3 2-3 3 3-4

Applicability rating: 1 high reliability, 2 high to moderate, 3 moderate, 4 moderate to low,5 low reliability

* Improved when using SCPT to get Vs

Robertson, 2014

Soil Behaviour Type (SBT)Q

t=

(qt-

svo

)/s’

vo

Fr = 100[fs/(qt-svo)]

Fine-grained

soils

Coarse -grained

soils

Robertson (1990)

Coarse-grained soilsessentially plot in

SBT zones 5, 6, 7 and8 on the normalized

SBT chart byRobertson (1990)

Approx. Ic < 2.60

Robertson, 2014

SBT from CPT

Low plastic finesHigh plastic fines

Clay like

Idriss & Boulanger, 2008

Transitionregion

Sand likeIc ~ 2.60

Robertson & Wride, 1998

Robertson, 2013

SBT from CPT

• Soil behavior influenced by fines content andplasticity of fine– e.g. soils with small amount of high plastic fines behave

more like a clay and soils with large amount of low plasticfines behave more like sand

• CPT Soil Behavior Type Index Ic capturescombined influence of fines content andplasticity of fines

Fines content alone can not capture correct soilresponse

Robertson, 2014

Schematic of soil loading around cone

Robertson, 2014

Generalized stress-strain relationship • Tip and sleeve resistancestrongly influenced byhorizontal effective stress,sh’

• Tip stress (qc) mustovercome any soildilatancy – high qc indense sands

• Easier to overcomedilatancy when grains aremore compressible (e.g.angular and/or carbonatesands)

Refusal reached when meangrain size, D50

D50 > dcone

qc influenced by grain sizewhen D50 > 0.3 dcone

sh’

dcone

Harder to compressgrains when

cemented

Thin layer effect

CPT data in ‘transition’when cone is moving fromone soil type to another whenthere is significant differencein soil stiffness/strength

CPT data within transitionzone will be misinterpreted

In a thin sand layer qc willnot reach full (correct) valuefor the sand (strength of sandwill be under estimated)

Ahmadi & Robertson, 2005 Robertson, 2014

10 cm2 cone

For qc to reach full value indense sand - layer must be~1m thick (t > 25dcone)

Geologic Context

• Most semi-empirical correlations are based oncase histories in ‘well behaved’ soils

– Mostly normally to lightly overconsolidated

– Relatively young (Holocene to Pleistocene-age)

– Silica based (quartz)

– Sedimentary soils

Robertson, 2014

Theory for CPT• Challenges:

– Major assumptions needed for:

• Geometry & boundary conditions

• Soil behavior

• Drainage conditions

• Real soil behavior very complex

• Semi-empirical correlations still dominate, butsupported by theory

– many correlations (in coarse-grained soils) limitedto ‘clean, young, uncemented, silica sands’

Robertson, 2014

Estimatingsoil unitweight

g/gw = 0.27 [log Rf] + 0.36 [log(qt/pa)] +1.236 Robertson, 2010

Robertson, 2013

Increasing unit weight

Relative Density, Dr

• Relative density, Dr is often used as anintermediate parameter

Dr = (emax – e)/(emax - emin)

• Uncertainty associated with emax and emin

• Strength and stiffness not always wellrepresented by Dr

• Most relationships between Dr and CPT basedon large calibration chamber (CC) testing

Calibration Chamber TestingControlled test environment tostudy link between CPT qc andrelative density Dr in cleansands

Limited to clean sandsLimited depositional environmentSamples very young (days)

Robertson, 2014

Modified from Mayne, 2009

Robertson & Campanella, 1983

Summary of Dr CC resultsRobertson, 2014

Mayne, 2009 = Qtn

(Dr)2 = Qtn

305 · Cc · COCR · CAGE

Cc = Compressibility Factor (+/- 30% in terms of Dr)

1.5 low compressibility0.5 high compressibilityControlled by grain size, shape and mineralogy

COCR = OCR0.18

CAGE = 1.2 + 0.05 · log(t/100), t in years

Qtn = (qt – σv) (Pa/σ’vo)0.5

Pa

Relative Density of SandsSimplified Approach (30% < Dr < 80%)

(Modified fromKulhawy & Mayne)

Robertson, 2014

Simplified Dr relationshipRobertson, 2014

Mayne, 2009

Qtn = 305 (Dr)2

= Qtn

Carbonate sandsRobertson, 2014

Modified fromMayne, 2009 = Qtn

Dr (%) = 0.87 Qtn

Qtn

/(D

r)2

(Dr)2 = Qtn/C

C increases with ageFor age < 1,000 yrs

C ~ 305

Effects of Age ofdeposit on CPT

tip stress, qc

Data from CANLEXfrozen samples

(Wride et al, 2000)

Robertson, 2014

AGE (years)

Jamiolkowski et al 1988

Kulhawy & Mayne, 1990

State Parameter, Y(better than Dr, since it accounts for stress level)

After Jefferies and Been, 1985Relative State Parameter index

After Boulanger, 2003

‘Loose’

(-) DenseDilative

‘Dense’

(+) LooseContractive

y

Robertson, 2014

State Parameter from CPT• Jefferies and Been (2006) summarized ~30yrs of

research related to evaluation of liquefaction using aCritical State Soil Mechanics (CSSM) approach

• Problem is complex & depends mainly on: in-situstresses, shear stiffness, shear strength, compressibility& plastic hardening

• Requires combination of in-situ tests (SCPT) and labtesting (reconstituted samples to get CSL)

• Based on extensive calibration chamber test results, fieldresults (frozen undisturbed samples), lab testing &numerical simulation – estimate of state from CPT

State Parameter

Soils with samestate have

similar behavior

Approx. contoursof soil state for

young, uncementedsoils

(shape controlled bysoil compressibility –

i.e. slope of CSL)

Increased resistanceto loading

Same in-situ StateDifferent penetration resistance

Robertson, 2014

State parameter & clean sand equivalent

Robertson, 2014

Based on liq. case historiesBased on CSSM theory & CC

Increased resistanceto loading

Increased resistanceto loading

~ 0.56 – 0.33 log Qtn,cs

Friction angle in sands

Mayne, 2006 Robertson, 2014

Young uncemented silica sands

Friction Angle in Sands

Based on link to StateParameter, y

young, uncemented coarse-grained soils

f ’ = fcv + 48 y

f’ = f cv+15.84 [log Qtn,cs]–26.88

*f cv very important and easy tomeasure on disturbed samples

f’ ~ 10 + fcv

f’ ~ 5+ fcv

f’ ~ 3 + fcv

f’ ~ fcv

Robertson, 2014

400

450

350

Mayne, 2006 (clean sands)

Seismic CPT

• Geophone incorporated into standard CPT

• Downhole seismic method

• Shear wave velocity, Vs, measured every 1meter

• Shear wave velocity measured in same soil ascone penetration resistance, qc

• Simple and reliable technique

Gmax = Go = ρ·(Vs)2 ρ = γ/g

Robertson, 2014

Seismic CPT

• 30 years experience (1983)

• Simple, reliable, and inexpensive

• Direct measure of soil stiffness

– small strain value, Go = ρ·Vs2

• Typically 1 meter intervals

• Combines qc and Vs profile in same soil

Robertson, 2014

Seismic CPT System Configuration

Robertson, 2014

Vs

Difficult or Non-textbook Ground

• Most existing published experience/researchbased on typical “ideal - well behaved”ground– young, uncemented: soft clay and clean silica sand

• Limited published experience/research on‘difficult - non-textbook’ ground– stiff fissured clays, soft rock, intermediate soils (silts),

calcareous soils, man-made ground, tailings, olderand/or cemented soils

Vs and CPT

• Vs controlled mainly by: state (relative density &

OCR), effective stresses, age and cementation

• CPT tip resistance, qt, controlled mainly by:state (relative density & OCR) and effective stresses,and to less degree by age and cementation

• Strong relationship between qt and Vs, butdepends mainly on age and cementation

– potential to estimate age/cementation using SCPT

Estimating age and/or cementation

After Eslaamizaad and Robertson, 1996 and Schnaid, 2005

Go/qt

Qtn

young & uncemented

High compressibility

Low compressibility

Age

Cementation/Bonding

Estimating Vs from CPTSoils withsame Vs1

have similar(small strain)

behavior

Based on extensivedatabase contours of

Vs1

for uncemented,Holocene -

Pleistocene age soils

Go = r (Vs)2

Increased resistanceto loading

Robertson, 2014Vs = [vs (qt – v)/pa]

0.5 (m/s) where vs = 10 (0.55 Ic + 1.68)

Example Vs measured vs estimated

Robertson, 2014Young uncemented soils – San Francisco

measured

estimated

E’ for uncemented silica sands based CC tests

After Bellotti et al., 1989

αE =

E = aE qc

Robertson, 2013

2 < aE < 20

Young’s Modulus E’

E’ = aE (qt - svo) ~ aE qc

aE = 0.015 [10 (0.55Ic + 1.68)]

2 < aE < 20

Robertson, 2013

Based on extension of therelationship between CPTand Vs for younguncemented coarse-grained soils

E = 2.5 G0 = 2.5 r (Vs)2

Robertson, 2009

Summary• CPT interpretation should be done within a

geology framework (i.e. understand thegeology)

• CPT can provide good estimate of a widerange of geotechnical parameters in mostcoarse-grained soils

– influenced by mineralogy, age and cementation

– SCPT helpful to identify age & cementation

• Best to view parameters as a profile (i.e.maintain the stratigraphy and variability)