Session Report

Interpretation of In-situ Tests

#2

Peter K. RobertsonSept 2016

Summary for Total Session

Summary for this Portion

SPT

• 3 papers related to the SPT:

– Nunez et al – Brazil

• Rotation speed for SPT-T

– Lukiantchuki et al – Brazil

• Undrained strength from N based on energy concepts

– Ibanez et al – Spain

• Estimating Vp from N in volcanic soils in Costa Rica

SPT

• Nunez et al – Brazil

– “Rotation speed for SPT-T”

• Seven sites tested, mostly residual soils in southern Brazil

• Rate of rotation (between 3 to 9 rpm) appears to have little

influences on the measured torque

• Recommend rate of rotation of 5 to 8 rpm

SPT-T has been around for over 25 years and is

still not popular outside of Brazil • Simple but somewhat crude test

• Many variables (e.g. soil plug inside sampler, etc.)

SPT

• Lukiantchuki – Brazil

– “Undrained strength from N using energy concepts ”

• Major assumptions:

– Convert dynamic force to static force

– Variable ‘Adhesion’ factor to get undrained strength

– No point resistance (all side friction – inside and outside sampler)

• Poor correlation

– Insensitive in soft clays (N < 5), since dynamic and static force to

sampler is essentially constant

SPT

• Lukiantchuki – Brazil

– “Undrained strength from N using energy concepts ”

• Major assumptions:

– Convert dynamic force to static force

– Variable ‘Adhesion’ factor to get undrained strength

– No point resistance (all side friction – inside and outside sampler)

• Poor correlation

– Insensitive in soft clays (N < 5), since dynamic and static force to

sampler is essentially constant

–WHY?

SPT

• Ibanez et al – Spain

– “Estimating Vp from N in volcanic soils in Costa

Rica”

• Can only apply in unsaturated soils (Vp ~ 1500m/s in most

saturated soils)

• Why Vp? Generally Vs is more meaningful to geotechnical

engineers

• Poor correlation found

SPT

• Ibanez et al – Spain

– “Estimating Vp from N in volcanic soils in Costa

Rica”

• Can only apply in unsaturated soils (Vp ~ 1500m/s in most

saturated soils)

• Why Vp? Generally Vs is more meaningful to geotechnical

engineers

• Poor correlation found

–WHY?

Stop using the SPT?

• Mayne et al (2009)

– “false sense of reality in the geot. engineer’s

ability to assess each and every soil parameter

from the single N-value”

Geot. engineers in the 21st century should

progressively STOP using this crude,

unreliable in-situ test

SPT - CPT

1 paper on correlation between SPT - CPT:

• Cunha et al – Brazil

– “SPT-CPT correlation in sedimentary tropical soils

silty sands in Brazil”– 88 SPT and 8 CPT from 1 site in NE of Brazil

– No energy measurements – assumed 60%

– Soils mostly colluviun/alluvium silty sands

– Large variability in SPT N values (less in CPT qc)

– Distance between SPT and CPT up to 35m!

– Average (qc/pa)/N60 = 3.9

• No comparison with existing SPT-CPT correlations

which would have predicted same value

CPT

• 10 papers related to the CPT:

– 3.5 on Dissipation test

– 2.5 on Rate effects

– 4.0 on Numerical methods

Dissipation Tests

• Odebrecht et al – Brazil

– “Predicting su from CPTu dissipation tests”

• Imre – Hungary

– “Comments of CPTu & DMT dissipation tests”

• Imre – Hungary

– “Dissipation tests in saline environment”

• Paniagua – Norway

– “Mono. & Dilatory dissipation tests in silt following CPTu

at variable rate”

Dissipation Tests

• Odebrecht et al – Brazil

– “Predicting su from CPTu dissipation tests”

• Case studies (building on Mantaras et al, 2014, 2015)

• su = Dumax / (4.2 (+/- 0.2) log Ir)

Dumax = max. excess pore pressure during dissipation

Dumax

Conceptually

insensitive to

OCR – based on

theory

Dissipation Tests

• Odebrecht et al – Brazil

• su = Dumax / (4.2 (+/- 0.2) log Ir)

• Karlsrud et al (1996) & Lunne et al (1998)

• su = Du / NDu - where NDu varies with Bq (i.e. OCR)

Dumax Du

Generally

Little difference between

Dumax ~ Du

in soft clays

Dissipation Tests

Odebrecht et al – Brazil

• su = Dumax / (4.2 (+/- 0.2) log Ir) ~ Dumax /( 7 to 12)

Karlsrud et al (1996) & Lunne et al (1998)

• su = Du / NDu ~ Du / (4 to 10)

Robertson (2012)• NDu = Bq * Nkt

• Bq = fn (OCR, Ir, St) and Nkt = fn (OCR, Ir, St)

• Nkt ~ 14 (+/- 2) then NDu varies from 3 to 14

Unsure of benefits since it requires a dissipation

test and knowledge of uo (i.e. long test)

Dissipation Tests

• Imre – Hungary

– “Comments of CPTu & DMT dissipation tests”

• Building on Imre et al (2010, 2011, 2014, 2015)

• Complex numerical solution with several variables

• Can handle monotonic and non-monotonic dissipations

(even from negative start)

• Auto non-linear inverse problem solution (software)

• Evaluated using CPTu and DMT from Ballina (Aus.)

• Can work on shorter dissipation test (t < t50) in real time

• COMPLEX method but shows promise

Dissipation Tests

• Imre – Hungary

– “Dissipation tests in saline environment”

• Building on Imre et al (2014a & b)

• Site in Hungary (Duna and Tisza Rivers)

• Upward saline groundwater flow changes soil behaviour

(i.e. increased compressibility & lower strength)

• Saline decreases t50 (increases ch)

Dissipation Tests (rate effects)

• Paniagua – Norway

– “Mono and Dilatory dissipations in silt following

CPTu variable penetration rate”

• Soil response can be contractive or dilative

• Two Norwegian silts with mono and dilatory dissipations

– Vassfjellet in lab and Halden in field

• Study influence of penetration rate on t50

• Evaluated different published methods to get t50

• No clear trend – likely a function of soil dilatancy

• Test procedures become important, esp. for short

dissipations (e.g. fixed rods)

Rate effects (CPT)

• Dienstmann et al – Brazil

– “Cavity expansion applied to variable rate CPT in tailings”

• Martinez et al - Italy

– “Influence of penetration rate on CPTu in silty soils”

• Paniagua – Norway

– “Mono. & Dilatory dissipation tests in silt following CPTU

at variable rate”

Rate effects (CPT)

• Dienstmann et al – Brazil

– “Cavity expansion applied to variable rate CPT in

tailings”

• Building on Dienstmann et al (2015)

• Non-linear poro-elastic model with analytical and

numerical solutions (14 parameters needed for solution)

• Compared with field data from tailings (gold) in Brazil

• Reasonable agreement but sensitive to some parameters

Du2/Du(max)

V = v d/ ch

Undrained

Drained

Rate effects (CPT)• Martinez et al – Italy

– “Influence of penetration rate on CPTu in silty sands”

• Variable rate CPTu in southern Po River (Bologna)

• Holocene-age clayey-sandy silt to silty-clayey sand

• Soil variability & dilatancy complicates comparing rate

effects

Martinez et al - Italy

Dissipation Tests/ Rate effects

DeJong and

Randolph (2012)

showed that

partial drainage

creates essentially

a non-unique

relationship

between t50 and ch

when t50 < 60s and

partial drainage

occurs

DeJong & Randolph, 2012

~60s

10cm2 cone at 2 cm/s

Teh & Houlsby 1991

Dissipation Tests/ Rate effects

DeJong and

Randolph (2012)

showed that

partial drainage

creates essentially

a non-unique

relationship

between t50 and ch

when t50 < 60s and

partial drainage

occurs

DeJong & Randolph, 2012

UNDRAINED

PARTIALLY

DRAINED ~60s

Essentially

Drained

~6s

10cm2 cone at 2 cm/s

V =10 V =1

DeJong &

Randolph 2012

Teh & Houlsby 1991

Rate effects (CPT)• Most studies have looked at soft contractive soils

• Dilative soils will tend to behave with opposite trends

Contractive

Dilative

Data from

Martinez et al - Italy

UndrainedDrained

u2qt

u0

Numerical

• Frost et al – USA

– “DEM of interaction between soils and in-situ tests”

• Ma et al - Australia

– “Effect of Ir on interpretation of interbedded clay using CPT”

• Debasis et al – India

– “FEM of CPT in weakly cemented sand”

• Carvalho et al – Brazil

– “Virtual T-bar tests using DEM”

Numerical

• Frost et al – USA

– “DEM of interaction between soils and in-situ tests”

• DEM to study CPT, DMT & KSB in sands

– Increase in forces/stresses when probed changes size/shape

• DEM to study rough sleeve on CPT

– Textured sleeve have higher stresses and engage more soil shear

with well defined shear surface

In practice, textured sleeves have major issue of wear,

especially in sands (i.e. all sleeves tend to become

smooth)

Numerical• Ma et al - Australia

– “Effect of Ir on interpretation of interbedded clay using

CPT”

• LDFE used to study layering in clay (soft-stiff-soft)

• If t2 > 10D measured qnet is correct (up to su2/su3 < 4)

su1

su2t

su3= su1

qnet2 = k qnet2m

Numerical

• Debasis et al – India

– “FEM of CPT in weakly cemented sand”

• Numerical and lab analysis on very weakly cemented sand

• Cementation on lab samples was very weak (almost no

difference from uncemented samples)?

• Likewise in FEM simulation

• Suggest stronger cementation for next study and improved

constitutive model for FEM and measure Vs on lab

samples

Numerical

• Carvalho et al – Brazil

– “Virtual T-bar tests using DEM”

• DEM to model clay – needed added field forces to model

electrochemical forces between clay particles and

agglomerated particles

• Modeled soft Brazil coastal clay (Guanabara Bay)

• Results show promise but model parameters are complex

and hard to evaluate.

Main points

• Growing interest and activity in CPT (>60% of papers)

• Short paper length is often a restriction for authors to

adequately review background and present their work

• Papers often focus on narrow picture of an issue and

sometimes miss the bigger picture (e.g. why and how

to apply)

• sometimes part of a larger study