Dr John J M Powell Geolabs Ltd

When or should ‘advanced’ laboratory testing be ‘routine’

Dr John J M Powell

Geolabs Ltd

‘Routine tests’

• Atterbergs

• Particle size, density, specific gravity

• Compaction, CBR • Shear box • Triaxial – UU – CU • Permeability

• IL oedometers, Rowe cells

‘Routine tests’

• The profession often have trouble even

getting these repeatable and of a consistent quality

Proficiency / Interlaboratory

Comparison Testing Scheme

12 14 16 18 20 22 24 26 40 45 50 55 60 65 70 Co n e Pen e tr ation (m m ) Moisture Conent (%) LAB 1 LAB 5 LAB 6 LAB 7 LAB 8 LAB 10 LAB 13 LAB 15 LAB 16 LAB 17 LAB 21 LAB 23 LAB 24 LAB 27 LAB 30 LAB 32 LAB 34 LAB 35 LAB 36

Proficiency / Interlaboratory

Comparison Testing Scheme

Proficiency / Interlaboratory

Comparison Testing Scheme

Proficiency / Interlaboratory

Comparison Testing Scheme

Advanced Tests

• Advanced triaxial, (a significant enhancement on the

standard effective stress capability); including features such

as local axial and radial strain, mid height pwp,

piezobenders and anisotropic stress control (CAU)

• Cyclic triaxial

• Cyclic and static simple shear

• Resonant column

• Don’t forget the CRS oedometer

But first

• So you want to get reliable parameters for

your design using laboratory testing!

• So you need samples, but not just any old

samples, they need to be representative in terms of structure and composition

Eurocodes

Quality and QA

• Quality in sampling

• Quality in transport and storage

• Quality in preparation and testing

• Quality in reporting

• Quality throughout!!

Tube sampling

Stages in sampling and preparing soil

specimen for laboratory test

Sources of tube sampling disturbance

Plugging _Jarring

Indentation fractures

Plugging Jarring Indentation

fractures

Sources of tube sampling

disturbance

Measured water content distributions across the diameter of tube samples of soft clay

Measured water content distributions across the diameter of tube samples of heavily

Canadian Sherbrooke block sampler -Rotation Control vertical

mopvement Water or _{bentonite mud}

Borehole 400 mm in diameter

Water or mud circulated at each cutting tool

Annular slot

Cutting tool every 120 degr.

Block sample being carved out

Block sampling

with Sherbrooke

sampler

Block sample cleaned and wrapped in plastic cling film

• All natural clays have developed some structure

• Degree of structure can be assessed by comparing behaviour of an undisturbed sample to that of a remoulded clay (eg. in oedometer tests)

• Soil structure is a result of several processes including, but not limited to: secondary compression, thixotropy, cementation, cold welding between soil particles (ageing--)

• Effect of sample disturbance is to partly or fully break down the structure of the soil sample – parameters measured by lab tests may not be representative for in situ conditions

Results of CRS

tests

Lierstranda clay 12,3 m depth Semilogarithmic scale

Clearly block sample gives a more stiff behaviour showing less sample

disturbance

Better definition of preconsolidation stress, p_c’

Comparison of IL and CRS Consolidation Data

Vertical Effective Stress '_v (kPa)

10 100 1000 V ert ical S tr ai n  v ( %) 0 10 20 30 (b)

Boston Blue Clay - Newbury Depth = 7.3 m

w = 53%, PI = 21, LI = 1.4

'_vo

CRS IL 24 hr.

Better definition of preconsolidation stress, p_c’, from CRS

UU triaxial compression tests on Laval and piston samples. Bothkennar Clay

Results from shearing phase of CAUC tests

Lierstranda clay from 6.1 m depth

Stress path diagram

(Lunne et al, 2001) Similar failure envelopes?

Unconfined compression tests on Ariake Clay (Tanaka and Tanaka, 1999)

Axial strain (%) 0 2 4 6 8 10 12 14 Comp res si ve Stres s (kPa) 0 10 20 30 40 Shelby tube ELE100 NGI54

Japanese Standard Piston Sherbrooke Laval 10m Shelby tube ELE 100 NGI 54

Japanese standard piston Sherbrooke sampler Laval sampler

Disturbance during specimen preparation

Stiff clays: distinction on basis of unconsolidated undrained triaxial compression ( -)/ 2   a r ( ’+ ’)/2 a r 0 ( -)/ 2   a r ( ’+ ’)/2 a r 0 ( -)/ 2   a r ( ’+ ’)/2 a r 0

Stiff Sandy Clays C = f(w)u Stiff Fissured plastic Clays C = f(p ’)u 0 Stiff Medium plastic Clays C = f(w , p ’)u 0

Conventional practice for sampling

stiff plastic clays

• Shell and auger boring, dry hole, cased to cut off

ground water entry

• Open drive tube sampling

• Unconsolidated undrained triaxial compression tests

for stress-strain-strength

Invariably large scatter in strength and stiffness parameters variously attributed to:

– fabric

– sample disturbance

– stress relief

– sample size

Results of conventional site investigation in London Clay

0 100 200 300 400 500 600

Undrained shear strength, C_u (kPa)

35 30 25 20 15 10 5 0 D e p th b e lo w t o p o f L o n d o n C la y ( m ) 0 20 40 60 80 SPT N (blows/300) 35 30 25 20 15 10 5 0

Initial effective stresses in rotary cores and thin wall tube samples of London Clay

Probably

between rotary foam and

Effects of sampling method in UU triaxial compression tests on Upper Mottled Clay, Lambeth Group

Evaluation of sample quality

• X- ray

• Comparison of tube sampling strains and yield strains

• Reconsolidation strains (esp in oed)

• Measurement of initial effective stress

• Comparison of in situ and laboratory

measurements of shear wave velocity/dynamic shear modulus

How can we then reduce effects of sample disturbance?

• Use the best sampler possible for the project

• Careful sample handling and testing – recompression

technique may to some extent “repair” the sample

• Trimming of sample to smaller diameter may help in

some cases but can also damage sample if not undertaken with great care (tubing vs hand trimming).

Sample disturbance effects

Conclusions:

• Sample disturbance(SD) can be very significant!

• Effect of SD is to partly or completely destroy structure

• SD has significant effects on deformation and strength

characteristics as measured in oedometer and triaxial tests

• e/e_o is a consistent measure of SD for soft clays

• SD effects can best be minimized by carefull choice of drilling and sampling methods

• Sample handling and consolidation techniques may reduce SD effects

In situ tests will also give essential input to choice of soil design parameters, but will not eliminate need for sampling and laboratory testing

Advanced Tests

• Advanced triaxial, (a significant enhancement on the

standard effective stress capability); including features such

as local axial and radial strain, mid height pwp,

piezobenders and anisotropic stress control (CAU)

• Cyclic triaxial

• Cyclic and static simple shear

• Resonant column

• Don’t forget the CRS oedometer

Advanced Tests

• Advanced triaxial, (a significant enhancement on the

standard effective stress capability); including features such

as local axial and radial strain, mid height pwp,

piezobenders and anisotropic stress control (CAU)

• Cyclic triaxial

• Cyclic and static simple shear

• Resonant column

• Don’t forget the CRS oedometer

Shearing Tests

• we often have conflicting requirements of our tests:

Strength – need large strains with minimum restraint while maintaining uniform stresses & strains in sample

Stiffness – need to apply and measure very small stress/strain changes

• triaxial apparatus is fairly unique in its ability to

Triaxial Test

Advantages

• drainage can be controlled

• complete stress state is known

(_a, _r, and U) and can be controlled

 ’ ’_a ’_r ’ Disadvantages:

• axi-symmetric loading – soil parameters depend on mode of loading

’_a

’_r ’_r

Triaxial testing CAUC

The most basic and useful geotechnical test

0 4 8 12 16 20 Axial strain, a, % 0 10 20 30 40 50 S h e a r s tr e s s ,  = ( a -r) /2 k P a

We now have

Excellent equipment that allows us to, • control:

– Axial stresses – Radial stresses – Closed loop

• measure:

– Accurate axial displacements – Radial displacements

– Mid ht pore pressures

– Small strain stiffnesses in varying directions – Volume changes

Mid-height pore pressure measurement

(Hight, 1982)

• use without lateral filter paper will lengthen tests considerably

Mid-height pore pressure measurement

(Hight, 1982)

Prebore hole

Local Strain Measurement - Axial

Hall Effect

(Clayton & Kathrush., 1986) LVDTs

(Cuccovillo & Coop, 1997)

• most accurate

• difficult to mount

• transducers generally only glued to membrane – pins no longer used

Inclinometers

(Jardine et al., 1984)

• sensitive to rigid body rotation of sample – need to take average of two readings on opposite sides of sample

• cannot be used for r

• accurate

LVDT-core

LVDT-body

Flexible wire Fixing screw

Radial strain belt Mount

Sample Right-angle connection

Submersible cable

Local Strain Measurement - Radial

(Klotz & Coop, 2002)

• single LVDT version or Hall effect • double LVDT or Hall effect version - allows larger _r -difficult to mount -BUT SPACE

Bender Elements

 shear plane wave travelling through an elastic isotropic or cross-anisotropic medium – measure elastic shear stiffness, G₀

Input Output

D

Piezoelectric Bender Elements

Kramer (1996) _{(Dyvik & Madhus, 1985)}

v = D/t_arr G₀ = rv2

First Arrival

Control of triaxial tests: feedback loop ’_a ’_r ’_r triaxial transducer output (voltage) computer data logger (analogue-digital conversion) transducer output (digital) controller command change of stress or strain

• automated control of tests much less common than data-logging

simple basic program

Setting it all up

Setting it all up

Larger cells

more space, large strains

•behaviour defined by the following parameters:

E’_v = vertical Young’s modulus E’_H = horizontal Young’s modulus

’_VH = Poisson’s ratio for influence of ’_V on _H

’_HV = Poisson’s ratio for influence of ’_H on _V

’_HH = Poisson’s ratio for influence of ’_H1 on _H2 or ’_H2 on _H1 G_VH = shear modulus in vertical plane

G_HV = shear modulus in vertical plane G_HH = shear modulus in horizontal plane

Anisotropy of Elastic Stiffnesses: Cross-Anisotropic Soil

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 -10 0 10 20 30 40 50 60 70 80 90 100 S tr ai n (%)

Change in mid-plane effective stress (kPa)

local axial external vol local vol

Stress Path

Isotropic

Stress Path

Anisotropic Consolidation

Stress Path

2nd _Anisotropic

Stress Path

-200 -150 -100 -50 0 50 100 150 0 50 100 150 200 250 300 350 400 450 Sh ea r s tr es s t (k Pa )

Mean effective stress, s' (kPa)

this stage previous stages aniso3 aniso2 aniso1 finish start

0.0001 0.001 0.01 0.1 1 10

dynamic methods

local gauges

conventional soil testing

Shear strain _s: % Sti ffn es s G

Typical strain ranges

Retaining walls Foundations

Tunnels

Measurement of Stiffness

q’ _a E_tan = dq/d_a E_sec = q/_a critical state

- gradient over odd number of points

- No. of points in regression depends on No. of number data points

recorded (use a fixed strain interval)

- plot stiffness against strain at central point

q’

_a

E_tan = dq/d_a

calculation of tangent stiffnesses

0 20 40 60 80 100 120 0.0001 0.0010 0.0100 shear strain (%) G ( M P a )

• tangents always more scattered than secant at small strains (also have more meaning)

natural London clay (Gasparre, 2005)

secant

0.0001 0.0010 0.0100 0.1000 1.0000 axial strain (%) 0 500 1000 1500 2000 2500 3000 G u ( M P a) local LVDTs external LVDT

• natural Greensand – very high G

• suction cap used & compliance

correction made

• strains prior to shearing small

• reconstituted kaolin – low G

0.0001 0.0010 0.0100 0.1000 axial strain (%) 0 40 80 120 G u ( M P a) local LVDTs external LVDT

Measurement of Stiffness – Examples of Tangent Stiffnesses

0 100 200 300 400 500 600 700 -4 -3 -2 -1 0 1 2 Lo c al s ec an t Y ou ng 's m od ul us ( MP a)

log(local axial strain (%))

0 100 200 300 400 500 600 700 -4 -3 -2 -1 0 1 2 Lo c al s ec an t Y ou ng 's m od ul us ( MP a)

log(local axial strain (%))

Anisotropy in London Clay

Sample quality assessment based on shear wave velocity

Using the equipment for Poisson’s ratio and

small strain stiffness of rock

Is it new or just commercially viable?

Summary

• there is much that can go wrong in conducting and interpreting tests But it can be done

• we should conduct and interpret tests within a chosen and

appropriate theoretical framework

• level of complexity of tests should be appropriate to theoretical framework and design method

• You need to know what you are specifying and what can be

realistically achieved, commercial vs research

I must say - thanks

• I wish to acknowledge the help from

– David Hight

– Tom Lunne

– Matthew Coop

For some of the slides contained in this presentation

Conclusions!

• Rubbish in – Rubbish out!

Conclusions!

• We now have a new level of testing

available to us which I believe should be

consider ‘routine (advanced) testing’ for use when projects warrant it and samples are of the right quality.

• Particularly relevant for modelling and in

And finally

Thank you for your attention