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Guidance for Specifying the Strength of

Deep-Mixed Ground

George Filz

Virginia Tech

Deep Mixing Short CourseDeep Foundations Institute

February 15, 2012

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Guidance for Specifying the Strength ofDeep-Mixed Ground

Flowchart for design and construction

Bench-scale testing

Field trial/demo columns

Specified strength vs design strength

Specification provisions

QC/QA and remedial measures

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Need for

field trial duringdesign?

Design

requirements

satisfied?

Need for

field demo bycontractor?

Loads and

performance

Site characterization

studies

Bench-scale mix

design testing

Published data and

prior experience

Establish design

strength

Analyses

Field

trial

Treatment

geometry

Yes

No

No

Yes

Prepare plans

and specs

with

field demo

Field demo

Construction

with on-going

contractor QC and

owner/engineer QA

Prepare plans

and specs

without

field demo

BiddingBidding

No Yes

Bench-scale

testingBench-scale

testing

Data Collection

Design

Procurement

Construction

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Need for

field trial duringdesign?

Design

requirements

satisfied?

Need for

field demo bycontractor?

Loads and

performance

Site characterization

studies

Bench-scale mix

design testing

Published data and

prior experience

Establish design

strength

Analyses

Field

trial

Treatment

geometry

Yes

No

No

Yes

Prepare plans

and specs

with

field demo

Field demo

Construction

with on-going

contractor QC and

owner/engineer QA

Prepare plans

and specs

without

field demo

BiddingBidding

No Yes

Bench-scale

testingBench-scale

testing

Data Collection

Design

Procurement

Construction

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Bench-Scale Testing: Purposes

Determine treatability of site soils with different binder types:

Cement Lime

Slag

Estimate amount of binder needed, which tends to increasewith increasing:

Water content

Organic content

Observe mixing, which is generally more difficult for stiff and

plastic soils than for soft soil or cohesionless soil

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Bench-Scale Testing: Methods

Obtain sufficient amount of representative soil from each

distinct soil type to be treated Protect soil from drying and from oxidation

Perform index property tests on soil: water content,

Atterberg limits, organic content

Prepare binder in dry or slurry form to match proposed

construction method

Blend binder with soil using a dough mixer

Pack mixed soil into plastic molds and seal

Cure in humid environment

Test in unconfined compression

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Bench-Scale Testing: Parameter Variations

Soil type

Binder types and ratios Water-to-binder ratio of slurry, for wet method

Binder amount

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Definitions: Dry Method

Binder factor = weight of dry binder per unit volume of

soil to be mixed Binder factor in-place = weight of dry binder per unit

volume of mixture (volume of soil to be mixed plus

volume of dry binder)

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Definitions: Wet Method

Water-to-binder ratio of the slurry = ratio of the weight of

water to the weight of dry binder in the slurry Volume ratio = ratio of the volume of the slurry to the

volume of the soil to be treated

Binder factor = weight of dry binder per unit volume of soil

to be mixed

Binder factor in-place = weight of dry binder per unit

volume of mixture (volume of soil to be mixed plus volume

of slurry) Total-water-to-binder ratio = ratio of the weight of the soil

water plus the slurry water to the weight of dry binder in

the mixture

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Bench-Scale Testing: Data Reduction

Plot unconfined compression strength versus curing time for

each batch, use smoothed curve to obtain 28-day strength Plot 28-day strength versus binder factor for each soil type,

binder type, and water-binder ratio

Plot 28-day strength versus total-water-to-binder ratio for

each soil type and binder type

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Need for

field trial duringdesign?

Design

requirements

satisfied?

Need for

field demo bycontractor?

Loads and

performance

Site characterizationstudies

Bench-scale mix

design testing

Published data and

prior experience

Establish design

strength

Analyses

Field

trial

Treatment

geometry

Yes

No

No

Yes

Prepare plans

and specs

withfield demo

Field demo

Construction

with on-going

contractor QC and

owner/engineer QA

Prepare plans

and specs

withoutfield demo

BiddingBidding

No Yes

Bench-scale

testingBench-scale

testing

Data Collection

Design

Procurement

Construction

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Field Trial/Demo Columns: Purposes

Determine mixing parameters that reliably produce the

specified unconfined compressive strength in the field Establish QC/QA procedures and documentation

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Field Trial/Demo Columns: Methods

Install field test columns at a location where a boring has

been made and the soil conditions are known Contractor will vary some of the following:

Binder type, often already established by lab testing

Tooling details (number, location, pitch of blades;number, location, and size of nozzles), may already

be established

Water-binder ratio, for wet method

Slurry and/or air pressure

Penetration and withdrawal rates

Rotation rate

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Field Trial/Demo Columns: Testing

Sampling:

Wet grab sampling Coring

Video log core hole

Testing: Unconfined compression Field load test entire column

Exhume portion of the column

Trial/demo columns subject to high level of testing to getas much information as possible from relatively few

columns

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Need for

field trial duringdesign?

Design

requirements

satisfied?

Need for

field demo bycontractor?

Loads and

performance

Site characterizationstudies

Bench-scale mix

design testing

Published data and

prior experience

Establish design

strength

Analyses

Field

trial

Treatment

geometry

Yes

No

No

Yes

Prepare plans

and specs

withfield demo

Field demo

Construction

with on-going

contractor QC and

owner/engineer QA

Prepare plans

and specs

withoutfield demo

BiddingBidding

No Yes

Bench-scale

testingBench-scale

testing

Data Collection

Design

Procurement

Construction

Design Strength

vsSpec Strength

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Simple Strength Characterization of

Deep-Mixed Ground

dmvcrdm qfffs

2

1

where sdm = the design shear strength of the deep-

mixed ground

fr = factor for residual strength

fc = factor for curing time

fv = factor for variability

qdm = the contract specified value ofunconfined compression strength of

the deep-mixed ground

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Factor for Residual Strength, fr

According to Japanese researchers, the residual strength

of treated soil, even under relatively low confiningpressures, is about 65% to 90% of the peak unconfined

compressive strength. Kitazume et al. (2000) used 80%:

fr= 0.8

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Factor for Curing Time, fc

Values are project-specific, depending on mixture

characteristics and time between mixing and loading

Based on review of several published sources, rates of

strength gain for cement, lime-cement, and slag-cement

mixtures, including laboratory and field cured, safe

values of fc are given by

Example: t = 90 days fc = 1.22

Project-specific values could be higher

375.0ln187.0 tfc

where t = time in days

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Variability of Deep-Mixed Ground

The coefficient of variation of unconfined compressive

strength ranged from 0.34 to 0.79, with an average value

of 0.56, for 14 data sets (7,873 data points) from 10 deepmixing projects in the U.S.

International data compiled by Larsson (2005) shows

similar results. For comparison, the coefficient of variation strength of a

natural clay deposit is typically in the range from 0.2 to 0.3

Roughly speaking, the strength of deep-mixed ground isabout twice as variable as the strength of natural clay

deposits

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0

0.2

0.4

0.6

0.8

1

0 50 100 150 200 250 300

Cumulative

Distribution

28-day Unconf ined Compressive Strength (psi)

Proposed spec: 90 psi,allowing 10% below

Actual data f rom LPV 111,

725 tests

Current spec: 120 psi,allowing 10% below

Safe DM strength distribution thatproduces a higher probability that theactual DM strength exceeds the applied

DM stress than the probability that theactual soil strength exceeds the appliedsoil stress

Cumulative Strength Distribution

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Variability of Deep-Mixed Ground: Key Issues

Recognize that high variability exists

Account for the variability in design by applying a

variability factor to relate the design strength to the

specified strength

Write a specification that allows a certain percentage of

specimens to fall below the specified strength, withoutany additional requirements for a minimum strength,

e.g., the strength of 9 out of 10 specimens must equal or

exceed 100 psi

During QC/QA, select representative specimens for

testing and dont focus on the weakest portions or

portions obviously damaged by coring or containing an

unrepresentative clod of unmixed soil

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Factor for Variability

fv = Design Strength/Specified StrengthValues of fv depend on:

Probability that the actual untreated soil strength exceeds

the assumed design strength of the untreated soil,ps

Coefficient of variation of the soil strength,

Vs

Probability that the actual deep-mixed ground strength

exceeds the specified deep-mixed ground strength,

pdm

Coefficient of variation of the deep-mixed ground strength,Vdm

Design value of the factor of safety,

Fd

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Factor for Variability

fv = Design Strength/Specified StrengthValues of fv depend on:

Probability that the actual untreated soil strength exceeds

the assumed design strength of the untreated soil,ps = 67%

Coefficient of variation of the soil strength,

Vs

= 0.25

Probability that the actual deep-mixed ground strength

exceeds the specified deep-mixed ground strength,

pdm = 70%

Coefficient of variation of the deep-mixed ground strength,Vdm = 0.6

Design value of the factor of safety,

Fd = 1.4

fv = 0.69

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0

0.2

0.4

0.6

0.8

1

0% 50% 100% 150% 200%

C

umulative

Distribution

Soil Strength as a Percentage of Mean Strength

ps = 0.67 67% probability

that the soil strength valuesare larger than the designstrength, and 33%probability that they aresmaller than the designstrength

Design soilstrength is

87.1% of themean soilstrength

Mobilized soilstrength is87.1/1.4 =62.2% of themean soilstrength

3.5% probabilitythat the soil

strength valuesare less than themobilzed strength

Lognormal distribution of soil strength, Vs = 0.25

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0

0.2

0.4

0.6

0.8

1

0% 50% 100% 150% 200%

C

umulativeDistribution

Deep-Mixed Ground Strength, Percent of Mean Strength

Mobilized DMstrength is31.5% of themean DMstrength

3.5% probability that the DM strengthvalues are less than the mobilized strength

pdm = 0.70 70% probabilitythat the actual DM strength

values are larger than thespecifed strength, and 30%probability that they are lessthan the specified strength

Specified DMstrength is64.1% of themean DMstrength

DM designstrength is1.4(31.5%) =

44.1% of themean DMstrength

DM design strength is fv = 44.1/64.1 =0.69 times the specified DM strength

Lognormal distribution of DM strength, Vs = 0.60

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Example

dmvcrdm qfffs2

1

fr = 0.8

fc = 1.22

fv = 0.69qdm = 150 psi

psf7,200psi50psi15069.022.18.02

1

dm

s

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0

0.2

0.4

0.6

0.8

1

0 50 100 150 200 250 300

CumulativeDistribution

28-day Unconf ined Compressive Strength (psi)

Proposed spec: 90 psi,allowing 10% below

Actual data f rom LPV 111,

725 tests

Current spec: 120 psi,allowing 10% below

Safe DM strength distribution thatproduces a higher probability that theactual DM strength exceeds the applied

DM stress than the probability that theactual soil strength exceeds the appliedsoil stress

Cumulative Strength Distribution

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Need for

field trial during

design?

Design

requirements

satisfied?

Need for

field demo by

contractor?

Loads and

performance

Site characterizationstudies

Bench-scale mix

design testing

Published data and

prior experience

Establish design

strength

Analyses

Field

trial

Treatment

geometry

Yes

No

No

Yes

Prepare plans

and specs

withfield demo

Field demo

Construction

with on-going

contractor QC and

owner/engineer QA

Prepare plans

and specs

withoutfield demo

BiddingBidding

No Yes

Bench-scale

testingBench-scale

testing

Data Collection

Design

Procurement

Construction

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Specification Provisions

Very important point: one size does not fit all. Project-

specific specification requirements should depend on:

Soil types

Facility type

Performance requirements

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Specification Provisions

Use a statistically based specification, e.g., 9 out of 10

specimens should exhibit a strength greater than 100 psi,

with no requirement to achieve some minimum strength

If a specimen fails because of a soil inclusion that is not

representative of proportional soil inclusion in the full-scale

column, allow a retest For every 5-ft core run, require that not more than 20%

consist of unmixed soil crossing the entire core diameter

plus unrecovered core

L d d

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Need for

field trial during

design?

Design

requirements

satisfied?

Need for

field demo by

contractor?

Loads and

performance

Site characterizationstudies

Bench-scale mix

design testing

Published data and

prior experience

Establish design

strength

Analyses

Field

trial

Treatment

geometry

Yes

No

No

Yes

Prepare plans

and specs

withfield demo

Field demo

Construction

with on-going

contractor QC and

owner/engineer QA

Prepare plans

and specs

withoutfield demo

BiddingBidding

No Yes

Bench-scale

testingBench-scale

testing

Data Collection

Design

Procurement

Construction

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Quality Control Operations and Documentation

QC = things that the contractor does to control the quality of

the work, including:

Binder composition and quality

Slurry preparation

Mixing equipment (mixing tools and arrangements, slurry

delivery ports, etc.)

Column location and verticality

Binder delivery rate

Mixing procedures (rotation rate, penetration andwithdrawal rates, slurry and/or air pressures, etc.)

Documentation of QC in daily reports

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Quality Assurance Operations and

Documentation

QA = things that the owner, engineer, and/or contractor do

to verify the quality of the work, including:

Slurry density and strength measurements

Visual observations of equipment operation

As-built surveying

Coring for mixing thoroughness, possibly with video

logging

Strength: tests on core samples, wet grab samples

Permeability: its a challenge

Documentation of QA in daily and weekly reports

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Coring Coring provides evidence of thoroughness of deep mixing,

as well as samples for strength testing

USACE requires 3% of deep-mixed elements to be cored Japanese practice is to core about 1% of deep-mixed

elements on large projects

One size does not fit all, but on most projects, 1 to 3% of

deep-mixed elements should be cored, with the high end ofthe range applying to projects that are smaller, have higher

uncertainty, and/or greater consequences of failure

Key point: Quality control, which is the means by which thecontract achieves a quality end product, is documented for

every deep-mixed element, and coring is a supplemental

activity to verify quality

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Wet-Grab Sampling Wet-grab sampling provides information about the

effectiveness of the delivered binder to develop strength in

the soil at the sampling location

Wet-grab sampling does not provide information about

homogeneity over the entire column depth

Wet-grab sampling can provide more samples at a lower

cost than coring, thereby permitting collection of more data Wet-grab sampling can provide early indication of the rate of

strength gain

Not all mixtures are equally amendable to wet-grabsampling, particularly plastic clays with lower water contents

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Permeability Most test approaches have problems

Core samples can contain cracks and may not represent

large-scale features Wet-grab samples dont represent in-situ mixing and curing

conditions

Coring and slug testing can produce cracks in otherwise

suitable cutoff walls

Pumping on one side of the cutoff wall and monitoring

response on the other side involves sophisticated

understanding of hydrogeology and careful analysis Pumping from a box-out section is expensive

More research about permeability requirements and testing

methods is needed

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Remedial Measures Re-mix immediately if QC data is suspect

Re-core the same column if core samples or wet-grab

samples fail Core adjacent columns on either side

Replace entire buttress

Propose alternate remediation method that achieve thedesign intent, subject to approval by the owner/engineer

Loads and

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Need for

field trial during

design?

Design

requirements

satisfied?

Need for

field demo by

contractor?

Loads and

performance

Site characterizationstudies

Bench-scale mix

design testing

Published data andprior experience

Establish design

strength

Analyses

Field

trial

Treatment

geometry

Yes

No

No

Yes

Prepare plans

and specs

withfield demo

Field demo

Construction

with on-going

contractor QC and

owner/engineer QA

Prepare plans

and specs

withoutfield demo

BiddingBidding

No Yes

Bench-scale

testingBench-scale

testing

Data Collection

Design

Procurement

Construction

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References

CDIT (Coastal Development Institute of Technology).

(2002). The deep mixing method: principle, design, and

construction. A.A. Balkema, Lisse, The Netherlands.

Filz, G., Adams, T., Navin, M., and Templeton, A.E.

(2012). "Design of Deep Mixing for Support of Levees

and Floodwalls," Proc. Grouting and Deep Mixing 2012,DFI and ASCE, in press.

Filz, G.M., and Navin, M.P. (2010). A Practical Method

to Account for Strength Variability of Deep-Mixed

Ground, GeoFlorida 2010: Advances in Analysis,

Modeling & Design, (GSP 199), ASCE, Reston, 8 p.

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References

Jacobson, J.R., Filz, G.M., and Mitchell, J.K. (2003).

Factors Affecting Strength Gain in Lime-Cement

Columns and Development of a Laboratory Testing

Procedure, Virginia Transportation Research Council.

vtrc.virginiadot.org/PubDetails.aspx?PubNo=03-CR16

Hodges, D.K., Filz, G.M., and Weatherby, D.E. (2008)."Laboratory Mixing, Curing, and Strength Testing of Soil-

Cement Specimens Applicable to the Wet Method of

Deep Mixing," CGPR Report #48, Virginia Tech Center

for Geotechnical Practice and Research.www.cgpr.cee.vt.edu