Behavioral Thresholds in Sand and Clay Mixtures: an Experimental and Numerical StudyDaniel Simpson, Graduate Research Assistant and T. Matthew Evans, Associate ProfessorOregon State University School of Civil and Construction Engineering

Even soils with appreciable fines contents may be susceptible to 

liquefaction. 

ReferencesFredlund, Murray D., D. G. Fredlund, and G. Ward Wilson. "An equation to represent grain‐size distribution." Canadian Geotechnical Journal 37.4 (2000): 817‐827.

Edmund, Hazell. Numerical and Experimental Studies of Shallow Cone Penetration in Clay. Diss. Ph. D. Thesis, Oxford University, UK, 2008.

http://www.dailyrecord.co.uk/news/uk‐world‐news/new‐zealand‐city‐of‐christchurch‐hit‐1105379

http://www.dataforensics.net/RAPID_CPT.php

http://www.homesinharmony.co.uk/sustainable‐solutions/info_8.html

New technologies (Ground source heat pump in this figure) depend on 

optimization of thermal and mechanical properties of soil. 

Results from existing in‐situ testing techniques (e.g., CPT) can be better understood with knowledge of fine content fraction 

influence on soil behavior.

Practical MotivationNearly all soils are comprised of mixtures of sand and fine particles. Behaviorunder mechanical and thermal loading of soil is strongly influenced (and in somecases, governed) by the ratio of sand to fine particles.

USCS Class

Specific Surface Area(SSA) m2/g

Specific Gravity Gs

LL PL

CL 8.6 2.60 49 24

Baseline Characterization of MaterialsThe studied sand‐clay mixtures were comprised of Ottawa 50/70 sand andKaolinite clay in varying ratios.

Modified fall cone used for consistency limit testing. Unique 

time‐history data was collected using the inexpensive web cam and the 

highly accurate LVDT. 

Ottawa 50/70 sand characteristics

Characteristics of Kaolinite Clay 

Typical Kaolinite particles

10−4

10−3

10−2

10−1

100

0

0.2

0.4

0.6

0.8

1

Particle Size (mm)

Fra

ctio

n P

assi

ng

80% Clay50% Clay20% ClaySand particles

Composite grain size distributions of 80%, 50%, and 20% clay InterpretationThe experimental results will be used to infer clay content ratios where behaviorchanges. The results will also be used to calibrate a Plaxis FEM of the fall cone testwhich will be used to predict additional behavior. Experimental and numericalstudies are supported by concurrent research by Coria and Evans (2014).

0 5000

5

10

15

20

25

30

35

40

45

qt (tsf)d

epth

(ft

)

0 0.1 0.20

5

10

15

20

Time (s)

Dis

p (

mm

)

Typical images captured during fall cone test

Time‐History of cone displacement from two data 

collection techniques

LVDT

Camera

None – trace amounts of Clay

(sand‐sand contact)

Small Amounts of clay (sand‐sand 

contact with clay )

Large fractions of Clay (sand particles suspended in clay)

Increasing Clay Content

Does USCS agree with physical transitions of sand‐on‐sand contact to sand‐on‐sand contact with a clay matrix to sand particles suspended in a clay matrix?

Sand Grain

Clay Matrix

SP/SW orSW‐SC/SP‐SC SC CL

? ?

0

100

200

Filt

er C

rite

rio

n

0

100

200

Cu

0 0.5 10

20

40

Cc

Clay Fraction

Effects of Clay Fraction on Gradation Properties

A better understanding of the fundamental behavior of soil will improve the effectiveness of new 

and existing geotechnical technologies.

0 0.2 0.4 0.6 0.8 10

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Clay Fraction

Co

mp

ress

ion

Ind

ex

CcCs

0 0.2 0.4 0.6 0.8 110

−2

10−1

100

101

Clay Fraction

Cv

(cm

2 /min

)

Over consolidatedNormally Consolidated

Compressibility and Oedometric TestingIncremental consolidation tests were performed on each of the mixtures using acomputer‐controlled consolidometer. A load duration of 60min (120 min for highclay content) was used (t100≈45‐90 min.) Tests were performed with a loadincrement ratio of 0.25 to precisely define the e‐log(σ') curve .

Pore Pressure Distribution during fall cone test (axisymmetric) after Hazell (2008).

• USCS classification may not indicate correct clay threshold limits for classification changes  when fines are classified as CL or ML

• An optimum fines content exists which is beneficial in many applications  (fill, earth structures, etc.) where standard practice might rejects material based on fines content. 

0 20 40 60 80 1000

10

20

30

40

50

Clay Fraction

Co

nsi

sten

cy L

imit

LLPLPI

Theoretical MotivationIt is well‐known that grain size distribution has a dominant effect on materialbehavior for coarse‐grained soils. In fine‐grained soils, behavior is governed byspecific surface area and particle mineralogy. We seek to answer the question,“Where does this change in behavior occur?” Though not considered explicitly,acknowledgement of this behavioral transition is implicitly included in manypractical geomechanics applications – Terzaghi’s filter criterion for dams, theUnified Soil Classification System (USCS), and the Kozeny‐Carmen hydraulicconductivity equation.

Consistency Limits of MixturesConsistency Limits of all Mixtures were measured with a modified fall coneapparatus. The fall cone is a direct measure of the static undrained shear strengthof a soil and is not prone to the same level of operator error as the Casagrandecup. An additional benefit is that there are closed‐form analytical solutions for thebearing capacity of a cone on the surface of a semi‐infinite elastoplastic halfspace,which allows for a more robust analysis of the measured data.

12% 50%

Mixtures were preconsolidated to 100kPa in the apparatus above and then loaded 

into the consolidometer below.

0 0.2 0.4 0.6 0.8 10

2

4

6

8

10

Clay Fraction

Sp

ecif

ic S

urf

ace

Are

a (m

2 /g) 0 0.2 0.4 0.6 0.8 1

0

1

2

3

4

5

Clay Fraction

Beh

avio

r

This StudyUSCS

Behavior

1 – Sand particles in contact with each other and relatively large void spaces exist2 – Sand particles in contact with each other but void spaces mostly filled with clay3 – Sand particles suspended in clay matrix but still interact with one another during straining4 – Sand particles suspended in clay matrix and little to no interaction occurs between sand particles during straining.

Specific Surface area of the mixture depends on the physical dimensions of the constituents and the  ratio of the mixture constituents.