Unsaturated Soil Mechanics:Principles and Applications

William J. LikosGeological Engineering

University of Wisconsin-Madison

March 13, 2015WISCONSIN GROUND WATER ASSOCIATION

2015 Annual State Conference

What the @?*& is unsaturated soil mechanics and why should I care?

William J. LikosGeological Engineering

University of Wisconsin-Madison

March 13, 2015WISCONSIN GROUND WATER ASSOCIATION

2015 Annual State Conference

Unsaturated Soil Mechanics andthe Great Pyramids?

Wall painting from 1880 B.C. on the tomb of Djehutihotep in southeastern Egypt (Newberry, 1895).

Colossal statue of Djehutihotep (7 m high) transported by172 workers using ropes and a slide.

Water being poured in the path of the sled.

Ritual or Unsaturated Soil Mechanics?

(from Fall et al., 2014, Phys. Rev. Letters, 112, 175502)

“...maybe the time is here to adopt

unsaturated techniques to more

accurately model the real world.”

ASCE Geo-strata

(Jan/Feb 2011)

• slopes

• compacted soils

• retaining walls

• excavations

• expansive soils

• shallow foundations

• pavement subgrades

• waste covers

• thermal backfills

Unsaturated soils in geotechnical practice…

Grain Scale Macro ScalePore Scale Field ScaleAtomic Scale

nm mm - mm cm m

Unsaturated soil mechanics from the atomic scale to the field scale

Precipitation-induced landslides

F (???)

F (???)

(Image: A. Gens)

R = 3mm5 μl DI water drop

Particle-scale mechanical testing(pull-apart tests)

1 2 3 4 5 6 7

8 9 10 11 12 13 14

Menisci formation, isopropanol washed, R = 0.8 mm, V = 0.5 μl and v = 1.27 mm/min

Liquid volume effect (DI water)

-2.0E-04

-1.8E-04

-1.6E-04

-1.4E-04

-1.2E-04

-1.0E-04

0 0.3 0.6 0.9 1.2

Max

imu

m M

eas

ure

d

Forc

e (N

)DI water volume (μl)

V=1.0ul

V=0.5ul

V=0.2ul

~0.61~0.30~0.18-3.8

-3.3

-2.8

-2.3

-1.8

-1.3

-0.8

-0.3

0.2

0.0 0.3 0.6 0.9 1.2 1.5 1.8 2.1

F/(T

s·R

)

D/R

V=0.2 ul #1

V=0.2 ul #2

V=0.2 ul #3

V=0.5 ul #1

V=0.5 ul #2

V=0.5 ul #3

V=1.0 ul #1

V=1.0 ul #2

V=1.0 ul #3

(R = 0.8 mm, v = 1.27 mm/min and # number of trials)

Velocity effect

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7F/

(Ts·

R)

D/R

v=0.5 mm/min

v=1.0 mm/min

v=1.5 mm/min

v=2.0 mm/min

-0.950

-0.940

-0.930

-0.920

-0.910

-0.900

0.1 1 10M

axim

um

me

asu

red

fo

rce

(mN

)

Velocity (mm/min)

~ 0.046

(v is the separation velocity, isopropanol washed beads – R = 3 mm, V= 5μl)

• X-Ray microfocus computed tomography (mCT)

• Synchrotron XCT (GSECARS), Argonne National Laboratory

• Industrial X-Ray FLASH CT (Washington State University)

• CsCl-doped water

• Voxel sizes: 10 mm -15 mm

• Microstructure features

• Grain size distribution

• Pore shape and size distribution

• Solid, liquid, and gas surface areas

• Particle contact coordination number

• Filled and unfilled pores

2D Slice 3D Reconstruction

Multiphase Microstructural Imaging

• Bead sizes: 0.5 mm – 1.2 mm• Column size: 18.6 mm (ID)

Dry Sand

S = 0.17

S = 0.40 S = 0.70 S = 0.80

Emerging heat transport applications…

Buried high-voltage power cables

(EPFL)

Shallow geothermal

systems

Geosynthetic heat

exchangers“energy” piles

Thermal Conductivity Dryout Curve

S = 0.17

S = 0.40

S = 0.80

S = 0

Modeling Coupled Heat & Moisture Flow

Moisture flow vectors

Lu, N. and Likos, W.J., 2004, Unsaturated Soil Mechanics

Wu, R., Tinjum, J.M., and Likos, W.J., 2014, “Coupled thermal conductivity dryout curve and soil–water characteristic curve in modeling of shallow horizontal geothermal ground loops,” Journal of Geotechnical and Geological Engineering, doi: 10.1007/s10706-014-9811-2.

Likos, W.J., 2014, “Effective stress in unsaturated soil: Accounting for surface tension and interfacial area,” Vadose Zone Journal, doi:10.2136/vzj2013.05.0095.

Likos, W.J., 2013, “Modeling thermal conductivity dryout curves from soil-water characteristic curves,” Journal of Geotechnical and Geoenvironmental Engineering, 10.1061/(ASCE)GT.1943-5606.0001078, 04013056.

Willson, C.S., Lu, N., and Likos, W.J., 2012, “Quantification of grain, pore, and fluid microstructure of unsaturated sand from X-ray computed tomography images,” Geotechnical Testing Journal, 35(6): 911-923.

Further Reading

Phone: 608-890-2662Email: likos@wisc.edu