Soil

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  • Topic: Soil mechanics, Water, Geotechnical engineering
  • Pages : 22 (1715 words )
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  • Published : November 30, 2012
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OUTLINE
• Introduction and Terminology

Advanced
Advanced Soil Mechanics &
Foundation Design
UNSATURATED SOIL
MECHANICS
A/Prof Hadi Khabbaz
Email: hadi.khabbaz@uts.edu.au

• Suction and Measurement Methods
• Effective Stress Concept for Unsaturated
Soils
• Soil Water Characteristic Curve (SWCC)
Ch
(SWCC)
• Permeability of Unsaturated Soils
• Shear strength of Unsaturated Soils
• Examples
• Conclusions

Room 2.511B

Introduction
&
Terminology

State whether the following statements are True or False
1. Soils below water tables are fully saturated.
Soils belo
tables are
sat
2. All soils above water table are unsaturated.
3. Compacted soils are generally unsaturated soils.
4. Shear strength of a saturated clayey sand is greater
than its unsaturated shear strength.

Why do we need
to understand
Unsaturated Soil
Mechanics?

Most of the natural soil systems are in arid and semiarid regions and soils subjected to seasonal moisture deficits are in an unsaturated state.
Who needs unsaturated soil mechanics?
Every geotechnical engineer needs to be aware of the
th
role played by the portion of soil with negative porepressure (i.e. generally the portion above the water table).
Many present solutions in soil mechanics and
foundation design are quite approximate and based on
saturated soils. More research is still required.

1

Stress Distribution during the desiccation of a soil

Stress Distribution during the desiccation of a soil

Certainly there is a need for an appropriate technology
for unsaturated

soil behaviour.

Such technology must:
1. Be practical
2. Not be too costly to employ
3. Have a sound theoretical basis
4. Run parallel in concept of conventional soil mechanics

2

Basic Differences
between Saturated and
Unsaturated Soils

Nature
Existing of Air

Engineering Behaviour
Negative Pore Water
Pressure

Air

Water

Solid Particles

Compression?
1/P

Compression?
Cf

Compression?
Cs

Three Phase System of Unsaturated Soils

3

Total Soil Suction
Soil suction is commonly referred to as the free energy
state of soil water. The free energy of soil water can be
measured in terms of the partial vapour pressure of soil
water.

Suction
&

The thermodynamic relationship between soil suction
and the partial pressure of the pore – water vapour can
be written as follows:

Measurement Methods

ψ=−

ψ=−

⎛u
RT
ln⎜ v
v w 0 ω v ⎜ uv 0







⎛u ⎞
RT
ln⎜ v ⎟
v w 0 ω v ⎜ uv 0 ⎟



A Simplified Formula:

⎛u ⎞
ψ (kPa ) = −135,000 ln ⎜ v ⎟
⎜u ⎟
⎝ v0 ⎠
Example:
Assume a soil has a relative humidity (RH%) of 98%
at 20°C. Find the total suction of this soil in kPa.

Relative Humidity:

RH =

ψ = −135,000 ln (0.98 ) = 2,727 kPa

uv
uv 0

Relationship between

Components of Soil Suction

Relative Humidity and Total Suction

ψ (kPa ) = (ua − u w ) + π
(ua − u w ) = matric suction
ua = pore air pressure

u w = pore water pressure
23

π = osmotic suction

4

Typical Suction Values for
Compacted Soils

Soil Suction Measurement
♥ The osmotic suction can be
determined from the electrical
conductivity of the pore fluid.
♥ The total suction can be
Th
measured with a psychrometer
♥ The total and matric suctions
can also be measured using the
filter paper method.

Matric Suction
A meniscus forms at the soil-air interface, due to the
surface tension, resulting in reduced vapour pressure
in the water.
Th
The vapour pressure decreases, becomes more
negative, and the matric suction pressure increases
as the radius of curvature of the meniscus decreases.
♥ The matric suction can be measured with pressure
plate apparatus and tensiometers.

(ua − u w )
27

Surface Tension at Soil-Water-Air Interface

28

Osmotic Suction
The presence of dissolved ions in water decreases
the soil vapour pressures, relative humidity, which
then increases the total soil...
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