Chapter (1)
Geotechnical Properties of Soil
Eng. Haytham Besaiso
1
EFFECTIVE STRESS
Stress and pressure are often used
interchangeably in geotechnical engineering.
Effective Stress Equation:
Many engineering analyses use the vertical
effective stress, also known as the effective
overburden stress.
 Total   '  u
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 Pore Water Pressure:
• For dry sand, the pore water pressure is zero.
This is because there is no water in the soil
pores and hence there is no water pressure.
• In this case with the pore water pressure u is
equal to zero, the total stress is equal to the
effective stress.
3
-
The usual case is that there will be water in the void
spaces between the soil particles.
-
For the condition of a soil below a groundwater table
and for hydrostatic water pressure
(i.e., no groundwater flow or excess pore water
pressures), the pore water pressure is:
u 
where
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w
zw
u :pore water pressure (psf or kPa)
 w :unit weight of water (62.4 pcf or 9.81 kN m3)
zw :depth below the groundwater table (ft or m)

For cases where there is flowing groundwater or
excess pore water pressure due to the
consolidation of clay, the pore water will not be
hydrostatic.
Engineering analyses, such as seepage analyses
or the theory of consolidation can be used to
predict the pore water pressure.
For some projects, piezometers can be installed
to measure the pore water pressure u in the
ground.
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Example
 Total   1 h1   2 h2
u   w h2 .
 '   Total  u   1 h1   2 h2   w h2   1 h1  ( 2   w )h2   1 h1   ' h2
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Settlement
In general, the total settlement S of a foundation
can be given as:
S = Se+ Sc +Ss
Where:
Se = elastic settlement
Sc = primary consolidation settlement
Ss = secondary consolidation settlement
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 Immediate settlement:
 The settlement that occurs immediately after applying
the loads on the soil.
1 2
Se  B
I p.
Es
Se : im m ediatesettlement.
 : Appliedpressure.
B : Width of thefoundationor diameterif it was circular.
 : Poisson' s ratio (0  0.5)
Es : Soil Modulusof elasticity.
I p : Factorgiven from tablesdepend on typeof foundation(flexibleor rigid) and also
theshape (circular or rectangular).
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Consolidation settlement
Consolidation:

According to Terzaghi (1943), “a decrease
of water
content of a saturated
soil without replacement of the water by air is called a
process of consolidation.”
 According to consolidation, soil can be classified to three
types:
1. Normally consolidated
2. Overconsolidated
3. Underconsolidated
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 Normally consolidated: A soil is called normally
consolidated if the present effective overburden
pressure is the maximum to which the soil has ever
been subjected .
 Overconsolidated (preconsolidated ): A soil is
called overconsolidated if the present effective
overburden pressure is less than the maximum
to which the soil was ever subjected in the
past..
 Underconsolidated: A soil is called
overconsolidated if the present effective
overburden pressure is more than the maximum
to which the soil was ever subjected in the past.
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* The overconsolidation ratio (OCR) is defined as the ratio of
the past effective pressure  c to the present overburden
'
'

pressure o , OCR = c/  o
- A normally consolidated soil has OCR = 1 and an
overconsolidated soil has OCR > 1.
- OCR values of 1-3 are obtained for lightly
overconsolidated soils.
- Heavily overconsolidated soils might have OCRs > 6 to 8.
- An underconsolidated soil will have OCR < 1. In this case
the soil is still consolidating
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Total primary consolidation settlement :* For normally consolidated soil:
 Cc H 
  o'   ' 
  log 

S C  
'
 1  eo 
 o

Cc : Compressio n index  0.009(LL - 10)
H : Thickness of layer under considrati on.
eo : Initial void ratio.
 o' : Over burned pressure (effective stress).
 ' : Added vertical pressure.
* For overconsolidated clay with  o'   <= c
  '   ' 
 CsH 

  log o
SC  

 '

 1  eo 
o


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'


* For overconsolidated clay with  0 '< c < o  
 CsH 
 c
  log  '
SC  
 1  eo 
 o
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  CsH 
  o'   ' 
  
  log 

'
  1  eo 
 o

Shear strength
• Soil strength is the resistance to mass deformation
developed from a combination of particle rolling, sliding,
and crushing and is reduced by any pore pressure that exists
or develops during particle movement.
• This resistance to deformation is the shear strength of the
soil as opposed to the compressive or tensile strength of
other engineering materials.
• The shear strength is measured in terms of two soil
parameters: cohesion "c" and angle of internal friction "φ“
• The strength parameters are often used as constants, but
they are quite dependent on the type of laboratory test,
previous stress history, and current state (particle packing,
14grain shape, and water content).
• As a consequence, obtaining accurate values is not a trivial
task, and the values obtained actually apply only to the current
soil state.
The general equation to calculate the shear
strength at failure is coulomb equation:
 f  c   tan 
 f : Shear strength at failure.
c : cohesion
 : vertical stress
 : Angle of internal friction
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By determining the values of  & at plane
(ab), if it lies at point A, it means that no shear
failure will occur at plane (ab), but if it lies at
point B it means that it will fail at plane (ab),
Point C can’t be achieved because the soil will be
failed before it.
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There are many laboratory tests to obtain
these parameters, the famous tests are:
• Direct shear test:
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• Triaxial shear test:
By changing  3 several times we can obtain more than
Mohr circle, the common tangent for these circles is
the shear failure envelope, by which we can find the
shear strength parameters
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SHEAR STRENGTH OF COHESIONLESS SOIL:
 The shear strength testing should be performed on
saturated soil specimens. This is because the shear strength
testing of partially saturated soil could overestimate the shear
strength if the soil should become wetter.
 Cohesionless soils can also be tested in a dry state and the
shear strength of the soil is then expressed in terms of the
friction angle  .
 In a comparison of the effective friction angle  ' from
drained direct shear tests on saturated cohesionless soil and
the friction angle f from direct shear tests on the same soil in a
dry state, it has been determined that  ' is only 1%to
2%lower than  .
 slight difference is usually ignored and the friction angle 
and effective friction angle ' are typically considered to
mean
the same thing for cohesionless soils.
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 In summary, for the shear strength of cohesionless
soils, c' =0 and the effective friction angle  ' depends on:
• Soil type : Sand and gravel mixtures have a higher effective
friction angle than nonplastic silts
• Soil density: For a given cohesionless soil, the denser the soil,
the higher the effective friction angle.
This is due to the interlocking of soil particles. It has been
observed that in the ultimate shear strength state, that the
shear strength and density of a loose and dense sand tend to
approach each other.
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• Grain size distribution: A well graded cohesionless soil will
usually have a higher friction angle than a uniform soil.
With more soil particles to fill in the small spaces between soil
particles, there is more interlocking and frictional resistance
developed for a well graded than for a uniform cohesionless soil.
• Mineral type, angularity, and particle size: Soil particles
composed of quartz tend to have a higher friction angle than soil
particles composed of weak carbonate.
Angular soil particles tend to have rougher surfaces and better
interlocking ability. Larger size particles, such as gravel size
particles, typically have higher friction angles than sand.
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• Deposit variability: Because of variations in soil types,
gradations, particle arrangements, and dry density values, the
effective friction angle is rarely uniform with depth. It takes
considerable judgment and experience in selecting an effective
friction angle.
* SHEAR STRENGTH OF COHESIVE SOIL *
In general the shear strength of cohesive soils tend to
be lower than the shear strength of cohesionless soils.
As a result, more shear induced failures occur in
cohesive soils, such as clays, than in cohesionless soils.
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Distinguish between:
Soil compaction and consolidation
Distinguish between:
Relative compaction and relative density
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Relative Density Equation
emax  emeasured
Rd (%) 
x100
emax  emin
Diagram below illustrates a
relative density of about 40 %
emin
dmax
emeasured
d measured
increasing density
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emax
d min
Relative density is sometimes used to describe
the state condition in cohesionless soils.
Relative density is defined in terms of natural,
maximum, and minimum void ratios e
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Relative compaction or percent compaction
R.C. 
d filed
d max laboratory
 100%
Correlation between relative compaction (R.C.) and
the relative density Dr
R.C.  80  0.2Dr
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COMPACTION
• Man made
• Volume reduction due to expulsion of air
• Sudden (Short duration)
• Dry density increases water content does not change
• Applicable for unsaturated soils
CONSOLIDATION
• Natural
• Volume reduction due to expulsion of water
• Gradual
• Dry density increases water content decreases
• Applicable for saturated soils
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The process of consolidation is often confused
with the process of compaction.
Compaction increases the density of an
unsaturated soil by reducing the volume of air in
the voids .
Objectives:
• Decrease future settlements
• Increase shear strength
• Decrease permeability
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However, consolidation is a time-related
process of increasing the density of a
saturated soil by draining some of the water
out of the voids .
Consolidation is generally related to finegrained soils such as silts and clays. Coarsegrained soils, such as sands and gravels, also
undergo consolidation but at a much faster
rate due to their high permeability. Saturated
clays consolidate at a much slower rate due to
their low permeability.
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All The Best
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