4. some soil basics
TRANSCRIPT
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Some Soil Basics
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Soil –The Oldest and
A Complex Engineering Material.
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Basic Soil Characteristics
Soil - a combination of
• solid mineral particles
• water
• air
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Solid
Air
Water
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Particle Size DistributionAustralian Standard AS 1289
2.0
0.06
0.002
(mm)Gravel
Sand
Silt
Clay
Limit to visibilitynaked eye
~0.08
~0.001 Colloidal sizedparticles
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There are many factors affecting the behaviour of a soil -
• how dense is the soil?
• how wet is the soil?
• how big are the particles? rounded or angular?
• what are the relative proportions of each component?
• ……………….
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SUCTION IN SOIL
(CAPILLARY EFFECT)
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Phenomenon of Capillary Tube(due to surface tension of water)
Hc
Where: T (surface tension) = 72.75 mN/m at 20oCρw (density of water) = 1,000 kg/m3
g (acceleration of gravity) = 9.8 m2/sr (radius of tube)
Hc =ρw g r
2T
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Phenomenon of Capillary Tube(due to surface tension of water)
Hc
r (mm) Hc (mm)10 1.481 14.8
0.1 1480.01 1480
Hc =ρw g r
2T
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Capillary Effect in Coarse Soil
WaterCoarse Soil(e.g. gravel)
HSaturated
Water TableLow suction in soil
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Capillary Effect in Fine Soil
WaterFine Soil(e.g. clay)
H
Saturated
Water Table High suction in soil
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Typical Capillary Rise in Various Soil Types
Soil type Typical Capillary Rise Coarse Sand 0m approx.
Fine Sand 2m approx.Silty Soil 10m approx.
Clayey Soil 50m approx.
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Measuring Soil Suction
• Using laboratory samples• Based on pressure (suction) equilibrium between
porous plate and soil sample• Use a range of suction to determine suction-moisture
characteristic curve
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Suction Plate
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Suction vs. Moisture Content (Different soils have different characteristic curves)
Water Content (%)
Suc
tion
(pF
unit)
Soil Type
1. Sand
2. Sandy Clay
3. Clay A
4. Clay B
5. Clay C
Finer
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MOISTURE FLOW IN
UNSATURATED SOIL
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Example - determine direction of moisture flow
sand to clay or
clay to sand ?
Sand Claym.c. = 15% m.c. = 28%
Unsaturated soils
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Suction vs. Moisture Content Characteristic Curves
Water Content (%)
Suc
tion
(pF
unit)
Sand
Clay
28%15%
2.5
4.0
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Example - determine direction of moisture flow
pF = 2.5 pF = 4.0Sand Clay
m.c. = 15% m.c. = 28%
Unsaturated soils
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Example - determine direction of moisture flow
pF = 2.5 pF = 4.0Sand Clay
m.c. = 15% m.c. = 28%
Note: in this case moisture flow from a drier soil (but lower suction)to a wetter soil (higher suction)
Direction of Moisture Flow
Unsaturated soils
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Example - Irrigation
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Example - Irrigation
Water spread out from saturated zone to surrounding unsaturated soil due to difference in suction
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Suction vs. Moisture Content Characteristic Curves
Water Content (%)
Suc
tion
(pF
unit) Soil being irrigated
28% 50%
2.5
3.9
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SWELLING & SHRINKAGE
OF SOIL
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Swelling of Soil -
A soil may swell (increases in volume) as it gets wetted and “absorb” water
Sand/Gravel vs. Clay
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Coarse Sand or Gravel
Dry
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Adding water
Coarse Sand or Gravel
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Adding water
Coarse Sand or Gravel
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Volume of soil -no change as
sand or gravel does not absorb
water
Coarse Sand or Gravel
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Clay
Dry
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Adding water(very slowly !)
Clay
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Soil volume -increases as it
gets wetted
Clay
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No further volume increases -
after clay absorbedall water it can
Clay
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Clay part
icle
Adsorbed water molecules
Clay particles (tiny minerals) are capable of attracting and holding water molecules on their surface because of its surface electrical charges
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For example -Ground Heave caused by Soil Swelling
Swelling of Soil can be a Engineering Problem
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1.5m Unstable clay ρ = 2200 kg/m3
• Prior to rainfall season: m.c. of clay = 14%
Stable Soil
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1.5m
• Prior to rainfall season: m.c. of clay = 14%
Stable Soil
1.5m
• After rainfall season: m.c. of clay increased to 16%
59mm ground heave
Unstable clay ρ = 2200 kg/m3
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Shrinkage of Soil -
A soil may shrink (decreases in volume) as it dries and loses water
Sand/Gravel vs. Clay
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Coarse Sand or Gravel
EvaporationNo soil volume loss -
only loss in water above soil
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Coarse Sand or Gravel
EvaporationVolume of soil -
no change as water evaporated directly from soil
pores
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Coarse Sand or Gravel
EvaporationVolume of soil -
no change as water evaporated directly from soil
pores
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Coarse Sand or Gravel
EvaporationVolume of soil -
no change as water evaporated directly from soil
pores
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No soil volume loss -only loss in water
above soil
Clay
Evaporation
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Soil volume start to change
as all surface water evaporated
Clay
Evaporation
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A reduction in soil volume -
as water evaporated from
soil pores
Clay
Evaporation
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Clay
EvaporationNo further
change in soil volume -
as all water evaporated from
soil pores
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Soil instability due to Desiccation
• Caused by overall volume reduction due to loss of soil water
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Desiccation of clay caused by excess drying
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Desiccation of clay caused by excess drying
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Desiccation of clay liner caused by excess drying
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Summary
For the non-clay sized fraction soils an understanding of behaviourcan be gained from a knowledge of the physical characteristics of the particles
The same cannot be said for clays –
Clays require a knowledge of formation atomic structure, exchange capacity and the physical/chemical (and biological) environment to adequately explain behavioural changes
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STRENGTH OF SOIL
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Definition of Strength• The ability of the material to resist imposed
forces
• More specifically - the maximum stress the material can sustained under – Tension, – Compression, or– Shear
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Different Boundary Conditions
Δσv
(a) Unconfined (b) Confined(strain ε2 = ε3 = 0)
Δσv
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Strength of soil increases with depth as confining pressure increases
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Relevance of Strength in Various Engineering Materials
• Steel - tensile and compressive strength• Concrete (and also rock) - compressive
strength• Soil - shear strength
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Examples of soil failure by shear
Note:At failure, maximum shear stress or “shear strength” develops along entire slip surface
Foundation FailureSlope Failure
Excavation
Embankment Load
Loading Unloading
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How to Measure Shear Strength of Soil
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Direct Shear Test
Soil Sample
Loading Plate
Shear Box
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Direct Shear Test
Soil Sample
Shear Plane (area A)
T
T
N
Loading Plate
Shear Box
Δ L - Displacement
Note that shear resistance (T) depends on N (confining stress)
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How shear strength affected by the presence of water ?
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Slope Example
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How can a slope fail?
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How can a slope fail?
Too steep
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How can a slope fail?
Too steep Too much load
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How can a slope fail?
Too steep Too much load Too wet
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Unsaturated Soil - Stable Slope
Slip surface
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N
T
Shear Plan
N = normal reaction force (inter-granular)
T = shear strength (or frictional resistance)
Unsaturated Soil
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Law of Friction
the higher N - the higher T
Note:
N is a function of weight of soil above and inter-granular suction or pressure (in this case suction since unsaturated)
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Saturated Soil - Failed Slope
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N
Shear Plan
Saturated - all round pore water pressure (instead of suction) tends to push soil grains apart - N reduced
T
Soil poresaturated with water
N = normal reaction force (inter-granular)
T = shear strength (or frictional resistance)
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Law of Friction
Normal force N reduces (from unsaturated to saturated)
Shear strength (resisting force) reduces (from unsaturated to saturated)
i.e. in geotechnical engineering terms -
a reduction in soil strength caused by an increase in pore water pressure