4. some soil basics

Some Soil Basics

Soil –The Oldest and

A Complex Engineering Material.

Basic Soil Characteristics

Soil - a combination of

• solid mineral particles

• water

• air

Solid

Air

Water

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

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?

• ……………….

SUCTION IN SOIL

(CAPILLARY EFFECT)

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

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

Capillary Effect in Coarse Soil

WaterCoarse Soil(e.g. gravel)

HSaturated

Water TableLow suction in soil

Capillary Effect in Fine Soil

WaterFine Soil(e.g. clay)

H

Saturated

Water Table High suction in soil

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.

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

Suction Plate

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

MOISTURE FLOW IN

UNSATURATED SOIL

Example - determine direction of moisture flow

sand to clay or

clay to sand ?

Sand Claym.c. = 15% m.c. = 28%

Unsaturated soils

Suction vs. Moisture Content Characteristic Curves

Water Content (%)

Suc

tion

(pF

unit)

Sand

Clay

28%15%

2.5

4.0


pF = 2.5 pF = 4.0Sand Clay

m.c. = 15% m.c. = 28%

Unsaturated soils


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

Example - Irrigation

Example - Irrigation

Water spread out from saturated zone to surrounding unsaturated soil due to difference in suction

Suction vs. Moisture Content Characteristic Curves

Water Content (%)

Suc

tion

(pF

unit) Soil being irrigated

28% 50%

2.5

3.9

SWELLING & SHRINKAGE

OF SOIL

Swelling of Soil -

A soil may swell (increases in volume) as it gets wetted and “absorb” water

Sand/Gravel vs. Clay

Coarse Sand or Gravel

Dry

Adding water


Volume of soil -no change as

sand or gravel does not absorb

water


Clay

Dry

Adding water(very slowly !)

Clay

Soil volume -increases as it

gets wetted

Clay

No further volume increases -

after clay absorbedall water it can

Clay

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

For example -Ground Heave caused by Soil Swelling

Swelling of Soil can be a Engineering Problem

1.5m Unstable clay ρ = 2200 kg/m3

• Prior to rainfall season: m.c. of clay = 14%

Stable Soil

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

Shrinkage of Soil -

A soil may shrink (decreases in volume) as it dries and loses water

Sand/Gravel vs. Clay


EvaporationNo soil volume loss -

only loss in water above soil


EvaporationVolume of soil -

no change as water evaporated directly from soil

pores

No soil volume loss -only loss in water

above soil

Clay

Evaporation

Soil volume start to change

as all surface water evaporated

Clay

Evaporation

A reduction in soil volume -

as water evaporated from

soil pores

Clay

Evaporation

Clay

EvaporationNo further

change in soil volume -

as all water evaporated from

soil pores

Soil instability due to Desiccation

• Caused by overall volume reduction due to loss of soil water

Desiccation of clay caused by excess drying

Desiccation of clay liner caused by excess drying

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

STRENGTH OF SOIL

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

Different Boundary Conditions

Δσv

(a) Unconfined (b) Confined(strain ε2 = ε3 = 0)

Δσv

Strength of soil increases with depth as confining pressure increases

Relevance of Strength in Various Engineering Materials

• Steel - tensile and compressive strength• Concrete (and also rock) - compressive

strength• Soil - shear strength

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

How to Measure Shear Strength of Soil

Direct Shear Test

Soil Sample

Loading Plate

Shear Box

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)

How shear strength affected by the presence of water ?

Slope Example

How can a slope fail?


Too steep


Too steep Too much load


Too steep Too much load Too wet

Unsaturated Soil - Stable Slope

Slip surface

N

T

Shear Plan

N = normal reaction force (inter-granular)

T = shear strength (or frictional resistance)

Unsaturated Soil

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)

Saturated Soil - Failed Slope

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)

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

4. some soil basics

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