Soil Mechanics A

1
Basic Definitions and Terminology
2
Va
Vw
Vs

• Soil is generally a three phase material
• Contains solid particles and voids
• Voids can contain liquid and gas phases

3
Va
Vw
Vs

• Soil is generally a three phase material
• Contains solid particles and voids
• Voids can contain liquid and gas phases

4
Va
Vw
Vs

• Soil is generally a three phase material
• Contains solid particles and voids
• Voids can contain liquid and gas phases

5
Units

• Length metres
• Mass tonnes (1 tonne 103 kg)
• Density t/m3
• Weight kilonewtons (kN)
• Stress kilopascals (kPa) 1 kPa 1 kN/m2
• Unit weight kN/m3
• Accuracy Density of water, rw 1 t/m3
• Stress/Strength to 0.1 kPa

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Weight and Unit weight

• Force due to mass (weight) more important than
  mass
• W M g
• Unit weight

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Weight and Unit weight

• Force due to mass (weight) more important than
  mass
• W M g
• Unit weight
• g r g

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Weight and Unit weight

• Force due to mass (weight) more important than
  mass
• W M g
• Unit weight
• g r g

z
sv r g z sv g z
sv
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Specific Gravity
This is defined by

• Gs _at_ 2.65 for most soils
• Gs is useful because it enables the volume of
  solid particles to be calculated from mass or
  weight

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Specific Gravity
This is defined by

• Gs _at_ 2.65 for most soils
• Gs is useful because it enables the volume of
  solid particles to be calculated from mass or
  weight

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Voids ratio

• It is not the actual volumes that are important
  but rather the ratios between the volumes of the
  different phases. This is described by the voids
  ratio, e, or porosity, n, and the degree of
  saturation, S.

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Voids ratio

• It is not the actual volumes that are important
  but rather the ratios between the volumes of the
  different phases. This is described by the voids
  ratio, e, or porosity, n, and the degree of
  saturation, S.
• The voids ratio is defined as

13
Voids ratio

• It is not the actual volumes that are important
  but rather the ratios between the volumes of the
  different phases. This is described by the voids
  ratio, e, or porosity, n, and the degree of
  saturation, S.
• The voids ratio is defined as
• and the porosity as

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Voids ratio

• It is not the actual volumes that are important
  but rather the ratios between the volumes of the
  different phases. This is described by the voids
  ratio, e, or porosity, n, and the degree of
  saturation, S.
• The voids ratio is defined as
• and the porosity as
• The relation between these quantities can be
  simply determined as follows
• Vs V - Vv (1 - n) V

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Voids ratio

• It is not the actual volumes that are important
  but rather the ratios between the volumes of the
  different phases. This is described by the voids
  ratio, e, or porosity, n, and the degree of
  saturation, S.
• The voids ratio is defined as
• and the porosity as
• The relation between these quantities can be
  simply determined as follows
• Vs V - Vv (1 - n) V
• Hence

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Degree of Saturation

• The degree of saturation, S, has an important
  influence on soil behaviour
• It is defined as

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Degree of Saturation

• The degree of saturation, S, has an important
  influence on soil behaviour
• It is defined as
• The phase volumes may now be expressed in terms
  of e, S and Vs
• Vw e S Vs Va Vv - Vw e
  Vs (1 - S)

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Degree of Saturation

• The degree of saturation, S, has an important
  influence on soil behaviour
• It is defined as
• The phase volumes may now be expressed in terms
  of e, S and Vs
• Vw e S Vs Va Vv - Vw e
  Vs (1 - S)
• Assuming Vs 1 m3, the following table can be
  produced

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Unit Weights

• The bulk unit weight

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Unit Weights

• The bulk unit weight
• The saturated unit weight (S 1)

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Unit Weights

• The bulk unit weight
• The saturated unit weight (S 1)
• The dry unit weight (S 0)

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Unit Weights

• The bulk unit weight
• The saturated unit weight (S 1)
• The dry unit weight (S 0)
• The submerged unit weight

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Moisture Content

• The moisture content, m, is defined as

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Moisture Content

• The moisture content, m, is defined as
• In terms of e, S, Gs and gw
• Ww gw Vw gw e S Vs
• Ws gs Vs gw Gs Vs

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Moisture Content

• The moisture content, m, is defined as
• In terms of e, S, Gs and gw
• Ww gw Vw gw e S Vs
• Ws gs Vs gw Gs Vs
• hence

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Example 1

• Distribution by mass and weight

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Example 1

• Distribution by mass and weight

• Distribution by volume (assume Gs 2.65)
• Total Volume V p r2 l

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Example 1

• Distribution by mass and weight

• Distribution by volume (assume Gs 2.65)
• Total Volume V p r2 l
• Water Volume

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Example 1

• Distribution by mass and weight

• Distribution by volume (assume Gs 2.65)
• Total Volume V p r2 l
• Water Volume
• Solids Volume

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Example 1

• Distribution by mass and weight

• Distribution by volume (assume Gs 2.65)
• Total Volume V p r2 l
• Water Volume
• Solids Volume
• Air Volume Va V - Vs - Vw

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• Moisture content

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• Moisture content
• Voids ratio

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• Moisture content
• Voids ratio
• Degree of Saturation

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• Moisture content
• Voids ratio
• Degree of Saturation
• Bulk unit weight

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• Moisture content
• Voids ratio
• Degree of Saturation
• Bulk unit weight
• Dry unit weight

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• Moisture content
• Voids ratio
• Degree of Saturation
• Bulk unit weight
• Dry unit weight
• Saturated unit weight
• Note that gdry lt gbulk lt gsat

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Example 2

• Volume and weight distributions

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Example 2

• Volume and weight distributions
• Dry unit weight,

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Example 2

• Volume and weight distributions
• Dry unit weight,
• Saturated unit weight

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Example 2

• Volume and weight distributions
• Dry unit weight,
• Saturated unit weight
• Moisture content (if saturated)

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