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LECTURE 6. Soil Physical (Mechanical) Properties – Bulk density, porosity, strength, consistency. Definitions…. Atterberg limits (H. Matengu) Soil strength (L. Olver) Soil dynamics (N. Davenport) Soil micromorphology (A. Pietersen). Physical properties:

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LECTURE 6

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Lecture 6 l.jpg

LECTURE 6

Soil Physical (Mechanical) Properties –

Bulk density, porosity, strength, consistency


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Definitions…

  • Atterberg limits (H. Matengu)

  • Soil strength (L. Olver)

  • Soil dynamics (N. Davenport)

  • Soil micromorphology (A. Pietersen)


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  • Physical properties:

    • “Characteristics of soil which can be measured by physical means and expressed in physical terms, such as colour, density, porosity, hydraulic conductivity, structure, texture and depth”

      – van der Watt & van Rooyen, Soil Science Society of South Africa

  • Mechanical properties:

    • “An expression of the materials which make up the soil…” – Pitty (1978)


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    Bulk Density…

    • Definition:

      “The mass of dry soil per unit bulk volume. Values range roughly from 1000 – 1800 kg.m-3, although higher values may be found in compacted soils.”

      – van der Watt & van Rooyen, Soil Science Society of South Africa

    • Is an expression of the proportions of solid and void in the soil matrix.

    • Strongly influenced by texture, sorting and organic matter content.

      • Why? How?

  • Why is bulk density important?

    • Influences permeability, drainage rate and penetration by roots and burrowing animals.


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    Porosity…

    • Definition:

      “The percentage volume of the soil occupied by pores and pore space.”

      • Effective pore space = part of the pore system through which fluids can move freely.

  • Strongly related to bulk density

    • Lower bulk density = higher total pore space.

  • Pore size

    • Can be divided into macropores (larger than 0.08mm), and micropores (smaller than 0.08mm).


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    Soil strength…

    • Definition:

      “A general term referring to the ability of a soil to resist deformation by applied forces, which could be any one of several types.”

    • Can be described in terms of cohesion, shear strength and adhesion.


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    • Cohesion:

      • “The attraction of a substance for itself; the mutual attraction among molecules or particles comprising a substance that allows it to cling together as a continuous mass”

      • Changes as soil dries out and bulk density increases.

      • Maximum bond when soils are wet with an extremely restricted amount of water (water retreats into micropores and creates a “suction” force).

      • Commonly measured as soil’s resistance to penetration (use of a penetrometer).

      • Strongly related to soil erodibility.

      • NB in engineering.


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    • Shear strength:

      “The maximum resistance to shearing stresses which a specimen or element of soil can withstand before failure occurs”

      • Depends on internal friction and cohesion


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    • Adhesion:

      “Refers to a molecular attraction which holds two dissimilar substances in contact, such as water and soil particles”

      • Increases as texture becomes finer.

      • Increases with higher soil organic matter content.

      • Adhesion between soil and a foreign object can be attributed to the water film between the 2 surfaces.

      • Soil “stickiness” occurs when cohesion is less than adhesion (this is dependent on moisture content).


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    Consistency…

    • The greater the amount of moisture in a soil, the more it can behave like a liquid

      • Less interaction between adjacent particles.

  • As water is added to dry soil, it passes through phases:

    • Solid

    • Semi-solid

    • Plastic

    • Liquid


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    • Between each of these states, there is a boundary/limit.

      • These known as the Atterberg limits

  • Shrinkage limit:

    • Limit between solid and semi-solid states.

    • Above this limit, semi-solid state is reached.

  • Plastic limit:

    • Limit between semi-solid & plastic states.

    • Soil becomes plastic and can be moulded into shapes.

    • Limit defined as the minimum moisture content at which the soil can be rolled into a thread of 3mm diameter without breaking up.

  • Liquid limit:

    • Limit between plastic and liquid states.

    • At this moisture content, the soil will flow under its own weight.


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