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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


Soil Physical (Mechanical) Properties –

Bulk density, porosity, strength, consistency

  • Atterberg limits (H. Matengu)
  • Soil strength (L. Olver)
  • Soil dynamics (N. Davenport)
  • Soil micromorphology (A. Pietersen)
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)
bulk density
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.
  • 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).
soil strength
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.
      • “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.
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

“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).
  • 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
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.