Soil Physical (Mechanical) Properties

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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 soils 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.