Transcript LECTURE 5 - Rhodes University

LECTURE 6
Soil Physical (Mechanical)
Properties –
Bulk density, porosity, strength,
consistency
Definitions…
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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…
• 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.
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).
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.
• 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.
• 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
• 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).
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:
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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.