Post on 16-Jan-2016
Soil Physics 2010
Outline
• Announcements
• Where were we?
• Soil Structure (particles)
Soil Physics 2010
Announcements• Reminder: Homework due Feb. 8
• Reminder: Exam Feb. 12
• Example exam is now posted. Don’t panic! I covered material in a different order that year, and the class was not dual-listed at the 400-level.
• Note: after homework is handed in, I re-post the file with the answers
Soil Physics 2010
Where were we?
Soil strength
Soil structure
Specifically, most soil structure exists because of cohesion
Soil Physics 2010
Soil structure
Everyone agrees that soil structure is important, but no one knows how to define it or measure it.
Soil structure has defeated more soil scientists than (probably) any other topic.
The state of the art in studying soil structure has barely advanced in 50 years.
Classification of structure:
PlatyBlockyCrumbGranularColumnarPrismaticAngularSubangular…
Granular
Soil Physics 2010
Classification of structure: Blocky
Soil Physics 2010
Classification of structure: Platy
Soil Physics 2010
Classification of structure: Prismatic
Soil Physics 2010
Classification of structure: Columnar
Soil Physics 2010
Soil Physics 2010
What is soil structure?
• Structure: the arrangement of parts
• Not just physical locations:
• Relationship between a particle and its neighborhood
• Connections: bonds, glue, load-bearing links
• A blueprint is about more than the location of each brick!
Geometry
Topology
Same as in earlier lecture: what is required to describe a porespace.
Soil Physics 2010
Figure & Ground
Why soil structure (particles)?
Particles & Pores
Figure and Ground
Dual networksTriangular & honeycomb
Dual networksVoronoi & Delaunay
Soil Physics 2010
Soil Physics 2010
Duals in 3D Space between barley grains. Grains were continuous; the porespace (dual) is also.
Soil Physics 2010
Structure implies not random
This might be a preferential arrangement
Based on chance, you shouldn’t find lots of this:
Clay should hang out with the other particles, too.
Why do clay quasicrystals (and other non-random structures)
form?
Soil Physics 2010
Drivers of structure (particles)
Gravity: if it can’t stand, it will fall
Stability: if it’s not stable, it will soon change
Climate, life, parent material
Water, heat, roots: different ways energy disturbs the soil, shaking it into a more stable configuration
Soil Physics 2010
Hierarchical structure
clay platelets → quasicrystals
quasicrystals → clay skins & bridges
…
microaggregates → crumbs
crumbs → aggregates
aggregates → peds
flocs, tactoids, cutans …
Structures are built from smaller structures:
Soil Physics 2010
Fragmentation systems
When an aggregate is dropped, there is usually a power-law distribution of pieces:
fdrrN
This is characteristic of fragmentation systems.
It implies that larger pieces are easier to break than smaller pieces.
rN
(r)
log(r)log[N
(r)]
slope = –df
Soil Physics 2010
Causes / consequences of hierarchical structure
• Small structures tend to be denser than large structures
• Small structures are more stable than large structures
• Bonds within and between small structures are stronger than bonds within and between large structures
• Spaces (pores) between large structures are bigger than those between small structures
In soil, these structures are called aggregates
Soil Physics 2010
Bonds in soil structure
Chemical bonds: covalenthydrogen
Physical bonds:Van der Waalssurface energy
Biological:hyphaeroot exudatesworm castsother yucky gooey stuff
FlocculationCementationAggregationCohesion / Adhesion
Soil Physics 2010
Aggregate properties
• Size, shape, distribution
• Strength versus physical forces
• Strength versus chemical forces
FragmentationDry sievingRupture
Wet sieving
Soil Physics 2010
Granular structure
Eventually, physicists try to treat everything as spheres…
Granular structure
… or something close, like M&Ms™
Soil Physics 2010
Soil Physics 2010
Sphere packing
Soil Physics 2010
Sphere packing
Soil Physics 2010
The main point of this sphere packing:
A (fairly) predictable pore size distribution results from randomly packing particles of a known size distribution
We call these “textural pores”
Structure generally produces pores that would not occur by random packing
We call these “structural pores”
Structure emerges from the particles and pores competing for stable arrangements