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Geomorphology Chapter 3: Soils

Geomorphology Chapter 3: Soils. Chapter 3: Soils “Perhaps the most important part of weathering is the development of soil.” -D. Easterbrook Soil makes plant life possible Plants make animal life possible. Chapter 3: Soils Definitions Processes Characteristics Classification.

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Geomorphology Chapter 3: Soils

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  1. Geomorphology Chapter 3: Soils

  2. Chapter 3: Soils • “Perhaps the most important part of • weathering is the development of soil.” • -D. Easterbrook • Soil makes plant life possible • Plants make animal life possible

  3. Chapter 3: Soils • Definitions • Processes • Characteristics • Classification

  4. Chapter 3: Soils • Definition of soil: • varies according to discipline • Geology • Soil science • Engineering

  5. What is Soil? • A. Depends on who you ask. • To a geologist:residual material formed in situ from weathering of a parent material–either bedrock or sediment. • not a sediment • Profile from alteration of parent material • residual vs. “transported” soils

  6. What is Soil? • A. Depends on who you ask. • To a soil scientist:organic-rich material that supports plant growth. • The life zone: intersection of hydrosphere, atmosphere, lithosphere, and biosphere. • soil scientists largely trained/work in agricultural related areas—very specialized. • consider economic significance of agricultural: • grains, fruits, vegetables, feed stock, ethanol, etc. • grains biggest U.S. export • important considerations: fertility, texture, drainage

  7. What is Soil? • A. Depends on who you ask. • To an engineer:any unconsolidated material above bedrock. • important considerations: • thickness • strength • water conditions • unimportant considerations: • origin • true soil or sediment • fertility

  8. General problems associated with various soil types • (engineering and environmental perspectives): • Clays • shrink and swell w/moisture changes (esp. montmorillonite: “fat clays”) • compressibility • sensitivity (e.g., liquefaction) • poor drainage / slow or negligible transport • Silt • easily erodable • loses strength when saturated • highly susceptible to frost heave and “piping” • Sand / Gravel • fine sands may behave similar to silts • good drainage / rapid transport • Organics • highly compressible, non-uniform

  9. Chapter 3: Soils • Characteristics • unconsolidated • porous • differentiated by depth into horizons • upper horizon(s) organic-rich • soft, weak, compressible(compared to rock)

  10. Chapter 3: Soils • Processes

  11. Climate T, H2O Parent material composition, structure, texture Vegetation influences runoff / erosion depends on climate Topography slope influences runoff / erosion Time weathering may reach equilibrium given enough time Soil Forming Factors

  12. Processes and Resulting Profile • differentiated by depth into horizons • (i.e., soil “profile”) Zone of leaching CaCO3 Clays, oxides Bk

  13. Chapter 3: Soils • Classification

  14. Classification • GeologyEngineering • Coarse-grained Granular or non-cohesive • (gravels and sands) ►Well sorted ► Poorly graded ►Poorly sorted ► Well graded • Fine-grained Cohesive • (silts and clays) Grouped by texture Grouped by texture Grouped by texture and plasticity Grouped by texture

  15. Example 1 42% sand 20% clay 38% silt “Loam” Example 2 10% sand 46% silt 44% clay ?

  16. 10% sand 46% silt 44% clay Silty Clay

  17. Unified Soil Classification System (USCS)

  18. 68% 28% 4% 0% gravel sand silt clay Sample 1

  19. 68% 28% 4% gravel sand silt / clay Sample 1 D60 = 3.5mm

  20. 68% 28% 4% gravel sand silt/clay Sample 1 D10 = 0.23mm

  21. Grain Size Analysis (cont.) • Sorting (Grading) Parameters • Uniformity coefficient (Cu) • Cu =D60 / D10 • = 3.5mm / .23mm • = 1.5

  22. Grain Size Analysis (cont.) • Sorting (Grading) Parameters • coefficient of concavity (Cc) • Cc =[ D30 ]2 / [ D10 x D60 ] • = [ 1.8 ]2/ [ 0.23 x 3.5 ] • = 4

  23. Unified Soil Classification System (USCS)

  24. Our sample is 68% gravel, 28% sand, 4% silt/clay

  25. Our sample is 68% gravel, 28% sand, 4% silt/clay Cu = 1.5, Cc = 4

  26. 17% 38% 45% gravel sand silt/clay Sample 2

  27. Our sample is 17% gravel, 38% sand, 45% silt/clay YES NO

  28. Our sample is 17% gravel, 38% sand, 45% silt/clay YES NO

  29. Our sample is 17% gravel, 38% sand, 45% silt/clay YES SC OR YES YES LAB FIELD

  30. Why didn’t we just use grain size curve to determine %silt vs %clay? 17% 38% 45% gravel sand silt/clay Sample 2 • Often don’t have curve for fraction smaller than silt; just %clay/silt combined • Clay/silt distinction is based as much on behavior (plastic or non-plastic) • as actual grain size distribution

  31. Clay Mineralology Activity = PI / %clay Non-expansive clays (low PI): kaolinite, illite Expansive clays (high PI): montmorillonite

  32. SAND

  33. SILT

  34. CLAY

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