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Lecture 23

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Lecture 23

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  1. Lecture 23 Soil Properties and Mechanics

  2. The Soil Profile • Soil Profile - An underground side view of the soil (ie. the sidewall of a backhoe pit, or other excavation). Views of soil profiles follow in subsequent slides. • Horizons - The soil is naturally organized into horizontal layers that may be viewed in a soil profile.

  3. Organic horizons • Oi - Largely undecomposed plant remains • Oe - Partially decomposed organic material • Oa - Highly decomposed organic material

  4. Mineral horizons (<20 - 30% OM) • A - mixture of organic matter and mineral soil . • E - eluviated leached horizon • B - illuviation • accumulation of organic matter and sesquioxides • (oxides and hydroxides of Fe and Al) • C - unconsolidated, unweathered material underlying the solum (A and B horizons). • R - rock

  5. The Complete Idealized Soil Profile Oi - litter layer organic horizons Oe - moderately decomposed Oa - humus layer

  6. The Complete Idealized Soil Profile Oi - litter layer organic horizons Oe - moderately decomposed Oa - humus layer topsoil A - organic matter accumulation E - leached horizon - loss of clay and sesquioxides to B horizons. R - bedrock

  7. The Complete Idealized Soil Profile Oi - litter layer organic horizons Oe - moderately decomposed Oa - humus layer topsoil A - organic matter accumulation E - leached horizon the solum B - horizon of illuviation; clay and sequioxide accumulation. subsoil C -Some C horizons weather from bedrock; residual soils develop from these. Other C horizons form from various geologic deposits such as alluvium in flood plains, glacial deposits, and lacustrine deposits in ancient lake beds. R - bedrock

  8. The Physical Soil-Four Soil Components:Idealized Surface Loam Soil • Air 25% • Water 25% • Mineral 45% • Organic 5%

  9. Soil Physical Properties • Soil texture • Soil structure • Particle density and bulk density • Pore space

  10. Soil Texture • Soil texture - relative proportion of sand, silt and clay.

  11. Primary Soil Particles • Sand: 2 mm - 0.05 mm • irregular size and shape • primarily quartz (silicaceous) • Silt: 0.05 mm - 0.002 mm [2 microns] • microsand particles • Clay < 0.002 mm (colloidal < 0.001 mm) • aluminosilicates • Fe and Al oxides

  12. 10 20 30 40 50 60 70 80 90 100 70 80 100 60 20 10 90 50 30 40 100 90 80 70 Silt % 60 Clay % 50 40 30 20 10 Sand %

  13. 10 20 30 40 50 60 70 80 90 100 70 80 100 60 20 10 90 50 30 40 100 90 80 70 Silt % 60 Clay % 50 40 30 20 10 Sand %

  14. 10 20 30 40 50 60 70 80 90 100 70 80 100 60 20 10 90 50 30 40 100 90 80 70 Silt % 60 50 Clay % 40 30 20 10 Sand %

  15. 10 20 30 40 50 60 70 80 90 100 70 80 100 60 20 10 90 50 30 40 100 90 80 70 Clay Silt % 60 Clay % 50 Silty Clay Sandy Clay 40 Silty clay loam Clay Loam 30 Sandy clay loam 20 Loam Sandy Loam Silt loam 10 Loamy sand Silt Sand Sand %

  16. 10 20 30 40 50 60 70 80 90 100 70 80 100 60 20 10 90 50 30 40 100 Find textural class: 32% Sand 36% Silt 32% Clay 90 80 70 Clay Silt % 60 Clay % 50 Silty Clay Sandy Clay 40 Silty clay loam Clay Loam 30 Sandy clay loam 20 Loam Sandy Loam Silt loam 10 Loamy sand Silt Sand Sand %

  17. 10 20 30 40 50 60 70 80 90 100 70 80 100 60 20 10 90 50 30 40 100 32% Sand 90 80 70 Clay Silt % 60 Clay % 50 Silty Clay Sandy Clay 40 Silty clay loam Clay Loam 30 Sandy clay loam 20 Loam Sandy Loam Silt loam 10 Loamy sand Silt Sand Sand %

  18. 10 20 30 40 50 60 70 80 90 100 70 80 100 60 20 10 90 50 30 40 100 32% Sand 36% Silt 90 80 70 Clay Silt % 60 Clay % 50 Silty Clay Sandy Clay 40 Silty clay loam Clay Loam 30 Sandy clay loam 20 Loam Sandy Loam Silt loam 10 Loamy sand Silt Sand Sand %

  19. 10 20 30 40 50 60 70 80 90 100 70 80 100 60 20 10 90 50 30 40 100 32% Sand 36% Silt 32% Clay 90 80 70 Clay Silt % 60 Clay % 50 Silty Clay Sandy Clay 40 Clay Loam Silty clay loam 30 Sandy clay loam 20 Loam Sandy Loam Silt loam 10 Loamy sand Silt Sand Sand %

  20. Field textures are estimated by feel as a moist sample is rubbed between thumb and forefinger

  21. sand loamy sand Soil Texture Terms SANDY SOILS

  22. sand loamy sand sandy loam fine sandy loam very fine sandy loam loam silt loam silt sandy clay loam silty clay loam clay loam Soil Texture Terms SANDY SOILS LOAMY SOILS

  23. sand loamy sand sandy loam fine sandy loam very fine sandy loam loam silt loam silt sandy clay loam silty clay loam clay loam sandy clay silty clay clay Soil Texture Terms SANDY SOILS LOAMY SOILS CLAYEY SOILS

  24. Generalized Influence of Texture: Sands • low: • water holding capacity, OM, fertility, shrink/swell, compaction potential • rapid: • water movement (drainage) = well aerated • susceptible to wind erosion (fine sands) • resists water erosion (except fine sands)

  25. Generalized Influence of Texture:Silt • medium: • water holding capacity, OM, fertility, shrink/swell, drainage = moderate aeration • very susceptible to wind and water erosion

  26. Generalized Influence of Texture:Clay • low drainage = poor aeration • high: • water holding capacity, OM, fertility, shrink/swell • dispersed clay very susceptible to wind and water erosion • aggregated clay NOT susceptible to wind and water erosion

  27. Consistence • The degree and kind of cohesion and adhesion that soil exhibits, and/or the resistance of soil to deformation or rupture under applied stress. Field evaluations of consistence usually include rupture resistance, stickiness, and plasticity.

  28. Consistence • Rupture resistance is a measure of the soil’s ability to withstand applied stress. For this test, moist soil is normally used. A naturally occurring soil aggregate is placed between the thumb and index finger. Pressure is slowly applied to estimate the amount of force that is required to rupture a soil aggregate.

  29. Consistence • Stickiness is the capacity of a soil to adhere to other objects. Stickiness is estimated at the moisture content that displays the greatest adherence when pressed between the thumb and forefinger. This normally occurs when the soil is quite wet.

  30. Consistence • Plasticity is the degree to which a reworked soil can be permanently deformed without rupturing. Plasticity is evaluated by forming a roll (wire) of soil that is 4 cm long.

  31. Shrink-swell Potential • Shrink-swell potential is a measurement of the amount of volume change that can occur when a soil wets and dries. Most of this volume change is due to the clay fraction of the soil. Clays swell when wet and shrink when dry. Soils high in 2:1 clays such as montmorillonite tend to have high shrink-swell potentials. Soils high in 1:1 clays such as kaolinite tend to have low shrink-swell potentials.

  32. Shrink-swell Potential • A soil with a highshrink-swell potential can cause severe problems when used for urban development unless the problem is recognized and proper engineering precautions taken. Shrinking and swelling can buckle roads, crack building foundations and walls, and even damage plant roots.

  33. Bulk Density • Bulk density is the mass of dry soil per unit volume, including the air space. • Bulk density has a major effect on the movement of air and water in soils. • Soils with high bulk densities are often compacted.

  34. Bulk Density • Soil compaction restricts rooting depth, which reduces the uptake of water and nutrients by plants. No Compaction Compaction (plow pan)

  35. Bulk Density • Compaction also decreases infiltration, thereby increasing runoff and the hazard of water erosion.

  36. Soil Structure • Soil structure - aggregation of primary particles into secondary units (aggregates or peds). Affects rates of water, gas, and energy exchange. • Soil structure can be altered easily by traffic (man, machine) when the moisture content is sufficient to promote sliding and packing. Soil texture is relatively stable, not easily altered.

  37. Why is it important? Infiltration/Permeability Soil Erosion Root penetration Others? Soil Structure

  38. Factors Influencing Soil Structure • Surface Development Factors • Biological activity • Organic Matter • Tillage • Subsurface Development Factors • Texture • Shrink-swell, wetting & drying cycles • Others?

  39. Benefits of Soil Structure • Increased porosity • Increased aeration • Increased infiltration • downward entry into the soil • Increased percolation • downward movement through soil

  40. Structure • The arrangement of primary soil particles into compound particles or aggregates. • The type and grade of structure plays an important role in the movement of water within soils.

  41. Structure • There are eight structural types commonly recognized in soil profiles: Granular, single grain, blocky, prismatic, columnar, platy, wedge, and massive.

  42. Structure • Granular - Rounded aggregates usually less than 1/4 inch in diameter. These rounded complexes lie loosely on the surface and are readily shaken apart. The aggregates are called granules and the pattern is called granular. This is the most common type of structure found in topsoil.

  43. Structure • Single grain – Each individual soil particle is separate and there is essentially no structure. This is only found in very sandy soils and is the type of structure commonly seen in sand dunes at the beach.

  44. Structure • Blocky - The original aggregates have been reduced to blocks, irregularly faced, and basically equal in height, width, and depth. Blocky structure is the most common type of structure seen in the subsoil (B horizon) in North Carolina.

  45. Structure • Prismatic – Characterized by vertical oriented aggregates or pillars with flat tops. These elongated columns vary in length with different soils. Prismatic structure is commonly seen in soils with high clay content and in horizons dominated by high shrink-swell clays.

  46. Structure • Columnar–Characterized by vertical oriented aggregates or pillars with rounded tops. These elongated columns with flat tops vary in length with different soils. Most commonly seen in soils that have a high sodium content in a dry climate.

  47. Structure • Platy – The aggregates are arranged in thin horizontal plates or sheets. This structure is commonly found in soil layers that have been compacted. Platy structure inhibits the downward movement of water.

  48. Structure • Wedge – The aggregates resemble wedges, thinner at one end and thicker on the other. This structure is seen in soils with a high clay content that is dominated by expansive clays such as montmorillonite. (2:1 clays)