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Soil Structure Presented by: Mr. Brian Oram, PG, PASEO Wilkes University GeoEnvironmental Sciences and Environmental Engineering Department Wilkes - Barre,

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Presentation on theme: "Soil Structure Presented by: Mr. Brian Oram, PG, PASEO Wilkes University GeoEnvironmental Sciences and Environmental Engineering Department Wilkes - Barre,"— Presentation transcript:

1 Soil Structure Presented by: Mr. Brian Oram, PG, PASEO Wilkes University GeoEnvironmental Sciences and Environmental Engineering Department Wilkes - Barre, PA

2 Quick Review

3 Sand + Silt + Clay = 100% Texture = Clay 20 % Sand 30 % Silt 50 % Clay

4 Mineral Soil Is never saturated with water for more than a few days and contains less than 20 % (by weight) organic carbon Is never saturated with water for more than a few days and contains less than 20 % (by weight) organic carbon If saturated with water for longer periods or artificially drained and excluding live roots, has an organic-carbon content of: If saturated with water for longer periods or artificially drained and excluding live roots, has an organic-carbon content of: Less than 18 % if the mineral fraction contains 60% or more clay; Less than 18 % if the mineral fraction contains 60% or more clay; Less than 12 %, if the mineral fraction contains no clays; or Less than 12 %, if the mineral fraction contains no clays; or Less than 12 + (clay percentage multiplied by 0.1) %, if the mineral fraction contains less than 60%. Less than 12 + (clay percentage multiplied by 0.1) %, if the mineral fraction contains less than 60%. Soil material containing more than the above organic carbon is an organic soil.

5 Why Is Understanding Soil Structure Important ?

6 Describing Soil Morphology for Mineral Soil Soil Texture and Coarse Fragment Content Soil Texture and Coarse Fragment Content Soil Structure (Type, Size and Grade) Soil Structure (Type, Size and Grade) Soil Color Soil Color Consistency Consistency Redoximorphic Features Redoximorphic Features Boundaries Boundaries

7 What is the Soil Structure? How do we Describe Structure? What is Going On in this Photo?

8 Soil Structure Structure is the naturally occurring arrangement of soil particles into aggregates (peds) that result from pedogenic processes. Structure is the naturally occurring arrangement of soil particles into aggregates (peds) that result from pedogenic processes. The grouping or arrangement of individual soil particles into a larger grouping. The soil peds are a cluster or grouping of sand, silt, clay, organic material, biological slimes, and fungal masses. The grouping or arrangement of individual soil particles into a larger grouping. The soil peds are a cluster or grouping of sand, silt, clay, organic material, biological slimes, and fungal masses. Three general groups Three general groups Natural Soil Structural Units (pedogenic structure) Natural Soil Structural Units (pedogenic structure) Structureless Structureless Artificial Earthy Fragments or Clods Artificial Earthy Fragments or Clods

9 Describing Soil Structure Type – shape of the structure Type – shape of the structure Size – relative size of the structure Size – relative size of the structure Grade – how well the structure is expressed in place. Grade – how well the structure is expressed in place. When described in a soil profile the full description of the structure is describe in the following order – grade, size, shape. Such as: weak medium subangular blocky (WHAT??)

10 Soil Structure Type Type Size Size Grade Grade

11 Soil Structural Types Granular Granular Blocky Blocky Angular Blocky Angular Blocky Subangular Blocky Subangular Blocky Platy Platy Wedge Wedge Prismatic Prismatic Columnar Columnar Massive Massive Single Grain Single Grain

12 Granular Small Polyhedrons or Spheroids Bounded by Curved or Irregular Surfaces Symbol for Structure (gr) Photo – Thank you Dr. Lindo - NCSU

13 Blocky Subangular or Angular Subangular Blocky – the three dimensions are about the same size, but polyhedrons are subrounded. Symbol (sbk) Angular Blocky – the three dimensions are about the same size, but edges are shape and faces appear flattened. Symbol (abk)

14 Platy – Flat and Tabular- Units Structure has a horizontal dimension that is longer or greater than the vertical dimension. The plates tend to parallel the surface. Typical Symbol (pl or PL) Photo on the Right – Thank you Dr. Lindo - NCSU

15 Wedge Structures Elliptical Interlocking lenses that terminate in acute angles, bounded by slickensides. This structure is Not limited to Vertic martials or Vertisols. Symbol: WEG Soil: Has a High Clay Content- with Expanding clays. Photo – Thank you Dr. Lindo - NCSU

16 Prismatic Structure Symbol pr or PR Typically associated with fragipan and other similar horizons. These are prismlike Structural units where the length and width is significantly Less than the depth. (Vertical Structures) These structures have flat tops or caps. Water movement tends to be associated with movement along the vertical contact between units. A soil with a prismatic structure may also have secondary structural units – Photo 2 has a platy secondary structure. Photo 2 Photo 1 Photo on the Left – Thank you Dr. Lindo - NCSU Z Y X

17 Columnar Symbol cpr or COL Prism-like structural units where the length of the unit in the x and y direction are much less than the z direction. The units tend to have rounded tops. X Y Photo on the Right – Thank you Dr. Lindo - NCSU Z

18 Structureless (0) Single Grain (non-coherent) Single Grain (non-coherent) Massive (coherent mass) Massive (coherent mass) Massive - Rock Controlled Fabric- May be a saprolite. Massive - Rock Controlled Fabric- May be a saprolite.

19 Single Grain Symbol sg or SGR Lacks Structure- Loose On rupture- more than 50 % of the mass is single Grain – example beach sand. Material is NON-Coherant.

20 Massive Symbol m or MA Structureless material that is coherent, but is not cemented.

21 Massive Rock Controlled Fabric Structureless material that is coherent, but is not cemented or weakly cemented In this case the material contains the original apperance of the bedrock material, but lacks the strength. Possible – Cr Horizon – Saprolite Photo on the Right – Thank you Dr. Lindo - NCSU

22 Bx horizons E Horizons Compacted Horizons and Bx Horizons

23 Soil Structure Type Type Size Size Grade Grade

24 Soil Structure Size Very Fine (Very Thin) Very Fine (Very Thin) Fine (Thin) Fine (Thin) Medium Medium Coarse (Thick) Coarse (Thick) Very Coarse (Very Thick) Very Coarse (Very Thick) Section of Size Criteria Depends on the Structural Units.

25 Soil Structure Size (Granular and Blocky) Image Source: NRCS, Ver. 2.0, 2002

26 Soil Structure Size (Platy and Prismatic) Image Source: NRCS, Ver. 2.0, 2002

27 Soil Structure Grade Structureless = 0 Structureless = 0 Single Grain (non-coherent) Single Grain (non-coherent) Massive (coherent) Massive (coherent) Massive - Rock Controlled Fabric Weak = 1 Weak = 1 Moderate = 2 Moderate = 2 Strong = 3 Strong = 3

28 Description of Structure Three class of grade (Excluding Structureless (0) ): Three class of grade (Excluding Structureless (0) ): 1) Weak (1)– the units are barely observable in place and when gently disturbed, the soil parts into a mixture of whole and broken units. 2) Moderate (2) – The units are well formed and evident in undisturbed soil. When disturbed, the soil material parts into a mixture of whole with some broken units. 3) Strong (3) – The units are distinct in undisturbed soil. They separate cleanly when the soil is disturbed and when removed the soil separates into whole units.

29 Describing Soil Structure Grade – how well the structure is expressed in place. Grade – how well the structure is expressed in place. Size – relative size of the structure. Size – relative size of the structure. Type – shape of the structure. Type – shape of the structure.

30 Soil Structure Presented by: Mr. Brian Oram, PG, PASEO Wilkes University GeoEnvironmental Sciences and Environmental Engineering Department Wilkes - Barre, PA Soils and Hydrogeological Work Jack Frost


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