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Soil Structure. Presented by: Mr. Brian Oram, PG, PASEO Wilkes University GeoEnvironmental Sciences and Environmental Engineering Department Wilkes - Barre, PA 18766 570-408-4619 http://www.water-research.net. Quick Review. Sand + Silt + Clay = 100%. 20 % Sand. 30 % Silt. 50 % Clay.

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Soil structure

Soil Structure

Presented by:

Mr. Brian Oram, PG, PASEOWilkes UniversityGeoEnvironmental Sciences and Environmental Engineering DepartmentWilkes - Barre, PA 18766570-408-4619

http://www.water-research.net



Sand + Silt + Clay = 100%

20 % Sand

30 % Silt

50 % Clay

Texture = Clay


Mineral soil
Mineral Soil

  • 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:

    • 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 + (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.


Why Is Understanding Soil Structure Important ?


Describing soil morphology for mineral soil
Describing Soil Morphologyfor Mineral Soil

  • Soil Texture and Coarse Fragment Content

  • Soil Structure (Type, Size and Grade)

  • Soil Color

  • Consistency

  • Redoximorphic Features

  • Boundaries


What is the Soil Structure?

How do we Describe Structure?

What is Going On in this Photo?


Soil structure1
Soil Structure

  • “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.

  • Three general groups

    • Natural Soil Structural Units (pedogenic structure)

    • Structureless

    • Artificial Earthy Fragments or Clods


Describing soil structure
DescribingSoil Structure

  • Type – shape of the structure

  • Size – relative size of the structure

  • 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??)


Soil structure2
Soil Structure

  • Type

  • Size

  • Grade


Soil structural types
Soil Structural Types

  • Granular

  • Blocky

    • Angular Blocky

    • Subangular Blocky

  • Platy

  • Wedge

  • Prismatic

  • Columnar

  • Massive

  • Single Grain


Granular
Granular

Small Polyhedrons or SpheroidsBounded by Curved or Irregular Surfaces

Symbol for Structure (gr)

Photo – Thank you Dr. Lindo - NCSU


Blocky subangular or angular
BlockySubangular 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)


Platy flat and tabular units
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


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


Prismatic structure symbol pr or pr
Prismatic Structure Symbol pr or PR

Photo 1

Photo 2

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.

X

Y

Z

Photo on the Left – Thank you Dr. Lindo - NCSU

A soil with a prismatic structure may also have secondary

structural units – Photo 2 has a platy secondary structure.


Columnar symbol cpr or col
ColumnarSymbol 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.

Y

X

Z

Photo on the Right – Thank you Dr. Lindo - NCSU


Structureless 0
Structureless (0)

  • Single Grain (non-coherent)

  • Massive (coherent mass)

  • Massive - Rock Controlled Fabric- May be a saprolite.


Single grain symbol sg or sgr
Single GrainSymbol sg or SGR

Lacks Structure- LooseOn rupture- more than

50 % of the mass is single

Grain – example beach sand.

Material is NON-Coherant.


Massive symbol m or ma
Massive Symbol m or MA

Structureless material that is coherent,

but is not cemented.


Massive rock controlled fabric
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


E HorizonsCompacted Horizons

and Bx Horizons

Bx horizons


Soil structure3
Soil Structure

  • Type

  • Size

  • Grade


Soil structure size
Soil Structure Size

  • Very Fine (Very Thin)

  • Fine (Thin)

  • Medium

  • Coarse (Thick)

  • Very Coarse (Very Thick)

Section of Size Criteria Depends on

the Structural Units.


Soil structure size granular and blocky

Image Source:NRCS, Ver. 2.0, 2002

Soil Structure Size (Granular and Blocky)


Soil structure size platy and prismatic

Image Source:NRCS, Ver. 2.0, 2002

Soil Structure Size (Platy and Prismatic)


Soil structure grade
Soil Structure Grade

  • Structureless = 0

    • Single Grain (non-coherent)

    • Massive (coherent)

    • Massive - Rock Controlled Fabric

  • Weak = 1

  • Moderate = 2

  • Strong = 3


Description of structure
Description of Structure

  • Three class of grade (Excluding Structureless (0) ):

  • Weak (1)– the units are barely observable in place and when gently disturbed, the soil parts into a mixture of whole and broken units.

  • 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.

  • 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.


Describing soil structure1
DescribingSoil Structure

  • Grade – how well the structure is expressed in place.

  • Size – relative size of the structure.

  • Type – shape of the structure.


Soil structure4

Soil Structure

Presented by:

Mr. Brian Oram, PG, PASEOWilkes UniversityGeoEnvironmental Sciences and Environmental Engineering DepartmentWilkes - Barre, PA 18766570-408-4619

http://www.water-research.net

Soils and Hydrogeological Work Jack Frost


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