Engineering Properties of Soils Soil Types

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Engineering Properties of Soils Soil Types

• Engineering Properties of Soils
• foundation for the project
• construction material (road embankments,earth
  dams)
• Soil Definition (Engineering)
• refers to all unconsolidated material in the
  earths crust, all material above bedrock
• mineral particles (sands, silts, clays)
• organic material (topsoil, marshes) air
  water

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Engineering Properties of Soils Soil Types

• Mineral Soil Particles
• weathering of rock from the crust of the earth
• physical weathering and chemical weathering
• Physical Weathering
• action of frost, water, wind, glaciers,
  plant/animals, etc. breaking particles away from
  original bedrock
• particles transported by wind, water, ice
  gtrounding and reducing their size
• soils formed are called granular soil type
• grains are similar to the original bedrock

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Engineering Properties of Soils Soil Types

• Chemical weathering
• occurs when water flows through rocks and leaches
  out some of the mineral components of the rock
• soils formed are called clays
• clay particles are mineral crystals that have
  very different properties from those of the
  original bedrock

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Engineering Properties of Soils Mineral Soils
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Engineering Properties of Soils Soil Types

• Granular and Cohesive soil types
• difference in engineering properties result from
  the large variation in size and shape of the
  grains
• Cohesive soil type (clays)
• grains are extremely small and flat
• the mass of a grain as a force is negligible when
  compared to the forces resulting from the surface
  properties of the grain

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Engineering Properties of Soils Soil Types

• Water Holding Capacity of Clays
• Shrinkage
• evaporation of exposed clays
• loading
• Expansion
• dry side may absorb moisture
• Structure of Clays
• deposited by settling out in lakes

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Engineering Properties of Soils Soil Types

• Structure of Clays
• surface charges forces grains to edge to side
  pattern
• flocculent structure as opposed to granular soils
  which are deposited in a denser configuration
  because the force of gravity on the mass of these
  grains is more important

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Engineering Properties of Soils Soil Types

• Clays have surface charges due to the very large
  surface area per gram of material
• Chemical composition results in
• negative charges along the sides of a grain
• positive charges at the ends of a grain
• Results of these surface properties
• water holding capacity of clays surface charges
  attract water
• structure of clay deposits

clay grain
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Engineering Properties of Soils Soil Types
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Engineering Properties of Soils Soil Types

• Clay Soils
• Small flat shape
• Negative/positive surface charges
• Bound water on the surface
• Different clay minerals are different in size
• Swelling clays absorb water into the crystal
  lattice
• Shrinkage due to evaporation or loading

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Engineering Properties of Soils Soil Types

• Granular Soils
• Larger grain sizes than clays
• Particles tend to be more or less spheres/cubes
• Bound water is small compared to overall mass
• Silt particles may not be visible to eye but tend
  to be gritty, have dull appearance and lack
  cohesion when dry

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Engineering Properties of Soils Soil Types

• Organic Soils
• Tend to be fibrous and/or amorphous
• Brown to Black in color
• High moisture holding capacity
• Water may run out when squeezed
• Dried organic soils may combust

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Engineering Properties of Soils Soil Types

• Silts are coarser than clays and not bond tightly
  together
• Silts are gritty, less plastic and dull when cut
• Dry Strength-silts loose apparent cohesion when
  dried
• Shaking test-saturated silt samples become denser
  water seeps to the surface - dilantancy

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Engineering Properties of Soils Mass-Volume
Relationships
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Engineering Properties of Soils Mass-Volume
Relationships

• Example 1-2
• A soil sample has a volume of 175cm3 and a total
  mass of 300g. Mass when dried is 230g. Relative
  density of the soil solids is 2.70. Find r, rD,
  w, e, S and n

• Given
• Mw M - MD 70g
• MD 230g
• M 300g
• V 175cm3

Air
Water Solids
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Engineering Properties of Soils Mass-Volume
Relationships

• Calculations
• Vw Mw/rw
• 70g/(1 g/cm3)
• 70 cm3
• VD MD/(RD x rw)
• 230g/(2.70 x 1 g/cm3)
• 85 cm3
• VA V- (VD Vw )
• 175 - 155 20 cm3

• VA 20 cm3
• VW 70 cm3
• VD 85 cm3
• V 175 cm3

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Engineering Properties of Soils Mass-Volume
Relationships

• Answer
• r M/V 300g/175cm3 1.71 g/cm3
• rD MD/V 230g/175cm3 1.31g/cm3
• w Mw / MD 70g/230 g 30.4
• e Vv / VD 90 cm3/85 cm3 1.06
• S VW /Vv 70 cm3/90 cm3 78
• n Vv /V 90 cm3/ 175 cm3 51

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Engineering Properties of Soils Mass-Volume
Relationships - Rules

• 1. Density is given assume total unit volume 1
  cm3 or 1 m3
• 2. Water content is given along with total
  density or total mass.
• Use MD M or rD r
• 1 w 1w
• 3. Void Ratio is given and RD assume a unit
  volume of soil solids VD 1 m3

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Engineering Properties of Soils Mass-Volume
Relationship

• Density Index
• Field soil condition referred to as loose or
  dense
• Density Index is insitu soils density relative
  to the maximum and minimum for that type of soil
• Assessing the stability of granular soils
• Known as relative density
• ID r DRY MAX x r D - r DRY MIN
• r D r DRY MAX - r DRY MIN

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Engineering Properties of Soils
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Engineering Properties of Soils Classification
Tests

• Two types of tests used in classifying soils
• Grain size, measures grain sizes
• Plasticity, measures grain types
• Grain Size
• grain size distribution curve
• Sieve analysis gravel and sand
• Hydrometer test for silt and clay

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Engineering Properties of Soils Classification
Tests

• Hydrometer Test
• Used to find the size of smaller grains to plot a
  grain size distribution curve
• Stokes Law
• particles in suspension settle out at a rate
  which varies with their size
• hydrometer measures the density of a soil-water
  mix at various times as the grain settles
• The size of particle to the center of the bulb
  can be calculated and density of the solution
  indicates the percentage of the sample still in
  solution

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Engineering Properties of Soils Sieve Analysis
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Engineering Properties of Soils Classification
Tests
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Engineering Properties of Soils Classification
Tests

• Grain Size Distribution Curve
• Shape
• Uniform soil is composed of mainly one size grain
• Well graded soil contains a wide range of grain
  sizes
• Effective Size
• Effective size is the grain size that only 10 of
  the grain sizes are finer than.
• The amount and type of fine grains in a soil are
  important in assessing the properties of that
  soil

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Engineering Properties of Soils Classification
Tests

• Grain Size Distribution Curve
• Uniformity Coefficient Cu
• indication of the shape of the curve and range of
  particle sizes that the soil contains
• Cu D60 / D10
• Coefficient of Curvature Cc
• indication of the shape of the curve.
• Cc (D30)2 / (D60 x D10)

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Engineering Properties of Soils Classification
Tests

• Curve A - Uniform Soil
• Curve B - Well Graded Soil
• Soil B
• Effective size .09 mm
• Cu 7/.0978
• Cc 1.12/(7x.09)1.9

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Engineering Properties of Soils Classification
Tests

• Textural Classification
• based entirely on grain size
• ASTM System
• Gravel larger than 4.75 mm (No. 4)
• Sand 4.75 mm - 0.075 mm (No.4-No. 200)
• Silt 0.075 mm - 0.005 mm
• Clay smaller than 0.005 mm
• Identify of the grains as classified

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Engineering Properties of Soils Classification
Tests
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Engineering Properties of Soils Classification
Tests

• Plasticity Test (grain type)
• Measures the amount of water that a soil absorbs
• Plastic Limit
• w/c where the soil behaves like a plastic
  material
• Liquid Limit
• w/c where the soil behaves more like a liquid
• Plasticity Index (Atterberg Limits Test)
• the range of w/cs where soil is plastic

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Engineering Properties of Soils Classification
Tests
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Engineering Properties of Soils Classification
Tests

• Atterberg Limits test measures the consistency of
  the soil deposit
• Soil A Wp25 Wl32 w/c35
• Soil B Wp40 Wl80 w/c45
• Atterberg Limits Test
• soil rolled into a thread Wp
• soil flow measured in a cup Wl

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Engineering Properties of Soils Soil
Classification

• Unified Soil Classification ASTM D2487
• general engineering purposes
• AASHTO Soil Classification
• highway engineering purposes
• classifies soils to its suitability as a
  subgrade