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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
earth’s crust, all material above bedrock”
• mineral particles (sands, silts, clays)
• organic material (topsoil, marshes) + air + water
Soils ENCI 579
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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
>rounding and reducing their size
– soils formed are called granular soil type
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– “grains are similar to the original bedrock”
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
Soils ENCI 579
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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
clay grain
• Results of these surface properties
– water holding capacity of clays surface charges attract
water
– structure of clay deposits
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Engineering Properties of Soils
Soil Types
Soils ENCI 579
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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
Soils ENCI 579
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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
Soils ENCI 579
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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
Given:
A soil sample has a volume Mw = M - MD = 70g
of 175cm3 and a total
MD = 230g
mass of 300g. Mass
Air
M
=
300g
when dried is 230g.
Water
V = 175cm3
Relative density of the
soil solids is 2.70. Find
r, rD, w, e, S and n
Soils ENCI 579
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
Soils ENCI 579
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
1+w
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 soil’s 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
rD
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
Soils ENCI 579
– Effective size =
.09 mm
– Cu = 7/.09=78
– Cc =
1.12/(7x.09)=1.927
Engineering Properties of Soils
Classification Tests
– Textural Classification
• based entirely on grain size
– ASTM System
•
•
•
•
Gravel
Sand
Silt
Clay
larger than 4.75 mm (No. 4)
4.75 mm - 0.075 mm (No.4-No. 200)
0.075 mm - 0.005 mm
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/c’s 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: Wp=25 Wl=32 w/c=35
– Soil B: Wp=40 Wl=80 w/c=45
• 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 it’s suitability as a subgrade
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