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
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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
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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
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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
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
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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
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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
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– 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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