Engineering Properties of Soils
Unified Soil Classification
2 ENCI 579
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Engineering Properties of Soils
Unified Soil Classification
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Engineering Properties of Soils
Unified Soil Classification
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Engineering Properties of Soils
Unified Soil Classification - Examples
• Soil #1
– 9.5mm (100%)
– 4.75mm (60%)
– 425mm (30%)
– 150mm (10%)
– 75mm (4%)
Cu = D60/D10 =
4.75/.150= 32
Cc = (D30)2/ (D60xD10
2/(.15x4.75)
=(.425)
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=0.25 SP
• Soil #2
– 4.75mm (88%)
– 425mm (28%)
– 75mm (9%)
Wp = 20
WL = 31
Ip = 31 - 20 = 11
Cu = 25
Cc= 1.6
SW-SC
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Engineering Properties of Soils
AASHTO Soil Classification
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Engineering Properties of Soils
AASHTO Soil Classification
• Example
–
–
–
–
38mm (100%)
2.00mm (65%)
425mm (45%)
75mm (30%)
WL = 35
IP = 21
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A-2-6
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Engineering Properties of Soils
Soil Water
• Types of water found in soil
– Free water or gravitational
• found below the water table
• free to flow under the forces of gravity
– Capillary water
• brought up through soil pores
• due to surface tension and found above water table
in certain soil conditions
– Attached water or held water
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• moisture film around soil grains
• quantity may be very large for clays
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Engineering Properties of Soils
Soil Water
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Engineering Properties of Soils
Soil Water
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Engineering Properties of Soils
Soil Water
• Water Flow Through Soils
– where q is the flow of water (cm3/s)
• I is the hydraulic gradient causing the flow
• I = H (head loss due to flow through soil)
L (length of path of flow through the soil)
• A is the cross sectional area of the flow path (cm2)
• k is the coefficient of permeability of average
velocity of water through the soil (cm/s)
– Darcy’s law can also be stated as
• q=kHA
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L
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Engineering Properties of Soils
Soil Water
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Engineering Properties of Soils
Soil Water
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Engineering Properties of Soils
Soil Water
• Determining Permeability of Soils
– Clean uniform sands
• Hazen’s formula k=(D10)2 where:
• k=coefficient of permeability (cm/s)
• D10 = effective size (mm)
– Sands
• Constant Head Permeability Test
– Fine sands and silts
• Falling Head Permeability Test
– Clays
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Engineering Properties of Soils
Soil Water
• Coefficient of Permeability (Sands)
• Darcy’s Law
q=k HA
L
• k = qL
HA
q = measured flow (cm3/s)
H = head loss
L = length of path (cm)
A = cross sectional area (cm2)
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Engineering Properties of Soils
Soil Water - Sands
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Engineering Properties of Soils
Soil Water - Fine sands / silts
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Engineering Properties of Soils
Soil Water - Fine sands / silts k
• K = La ln (h1/ h2)
TA
a = area of the standpipe
A = area of sample
T = time
L = length of sample
h1,h2 = initial and final heads
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• For fine sands/silts, small
flows
• Used when the quantity of
flow would be too small to
measure properly by a
constant head permeability
test
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Engineering Properties of Soils
Soil Water - Capillary Rise
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Engineering Properties of Soils
Soil Water - Capillary Water
• Water that rises in tubes or pore spaces due to
surface tension
• hc varies inversely with d
• hc can be determined by: surface tension force =
force due to gravity of the volume of water
S.T. x pd = pd2/4 x hc x g x rw
hc = 4 x S.T.
d x g x rw
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Engineering Properties of Soils
Soil Water - Capillary Water Cont’d
Example - For Water:
Using: S.T. = 0.075g/cm
g rw = 1 g/cm3
hc (cm) = 0.3
d (cm)
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Engineering Properties of Soils
Soil Water
• Typical Values of capillary rise
– Sands 0-1 meters
– Silts 1-10 meters
– Clays over 10 meters
• Pore sizes in soils are similar to tubes
• pore sizes vary greatly with different soils and
therefore difficult to measure
• Estimate by 20 % of the effective size D10
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Engineering Properties of Soils
Soil Water
• Surface Tension in Soil Water
– Soil is saturated above the groundwater table
• difficulty in establishing ground water table
– Apparent Cohesion in sands and silts
• mistakenly indicating a clay material
– Frost Heaving
• water in large pores freeze
• water in smaller pores not frozen drawn to ice
crystal freezes enlarging the ice crystal
• capillary water moves up pore spaces to replace
smaller water particles
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• continuous process
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Engineering Properties of Soils
Soil Strength and Settlement
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Engineering Properties of Soils
Soil Strength and Settlement
• Shear strength is shear
stress resisting failure
along a plane
• Shear stress (t ) varies
with mass of the block
or normal stress (s)
tan f = t/s
t = s tan f
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t = shearing resistance
s = normal stress on plane of
failure
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f = angle of internal friction
Engineering Properties of Soils
Soil Strength and Settlement
– Clays
• shear strength is due to cohesion forces between the
grains t = c
– Granular soils
• shear strength results from friction between the
grains along the shearing plane t = s tan f
– Mixed Soils
• shear strength is due to both cohesion and friction
t = c + s tan f
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Engineering Properties of Soils
Soil Strength and Settlement
• Shear strength in soils
can be measured by a
number of tests
• At failure, ef (strain at
failure) is used to
correct the cross
sectional area
Af = Ao
1- ef
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• Unconfined
Compressive Test clays
– strain (change in
length) and load at
failure are measured
– Unconfined
Compressive strength=
qu = Max Load
Af
 Shear Strength
(cohesion)
t = qu/2
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Engineering Properties of Soils
Soil Strength and Settlement
• Shear plane develops
in stiff samples at 5560° with horizontal
Note: Soft saturated
clays, bulging may
occur
ef = 0.15
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Engineering Properties of Soils
Soil Strength and Settlement
• Direct Shear Test
any soil type
– Maximum value shear force is measured
– Stresses at failure, t s are calculated
 s = N/A t = max shear stress / A
– Cohesionless Soils
• Calculate t , s
• shear strength
f = arctan (t / s)
t = s tan f
– Soft Clays
• shear strength = shear stress recorded
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Engineering Properties of Soils
Soil Strength and Settlement
• Mixed Soils
– two tests required
• Test #1
 s1
 t1
t
(kPa)
• Test #2
 s2
 t2
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t = c + s tanf
f
c
s (kPa)
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Engineering Properties of Soils
Soil Strength and Settlement
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Engineering Properties of Soils
Soil Strength and Settlement
• Triaxial Compression Test any soil type
– Cell pressure applied to simulate field conditions s3
– Clays are often tested in a quick shear test without
drainage or water pressure measurements
– s3 is applied
– axial load s1 is applied
– axial load to failure (s1-s3)
• Calculate A0, ef (ratio of original length)
• Af =A0
qq = (s1-s3)
t = qq
1-ef
Af Compressive 2
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strength
Shear strength
/cohesion 31
Engineering Properties of Soils
Soil Strength and Settlement
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Engineering Properties of Soils
Soil Strength and Settlement
• Settlement Failures
– Amount Fn (compressibility of the soil)
• rearrangement of soil grains to a denser thinner layer
• usually involves squeezing out of water when a load
is applied
– Clays
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• may have a loose structure and a high voids and
moisture content and can be compressed
considerably
• Due to the extremely slow movement of water in
clays time for settlement could take years
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Engineering Properties of Soils
Soil Strength and Settlement
• Settlement Failures
– Granular Soils
• grains are usually in close contact
• Settlement usually takes place as the load is applied
and does not lead to long term settlement problems
• Consolidation Test
– measures the amount and rate of
compressibility
• sample placed in cell and loaded measured over time
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