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Engineering Properties of SoilsUnified Soil Classification
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Engineering Properties of SoilsUnified Soil Classification
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Engineering Properties of SoilsUnified Soil Classification
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Engineering Properties of SoilsUnified Soil Classification - Examples
• Soil #1– 9.5mm (100%)– 4.75mm (60%)– 425m (30%)– 150m (10%)– 75m (4%)
Cu = D60/D10 = 4.75/.150= 32
Cc = (D30)2/ (D60xD10 =(.425)2/(.15x4.75)=0.25 SP
• Soil #2– 4.75mm (88%)– 425m (28%)– 75m (9%)
Wp = 20
WL = 31
Ip = 31 - 20 = 11
Cu = 25
Cc= 1.6 SW-SC
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Engineering Properties of SoilsAASHTO Soil Classification
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Engineering Properties of SoilsAASHTO Soil Classification
• Example– 38mm (100%)
– 2.00mm (65%)
– 425m (45%)
– 75m (30%)
WL = 35
IP = 21 A-2-6
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Engineering Properties of SoilsSoil 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• moisture film around soil grains• quantity may be very large for clays
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Engineering Properties of SoilsSoil Water
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Engineering Properties of SoilsSoil Water
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Engineering Properties of SoilsSoil 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 = k H A
L
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Engineering Properties of SoilsSoil Water
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Engineering Properties of SoilsSoil Water
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Engineering Properties of SoilsSoil 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• Consolidation Test
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Engineering Properties of SoilsSoil Water
• Coefficient of Permeability (Sands)• Darcy’s Law q = k H A
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 SoilsSoil Water - Fine sands / silts
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Engineering Properties of SoilsSoil Water - Fine sands / silts k
• K = La ln (h1/ h2)
TAa = area of the standpipe
A = area of sample
T = time
L = length of sample
h1,h2 = initial and final heads
• 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 SoilsSoil Water - Capillary Rise
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Engineering Properties of SoilsSoil 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 d = d2/4 x hc x g x w
hc = 4 x S.T.
d x g x w
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Engineering Properties of SoilsSoil Water - Capillary Water Cont’d
Example - For Water:
Using: S.T. = 0.075g/cm
g w = 1 g/cm3
hc (cm) = 0.3
d (cm)
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Engineering Properties of SoilsSoil 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 SoilsSoil 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
• continuous process
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Engineering Properties of SoilsSoil Strength and Settlement
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Engineering Properties of SoilsSoil Strength and Settlement
• Shear strength is shear stress resisting failure along a plane
• Shear stress ( ) varies with mass of the block or normal stress (
tan = / = tan = shearing resistance
= normal stress on plane of failure
= angle of internal friction
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Engineering Properties of SoilsSoil Strength and Settlement
– Clays• shear strength is due to cohesion forces between the
grains c
– Granular soils• shear strength results from friction between the
grains along the shearing plane = tan
– Mixed Soils• shear strength is due to both cohesion and friction
= c + tan
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Engineering Properties of SoilsSoil Strength and Settlement
• Shear strength in soils can be measured by a number of tests
• At failure, f (strain at failure) is used to correct the cross sectional area
Af = Ao
1- f
• Unconfined Compressive Test clays
– strain (change in length) and load at failure are measured
– Unconfined Compressive strength=
qu = Max Load
Af
Shear Strength (cohesion)
= qu/2
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Engineering Properties of SoilsSoil Strength and Settlement
• Shear plane develops in stiff samples at 55-60° with horizontal
Note: Soft saturated clays, bulging may occur
f = 0.15
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Engineering Properties of SoilsSoil Strength and Settlement
• Direct Shear Test any soil type
– Maximum value shear force is measured
– Stresses at failure, are calculated max shear stress / A
– Cohesionless Soils• Calculate , = arctan ( / )
• shear strength = tan
– Soft Clays• shear strength = shear stress recorded
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Engineering Properties of SoilsSoil Strength and Settlement
• Mixed Soils– two tests required
• Test #1 1
1
• Test #2 2
2 = c + tan
c
(kPa)
(kPa)
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Engineering Properties of SoilsSoil Strength and Settlement
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Engineering Properties of SoilsSoil Strength and Settlement
• Triaxial Compression Test any soil type
– Cell pressure applied to simulate field conditions 3
– Clays are often tested in a quick shear test without drainage or water pressure measurements
– 3 is applied
– axial load 1 is applied
– axial load to failure (1-3)
• Calculate A0, f (ratio of original length)
• Af =A0 qq = (1-3) = qq
1-f Af 2
Compressive strength
Shear strength /cohesion
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Engineering Properties of SoilsSoil Strength and Settlement
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Engineering Properties of SoilsSoil 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• 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 SoilsSoil 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