Engineering Properties of Soils Unified 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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• 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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• 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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• 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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• 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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• 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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• 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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– 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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• 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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• 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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• 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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• Mixed Soils– two tests required

• Test #1 1

1

• Test #2 2

2 = c + tan

c

(kPa)

(kPa)

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• 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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• 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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• 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

Engineering Properties of Soils Unified Soil Classification

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