3 SG Strength

8/13/2019 3 SG Strength

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Mater ial Strength


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Subgrade Strength/Stiffness

California Bearing Ratio (CBR)

Resistance Value (R-Value)

Resilient Modulus (MR)

Modulus of Subgrade Reaction (K)


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CBR: California Bearing Ratio Test.

Developed by The California State Highways

Department in 1930. Resistance of the material to uniaxial

penetration.

Measure of soil shear strength relative tostandard crushed stone material.

Field and laboratory test.


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Used in Pavement Design

Performed on unbound layers:

Subgrade layer, Subbase layer

base layer.


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Load a piston (area = 3 in2) ata constant rate (0.05 in/min)

Record Load every 0.1 in

penetration

Total penetration not toexceed 0.5 in.

Draw Load-Penetration

Curve.


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CBR Test Equipment

Soaking Samples for 4 daysmeasure swelling and CBRTypical Testing Machine

SurchargeWeights

Surcharge weights are added

during testing and soaking to:

Simulate the weight of

pavement.

Prevent heaving up

around the piston.

Piston


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CBR Calculation

RocksStandardofStressorLoadSoilofStressorLoad100CBR

Calculate CBR at 0.1 in (25 mm ) and 0.2 in (50 mm ) deformation

then use theMaximumvalue as the design CBR.

Penetration 0.1 (2.5 mm) 0.2 (5.0 mm)

Load of Standard Rocks (Ib) 3000 4500

Load of Standard Rocks (kN) 13.24 19.96

Stress of Standard Rocks (KPa) 6895 10342

Stress of Standard Rocks (psi) 1000 1500

Loads and Stresses Corresponding to 0.1 and 0.2 inches

Penetration for the Standard Rocks


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CBR Curves

0

100

200

300

400500

600

700

800

0 0.1 0.2 0.3 0.4 0.5 0.6

Penetration (in)

Load(Ib)

Wrong Curve

Standard Curve

Need correction


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CBR Curve Correction

0

100

200

300

400

500

600

700

0 0.1 0.2 0.3 0.4 0.5 0.6

Penetration (in)

Load(Ib)

0.0 0.1 0.2


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Influence of Moisture upon CBR

0

100

200

300

400

500

600

700

0 0.1 0.2 0.3 0.4 0.5 0.6

Moisture Content

CB

R

Use relevant value of moistu re content when assess ing so i ls

under laboratory condi t ions.


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Developed by California Division of Highways:

1940s

Measures frictional resistance of granular

material to deformation

Uses the Hveem Stabilometer

Tests material in a saturated condition (worst

case scenario


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Stabilometer


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Ph

Pv

R-value Test (ASTM D2844)

1)1)(5.2

(

100100

2

h

v

P

P

D

R Pv= applied

vertical pressure

(typically 160 psi)

Ph= transmitted

horizontal pressure

D2 = displacement

of stabilometer

fluid necessary to

increase horizontal

pressure from 5 to

100 psi.


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Typical R-Value Ranges

General Soil Type USCS Soil Type R-Value Range

Clean gravelsGW

3080GP

Gravels with finesGM

3080GC

Clean sandsSW

1050SP

Sands with finesSM

2060SC

Silts and clays

ML 5

20CL 520

OL < 7

MH 520

CH 520

OH < 7


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Resilient Modulus (MR)

Measures stiffness of the material under repeated load.

Determines the load carrying capacity of the material.

Used for HMA as well as unbound materials

Uses a repeated load triaxial test.

Used in most modern methods of pavement design.

r

RM

31

straineRecoverabl

stressDeviator 1

3

2

3

1


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Triaxial Test Equipment

Inside Rods

Load Cell

ChamberSoil Specimen

Bottom Platen

Top Platen

Frame

External LVDTLoading Piston

Cell Pressure Inlet

LVDT ClampLVDT


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Typical Stress Strain Response During one

Loading Cycle

0.0

5.0

10.0

15.0

20.0

25.0

30.0

0.0 0.5 1.0 1.5

Time (sec)

Stress

(psi)

0.000

0.004

0.008

0.012

0.016

0.0 0.5 1.0 1.5

Time (sec)

Strain

(in

/in)

Stress vs. Time

Strain vs. Time

pr

DwellLoading

Unloading


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Resilient Modulus

Animation from University of Tokyo Geotechnical Engineering Lab

2 14 16 2 14 16

load restTime

Load

r= L/L

ASU Advanced Pavement Laboratory


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Nonlinear Material Behavior:

Coarse-Grained Soils

Bulk stress: q= 1+ 2+ 3 K1, K2are material constants

K1> 0

K2 0 (stress-stiffening)

log MR

log

K1

K2

21

KR KM q


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Nonlinear Material Behavior:

Fine-Grained Soils

Octahedral shear stress:

K3, K4are material constants K3> 0

K4 0 (stress-softening)

K3 K4

log MR

log oct

43

KoctR KM


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Combined Stress Dependence of MR

(NCHRP 1-37A)

Bu lk (Confin ing) Stress

Stiffening term (k2

> 0)

Dominates for coarse granular

soils(base, subbase)

Shear (Deviato ric) Stress

Softening Term (k3< 0)

Dominates for fine-grained

soils(subgrade)

32

11

k

a

oct

k

aaR

PPpkM

q


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Effect of Stress on MR

log MR

log

log MR

log octBulk Stress Sti ffening Shear Stress Softening

q= 1+ 2+ 3oct= Octahedral shear stress= I= Bulk stress = Firststress invariant

Coarse Materials Fine Materials


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Effect of Moisture/Density on MR

log MR

S dry

log MR

Moisture Softening Density Stiffening


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Correlations

Conversions between CBR, R-value, MR

Important points:

No direct correlation

Each test measures a fundamentally different property

Developed correlations are only for limited data sets


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Correlations (CBR MR)

CBRMR 1500Origin: Heukelom and Klomp (1962)Limitation: Fine-grained non-expansive soils with soaked CBR 10

64.02555 CBRMR

Origin: NCHRP 1-37A

Mechanistic Design GuideLimitation: not stated

Units: CBR %

MR, psi


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Correlations

555

11551500ValueR

CBR

Origin: HDOT

Limitation: Fine-grained non-expansive soils with soaked CBR 8

ValueR5551000 RM

Origin: 1993 AASHTO GuideLimitation: Fine-grained non-expansive soils with R 20


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Correlation Example

MRvs. R-value for some Washington State soils

MR

R-Value


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50 6040302015108654321.5

1 2 3 4 5 10 15 20 40 60 80 100

1008060403020151054321

Poor Medium Good ExcellentSubgrade Soil

Category

Unified Soil

Classification

AASHTO Soil

Classification

Mr(ksi)

CBR (%)

R- Value

A-1-b

A-2-7

A-3

A-4

A-5

A-6

A-7-6

CH

MH

CL

ML

SW

GM

GC

GP - GM

GP - GC

GW - GM

GW - GC

GP

GW

SMSC

SP - SM

SP - SC

SW - SM

SW - SC

SP

A-1-a

A-7-5

A-2-5

A-2-4

A-2-6

MRCorrelationsw/ I ndex

Properties and

Soil Classif ication

(NCHRP 1-37A)


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Plate Loading Test

Measure supporting power of subgrades,

subases, bases and a complete pavement.

Field test.

Data from the test are applicable for design of

both flexible and rigid pavements.

Results might need some corrections.


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Plate Loading Test

Plate Loading TestPlate Loading Test Schematic

Reaction

Pressure

Gauge

3 Deflection

Dials

Reaction

for Dial

Hydraulic

Jack

30f Plate24f Plate

18f Plate

Tested Layer

12f Plate

Reaction


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Effect of Plate Size

p= n+ m(P/A)

p = Unit load (stress)

n, m = Empirical values obtained by test

P/A = Perimeter over area

n

m

P/A

p


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Modulus of Subgrade Reaction (k)

K = modulus of subgrade reaction

P = unit load on the plate (stress) (psi)

D= deflection of the plate (in)

For design use stress P = 10 ps i (68.95 kN/m2)

PK

Required for rigid pavement design.

Str

ess,psi

10 psi

Deformation, in


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Corrections for K

Correction due to saturation (worst case

scenario).

Correction due to bending of the plates.


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Correction Due to saturation

us

us KK

DD

Ks= modulus of subgrade reaction corrected for saturation

Ku= field modulus of subgrade reactionDu/Ds= ratio of the deflection in the unsaturated and saturated tests

Deformation

Stres

s10 psi

us

10 psi

Stress

Deformation

Saturated

Condition

Field

Condition


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Correction due to Bending of the Plates

Some bending of the plates might occur Whenmaterials of high modulus are tested.

Use chart for correction of k for plate bending.

K (pci)

Kcorrected(pci)


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Basic Plate Loading Test Types

Deformation

Stress

Deformation

Stress

Accumulated Plastic

DeformationElastic

Rebound

Repeated Load


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3 SG Strength

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