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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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California Bearing Ratio (CBR)
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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California Bearing Ratio (CBR)
Used in Pavement Design
Performed on unbound layers:
Subgrade layer, Subbase layer
base layer.
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California Bearing Ratio (CBR)
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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Resistance Value (R-Value)
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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Resistance Value (R-Value)
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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