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Material Strength

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Subgrade Strength/Stiffness California Bearing Ratio (CBR) Resistance Value (R-Value) Resilient Modulus (M R ) 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 Resistance of the material to uniaxial penetration. Measure of soil shear strength relative to standard 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 in 2 ) at a constant rate (0.05 in/min) Record Load every 0.1 in penetration Total penetration not to exceed 0.5 in. Draw Load-Penetration Curve.

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CBR Test Equipment Soaking Samples for 4 days measure swelling and CBR Typical Testing Machine Surcharge Weights 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 Calculate CBR at 0.1 in (25 mm) and 0.2 in (50 mm) deformation then use the Maximum value as the design CBR. Penetration0.1 (2.5 mm)0.2 (5.0 mm) Load of Standard Rocks (Ib) Load of Standard Rocks (kN) Stress of Standard Rocks (KPa) Stress of Standard Rocks (psi) Loads and Stresses Corresponding to 0.1 and 0.2 inches Penetration for the Standard Rocks

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CBR Curves

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CBR Curve Correction

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Influence of Moisture upon CBR Use relevant value of moisture content when assessing soils under laboratory conditions.

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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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PhPh PvPv R-value Test (ASTM D2844) P v = applied vertical pressure (typically 160 psi) P h = 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 TypeUSCS Soil TypeR-Value Range Clean gravels GW 30 – 80 GP Gravels with fines GM 30 – 80 GC Clean sands SW 10 – 50 SP Sands with fines SM 20 – 60 SC Silts and clays ML5 – 20 CL5 – 20 OL< 7 MH5 – 20 CH5 – 20 OH< 7

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Resilient Modulus (M R ) 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

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Triaxial Test Equipment

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Typical Stress Strain Response During one Loading Cycle Stress vs. Time Strain vs. Time p r Dwell Loading Unloading

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Resilient Modulus Animation from University of Tokyo Geotechnical Engineering Lab loadrest Time Load r = L/L ASU Advanced Pavement Laboratory

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Nonlinear Material Behavior: Coarse-Grained Soils Bulk stress: = K 1, K 2 are material constants K 1 > 0 K 2 0 (stress-stiffening) K1K1 K2K2

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Nonlinear Material Behavior: Fine-Grained Soils Octahedral shear stress: K 3, K 4 are material constants K 3 > 0 K 4 0 (stress-softening) K3K3 K4K4 log M R log oct

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Combined Stress Dependence of M R (NCHRP 1-37A) Bulk (Confining) Stress Stiffening term (k 2 > 0) Dominates for coarse granular soils (base, subbase) Shear (Deviatoric) Stress Softening Term (k 3 < 0) Dominates for fine-grained soils (subgrade)

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Effect of Stress on M R Bulk Stress Stiffening Shear Stress Softening = oct = Octahedral shear stress = I = Bulk stress = First stress invariant Coarse Materials Fine Materials

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Effect of Moisture/Density on M R Moisture Softening Density Stiffening

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M R Model Including Moisture and Density S = degree of saturation d = dry unit weight k S, k = regression coefficients a,b = constants (function of soil type) P a = atmospheric pressure S = degree of saturation d = dry unit weight k S, k = regression coefficients a,b = constants (function of soil type) P a = atmospheric pressure (NCHRP 1-37A)

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Correlations Conversions between CBR, R-value, M R 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 M R ) Origin: Heukelom and Klomp (1962) Limitation: Fine-grained non-expansive soils with soaked CBR 10 Origin: NCHRP 1-37A – Mechanistic Design Guide Limitation: not stated Units: CBR % M R, psi

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Correlations Origin: HDOT Limitation: Fine-grained non-expansive soils with soaked CBR 8 Origin: 1993 AASHTO Guide Limitation: Fine-grained non-expansive soils with R 20

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Correlation Example M R vs. R-value for some Washington State soils MRMR R-Value

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M R Correlations w/ Index Properties and Soil Classification (NCHRP 1-37A)

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Default M R for USCS Classes (NCHRP 1-37A)

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Default M R for AASHTO Classes (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 Test Schematic Reaction Pressure Gauge 3 Deflection Dials Reaction for Dial Hydraulic Jack 30 Plate 24 Plate 18 Plate Tested Layer 12 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) = deflection of the plate (in) For design use stress P = 10 psi (68.95 kN/m 2 ) Required for rigid pavement design. Stress, 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 K s = modulus of subgrade reaction corrected for saturation K u = field modulus of subgrade reaction u / s = ratio of the deflection in the unsaturated and saturated tests Deformation Stress 10 psi u s Stress Deformation Saturated Condition Field Condition

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Correction due to Bending of the Plates Some bending of the plates might occur When materials of high modulus are tested. Use chart for correction of k for plate bending. K (pci) K corrected (pci)

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Basic Plate Loading Test Types Deformation Stress Time Deformation Rate Static Load

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Basic Plate Loading Test Types Deformation Stress Deformation Stress Accumulated Plastic Deformation Elastic Rebound Repeated Load

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