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Material Strength Local subgrade settlement failure (from NHI Highway Materials Engineering course)

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Presentation on theme: "Material Strength Local subgrade settlement failure (from NHI Highway Materials Engineering course)"— Presentation transcript:

1 Material Strength Local subgrade settlement failure (from NHI Highway Materials Engineering course)

2 Subgrade Strength/Stiffness
California Bearing Ratio (CBR) Resistance Value (R-Value) Resilient Modulus (MR) Modulus of Subgrade Reaction (K) Each measure a different thing

3 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 to standard crushed stone material. Field and laboratory test.

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

5 California Bearing Ratio (CBR)
Load a piston (area = 3 in2) 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.

6 CBR Test Equipment Soaking Samples for 4 days measure swelling and CBR
Piston Surcharge Weights Allow soaked samples to drain water for 15 min. then perform the CBR test. Min. surcharge weight = 10 lb Surcharge weights are added during testing and soaking to: Simulate the weight of pavement. Prevent heaving up around the piston. Soaking Samples for 4 days measure swelling and CBR Typical Testing Machine

7 CBR Calculation Loads and Stresses Corresponding to 0.1 and 0.2 inches Penetration for the Standard Rocks 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 Calculate CBR at 0.1 in (25 mm) and 0.2 in (50 mm) deformation then use the Maximum value as the design CBR.

8 CBR Curves Need correction because the piston is not exactly perpendicular with the soil sample.

9 CBR Curve Correction 0.0 0.1 0.2

10 Influence of Moisture upon CBR
CBR is evaluated at equilibrium moisture condition Use relevant value of moisture content when assessing soils under laboratory conditions.

11 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

12 Resistance Value (R-Value)

13 R-value Test (ASTM D2844) Pv Ph
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. Pv Ph

14 Typical R-Value Ranges
General Soil Type USCS Soil Type R-Value Range Clean gravels GW 30 – 80 GP Gravels with fines GM GC Clean sands SW 10 – 50 SP Sands with fines SM 20 – 60 SC Silts and clays ML 5 – 20 CL OL < 7 MH CH OH

15 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. s1 s3 s3 s2 s1

16 Triaxial Test Equipment

17 Typical Stress Strain Response During one Loading Cycle
Dwell Unloading Strain vs. Time ep er Stress vs. Time Linearity: stress strain relationship, Viscosity: time dependency of strain under constant stress, Plastic or elastic: degree to which the material can rebound or recover strain after stress removal.

18 Resilient Modulus er = DL/L load rest Time Load
2 14 16 load rest Time Load er = DL/L ASU Advanced Pavement Laboratory Animation from University of Tokyo Geotechnical Engineering Lab

19 Nonlinear Material Behavior: Coarse-Grained Soils
Bulk stress: q = s1 + s2 + s3 K1, K2 are material constants K1 > 0 K2 ≥ 0 (stress-stiffening) K2 K1

20 Nonlinear Material Behavior: Fine-Grained Soils
Octahedral shear stress: K3, K4 are material constants K3 > 0 K4 ≤ 0 (stress-softening) log M R K3 K4 log t oct

21 Combined Stress Dependence of MR
(NCHRP 1-37A) Bulk (Confining) Stress Stiffening term (k2 > 0) Dominates for coarse granular soils (base, subbase) Shear (Deviatoric) Stress Softening Term (k3 < 0) Dominates for fine-grained soils (subgrade)

22 Bulk Stress  Stiffening Shear Stress  Softening
Effect of Stress on MR Coarse Materials Fine Materials Bulk Stress  Stiffening Shear Stress  Softening q = s1 + s2 + s3 q = I = Bulk stress = First stress invariant toct = Octahedral shear stress

23 Effect of Moisture/Density on MR
Moisture  Softening Density  Stiffening

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

25 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 Direct correlation is like centigrade to farenheit, or liters to gallons CBR = strength and resistance to penetration R-value = frictional resistance to permanent deformation MR = stiffness or recoverable elastic movement

26 Correlations (CBR  MR)
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  % MR,  psi

27 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

28 Correlation Example MR R-Value
MR vs. R-value for some Washington State soils MR R-Value

29 MR Correlations w/ Index Properties and Soil Classification
(NCHRP 1-37A)

30 Default MR for USCS Classes
(NCHRP 1-37A)

31 Default MR for AASHTO Classes
(NCHRP 1-37A)

32 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.

33 Plate Loading Test Plate Loading Test Schematic Plate Loading Test
Reaction Reaction Pressure Gauge Hydraulic Jack 12″ f Plate 3 Deflection Dials 18″ f Plate 24″ f Plate Reaction for Dial 30″ f Plate To minimize bending, series of plates are used Apply load through the hydraulic jack and the reaction beam (use heavy equipment), EX: loader Measure the deflection of the plates at at least three points Draw the load deflection relationship Tested Layer Plate Loading Test Schematic Plate Loading Test

34 Effect of Plate Size p = n + m (P/A) p m n P/A p = Unit load (stress)
n, m = Empirical values obtained by test P/A = Perimeter over area p To determine m, n tests must be made on each soil using two different sizes of bearing plates using same deflection m n P/A

35 Modulus of Subgrade Reaction (k)
Required for rigid pavement design. Stress , psi 10 psi D 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 psi (68.95 kN/m2) Deformation, in

36 Corrections for K Correction due to saturation (worst case scenario).
Correction due to bending of the plates.

37 Correction Due to saturation
Ks = modulus of subgrade reaction corrected for saturation Ku = field modulus of subgrade reaction Du/Ds = ratio of the deflection in the unsaturated and saturated tests Field Condition Saturated Condition Stress 10 psi Du 10 psi Stress Deformation Run laboratory consolidation or simple compression test on subgrade soil samples at both the field moisture content and dry density as well as saturated condition. Determine the ratio of the measured deflection in both tests corresponding to 10 psi Ds Deformation

38 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) Kcorrected (pci)

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

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

41 To do a good job you need to study Hard, don’t you?!
Importance of subgrade uniformity (from NHI Highway Materials Engineering course) To do a good job you need to study Hard, don’t you?!

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