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)
Stabilometer

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 Load2 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?!