1. Pavement Design

2. I. Pavement Design A. Overview Degree of curvature
“Principal cause of pavement failure shown above—not the blacktop”

3. B. California Bearing Ratio (CBR)
I. Pavement Design
B. California Bearing Ratio (CBR)
“How to build a road!”

4. B. California Bearing Ratio (CBR)
I. Pavement Design
B. California Bearing Ratio (CBR)
1. The California bearing ratio (CBR) is a penetration test for evaluation of the mechanical strength of road subgrades and basecourses. It was developed by the California Department of Transportation. 4

5. I. Pavement Design
B. California Bearing Ratio (CBR)
The California bearing ratio (CBR) is a penetration test for evaluation of the mechanical strength of road subgrades and basecourses. It was developed by the California Department of Transportation.
2. The test is performed by measuring the pressure required to penetrate a soil sample with a plunger of standard area. The measured pressure is then divided by the pressure required to achieve an equal penetration on a standard crushed rock material.

6. 3. “The Test” I. Pavement Design B. California Bearing Ratio (CBR)
Take load readings at penetrations of:
“the result”
0.025” ……………70 psi
0.05”…………… psi
0.1”……………….220 psi
0.2”……………….300 psi
0.4”……………….320 psi
6” mold
“Achieve OM &MD”
Penetrations of 0.05” per minute

7. 4. Plot the Data

8. 5. Determine the percent of compacted crushed stone values for the 0
5. Determine the percent of compacted crushed stone values for the 0.1 and 0.2
penetration.
“The Gold Standard” for CBR
for 0.1” of penetration, 1000 psi
for 0.2” of penetration, 1500 psi
Example above:
for 0.1” of penetration, 220 psi
for 0.2” of penetration, 300 psi
The standard material for this test is crushed California limestone

9. 5. Determine the percent of compacted crushed stone values for the 0
5. Determine the percent of compacted crushed stone values for the 0.1 and 0.2
penetration.
Example psi = CBR
Standard psi
220 psi = .22, or 22%
1000 psi
300 psi = .20, or 20%
1500 psi
CBR of material = 22%
“The Gold Standard” for CBR
for 0.1” of penetration, 1000 psi
for 0.2’ of penetration, 1500 psi
Example above:
for 0.1” of penetration, 220 psi
for 0.2” of penetration, 300 psi

10. 5. Determine the percent of compacted crushed stone values for the 0
5. Determine the percent of compacted crushed stone values for the 0.1 and 0.2
penetration.
Example psi = CBR
Standard psi
220 psi = .22, or 22%
1000 psi
300 psi = .20, or 20%
1500 psi
CBR of material = 22%
“The Gold Standard” for CBR
for 0.1” of penetration, 1000 psi
for 0.2’ of penetration, 1500 psi
Example above:
for 0.1” of penetration, 220 psi
for 0.2” of penetration, 300 psi
Use 0.1” of penetration, unless 0.2” is the greater value.
If so, then rerun the test, taking the higher of the two values from this second trial

11. 5. Determine the percent of compacted crushed stone values for the 0
5. Determine the percent of compacted crushed stone values for the 0.1 and 0.2
penetration.
In General:
The harder the surface, the higher the CBR rating.
A CBR of 3 equates to tilled farmland,
A CBR of 4.75 equates to turf or moist clay,
Moist sand may have a CBR of 10.
High quality crushed rock has a CBR over 80.
The standard material for this test is crushed California
limestone which has a value of 100.
Example psi = CBR
Standard psi
220 psi = .22, or 22%
1000 psi
300 psi = .20, or 20%
1500 psi
CBR of material = 22%,
or “22”
“The Gold Standard” for CBR
for 0.1” of penetration, 1000 psi
for 0.2’ of penetration, 1500 psi
Example above:
for 0.1” of penetration, 220 psi
for 0.2” of penetration, 300 psi

12. Potential Corrections to the
Stress-Penetration Curves

13. I. Pavement Design
C. The Mechanics of the Design

14. C. The Mechanics of the Design Determine
I. Pavement Design
C. The Mechanics of the Design
Determine
The CBR values of the subgrade
The type of use expected (runways vs. taxiways) 14

15. C. The Mechanics of the Design Determine
I. Pavement Design
C. The Mechanics of the Design
Determine
The CBR values of the subgrade
The type of use expected (runways vs. taxiways)
The expected wheel load during service
Types of CBR materials available for the construction 15

16. C. The Mechanics of the Design 2. Primary Goals
I. Pavement Design
C. The Mechanics of the Design
2. Primary Goals
Total strength of each layer only as good as what is beneath it
Therefore, must meet minimum thickness requirements
“Don’t break the bank”
Use less inexpensive CBR materials when allowed while not shortchanging the project’s integrity

17. C. The Mechanics of the Design 3. An example
I. Pavement Design
C. The Mechanics of the Design
3. An example
A compacted subgrade has a CBR value of 8. What is the minimum pavement thickness if it is to support a taxiway pavement designed to support a 80,000 lb airplane (40,000 wheel load)?

18. “ a point on the curve for a
given CBR material represents
the minimum thickness of
pavement courses that will
reside above it, in order to
maintain stability

19. CBR subbase of 8,
Taxiway, and wheel load
of 40,000 lb
23 inches
“23 inches of
total earth material
and pavement”

20. C. The Mechanics of the Design 3. An example
I. Pavement Design
C. The Mechanics of the Design
3. An example
A compacted subgrade has a CBR value of 8. What is the minimum pavement thickness if it is to support a taxiway pavement designed to support a 80,000 lb airplane (40,000 wheel load)
What is the optimal pavement thickness (wearing surface)?
What is the optimal CBR value of upper 6 inches?
23 inches

21. C. The Mechanics of the Design 3. An example
I. Pavement Design
C. The Mechanics of the Design
3. An example
A compacted subgrade has a CBR value of 8. What is the minimum pavement thickness if it is to support a taxiway pavement designed to support a 80,000 lb airplane (40,000 wheel load)
What is the optimal pavement thickness (wearing surface)?
What is the optimal CBR value of upper 6 inches?
Wearing Surface
0-15k…….....2”
>15k-40k…..3”
>40k-55k…..4”
>55k-70k…..5”
>70k……..…6”
Wheel Pound Loads CBR Value
15,000 or less 50
15k-40k 65
40k-70k 80
70k-150k 80+

22. C. The Mechanics of the Design 3. An example
I. Pavement Design
C. The Mechanics of the Design
3. An example
A compacted subgrade has a CBR value of 8. What is the minimum pavement thickness if it is to support a taxiway pavement designed to support a 80,000 lb airplane (40,000 wheel load)
What is the optimal pavement thickness (wearing surface)?
What is the optimal CBR value of upper 6 inches?
23 inches
3 inches
6 inches of CBR 65/80
Wearing Surface
0-15k…….....2”
>15k-40k…..3”
>40k-55k…..4”
>55k-70k…..5”
>70k……..…6”
Wheel Pound Loads CBR Value
15,000 or less 50
>15k-40k 65
>40k-70k 80
>70k-150k 80+

23. C. The Mechanics of the Design 3. An example3” 6”
V. Pavement Design
C. The Mechanics of the Design
3. An example
A compacted subgrade has a CBR value of 8. What is the minimum pavement thickness if it is to support a taxiway pavement designed to support a 80,000 lb airplane (40,000 wheel load)
What is the optimal pavement thickness (wearing surface)?
What is the optimal CBR value of upper 6 inches?
CBR = 80
23 inches
3 inches
6 inches of CBR 65/80
Wearing Surface
0-15k…….....2”
>15k-40k…..3”
>40k-55k…..4”
>55k-70k…..5”
>70k……..…6”
Wheel Pound Loads CBR Value
15,000 or less 50
>15k-40k 65
>40k-70k 80
>70k-150k 80+

24. C. The Mechanics of the Design 3. An example3” 6”
V. Pavement Design
C. The Mechanics of the Design
3. An example
A compacted subgrade has a CBR value of 8. What is the minimum pavement thickness if it is to support a taxiway pavement designed to support a 80,000 lb airplane (40,000 wheel load)
What is the optimal pavement thickness (wearing surface)?
What is the optimal CBR value of upper 6 inches?
What can we use for the remainder of thickness?
CBR = 80
23 inches
3 inches
6 inches of CBR 65/80
Wearing Surface
0-15k…….....2”
>15k-40k…..3”
>40k-55k…..4”
>55k-70k…..5”
>70k……..…6”
Wheel Pound Loads CBR Value
15,000 or less 50
>15k-40k 65
>40k-70k 80
>70k-150k 80+

25. Need = 9” minimum
thickness 25

26. CBR = 27 for
remainder of base
(14”) 26

27. Given: Same CBR subgrade as
before
Materials available of:
CBR=30, 80
Determine:
Optimal thickness of each
layer while minimizing costs

28. Given: Same CBR subgrade as
before
Materials available of:
CBR=30, 80
Determine:
Optimal thickness of each
layer while minimizing costs
CBR of 30 needs minimum of 9”
of pavement courses above it.

29. Given: Same CBR subgrade as
before
Materials available of:
CBR=30, 80
Determine:
Optimal thickness of each
layer while minimizing costs
CBR of 30 needs minimum of 9”
of pavement courses above it.
3” of wearing surface
6” of CBR 80 in upper 6”

30. Given: Same CBR subgrade as before Materials available of: CBR=30, 80
Determine:
Optimal thickness of each
layer while minimizing costs
CBR of 30 needs minimum of 9”
of pavement courses above it.
3” of wearing surface
6” of CBR 80 in upper 6”
14” of CBR 30 30

31. Another Example:
Given: Same CBR subgrade as
before
Materials available of:
CBR=15, 30, 80
Determine:
Optimal thickness of each
layer while minimizing costs

32. Given: Same CBR subgrade as before Materials available of:
Another Example:
Given: Same CBR subgrade as
before
Materials available of:
CBR=15, 30, 80
Determine:
Optimal thickness of each
layer while minimizing costs
3” of wearing surface
6” of CBR 80 in upper 6” 32

33. Another Example:
Given: Same CBR subgrade as
before
Materials available of:
CBR=15, 30, 80
Determine:
Optimal thickness of each
layer while minimizing costs
3” of wearing surface
6” of CBR 80 in upper 6”
A CBR of 15 requires X” above it

34. Another Example:
Given: Same CBR subgrade as
before
Materials available of:
CBR=15, 30, 80
Determine:
Optimal thickness of each
layer while minimizing costs
3” of wearing surface
6” of CBR 80 in upper 6”
A CBR of 15 requires 15” above it

35. Another Example:
Given: Same CBR subgrade as
before
Materials available of:
CBR=15, 30, 80
Determine:
Optimal thickness of each
layer while minimizing costs
3” of wearing surface
6” of CBR 80 in upper 6”
A CBR of 15 requires 15” above it
A CBR of 30 requires X” above it

36. Another Example:
Given: Same CBR subgrade as
before
Materials available of:
CBR=15, 30, 80
Determine:
Optimal thickness of each
layer while minimizing costs
3” of wearing surface
6” of CBR 80 in upper 6”
A CBR of 15 requires 15” above it
A CBR of 30 requires 9” above it

37. CBR materials available: 80, 30, 15
Your turn….
Subbase of CBR=7,
50,000 lb loads for a taxiway
CBR materials available: 80, 30, 15
Design the pavement with attention paid to optimizing costs and stability 37

38. CBR materials available: 80, 30, 15
Your turn….
Sub base of CBR=7,
50,000 lb loads for a taxiway
CBR materials available: 80, 30, 15
Design the pavement with attention paid to optimizing
costs and stability
Solution:
Total Thickness: 28”
Wearing Surface Thickness: 4”
Upper 6” of CBR=80
CBR 30 of 7”
CBR 15 of 11” 38

39. Homework:
Subbase of CBR=15,
70,000 lb loads for a runway
CBR materials available: 80, 40, 20
Design the pavement with attention paid to optimizing costs and stability