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Pavement Design

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**I. Pavement Design A. Overview Degree of curvature**

“Principal cause of pavement failure shown above—not the blacktop”

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**B. California Bearing Ratio (CBR)**

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

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**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

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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.

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**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

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4. Plot the Data

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**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

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**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

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**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

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**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

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**Potential Corrections to the**

Stress-Penetration Curves

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I. Pavement Design C. The Mechanics of the Design

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**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

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**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

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**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

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**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)?

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**“ 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

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

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**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

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**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+

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**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+

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**C. The Mechanics of the Design 3. An example**

3” 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+

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**C. The Mechanics of the Design 3. An example**

3” 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+

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Need = 9” minimum thickness 25

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

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**Given: Same CBR subgrade as**

before Materials available of: CBR=30, 80 Determine: Optimal thickness of each layer while minimizing costs

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**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.

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**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”

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**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

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

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**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

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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

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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

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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

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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

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**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

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**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

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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

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