Pavement Design

I. Pavement Design

A. Overview

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

Degree of curvature

I. Pavement Design

B. California Bearing Ratio (CBR)

“How to build a road!”

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.

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.

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.

I. Pavement Design

B. California Bearing Ratio (CBR)

3. “The Test”

Take load readings at penetrations of: “the result” 0.025” ……………70 psi0.05”……………...115 psi0.1”……………….220 psi0.2”……………….300 psi0.4”……………….320 psi

Penetrations of 0.05” per minute

“Achieve OM &MD”6” mold

4. Plot the Data

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0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

Penetration (inches)

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5. Determine the percent of compacted crushed stone values for the 0.1 and 0.2penetration.

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0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

Penetration (inches)

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“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

5. Determine the percent of compacted crushed stone values for the 0.1 and 0.2penetration.

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0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

Penetration (inches)

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“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

Example psi = CBRStandard psi

220 psi = .22, or 22%1000 psi

300 psi = .20, or 20%1500 psi

CBR of material = 22%

5. Determine the percent of compacted crushed stone values for the 0.1 and 0.2penetration.

0

50

100

150

200

250

300

350

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

Penetration (inches)

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Pis

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si)

“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

Example psi = CBRStandard psi

220 psi = .22, or 22%1000 psi

300 psi = .20, or 20%1500 psi

CBR of material = 22%

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

5. Determine the percent of compacted crushed stone values for the 0.1 and 0.2penetration.

“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

Example psi = CBRStandard psi

220 psi = .22, or 22%1000 psi

300 psi = .20, or 20%1500 psi

CBR of material = 22%, or “22”

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.

Potential Corrections to the Stress-Penetration Curves

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0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

Penetration (inches)

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I. Pavement Design

C. The Mechanics of the Design

I. Pavement Design

C. The Mechanics of the Design

1. Determine• The CBR values of the subgrade• The type of use expected (runways vs.

taxiways)

I. Pavement Design C. The Mechanics of the Design1. 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

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

I. Pavement Design

C. The Mechanics of the Design

3. An exampleA 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)?

“ a point on the curve for agiven CBR material representsthe minimum thickness ofpavement courses that willreside above it, in order tomaintain stability

CBR subbase of 8,Taxiway, and wheel load of 40,000 lb

23 inches

“23 inches of total earth materialand pavement”

I. Pavement Design

C. The Mechanics of the Design

3. An exampleA 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

I. Pavement Design

C. The Mechanics of the Design

3. An exampleA 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?

Wheel Pound Loads CBR Value 15,000 or less 50 15k-40k 65 40k-70k 80 70k-150k 80+

Wearing Surface0-15k…….....2”>15k-40k…..3”>40k-55k…..4”>55k-70k…..5”>70k……..…6”

I. Pavement Design

C. The Mechanics of the Design

3. An exampleA 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?

Wheel Pound Loads CBR Value 15,000 or less 50 >15k-40k 65 >40k-70k 80 >70k-150k 80+

Wearing Surface0-15k…….....2”>15k-40k…..3”>40k-55k…..4”>55k-70k…..5”>70k……..…6”

23 inches

3 inches6 inches of CBR 65/80

V. Pavement Design

C. The Mechanics of the Design

3. An exampleA 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?

Wheel Pound Loads CBR Value 15,000 or less 50 >15k-40k 65 >40k-70k 80 >70k-150k 80+

Wearing Surface0-15k…….....2”>15k-40k…..3”>40k-55k…..4”>55k-70k…..5”>70k……..…6”

23 inches

3 inches6 inches of CBR 65/80

CBR = 80

3” 6”

V. Pavement Design

C. The Mechanics of the Design

3. An exampleA 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?Wheel Pound Loads CBR Value 15,000 or less 50 >15k-40k 65 >40k-70k 80 >70k-150k 80+

Wearing Surface0-15k…….....2”>15k-40k…..3”>40k-55k…..4”>55k-70k…..5”>70k……..…6”

23 inches

3 inches6 inches of CBR 65/80

CBR = 80

3” 6”

Need = 9” minimum thickness

CBR = 27 for remainder of base(14”)

Given: Same CBR subgrade as before

Materials available of:CBR=30, 80

Determine:Optimal thickness of each layer while minimizing costs

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.

Given: Same CBR subgrade as before

Materials available of:CBR=30, 80

Determine:Optimal thickness of each layer while minimizing costs

3” of wearing surface6” of CBR 80 in upper 6”

CBR of 30 needs minimum of 9”of pavement courses above it.

Given: Same CBR subgrade as before

Materials available of:CBR=30, 80

Determine:Optimal thickness of each layer while minimizing costs

3” of wearing surface6” of CBR 80 in upper 6”14” of CBR 30

CBR of 30 needs minimum of 9”of pavement courses above it.

Another Example:Given: Same CBR subgrade as before

Materials available of:CBR=15, 30, 80

Determine:Optimal thickness of each layer while minimizing costs

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 surface6” of CBR 80 in upper 6”

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 surface6” of CBR 80 in upper 6”

A CBR of 15 requires X” above it

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 surface6” of CBR 80 in upper 6”

A CBR of 15 requires 15” above it

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 surface6” of CBR 80 in upper 6”

A CBR of 15 requires 15” above itA CBR of 30 requires X” above it

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 surface6” of CBR 80 in upper 6”

A CBR of 15 requires 15” above itA CBR of 30 requires 9” above it

Your turn….

Subbase of CBR=7,50,000 lb loads for a taxiwayCBR materials available: 80, 30, 15

Design the pavement with attention paid to optimizing costs and stability

Your turn….

Sub base of CBR=7,50,000 lb loads for a taxiwayCBR 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=80CBR 30 of 7”CBR 15 of 11”

Homework:

Subbase of CBR=15,70,000 lb loads for a runwayCBR materials available: 80, 40, 20

Design the pavement with attention paid to optimizing costs and stability