# Presentation: 720805

## Alpha Factor Determination for 6-Wheel Gears

- Gordon Hayhoe, AAR-410, FAA William J. Hughes Technical Center, Atlantic City, New Jersey, U.S.A.
- Need for evaluation
- Full-scale test structures and results
- Procedure for calculating alpha factors
- Alpha factor Proposals for consideration by ICAO
- Implications for thickness design

- February 2, 2005

## B-777 Six-Wheel ACNs

- For flexible pavements, the ACNs initially computed for B-777 6-wheel gears appeared to be  unreasonably high.
- The FAA had similar concerns about the existing CBR method for 6-wheel gears.
- A380 also has 6-wheel body gears.

- February 2, 2005

## Alpha Factors – MWHGL Data

- February 2, 2005

## Interim 6-Wheel Alpha Factor at 10,000 Coverages

- 4-Wheel alpha = 0.825
- “Original” 6-wheel alpha = 0.788 (inception to 1995)
- “Interim” 6-wheel alpha = 0.72 (1995 to present)
- Current 12-wheel alpha = 0.722

- February 2, 2005

## Alpha Factors – MWHGL Data

- C5-A as one 12-wheel gear

- C5-A as two 6-wheel gears

- February 2, 2005

## National Airport Pavement Test Facility (NAPTF) for 6-Wheel Tests

- Joint FAA and Boeing.
- Testing is funded and conducted entirely by the FAA.
- Tests run on flexible test items to compare 4-wheel and 6-wheel gears.
- Construction cycles CC1 etc.

- February 2, 2005

## NAPTF Construction Cycles

-  CC1 = original construction.
- Conventional and stabilized base flexible on low-strength subgrade (LFC and LFS).
- Conventional and stabilized base flexible on medium-strength subgrade (MFC and MFS).
- CC2 = rigid pavements, trafficking completed.
- CC3 = flexible pavement reconstruction with four conventional test items, trafficking and posttraffic testing completed.

- February 2, 2005

## CC3 Test Pavements - Profile

- Direction of Traffic

- February 2, 2005

## North, 6-Wheel Track

*North, 6-Wheel Track*

**LFC1**

**LFC2**

**LFC3**

**LFC4**

- February 2, 2005

## Trench in LFC2 Flexible

- February 2, 2005

## Computation of Alpha Factor

- Pass/Coverage ratios calculated from surface coverages in test wander pattern:
- 4-Wheel = 2.36 for CC3 and 2.06 for CC1
- 6-Wheel = 1.57
- Subgrade CBR = trench measurements.
- Total structure thicknesses are known.
- Contact area = 265 square inches.
- Compute Alpha using COMFAA.

- February 2, 2005

## CBR Equations

- Post-MWHGL equation:
- t =  (Ac)0.5 [-0.0481 – 1.1562 (log CBR/P)
- 	– 0.6414 (log CBR/P)2 – 0.473 (log CBR/P)3]

- Pre-MWHGL equation:

- Solve the Post-MWHGL equation for 

- OR

*t ****=*** Total Thickness
*P ****=**** *ESWL

- February 2, 2005

## Change the Input Alpha until the design thickness is equal to the test structure thickness. 

- February 2, 2005

## MWHGL Subgrade CBR Measurements

- The CBR of the subgrade for each MWHGL test item was calculated from all available measurements:
- After construction, before traffic.
- Trench and pit after traffic at surface, 12-inch, and 24-inch depth.

- February 2, 2005

## Summary of NAPTF Flexible Pavement Full-Scale Test Results

- * Extrapolated from rut depth curve
-   ** ****Bold** = corrected values

- February 2, 2005

## NAPTF and MWHGL Alpha Factor Results(No conversion of NAPTF to MWHGL structures)

- February 2, 2005

## LEDFAA 1.3 Flexible Failure Model

- February 2, 2005

## NAPTF versus MWHGL Test Results

- NAPTF pavements tended to last longer than MWHGL pavements. Possible reasons for this are:
- Indoor NAPTF operation means lower asphalt temperatures.
- NAPTF asphalt and base layers are thicker.
- NAPTF subbase material is of higher quality (strength – screenings versus uncrushed aggregate).

- February 2, 2005

## Procedure for Converting NAPTF Structures to Equivalent MWHGL Structures (Example)

- (a) real structure, 29.0 in.
- (b) convert 2 in. AC to 3.2 in. CA (E.F. 1.6)
- (c) add 3.2 in. CA to exist. 8 in. CA = 11.2 in. CA
- (d) convert 5.2 in. CA to 8.3 in. SQS (E.F. 1.6)
- (e) convert 16 in. HQS to 19.2 in. SQS (E.F. 1.2)
- (f) equivalent MWHGL structure, 36.5 in.

- Steps:

- February 2, 2005

## NAPTF Flexible Pavement Equivalent Thicknesses and Alpha Factors

- February 2, 2005

## NAPTF and MWHGL Alpha Factor Results(With conversion of NAPTF to MWHGL structures)

- February 2, 2005

## NAPTF and MWHGL Alpha Factor Results

- No structure conversions and C5-A as two 6-wheel gears

- NAPTF structures converted to equivalent MWHGL structures (SQS = 1.6 x CA) and C5-A as two 6-wheel gears

- February 2, 2005

## 4- and 6-Wheel Alpha Factors forBase-to-Subbase Equivalency = 1.4

**Alpha factor quadratic curve fit intercepts at 10,000 coverages:**
**4-wheel ******** = 0.806**
**6-wheel ******** = 0.7178**
**From MWHGL report:**
**4-wheel ******** = 0.825**
**6-wheel ******** = 0.788**

- February 2, 2005

## 4- and 6-Wheel Alpha Factors forBase-to-Subbase Equivalency = 1.6

**Alpha factor quadratic curve fit intercepts at 10,000 coverages:**
**4-wheel ******** = 0.832**
**6-wheel ******** = 0.7295**
**From MWHGL report:**
**4-wheel ******** = 0.825**
**6-wheel ******** = 0.788**

- February 2, 2005

## Subbase Equivalency Factors

- Burns, C.D., R.H. Ledbetter, and R.W. Grau.
- “Study of Behavior of Bituminous-Stabilized Pavement Layers,” Miscellaneous Paper S-73-4, U.S. Army Engineer Waterways Experiment Station, Vicksburg, Mississipi, March 1973.
- Bituminous stabilized base, asphalt base, bituminous stabilized subbase.

- February 2, 2005

## Subbase Equivalencies for 12-Wheel Traffic

- BLS stabilized layers replaced by MWHGL equivalent thicknesses

- February 2, 2005

## Subbase Equivalencies for 12-Wheel Traffic

- BLS stabilized layers replaced by MWHGL equivalent thicknesses

- February 2, 2005

## Alpha Factor Results - Discussion

- Conversion of NAPTF structures gives better agreement with MWHGL test results.
- This indicates that extra conservatism for subgrade protection has been built into the design procedure by increasing minimum thickness requirements for surface (5 in versus 3 in) and base (8 in versus 6 in) without reducing total thickness.
- If 150/5320-6D is used to calibrate LEDFAA then LEDFAA is also conservative.

- February 2, 2005

## MWHGL Designs versus Current FAA CBR Designs

- The MWHGL alpha factor curves give design thicknesses for structures with 3-in asphalt and 6-in base, and for material properties the same as the MWHGL test materials.
- Thickness designs for other layer thicknesses and properties must be converted to MWHGL compatible structures to give the same level of subgrade protection.

- 0.87 x 33 in

- 1.15 x 28.7 in

- February 2, 2005

## Alpha Factor Results - Discussion

- But, overconservative thicknesses for subgrade protection may provide other benefits for operation with heavy aircraft loads.
- Safety factor for structural failure.
- Compaction rutting in base and subbase materials.
- Fatigue cracking of stabilized layers.
- LEDFAA and FEDFAA are therefore being calibrated against -6D designs (5 and 8+ in), not MWHGL designs (3 and 6 in).

- February 2, 2005

## LEDFAA 1.3 Flexible Failure Model

- February 2, 2005

## North, 6-Wheel Track

*North, 6-Wheel Track*

**LFC1**

**LFC2**

**LFC3**

**LFC4**

**Subgrade CBR = 3.3**

- February 2, 2005

## LFC1 Center Line, 6-Wheel Track

**LFC1**

**CBR = 4.3**

- February 2, 2005

## CC-3 PHASE-2: LFC-1 CL TRAFFIC TESTS

_**CC-3 PHASE-2: LFC-1 CL TRAFFIC TESTS**_

- Pass No = 0
- Pass No = 66
- Pass No = 132
- Pass No = 198
- Pass No = 264
- Pass No = 330

- February 2, 2005

## CC-3 PHASE-2 LFC-1 CL TRAFFIC TEST RESULTS

- February 2, 2005

## CC3-LFC1 Traffic Results Summary

- A relatively small change in subgrade CBR can produce a very significant change in the magnitude and character of flexible pavement structural performance.
- Very large deformations can occur at, say, 5 passes, even when the life to the failure criterion is as large as 100 passes.
- This is the basis for the 240 coverage requirement in Engineering Brief No. 65, “Minimum Requirements to Widen Existing 150‐Foot Wide Runways for Airbus A380 Operations.”

- February 2, 2005