1. Erol Tutumluer & In Tai Kim University of Illinois, Urbana-Champaign
NAPTF Materials Testing: Permanent Deformation Behavior of Airport Pavement Base and Subbase Courses
Erol Tutumluer & In Tai Kim
University of Illinois, Urbana-Champaign
November 9, 2005

2. Introduction Rutting of Aggregate Layers
The only failure mechanism of Unbound Aggregate base/subbase layers – The Performance Indicator!..
High Priority: Need to establish laboratory and field tests to characterize/predict permanent deformation behavior
FAA’s Full Scale Airport Pavement Tests
Considerable rutting in base and subbase layers under 6-wheel and 4-wheel gear loadings before subgrade was affected

3. FAA’s Full Scale Test Facility (NAPTF)
Low & Medium strength flexible sections (5 to 10 inches Asphalt & CBR 4 to 8 subgrade soils) failed with up to 4 inches ruts
Highest contribution to permanent deformations often from
4 to 30 inches thick P209 base, or
12 to 36 inches thick P154 subbase
FAA has constructed full-scale test facility, named National Airport Pavement Test Facility, at Atlantic city, New Jersey. It allows many combinations of aircraft gear loading. First round testing of the constructed pavement test sections was completed in Two gear configurations, b777 and b 747 and up to 65,000 lbs for single wheel were applied.
In Flexible pavement sections, rutting of up to 4 inches permanent deformations were observed on the surface.
Furthermore, multi depth deflectometer (MDD) data and test section forensic analyses showed significant rutting was found in both P209 base and P154 subbase layers
There were 9 sections tested in first round trafficking. 6 flexible pavement and 3 rigid pavement sections. Here are the crosssections of 6 flxible test sections.
6-wheel (B777) & 4-wheel (B747) Gear Assemblies

4. NAPTF Trafficking Results – LFC
Low-Strength Subgrade
127-mm P-401 Surface
203-mm P-209 Base
914-mm P-154
Subbase
LFC
Wheel Load: 45,000-lbs (20.4 metric tonnes) per wheel
After 20,000 passes : 65,000-lbs (29.5 metric tonnes) per wheel
(Garg, 2003)

5. Need for Research
Knowledge is required of the relative contribution of the aggregate layers to the total permanent deformation of the airport pavement structure
Current standard laboratory test procedures, such as the AASHTO T307-99, are not adequate for evaluating permanent deformation behavior of granular geomaterials because
Heavy aircraft wheel loads (higher than highway truck wheel loads) causing high pavement layer stresses not accounted for
Moving wheel load conditions resulting in the rotation of principal stress directions & variable confining and deviator stresses not accounted for
Considering those heavy aircraft weight, more specific permanent deformation analysis in each layers is required.
For that concern, laboratory test is much easier and cheaper than full-scale test, if it is valid.
However, current laboratory test procedures, such as the AASHTO , are based on simulating highway loading conditions, so inadequate for evaluating permanent deformation behavior of airport pavement granular layers.
There are two limitations.
First, much heavier aircraft wheel loads are applied on airport pavements.
And second, the loading is not static, but moving wheel-loading conditions.

6. Objectives
Develop Permanent Deformation Test Procedures for Airport Pavement Loading Conditions
High wheel load stresses!..
Investigate Effects of Stress Path Loading under Moving Wheel Loads
Investigate Loading and Material Property Related Factors on Permanent Deformation Accumulation
Develop Prediction Models
Validate Model Performances using NAPTF Data
Prepare A Set of Recommendations for Evaluating Granular Base/Subbase Rutting Potentials Including Strength Criteria

7. Predicting Field Stress States
MFC Section Analysis Result for 45,000 lb Wheel Load 30 50
100
150
200
250 sv
(psi)
Depth (in.)
GT_Pave Iso Solution
GT_Pave Aniso solution
ILLI-PAVE Result AC
Granular Base
P-209
Granular Subbase
P-154
Medium Strength Subgrade Soil
5.1 13
25.1
127.95
118.84
116.5
53.19
65.88
47.69
20.02
17.44
23.12
Contact Pressure : 182 psi
Nonlinear finite element analyses were performed on the NAPTF medium strength pavement section to compute the field stresses acting on these granular layers under 43 kip single wheel loading. Comprehensive analyses indicated that stresses up to 127 psi in P209 and 65 psi in P154 under a B777 single wheel were predicted in the medium strength pavement section.

8. Predicting Field Stress States
MFC Section Analysis Result - Field Stress Ratios (s1/s3)
182 psi
Distance (in.)
1.3 5 10 15 20 25 AC
5.1 13
This shows a contour plot of the principal stress ratios predicted from the anisotropic analysis of the granular layers in the NAPTF MFC section.
Depth (in.)
25.1
Medium Strength Subgrade Soil
GT_Pave Anisotropic Solution

9. NAPTF Moving Wheel Stress Paths
FAA – National Airport Pavement Test Facility
Compression
LFS sections
Interestingly, the actual stress regime experienced by pavement elements under a rolling wheel is consisting of the combination of several complex stress paths, not single stress path utilized in current laboratory test procedure, as a moving wheel is approaching and departing. Therefore, a proper laboratory test protocol is required to simulate the effect of principal stress rotation on permanent deformation development.
Extension

10. Laboratory Investigation of Permanent Deformation Behavior
► Advanced Test Equipment: UI-FastCell
Compression and Extension Stress States
Constant (CCP) & Variable (VCP) Confining Stress Paths

11. Laboratory Test Program P209 & P154
static
1d (dynamic)
CCP
► Constant & Variable Confining
Pressure (CCP & VCP) Tests
Stress Ratio s1/s3 = 4
Stress Ratio s1/s3 = 6
Stress Ratio s1/s3 = 8
Stress Ratio s1/s3 = 10
s1d (kPa)
s3 (kPa)
62.1
20.7
96.6
144.9
186.3
103.5
34.5
172.5
241.5
310.5
165.6
55.2
276.0
386.4 
207.0
69.0
345.0
69.5

12. Variable Confining Pressure (VCP) Test Program
Moving wheel load x
Stresses sv
Vertical stress t sh
Extension
Extension
So far, I talked about CCP test program which didn’t consider principal stress rotation.
Then, let’s move to VCP test program
Horizontal
stress
Time
Shear stress t
Typical pavement element z

13. VCP Test Program m 1d q s3d = 0 CCP Static failure VCP: s3d  0 3d 
VCP ( s3d & s1d ) 1 m
p = (s1d+2s3d)/3 + p0= q/3
Compression
q = s1d- s3d
In general, the stress path slope for the standard constant confining pressure(CCP) tests, such as AASHTO , takes a constant value of 3.0. For variable confining pressure (VCP) tests, the stress slope varies generally from -1.5 to 3. Various stress paths cause different loading effects on pavement elements, which are not yet fully studied and understood to explain permanent deformation accumulation p0
Extension
m = Dq / Dp
= slope of stress path -3 2
CCP: Constant Confining Pressure,
m = 3, s3d = 0
(SHRP P46)
s1d = 0
- q

14. Stress Path Slope (m) = -1
VCP Test Program
Stress Path Slope (m) = 1.5
(compression states)
Stress Path Slope (m) = 0
Stress Path Slope (m) = -1
(extension states) 3
(kPa)
1d
3d
20.67
72.62
18.12
65.46
15.43
61.73
120.8
30.18
108.9
25.63
102.7
168.9
42.24
152.3
35.9
143.6
217.9
54.43
196.4
46.3
185.1
34.45
201.8
50.43
181.9
42.86
171.5
282.1
70.48
254.2
59.94
239.7
362.3
90.6
326.6
76.96
307.9
55.12
193.4
48.37
174.3
41.06
164.3
322.6
80.61
290.8
68.56
274.2
451
112.7
406.5
95.84
383.3
68.9
241.6
60.36
217.7
51.33
205.3
402.9
100.7
363.1
85.57
342.4
39 stress states were used to evaluate the effects of stress path slopes (m) and lengths (L) on the permanent deformation. Two dynamic stresses are properly applied at the same time in the horizontal and also vertical (1d) directions according to the simulated field stress states. As such, the VCP tests offer the capability to apply a wide combination of stress paths by pulsing cell pressure and vertical deviator stress.

15. Permanent Deformation (Strain) Model Development (CCP)
CCP Models
Materials
Regression Correlation Coefficient (R2)
P209
P154
Model 1 : p = A*3B*NC
0.03
0.04
Model 2 : p = A*dB*NC
0.50
0.42
Model 3 : p = A*(1/3 )B*NC
0.62
0.78
Model 4 : p = A*dB *C *ND
0.86
0.85
Various mathematical forms such as linear, nonlinear, logarithmic, hyperbolic, were investigated using multiple regression analyses. Considering the typical exponential growth of permanent strains with number of load applications (N) in the triaxial tests, the power or logarithmic functions were found to be the most suitable. Four such models studied are listed in Table 5–1. The bulk stress term (θ) is simply the first stress invariant eaqual to the sum of σ1 and twice the confining pressure σ3. Model 4 gave the highest correlation coefficients, R2s.
* Based on CCP test database
Commonly Used Resilient Modulus Model MR = X * Y * dZ

16. Permanent Deformation Model Development (VCP)
Based on P209/P154 CCP and VCP test results
Model 1
Model 2
Model 3
Model 4
Model 5
Model 6
Model 7
a, b, c, d, and e : regression parameters
Based on the extensive laboratory testing database established from UI-FastCell permanent deformation testing of the P209/P154 base and subbase course materials under the various constant and variable confining pressure (CCP and VCP) test conditions, seven models were developed to account for hydrostatic confining pressure (s), dynamic stresses in both axial (1d) and radial (3d) directions, stress path length (L), stress path slope (m), and number of load applications (N) as shown in Table 5–2. The model performances were compared to predict the axial permanent deformation (p) behavior of the P209 and P154 aggregates. Due to the complex loading regimes followed especially in VCP testing, models had to be analyzed simultaneously using the static and dynamic components of the applied stresses.
ss: Static confining pressure
s1d: Vertical dynamic stress
s3d: Horizontal dynamic stress
L: Stress path length
m: Stress path slope
a, b, c, d, & e: regression
parameters

17. Permanent Deformation Model Development – CCP & VCP
Model No.
R2 Values for
All data
R2 Values for Stress Path Slope (m) -1
1.5
3 (CCP)
P209 FAA Base Material 1
0.02
0.17
0.11
0.16
0.12 2
0.56
0.24
0.70
0.47
0.86 3
0.41
0.42
0.72
0.32
0.87 4
0.80
0.38
0.78
0.53
0.96 5
0.05
0.71
0.50
0.84 6
0.73 - 7
P154 FAA Subbase Material
0.35
0.03
0.04
0.46
0.62
0.44
0.52
0.34
0.60
0.90
0.79
0.65
. The R2 values for m = -1 were in general the lowest possibly due to the high noise and fluctuations in the recorded triaxial data. The best model performances were obtained for the m = 3 CCP tests resulting in the highest R2 values. In general, Model 4, accounting for both static and dynamic stresses in both axial and radial directions, showed better correlations than those achieved from other models employing only single dynamic stress (either axial or radial dynamic stress) or no dynamic stress. Rather low regression correlation values (R2s), less than 0.5, were also obtained for the intermediate stress path slopes, m = -1 and Nevertheless, Models 6 and 7, which properly account for the various static and dynamic stress states and stress path loading conditions, gave relatively high R2s on all data.

18. Model Development & Validation
NAPTF
Load Wander
Patterns
Calculate stress
states for each
wander position

19. Model Development & Validation
LFC P154 subbase layer

20. Model Development & Validation
Calculate probability according wander distribution
Odd-Numbered Passes: Carriage Moves West to east
Even-Numbered Passes: Carriage Moves East to West
Normal Distribution ( s = 30.5 in.)
63,64
64,66
61,62
51,52
59,60
53,54
57,58
55,56
43,44
45,46
41,42
47,48
39,40
49,50
37,38
19,20
35,36
21,22
33,34
23,24
31,32
25,26
29,30
27,28
1,2
17,18
3,4
15,16
5,6
13,14
7,8
11,12
9,10
Track No. : -4 -3 -2 -1 1 2 3 4
9.843 in
(250 mm)
typical

21. Model Development & Validation8
2) Not matching trends 7
1) Over predicted 6 e
0.0632
= N P 5 e
0.7152
= N P
Permanent Deformation, mm 4 3
Measured 2
Predicted
Power (Measured) 1
Power (Predicted)
LFC P154 subbase layer
5000
10000
15000
20000
25000
No. of Passes

22. VCP Model Validation with NAPTF Trafficking Results
Prediction of permanent deformations
with VCP models
What do we need?
(1)
Estimate various stress states under moving wheels in the traffic direction
(2)
Advanced dp models to account for the varying stress states

23. VCP Model Validation with NAPTF Trafficking Results
VCP Model dp Prediction Concept
“A moving wheel loading consists of five sequential (15) load locations”
Stress path slope = -1
4 1d = 3d
Stress path slope = 0
1d = 3d
Stress path slope = 3
3d = 0
Stress path slope = 0
1d = 3d
Stress path slope = -1
4 1d = 3d 1 2 3 4 5
The granular layer was divided into six sublayers and the permanent strain in each sublayer was computed by the model individually using the average stress states predicted at mid-depth of that sublayer, according to five different wheel positions. The predicted permanent strain in each sublayer, i.e., the summation of the strains obtained considering five load locations, was then multiplied by the thickness of that sublayer. The total granular layer permanent deformation was finally obtained as the summation of all the sublayer deformations, as explained in detail in Figure 7–12. }
P154 subbase
6 sublayers
* Layer 1: Top layer
Pavement elements

24. VCP Model Validation with NAPTF Trafficking Results
p, layer 1 =  p,1+  p,2 +  p,3 +  p,4 +  p,5
p, layer 2 =  p,1+  p,2 +  p,3 +  p,4 +  p,5
Summation of permanent strains
from 5 wheel load locations
p, layer 3 =  p,1+  p,2 +  p,3 +  p,4 +  p,5
p, layer 4 =  p,1+  p,2 +  p,3 +  p,4 +  p,5
p, layer 5 =  p,1+  p,2 +  p,3 +  p,4 +  p,5
p, layer 6 =  p,1+  p,2 +  p,3 +  p,4 +  p,5 
p, Total = p, layer 1+ p, layer 2 + p, layer 3 +
p, layer 4 + p, layer5 + p, layer6

25. VCP Model Validation with NAPTF Trafficking Results
Figure 7–13 shows the permanent deformations predicted from 6 sublayers using the VCP model 4. The deformations in the upper layers were usually higher than those obtained in the lower portion of the LFS P209 base layer. This is due to the fact that the upper part usually experiences higher stress states compared to the lower portion and also the prediction models were highly nonlinear stress dependent.

26. VCP Model Validation with NAPTF Trafficking Results
VCP Prediction
CCP Prediction
Figure 7–14 compares the prediction performances of the CCP model 4 and VCP model 4 with the measured ruts in the LCF P154 subbase layer. As expected, the predictions considering moving wheel load condition produced much higher permanent deformations than those obtained form the CCP type predictions. This is in accordance with the previous findings that principal stress rotations cause higher permanent deformation accumulations under actual trafficking (Brown and Brodrick, 1999; Hornych et al., 2000).
Measured

27. Factors Affecting Permanent Deformation Behavior
Laboratory Testing versus NAPTF Testing
Compacted with
vibratory compactor
Unconditioned virgin specimen
Loaded with 0.1-sec
(equivalent to 50 km/hr)
load duration
Trafficked at 8 km/hr (0.5-sec
load duration) with aircraft gear
Previous loading of base
and subbase layers during
pavement construction
and slow moving load test
(response test)
Load Pulse Duration and Stress History effects are involved

28. Factors Affecting Permanent Deformation Behavior
p = A N B
‘B’ is Constant and ‘A’ Varies
‘A’ is Constant and ‘B’ Varies
. In this equation, ‘ep’ is the permanent axial strain, ‘N’ is the number of load applications at that stress state, and finally, ‘A’ and ‘B’ are the model parameters obtained from regression analyses. In terms of the model parameters ‘A’ and ‘B’, the salient points of difference between the trend lines are highlighted in Figure 7–15.
‘A’ is related to the magnitude of
permanent strain at first cycle
‘B’ is related to the rate of
permanent strain accumulation

29. Factors Affecting Permanent Deformation Behavior
The Effect of Load Duration
Confining Pressure = 21 kPa
~ 40%
higher
Load Duration
0.1 sec vs
0.5 sec
The first one was tested under 63-kPa axial pulse loading with 0.1-second load duration, and after 72,000 cycles, the repeated stress level was increased to 336 kPa for another 72,000 cycles. The second specimen was pulsed with 0.5-second load duration at the same stress levels and number of load cycles as the previous one. As shown in Figure 7–17, the loading due to 0.5-second load duration accumulated approximately 40% more permanent deformations in the specimen than the 0.1-second load duration did. This was indeed an important finding indicating that the load duration applied to specimens during testing has to be chosen in accordance with the field trafficking speed for an accurate prediction of rutting potential. The differences in the permanent deformations produced by the 0.1-second and 0.5-second load pulse durations after 144,000 load cycles can be related to both load pulse duration and stress history effects at the same time (see Figure 7–17).

30. Factors Affecting Permanent Deformation Behavior
The Effect of Stress History
Loaded at 21 kPa Confining Pressure
and 336 kPa Axial Loading
Unconditioned specimen
p = N
Increase
Specimen conditioned at
21 kPa confining pressure and 105 kPa axial deviator stress
Decrease
p = N
Three different P154 specimens were prepared and tested in the laboratory at the constant confining pressure of 21 kPa to investigate the so-called stress history effects on the P154 aggregate permanent strain behavior. Specimen 1 was not conditioned, whereas, specimens 2 and 3 were applied initial pulsed vertical stresses of 105 kPa and 336 kPa, respectively. Then, all three specimens were loaded at a dynamic vertical stress of 336 kPa. Figure 7–18 shows the test results with the ‘A’ parameter, which is the deformation at the first cycle, drastically increasing for the unconditioned virgin specimen and decreasing down to almost insignificant levels for the heavily conditioned third specimen. Also, the considerably higher value of the ‘B’ parameter obtained in the power function model can be clearly seen for the heavily conditioned third specimen. As a result, one can easily relate the small ‘A’ and the large ‘B’ parameters seen in the measured permanent deformation trends of the NAPTF granular base/subbase layers to the stress history effects.
Specimen conditioned at
21 kPa confining pressure and 336 kPa axial deviator stress
p = N

31. Factors Affecting Permanent Deformation Behavior
Load path (stress history) effect

32. Factors Affecting Permanent Deformation Behavior
If a model does not consider the effect of stress history, the trend is quite different from the measured one.
From their research study, Figure 7–19 shows predicted and measured unbound aggregate permanent strains with clear indications of stress history effects. The experimental findings represented by data points indicate more of linear type permanent deformation accumulations as stress levels increase for about three times. In other words, as stress history builds up in a specimen, the trend of permanent strain/deformation accumulation becomes more of a linear type, similar to the reported NAPTF full scale testing granular base/subbase permanent deformation trends.
(El abd et al., 2004)

33. Permanent Deformation Prediction Considering NAPTF Stress History Effects
A new set of specimens were tested to adequately account for NAPTF stress history effects
Slow Moving Response Tests with 36,000-lb wheel loads at 0.54 km/h
Only 200 load cycles were applied to simulate slow moving response test for conditioning specimens
Stress history ratios used in computing A & B adjustment factors (36,000 lbs / 45,000 lbs = 0.8)

34. Permanent Deformation Prediction Considering NAPTF Stress History Effects
Adjustment factor for ‘A’
Stress history ratio
Adjustment factor for ‘B’
Stress history ratio

35. Permanent Deformation Prediction Considering NAPTF Stress History Effects
NAPTF LFC P154 Subbase Layer Permanent Deformation Prediction
with Model Parameter New Adjustment Factors

36. Permanent Deformation Prediction Considering NAPTF Stress History Effects
NAPTF LFC P154 Subbase Layer Permanent Deformation Prediction
with Model Parameter New Adjustment Factors

37. Permanent Deformation Prediction Considering NAPTF Stress History Effects
S : Stress History Effects
L : Load Duration Effects
Measured Permanent Deformation
VCP Prediction considering S + L
VCP Prediction considering S
CCP Prediction considering S + L
CCP Prediction considering S

38. Development of Permanent Deformation Test Procedure
To investigate rutting potentials of granular materials,
applied stress states should reach
a shear stress ratio of up to 0.9
As defined by seyhan and tutumluer, shear stress ratio can be the one of those. At any applied stress state, a shear stress ratio will have to give a certain fraction of the shear strength tmax of the material Accordingly, the lower the shear stress ratio tf/tmax is achieved from stabilization, it will be less likely for the material to fail in shear type failur.

39. Development of Permanent Deformation Test Procedure (CCP)
Confining Pressure,
s3 (kPa)
Shear Stress Ratio, tf /tmax
Axial Deviator Stress,
d (kPa) 21
0.3 / 0.5 / 0.7 / 0.9
Solution of the System of Two Equations Below 35 69
105
The stress states applied in the laboratory should represent different load levels considering a range of shear stress ratios (f / max). The number of the stress levels to apply depends on the application. To cover the shear stress ratios up to 90 percent of maximum shear strength, the deviator stresses can be computed according to 4 different confining pressures and axial deviator stress pairs as recommended in Table 8–17. The cyclic loading should be applied at least 10,000 cycles. If the loading stress is already known or anticipated of the pavement, conditioning of the specimens must be considered for the best predictions of rutting potential.
Where:

40. Development of Permanent Deformation Test Procedure (VCP)3
Deviator
Stress, q
Mean Normal Stress, p
Stress Path Slope,
m = 3
m = 0
m = - 1 T he
stress path lengths determined
in the CCP test procedure
are kept constant throughout
the VCP test procedure
Stress Path Length, L
the total deformation accumulated by 5 different positions represented by stress path slopes, 3, 0, and -1, in the traffic direction of a moving wheel. It is recommended to conduct VCP testing for additional stress path slopes (m) of 0 and -1 after the completion of the CCP test procedure covering the stress path slope (m) of 3 as illustrated in Figure 8–5. The dynamic stresses for each stress path slope in the axial and horizontal directions are determined by the CCP stress states computed based on shear stress ratios.
5 load locations for VCP Prediction
include stress path slope -1 and 0

41. Research Findings
Large stresses and stress ratios (vertical to horizontal) as high as 10 were computed in NAPTF base/subbase at the centerline of loading from Finite Element analyses
Permanent deformation tests were conducted and characterization models were developed based on the CCP, VCP, and multiple stress path test results
The magnitude of the applied dynamic stress in the vertical direction affected predominantly the accumulation of permanent deformations in the CCP tests
The VCP models produced a higher degree of accuracy by properly accounting for complex loading regimes

42. Research Findings
From VCP and multiple stress path (realistic approaching and departing wheel loading condition) tests
A higher stress path slope produced higher axial and shear permanent strains but lower volumetric strain (linked to NAPTF results under moving wheel loads)
Both the volumetric and deviatoric strains obtained from the multiple stress path tests were consistently higher than those from the single path tests
The multiple stress path loading caused significantly higher permanent deformations or damage especially in the somewhat loose base/subbase layers

43. Research Findings
To predict NAPTF P209/P154 base/subbase permanent deformations, stress history adjustment factors were developed using conditioned/pre-loaded specimens to correct for the stress history effects
The predictions were improved with the use of adjustment factors
The corrected predictions from VCP models were much closer to the measured NAPTF ruts
To accurately predict the behavior of permanent deformation in the field, one should closely follow pavement construction and trafficking load levels, load combinations

44. Research Findings/Accomplishments
A new granular base/subbase permanent deformation test procedure was proposed to take into account the effects of
Heavy wheel loads: applying stresses up to 90 % of the shear strength
Moving wheel loads: considering three different stress path slopes in VCP testing
Load pulse duration: in accordance with field trafficking speed
Stress history: preconditioning of specimens
Major Accomplishments
PhD Dissertation of Dr. In Tai Kim – August/October 2005
Final Project Report – CEAT Report No. 28

45. Current/Future Research Focus
Investigate the NAPTF trafficking dynamic response database to understand complicated recovered & unrecovered pavement deformation behavior due to various combinations of applied
Load magnitudes and loading sequences (stress history effects)
Trafficking speeds (load duration effects)
Traffic directions (shear stress reversals)
Gear spacing or interaction
Wander positions and wander sequences (order of 66 loadings)
Based on the proposed test procedure, fully develop a permanent deformation test procedure for evaluating airport pavement granular base/subbase layer rutting potential
Study CC3 test section subbase rutting performances
Establish granular layer thickness/performance equivalencies

46. NAPTF trafficking dynamic response
unrecovered !..

47. NAPTF trafficking dynamic response
Base/Subbase
Contractive &
Dilative
Behavior
A large scatter is observed in the permanent deformation accumulation in the P-154 subbase layer of MFC section, especially after 5,000 passes (for example, see Figure 9.24). This is due to the continual “contractive/dilative” response of the P-154 granular layer resulting from the effect of gear wander. This was pointed out in Chapter 8 while discussing the MDD transverse distribution results. This behavior is also reflected in the development of permanent deformation at the surface. Interestingly, the accumulation of permanent deformation in the subgrade shows less scatter than the granular layers even though the subgrade rutting magnitudes are higher.
In Figure 9.36, the Residual or Unrecovered displacements in the pavement and subgrade layers is shown during the first 1,400 passes. The Residual responses show higher magnitudes in the pavement layers than in the subgrade. Note that the pavement layer shows a strong contractive/dilative Residual response behavior. Even though the Subgrade exhibits contractive/dilative Residual response behavior, it is mainly a contractive response. Thus, the NET permanent deformation which is equal in magnitude to the differences between the sequential dilative and contractive Residual responses will be lower in the case of the pavement layers while the cumulative contractive Residual responses produces higher rutting magnitude in the subgrade (see Figure 9.37). A closer view of the contractive/dilative Residual response behavior in the pavement and subgrade layers is shown in Figure 9.38 for one wander cycle. The maximum Residual response occurs when one of the wheel groups passes directly over the MDDs.
The phenomenon of contractive/dilative permanent deformation behavior for a sequence of repetitive loads with different wander positions have been observed in previous full-scale airport flexible pavement trafficking tests (Ledbetter, 1977; Ahlvin et al, 1971; Crockford et al, 1990; Webster, 1992). Gomez-Ramirez and Thompson (2002) observed this phenomenon during the NAPTF response tests. It was also reported by Hayhoe and Garg (2002) based on their analysis of subgrade strains measured in the MFC section.

48. NAPTF trafficking dynamic response
Traffic Direction Effect

49. CC3 Test Sections
Test section MFC “failed” after 12,000 passes with 4 to 6 inches of rutting. AC surface cracking were observed inside the traffic lane. In the MFS section, localized failure occurred in the B777 traffic lane towards the West end. At 19,900 passes, 3.5 inches of rut depth was observed with upheaval outside the traffic path. Trafficking was terminated on the B777 traffic lane, but it continued on the B747 traffic lane. The B747 lane “failed” at 30,000 passes. The LFC and LFS sections showed few signs of genuine distress even after 20,000 passes and therefore the wheel loading was increased from 45,000-lb to 65,000-lb. The trafficking was terminated in the low-strength test sections after 28,000 passes of 65,000-lb. While the MFC and MFS sections “failed” at the subgrade level, the LFC and LFS sections failed in the surface layers, signifying tire pressure or other upper layer failure effects, but not subgrade level failure (Gervais et al, 2003).
This led to the removal of all of the pavement structures from the low-strength subgrade and the reconstruction of four pavement sections (LFC1, LFC2, LFC3, and LFC4) for retesting. These reconstructed test sections (see Figure 15.1) are referred to as Construction Cycle 3 or CC3. In constructing the CC3 test sections, the top portion (30 inches) of the low-strength subgrade was reworked. The reworked subgrade had a CBR of 3 and the existing subgrade had a CBR of 5.5. The LFC1 and LFC2 sections were designed to “fail” early whereas the LFC3 and LFC4 sections were designed to last normal pavement life (see Table 15.1).

50. CC3 Rutting Results 1 2 3 4 5 6 7 8 9 10
100
1,000
10,000
100,000 N
Surface Rut Depth (in.)
6-Wheel
4-Wheel
LFC1
LFC4
LFC3
LFC2
65-Kip
55-Kip
34-in. P154
24-in. P154
43-in. P154
16-in. P154
The LFC1 section showed rapid accumulation of rutting and “failed” at 90 passes on the 6-wheel traffic path and at 122 passes on the 4-wheel traffic path. In LFC2, rutting accumulated at a rapid rate with rut depths reaching 4 inches in both the traffic lanes within 400 passes. After 500 passes, the rut depth accumulations in the 6-wheel side and 4-wheel side diverged with the 6-wheel side increasing more rapidly and reaching “failure” at 1,100 passes. The 4-wheel traffic path reached “failure” around 3,000 passes. The same trend is seen in the LFC3 and LFC4 sections with the divergence in rutting curves occurring around 7,000 passes. The increase in wheel load from 55-kip to 65-kip at 4,000 passes is shown in Figure where the rate of rutting increases rapidly. Before increasing the wheel load to 65-kip, the MWHGL wander pattern was tried on the LFC3 and LFC4 test sections.

51. CC3 Trench Study LFC1 & LFC2 Rutting in P-209 Base and P-154 Subbase
Increasing the P-154 subbase thickness did not solve the problem of rutting
Rutting in P-209 Base and P-154 Subbase

52. CC3 Trench Study LFC2 – Centerline testing (surface rutting)
Rut depth after 198 passes
Channelized only
up to 198 passes
Rut depth after 858 passes

53. CC3 Trench Study LFC2 – Centerline testing (858 passes)
Channelized only
up to 198 passes
P154 Subbase Layer
1 ~ 1.5 inches
P209 Base Layer
0.6 ~ 1 inches