Mechanistic-Empirical Design of Chip- sealed Roads in SA. Christchurch workshop presentation 21/22 November 2002

Transcription

1 Mechanistic-Empirical Design of Chip- sealed Roads in SA Christchurch workshop presentation 21/22 November 2002

2 Structure of presentation Pavement materials Current SAMDM Variability and accuracy Distress of chip-sealed LVRs Recent SA developments Stress regimes Resilient response Permanent deformation response Conclusions

3 Pavement materials None Bituminous binder Low Intermediate 0,5 2,0 % 2,0 4,0 % High 4,0 7,0 % Concrete Very high 8,0 10,0 % Greater permanent deformation resistance Strongly cemented material C2 to C1 Lightly cemented material C4 to C3 Modified material Unbound material High quality crushed stone Moderate quality natural gravel Poor quality natural gravel Brittle Presumed economically behaviour not viable Cemented foamed bitumen and emulsion-treated material Asphalt concrete Foamed bitumen and and half-warm Emulsion-treated foamed bitumen material mixes Stress Temperature dependent dependent, behaviour visco-elastic behaviour High 3,0 5,0 % Intermediate 1,5 3,0 % Low 0,5 1,5 % None Strength Cement Catalyst Improved flexibility

4 Current SAMDM 1. System geometry input 2. Load characterization 3. Material input parameters: Resilient properties Strength properties Critical layer approach Bearing capacity estimate for terminal distress Analysis technique Static, linear-elastic elastic multi- layer analysis Pavement response σ and ε Critical parameter depends on Material type Mode of failure Structural analysis model: Pavement response and Pavement performance model: Transfer function Pavement bearing capacity estimate Adequate? Yes Final pavement design No Design iterations

5 Current SAMDM: Transfer functions Asphalt fatigue Strain at bottom of layer Fatigue Unbound base/subbase layers Factor of safety (1/Stress Ratio) Permanent deformation Foamed-bitumen treated Strain at bottom and Stress Ratio Effective fatigue and Permanent deformation Lightly cemented material Subgrade Vertical strain Permanent deformation

6 Sources of variability 2150 Variation in Dry Density 170 Variation in Layer Thickness 140 Dynamic Axle Load Streams Dry Density (kg/m 3 ) Spec: 98% of 1989 kg/m 3 Dry Density (kg/m 3 ) Spec: 150 mm Dynamic axle load (kn) to 90 kn 90 to 100 kn 100 to 110 kn 110 to 120 kn Distance (m) Distance (m) Distance (m) or time (s) Variation in Saturation 60 Saturation (%) Resilient modulus and shear strength Analysis model Response Parameter Distance (m) Variation in Particle Size Distribution 2.25 Dependent variable Transfer Function Grading modulus Stress Ratio Distance (m) Independent variable

7 Variability and accuracy True mean of a variable Model estimate of the mean Accuracy True variation Model variation

8 Distress of chip-sealed roads Patching Base layer material quality Material shear strength close to minimum requirement Slight changes in moisture condition shear failure Permanent Deformation Minimal protection of subgrade Differential permanent deformation caused by spatial variability of Material parameters Layer thickness Dynamic loading Experienced by road user as bad riding quality

9 Recent developments in SA and future directions Thickness performance Still split the elastic and plastic response modeling Move towards plastic strain transfer functions All layers contribute towards the total rut Different design approaches for structural layers and subgrade Calibrate all models for the effect of density and saturation Resilient modulus model Shear strength models Permanent deformation model Heavy Vehicle Simulator and triaxial testing

10 Stress regimes: Subgrade and structural layers Wearing course Base layer Subbase layer Upper selected subgrade Lower selected subgrade Pavement structural layers High shear stresses Large strains Pavement foundation or subgrade: Low shear stresses Small strains In situ subgrade

11 Subgrade design

12 Subgrade design: Model development HVS tests: Multi Depth Deflectometer data Elastic response: Direct calculation (ε v δ s ) Back-calculation (ε v σ v ) Plastic response: Non-linear regression model Subgrade behaviour: Resilient modulus Characteristics of permanent deformation Permanent deformation design model

13 HVS testing: MDD installation Pavement structure MDD installation MDD modules at layer interfaces Anchor at 2,5 to 3 m MDDmodule Reference point anchored at 2,5 to 3 m

14 Analysis process: Depth deflection bowls 256 data points on each bowl Peak deflections DEFLECTION (mm) Depth (mm) 60 mm 200 mm 375 mm 550 mm 800 mm -0.5 Direction of wheel movement DISTANCE

15 Analysis process: Depth deflection profile Direct calculation Average vertical strain between MDD modules Elastic subgrade deflection Back-calculation calculation At least 2 modules in subgrade Estimate depth to zero deflection Vertical stress and strain at top of subgrade DEFLECTION (mm) HVS load repetitions 1000 DEPTH (mm)

16 Elastic response: Resilient modulus 14 Ferricrete (wet) Count Mr (MPa)

17 Calcrete Calcrete Sandy gravel Sand/clay Sand/limestone Elastic response: Elastic response: Resilient modulus Material Type Mr (MPa) Ferricrete (wet) Ferricrete (dry) Sandstone Berea Red

18 Analysis process: Permanent MDD displacement Initial bedding-in Eventual linear rate of displacement Non-linear regression model PD = mn + a(1 e -bn ) a = bedding-in m = linear displacement rate Permanent MDD Displacement (mm) Depth (mm) 3 80mm mm mm 660mm 900mm HVS Wheel-load Repetitions Millions

19 Analysis process: Permanent MDD displacement Permanent MDD displacement a a(1 e -bn ) a mn 1 m Number of load repetitions, N

20 Analysis process: Permanent MDD displacement PD (mm) 0,700 0,600 0,500 0,400 0,300 0,200 0,100 0, R 2 = 0,989 SEE = 0,014 Model Data HVS load repetitions

21 Analysis process: Permanent MDD displacement 1.0E+09 Bearing capacity (repetitions) 1.0E E E kn 60 kn 70 kn 100 kn 1.0E Vertical strain from MDD deflections (microstrain)

22 Analysis process: Permanent MDD displacement 1.0E+09 Bearing capacity (repetitions) 1.0E E E E Vertical stress (kpa) 40 kn 60 kn 70 kn 100 kn

23 Analysis process: Permanent MDD displacement 1.0E+09 Bearing capacity (repetitions) 1.0E E E kn 60 kn 70 kn 100 kn 1.0E Subgrade elastic deflection (micron)

24 Subgrade Design: Permanent deformation model 1.00E+09 5 mm subgrade deformation R 2 = 0,672 SEE = 0, E+08 Cycles 1.00E E E Subgrade elastic deflection (micron) Model G6 G7 G8 G10

25 The Design of Unbound Structural Pavement Layers

26 Elastic response: Resilient modulus Resilient Modulus is a function of Material type (grading) Relative Density Saturation Confinement Stress Ratio log M r = RD 0.76 S σ 0. 41SR R 2 = SEE =

27 Pavement response: Unacceptable errors in stress calculations Model contradicts observed behaviour under the HVS

28 Shear strength is a function of tu ra t io n R tive a l e sity n e d Relativ e dens ity ion r at Sa tu Material type (grading) Relative Density Saturation ( ) Sa gle Friction an 180 ) Cohesion (kpa Static shear strength

29 Plastic response: Permanent Deformation Permanent deformation is a function of Stress Ratio Repetitions (N) Möhr-Coulomb failure envelope Internal friction angle, Cohesion a c a w a m b c b w b m wa - a c ma - a c Stress Ratio = = wb - b c mb - b c SR = σ σ σ a 1 3 m 1 σ3 = 2 σ tan 3 45 o σ σ a 1 3 φ + C + + φ o 1 2 tan

30 Concluding remarks: Subgrade design Critical parameter Subgrade Elastic Deflection Resilient response LE-theory can model deflection well enough Calibrate stiffness models from HVS MDD data Permanent deformation Model developed from HVS MDD data for a range of subgrade materials dual wheel loads from 40 to 100 kn

31 Concluding remarks: Design of unbound structural layers Too much emphasis on stress condition in past Density and saturation as important in determining pavement response Have to improve accuracy of stress and strain calculations

32 Concluding remarks: Design of unbound structural layers Critical parameter Stress Ratio Resilient response M r = f (Grading, RD, S, Confinement, Shear Stress Ratio) Shear strength Shear strength = f (Grading, RD, S) Permanent deformation PS = f (N, Stress Ratio)