South African Mechanistic Design Method …

1 South African Mechanistic Design Method ( samdm ) Hechter Theyse PMC Louw Kannemeyer - SANRALW ynand Steyn - CSIRS lide 2 Historical Overview samdm The samdm used since 1980s in various forms Any Method is better than no Method National DoT insisted that consultants Design using samdm Attempt to introduce more Science and reduce Art ELSYM 5 samdm used for developing Design catalogues in 1984 and 1995 TRH 4 Wide-scale Design software implementation since 1995 MEPADS, Rubicon, Cerano Further development of engineering models slowed down because ofa lack of funding since early 1990sSlide 3 Typical SA pavement and SAMDM35mm Wearing course150 mm Crushed stone base150 mm Cemented subbase150 mm Granular upper selected subgradeIn situ subgrade150 mm Granular lower selected subgradeSA pavement StructureCurrent ME Damage ModelAsphalt Fatigue Freeme 1970sPermanent Deformation FOS Maree 1970s to 1980sEffective Fatigue and Crushing FailureDe Beer 1980sVertical Strain CriteriaDorman and Metcalf 1965 Vertical Strain

2 CriteriaDorman and Metcalf 1965 Vertical Strain CriteriaDorman and Metcalf 1965 Current samdm has number of limitations, no damage models for plastic deformation in Asphalt layers, number of models outdated, etc, etc Slide 4 samdm - Current status Summary Classical ME Design Method - single estimate of bearing capacity Critical layer approach distress mechanisms disconnected Separated resilient response and damage models Material resilient response Recommended Mrand Poisson s Ratio values Conflict between slow and dynamic test results Users are disillusioned with the Method Counter-intuitive and inadmissible results Extreme sensitivity of the Method to input data Inconsistent input Resilient response (FWD, MDD, Laboratory)

3 Strength parameters Statements made that ME- Design is not possible due to: Too many unexplained effects (chaos) Getting the right answers for the wrong reasons ( samdm correctly predicted expected life, but predicted failure layer as being subgrade, yet it actually is base !) samdm require extensive revision !!!Slide 5 samdm Revision Overall objective To develop a Design Method that is: Accurate (theory must agree with reality) Impartial in terms of pavement type selection Unbound (Crushed stone, natural gravel) Stabilised (Cement, Foamed-bitumen, Emulsified-bitumen) HMA Concrete (not included in flexible pavement Design R&D process) pavement Design Task Group Submitted R&D framework in November 2005 Characteristics of new pavement Design Method R&D topics Demand analysis (Traffic and environment)

4 Material resilient response models pavement resilient response models Damage models Probabilistic and recursive schemes Each R&D topic have a number of identified R&D needs Each R&D need translated into one or more detailed project briefs to address the need November 2006 TheoryReality=Slide 6 New South African pavement Design Method (SAPDM) Two components Information system based on past performance Mechanistic -Empirical analysis system Mechanistic -Empirical analysis system Two components engineering models Simulation schemes Based on separated response analysis Resilient response and damage models Static resilient pavement response analysis Damage modelling Stiffness reduction for bound layers Material resilient response and damage model calibration Field variables Temp, MC, Dens.

5 7 SAPDM Integration of Design MethodsDesign MethodYoung professionalSeasoned professionalDesign specialistDesign scenario: Routine and preliminary Design Low risk Low Design experience Use only known materials Conventional material classificationDesign scenario: Important Design Medium risk Seasoned professional designer Test input parameters for materialsDesign scenario: Very important Design , high risk Special investigations Specialist designer Unusual materials Test input and performance of materialsUserDesign applicationPerformance based information systemME- Design componentDesign MethodYoung professionalSeasoned professionalDesign specialistDesign scenario: Routine and preliminary Design Low risk Low Design experience Use only known materials Conventional material classificationDesign scenario.

6 Important Design Medium risk Seasoned professional designer Test input parameters for materialsDesign scenario: Very important Design , high risk Special investigations Specialist designer Unusual materials Test input and performance of materialsUserDesign applicationPerformance based information systemME- Design componentSlide 8 SAPDM - engineering Models of M-E ComponentTemperatureMoisturecontentDensi tyGradingAtterberglimitsBindercontentBin derpropertiesGeometryAxle loadhistogramOtherOtherContact stresshistogramGradingFixedloadFixedcont act stressMr = ConstantMr= f(Temp)Mr= f(Dens,saturation)Mr= f(Bulk and shearstress)Mr= f(Strain)

7 Linearvisco-elasticOther?UCSS tress and strain at breakTime/previousloadingHMAU nboundStabilizedMaterial dataTrafficdataResilientresponseanalysis Input layersResilientresponse modelsDamage modelsFatiguePlastic strain/permanent deformationTop-down crackingThermal crackingHMAU nboundPlastic strain/permanent deformationOtherOtherStabilizedStiffness reductionPlastic strain/permanent deformationOtherCrushingSubgradePlastic strain/permanent deformationOtherStructuralcapacityestima teComputersolutionTemperatureMoisturecon tentDensityGradingAtterberglimitsBinderc ontentBinderpropertiesGeometryAxle loadhistogramOtherOtherContact stresshistogramGradingFixedloadFixedcont act stressMr = ConstantMr= f(Temp)Mr= f(Dens,saturation)Mr= f(Bulk and shearstress)Mr= f(Strain)Linearvisco-elasticOther?

8 UCSS tress and strain at breakTime/previousloadingHMAU nboundStabilizedMaterial dataTrafficdataResilientresponseanalysis Input layersResilientresponse modelsDamage modelsFatiguePlastic strain/permanent deformationTop-down crackingThermal crackingHMAU nboundPlastic strain/permanent deformationOtherOtherStabilizedStiffness reductionPlastic strain/permanent deformationOtherCrushingSubgradePlastic strain/permanent deformationOtherStructuralcapacityestima teComputersolutionSlide 9 Summary of issues Demands Traffic - Traffic volume, axle load and contact stress information system Environment Information system for country (temperature, rainfall etc)

9 Material resilient response models Static vs dynamic tests and analyses pavement resilient response model Improved load characterisation and interface conditions Sub-layering stress, density, saturation, temperature Effective stress in unbound granular materials Damage models Appropriate critical parameter Proper material strength characterisation No log-log damage models improved computing capabilities Include effect of field variablesSlide 10 SAPDM Revision - Fundamental Principles Validate and calibrate Method thoroughly before release: Reality checks Benchmark data sets LTPP data Ensure input and theory agree.

10 Understand boundary conditions and fundamentals of tests Extract the correct input from the test Don t fit performance data to a specific damage model formulation formulate the damage model according to trends in the performance data Critical parameter selection Damage model shape Slide 11 The accuracy of the Design Method is determined by the accuracy of the engineering models included in the Method Simulation schemes Mirror the characteristics of real-life behavior and performance Present data in a handy format Do nothing to improve accuracy Supplement the Design Method with comprehensive engineering practice guideline documentsSAPDM Revision - Fundamental PrinciplesSlide 12 SAPDM Revision Current Status South African pavement Design Method Process Phase 1 Develop Detailed Project Briefs November 2006 Phase 2 - Inception Phase July 2007 Investigate available solutions Finalize project methodology Finalize cost and resource allocation Inception Report Peer Review November 2007 Phase 3 Project Delivery - Anticipated Start Date February 2008