NOTTINGHAM DESIGN METHOD

Transcription

1 NOTTINGHAM DESIGN METHOD Dr Andrew Collop Reader in Civil Engineering University of Nottingham CONTENTS Introduction Traffic Design temperatures Material properties Allowable strains Asphalt thickness design Corrections Example

2 INTRODUCTION Chart based approach (based on computer analysis) Simplified structure: 3 layers Standard dual wheel load (40kN) Fixed Poisson s ratio s Standard granular layer (200mm, 100MPa) SIMPLIFIED STRUCTURE

3 DESIGN VARIABLES 3 design variables Asphalt thickness Asphalt stiffness modulus Elastic stiffness of subgrade Correction for separate wearing course Correction for non-standard (thickness) granular layer FLOW CHART

4 TRAFFIC Average vehicle speed required From knowledge of road situation Err on the slow side (more conservative) Cumulative number of standard axles also required STANDARD AXLES Method 1: Estimated from procedure developed at TRL 1. Estimate initial daily number of CVs in one direction (C 0 ) and expected percentage growth rate (r) 2. Calculate mid-life flow (C m ) using (x = design life in years) C m = C 0 ( r) 0.5x

5 STANDARD AXLES 3. Calculate proportion of CVs using slow lane at mid-life (P) (P = 1 for single carriageways) P = x 10-5 C m 4. Calculate cumulative number of CVs using slow lane during design life C c = 365 P C 0 [( r) x 1] 0.01r STANDARD AXLES 5. Convert to the number of million standard axles (msa) N = D C c x D is the damage factor determined from (y = year of opening + 0.5x 1945) D = y (0.92 y ) (3.9 C m /1550 )

6 METHOD 2: CHART STANDARD AXLES Method 3: Use Equivalence Factor (EF) approach (needs detailed data) 1. EF determined using (W is in kn, 80kN = standard axle) EF w = (W / 80) 4

7 STANDARD AXLES 2. Consider initial traffic loading spectrum for 1 day and calculate equivalent number of std axles (A w = number of axles of load! In 1 day) N 0 = Σ (A w EF w ) STANDARD AXLES 2. Cumulative number calculated from N = P N 0 [( r) x 1] r P = x 10-5 N m N m = N 0 ( r) 0.5x

8 FLOW CHART SUBGRADE STIFFNESS Resilient modulus required (elastic modulus) Approximate procedure adopted Based on CBR Value inmpa E 3 = 10 x CBR Alternative for cohesive soils (Ip = plasticity Index Percentage) E 3 = 70 - I p

9 FLOW CHART DESIGN TEMPERATURES Based on Annual Average Air Temperature (AAAT) Rutting temperature takes into account diurnal variations in both temperature and traffic loading T rut = 1.47 x AAAT

10 DESIGN TEMPERATURES Fatigue temperature also takes into account cumulative damage effects T fat = 1.92 x AAAT FLOW CHART

11 BITUMEN STIFFNESS Loading time Temperature Binder properties Softening Point (SP) Penetration Penentration Index (PI) calculated from SP and Pen LOADING TIME Estimated from average commercial vehicle speed and asphalt layer thickness Approximate relationship also used T (secs) 1 / V (km/hr)

12 BITUMEN STIFFNESS LOADING TIME

13 ASPHALT STIFFNESS Calculated from bitumen stiffness and volumetrics (VMA) Valid for traffic loading only (elastic region) ASPHALT STIFFNESS

14 FLOW CHART ASPHALT STRAIN Allowable tensile asphalt base strain calculated (either to critical conditions of failure) Depends on binder grade (SP), volume of binder & traffic loading

15 ASPHALT STRAIN SUBGRADE STRAIN Allowable compressive subgrade strain calculated (either to critical conditions of failure) Depends on type of asphaltic material and traffic loading

16 SUBGRADE STRAIN SUBGRADE STRAIN Rut factor used for materials other than Hot Rolled Asphalt (HRA) Hot Rolled Asphalt (HRA) = 1 Dense Bitumen Macadam (DBM) = 1.56 Modified HRA = 1.37 Modified DBM = 1.52

17 FLOW CHART ASPHALT THICKNESS Determine asphalt layer thickness required Depends on Mixture stiffness Subgrade stiffness Asphalt strain Subgrade strain 2 thickness calculated (deformation and fatigue) Minimum is design thickness

18 ASPHALT THICKNESS FLOW CHART

19 GRANULAR CORRECTION Reduce design thickness if granular sub-base thickness > 200mm Deformation h 1 = h 2 [ E E 3 ] 300 Fatigue h 1 = h 2 [ E E 3 ] 300 SURFACING CORRECTION Reduce design thickness if type and thickness of surfacing known (rutting design temperature only) H b = h w E w E 1

20 SURFACING CORRECTION WORKED EXAMPLE

21 WORKED EXAMPLE Traffic Data Initial volume = 1,500 CVs/day, single carriageway Growth rate = 3%/annum Life = 20 years Average speed of CVs = 60km/hr AAAT = 9 o C Year road opened to traffic = 1984 WORKED EXAMPLE Soil plasticity index = 38% Carry out detailed design calculations to failure using 4 typical bases (Table 1) for a pavement with a 200mm granular sub-base.

22 SOLUTION For deformation, design temperature = 1.47 x 9 = 13.2 o C For fatigue, design temperature = 1.92 x 9 = 17.3 o C Cumulative number of CVs = 14.7msa Damage Factor = 2.72 Design Traffic = 2.72 x 14.7 = 40msa Elastic subgrade stiffness = = 32 MPa (say 30MPa) Asphalt stiffness SOLUTION

23 SOLUTION Maximum allowable asphalt strain (µε) HRA 130 DBM 54 Modified HRA 120 Modified DBM 108 Maximum allowable subgrade strain (µε) HRA 161 DBM 182 Modified HRA 175 Modified DBM 181 SOLUTION Minimum layer thicknesses