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NOTTINGHAM DESIGN METHOD

Dr Andrew CollopReader in Civil EngineeringUniversity of Nottingham

CONTENTS• Introduction• Traffic • Design temperatures• Material properties• Allowable strains• Asphalt thickness design• Corrections• Example

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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

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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

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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 AXLESMethod 1: Estimated from procedure

developed at TRL1. Estimate initial daily number of CVs

in one direction (C0) and expected percentage growth rate (r)

2. Calculate mid-life flow (Cm) using (x = design life in years)

Cm = C0 (1 + 0.01 r)0.5x

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STANDARD AXLES3. Calculate proportion of CVs using

slow lane at mid-life (P) (P = 1 for single carriageways)

P = 0.97 – 4 x 10-5 Cm

4. Calculate cumulative number of CVs using slow lane during design life

Cc = 365 P C0 [(1 + 0.01r)x – 1]0.01r

STANDARD AXLES5. Convert to the number of million

standard axles (msa)

N = D Cc x 10-6

6. D is the damage factor determined from (y = year of opening + 0.5x –1945)

D = 0.35 - 0.26 0.93y + 0.082 (0.92y + 0.082) (3.9Cm/1550)

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METHOD 2: CHART

STANDARD AXLESMethod 3: Use Equivalence Factor (EF)

approach (needs detailed data)1. EF determined using (W is in kN,

80kN = standard axle)

EFw = (W / 80)4

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STANDARD AXLES2. Consider initial traffic loading

spectrum for 1 day and calculate equivalent number of std axles (Aw = number of axles of load ! In 1 day)

N0 = Σ (Aw EFw)

STANDARD AXLES

N = 0.0365 P N0 [(1 + 0.01r)x – 1]r

2. Cumulative number calculated from

P = 0.97 – 4 x 10-5 Nm

Nm = N0(1 + 0.01r)0.5x

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FLOW CHART

SUBGRADE STIFFNESS• Resilient modulus required (elastic

modulus)• Approximate procedure adopted• Based on CBR• Value in MPa

E3 = 10 x CBR• Alternative for cohesive soils (Ip =

plasticity Index Percentage)

E3 = 70 - Ip

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FLOW CHART

DESIGN TEMPERATURES• Based on Annual Average Air

Temperature (AAAT)• Rutting temperature takes into

account diurnal variations in both temperature and traffic loading

Trut = 1.47 x AAAT

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DESIGN TEMPERATURES• Fatigue temperature also takes into

account cumulative damage effects

Tfat = 1.92 x AAAT

FLOW CHART

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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)

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LOADING TIME

BITUMEN STIFFNESS

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ASPHALT STIFFNESS• Calculated from bitumen stiffness

and volumetrics (VMA)• Valid for traffic loading only (elastic

region)

ASPHALT STIFFNESS

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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

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ASPHALTSTRAIN

SUBGRADE STRAIN• Allowable compressive subgrade strain

calculated (either to critical conditions of failure)

• Depends on type of asphaltic material and traffic loading

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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

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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

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ASPHALTTHICKNESS

FLOW CHART

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GRANULAR CORRECTION• Reduce design thickness if granular

sub-base thickness > 200mm

∆h1 = ∆h2 [53 – 2.8 E1 + 0.5 E3]300

∆h1 = ∆h2 [26.5 – 0.5 E1 - 0.23 E3]300

Deformation

Fatigue

SURFACING CORRECTION• Reduce design thickness if type and

thickness of surfacing known (rutting design temperature only)

Hb = hw EwE1

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SURFACING CORRECTION

WORKED EXAMPLE

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WORKED EXAMPLETraffic Data• Initial volume = 1,500 CVs/day, single

carriageway• Growth rate = 3%/annum• Life = 20 years• Average speed of CVs = 60km/hr• AAAT = 9oC• 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.

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SOLUTION• For deformation, design temperature

= 1.47 x 9 = 13.2oC• For fatigue, design temperature

= 1.92 x 9 = 17.3oC• Cumulative number of CVs = 14.7msa• Damage Factor = 2.72• Design Traffic = 2.72 x 14.7 = 40msa• Elastic subgrade stiffness = 70 – 38

= 32 MPa (say 30MPa)

SOLUTION• Asphalt stiffness

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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