1
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
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 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
5
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)
6
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
7
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
8
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
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
Trut = 1.47 x AAAT
10
DESIGN TEMPERATURES• Fatigue temperature also takes into
account cumulative damage effects
Tfat = 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
LOADING TIME
BITUMEN STIFFNESS
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
ASPHALTSTRAIN
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
ASPHALTTHICKNESS
FLOW CHART
19
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
20
SURFACING CORRECTION
WORKED EXAMPLE
21
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.
22
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
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
Top Related