TRANSPORTATION ENGINEERING-II
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Transcript of TRANSPORTATION ENGINEERING-II
TRANSPORTATION ENGINEERING-II
AASHTO 1993Flexible Pavement Design Equation
AASHTO DESIGN METHOD
• The basic objective of this test was to determine significant relationship between the no. of repetition of specified axle loads (of different magnitude and arrangement) and the performance of different thickness of pavement layers.
AASHTO DESIGN METHOD CONSIDERATIONS
• Pavement Performance• Traffic• Roadbed Soil• Materials of Construction• Environment• Drainage• Reliability• Life-Cycle Costs• Shoulder Design
STEPS FOR DESIGNING
• The AASHTO design method states that:
• “The function of any road is to carry the vehicular traffic safely and smoothly from one place to another”.
• Following are the different steps followed in AASHTO design method while designing the pavement.
• Measuring Standard Axle Load• Predicting Serviceability• Performance• Present Serviceability Rating (PSR)
• Present Serviceability Index• Terminal Serviceability• Regional Factor• Structural Number• Soil Support• Reliability• Over all Standard Deviation• Resilient Modulus
Standard Axle Load (ESAL’s)• “An axle carrying a load of 18Kips and causing
a damaging effect of unity is known as Standard Axle Load”.
Serviceability• “Ability of a pavement to serve the traffic for
which it is designed”.
Performance• “Ability of a pavement to serve the traffic for a
period of time”. Performance is interpreted as trend of serviceability with time.
Present Serviceability Rating• To define PSR, the AASHO
constituted a panel of drivers belonging to different private and commercial vehicles. They were asked to
• Rate the serviceability of different section on a scale of 0-5.
• Say whether the sections were acceptable or not.
Very Good
Good
Fair
Poor
Very Poor
Present Serviceability Index (ISI)• The prediction of PSR from these physical
measurements is known as PSI and defined as “Ability of a pavement to serve the traffic for which it is designed”. Normally the value is taken as 4.
• PSI value depends on the following factors;• Measurement of longitudinal surface irregularities• Degree of cracking• Depth of rutting in the wheel paths
Terminal Serviceability Index (ISI)• “The lowest serviceability that will be tolerated
on the road at the end of the traffic analysis period before resurfacing or reconstruction is warned”.
• Its usual value is 2 for roads of lesser traffic volume and 2.5 for major highways.
Basic design equation for Terminal Serviceability is Pt= Gt-{log (Wt)-log (p)}
=0.4+{0.081(L1+L2)3.23}/{(1+SN)5.19+L23.23}
• log (p)= 5.93 + 9.36log(SN+1)-4.79log (L1+L2)+ 4.33log(L2)
• Gt=a logarithmic function of the ratio of the loss in serviceability at time t to the potential loss taken to a point where pt=1.50
• p=a function of design and load variables that denotes the expected number of axle load applications to a pt=1.5
= a function of design and load variables that influence the shape of the p Vs W serviceability curve.
• Wt=axle load applications at the end of the time t• L1=load on one single axle or on one tendon axle set, in
kg• SN= Structural Number of pavement
• Regional factorIt is a factor which helps the use of the
basic equations in a climatic condition other than the ones prevailing during the road test. Its values are:
• Road bed material frozen to a depth of 5 in or more (winter)
• Road bed material dry (Summer and fall)• Road bed material wet (spring thaw)
• Structural NumberAn index number that represents the overall
pavement system structural requirements needed to sustain the design traffic loading for the design period. Analytically, the SN is given by:
SN=a1D1M1+a2D2M2+a3D3M3
Where • D1,D2,D3 = thickness in inches respectively of
surfacing, base and sub-base.• a1,a2,a3 = coefficients of relative strength.
a1 = 0.2 for road bricks
0.44 for plant mix
0.45 for the sand asphalt
a2 = 0.07 for sandy gravel
0.14 for crushed stone
a3 = 0.11 for sandy gravel
0.50 to 0.10 for sandy soil
M1, M2,M3 = drainage coefficients
M1 = 1 shows good drainage conditions
Soil Support• Its value depends on the CBR value of the
layer.
ReliabilityIt is defined as “probability that serviceability will be maintained at adequate levels from a user point of view, through out the design life of the facility”
• Overall Standard DeviationIt takes in to account the designer’s ability to estimate the variation in 18K Equivalent Standard Axle Load.
• Resilient ModulusIt is defined as
Mr = Repeated Axial Stress / Total Recoverable Axial Strain
Mr=CBR x 1500
AASHTO DESIGN EQUATION
This equation is widely used and has the following form:
Log10(W18)=Zr x So+ 9.36 x log10(SN + 1)-0.20+(log10((ΔPSI)/(4.2-1.5)) /(0.4+(1094/(SN+1)5.19)+2.32x log10(MR)-8.07
where:
W18=predicted number of 80 KN (18,000 lb.) ESAL’s ZR=standard normal deviate
So=combined standard error of the traffic prediction and performance prediction
SN=Structural Number (an index that is indicative of the total pavement thickness required)
SN=a1D1M1 + a2D2m2 + a3D3m3+...ai =ith layer coefficientdi =ith layer thickness (inches)mi =ith layer drainage coefficientΔ PSI =difference between the initial design serviceability index, po, and the design terminal serviceability index, pt
MR =sub-grade resilient modulus (in psi)
Nomo-graph
1993 AASHTO Structural Design
Step-by-Step
Step 1: Traffic Calculation
Total ESALs• Buses + Trucks• 2.13 million + 1.33 million = 3.46 million
Step 2: Get MR Value
• CBR tests along Kailua Road show:– CBR ≈ 8
• MR conversion
psiCBRM R 000,12815001500
psiCBRM R 669,9825552555 64.064.0
AASHTO Conversion
NCHRP 1-37A Conversion
Step 3: Choose Reliability
Arterial Road• AASHTO Recommendations
Functional ClassificationRecommended Reliability
Urban Rural
Interstate/freeways 85 – 99.9 85 – 99.9
Principal arterials 80 – 99 75 – 95
Collectors 80 – 95 75 – 95
Local 50 – 80 50 – 80
WSDOT
95
85
75
75
Choose 85%
Step 3: Choose Reliability
Reliability ZR
99.9 -3.090
99 -2.327
95 -1.645
90 -1.282
85 -1.037
80 -0.841
75 -0.674
70 -0.524
50 0
Choose S0 = 0.50
Step 4: Choose ΔPSI
Somewhat arbitrary• Typical p0 = 4.5
• Typical pt = 1.5 to 3.0
• Typical ΔPSI = 3.0 down to 1.5
Step 5: Calculate Design
Decide on basic structureResilient Modulus (psi)
Layer a Typical Chosen
HMA 0.44 500,000 at 70°F 500,000
ACB 0.44 500,000 at 70°F 500,000
UTB 0.13 20,000 to 30,000 25,000
Aggregate 0.13 20,000 to 30,000 25,000
Step 5: Calculate Design
Preliminary Results• Total Required SN = 3.995• HMA/ACB
• Required SN = 2.74• Required depth = 6.5 inches
• UTB and aggregate• Required SN = 1.13• Required depth = 9 inches
Step 5: Calculate Design
Apply HDOT rules and common sense• HMA/ACB
• Required depth = 6.5 inches• 2.5 inches Mix IV (½ inch Superpave)• 4 inches ACB (¾ inch Superpave)
• UTB and aggregate• Required depth = 9 inches• Minimum depths = 6 inches each
– 6 inches UTB– 6 inches aggregate subbase
Comparison
Layer California AASHTO
HMA Surface 2.5 inches 2.5 inches
ACB 7.0 inches 4.0 inches
UTB 6.0 inches 6.0 inches
Aggregate subbase 6.0 inches 6.0 inches