Problem 19: Pavement Design (15 points) A three-layer (surface course HMA over g
ID: 1866921 • Letter: P
Question
Problem 19: Pavement Design (15 points) A three-layer (surface course HMA over granular base over subbase) flexible pavement section is being designed for a new roadway. The calculated ESAL value is 8,000,000. Information about the materials for each layer of the pavement is presented in the table below: Surface Course HMA (Layer 1) Elastic Modulus 320,000 psi Granular Base Subbase Layer 2) CBR 55 Layer 3) R-value 50 The pavement will be designed assuming a reliability of 95%, Standard Deviation (SO) of 0.35. the MR for the existing soil to be 7.5 ksi, and the design serviceability loss (APSI) to be 1.0 Moisture modifiers are assumed to be 1.0 a) The Traffic Index (Ti) value of this roadway (assuming a lane distribution factor 1 b) The thickness required of each layer using the AASHTO 1993 design method Determine the following:Explanation / Answer
Step 1:
Given data:
E.S.A.L=8x106
H.M.A Modulus of resilience=320,000 psi
Granular base CBR=55
Sub base R value=50
Reliability=95%
Standard deviation=35%
Loss in servicability index=1.0%
Step 2:
Calculate the Traffic index
Traffic index=9.0(E.S.A.LxL.D.F/106)0.119
T.I=9.0(8x106/106)0.119
T.I=11.52
Step 3:
Calculate the design thickness of pavement by AASHTO and using structural number concept
First calculate the modulus of resilience of base and sub-base
MR=25000 PSI for base (From AAHSTO Charts)
MR=12000 PSI for sub base (From AASHTO Charts)
MR=7500 psi For soil (Given in the question)
Step 4:
For a reliability of 95%, standard deviation of 0.35 let us calculate the S.N based on the modulus of resilience of each and every individual layer, in flexible pavement, the S.N is a function of the modulus of resilience of the next layer, that means for H.M.A (Hot mix asphalt) we will use modulus of resilience of base for base we use modulus of resilience of sub base and for sub base we use modulus of resilience of soil
SN1=5
SN1=a1D1
5=0.35D1
D1=14.28 in
SN2=5.5
5.5=a1D1+a2D2
D2=SN2-SN1/(a2m2)
D2=5.5-5.0/(0.12x0.8)
D2=5.2 in (Provide 6 in)
SN2*=0.12x6+14.28x0.35
SN2*=5.718
SN3=a1d1+a2d2+a3d3
d3=SN3-SN2/(a3m3)
d3=6.3-5.718/(0.09x0.8)
d3=8.08 in
Note: Here a1, a2 and a3 are the coefficients which can be obtained from AASHTO Charts and m2 and m3 are the drainage coefficients used
Step 5:
Conclusion:
Provide 14.28 in of H.M.A layer
Provide 6 in of base layer
Provide 8 in of sub base layer