n of a Florida State Road is being planned The current pavem

n of a Florida State Road is being planned. The current pavement A proposed thickness and characteristics of the foundation layers are shown in the attached boring log. The existing pavement consists of an asphalt layer, a sandy-limerock base, and a sandy subbase. The subgrade consists of organic soils and peat. This pavement was constructed many ycars ago based on pure experience MODULEI: AASHTO APPROACH New information is now available on traffic and material properties. Based on that, check the adequacy of the existing design, using the AASHTO 1993) approach. To do that, re-design the thicknesses of a new 2-lane flexible pavement system with asphalt, base, and subbase layers using the existing materials, and compare with the existing layer thicknesses. Following information is available: Traffic: The average daily truck traffic is 5000 along the 2-lane highway. Assume a 5% growth rate for the traffic. Traffic counts are given in terms of number of axles per 100 trucks as follows # of Single axles/100 #of Tandem axles/100 trucks load (kips)trucks #3 75.3 29.9 10.5 10 12 14 4.2 0.5 1.5 2.0 3.6 4.2 18 20 1.0 8.0 24 Subgrade: Seasonal data shows that the resilient moduli of the subgrade soils from January through December are: 20000, 20000, 6000, 6000, 6000, 9000, 9000, 9000, 9500, 9500, 8000, and 20000 psi Base and Subbase: Assume properties based on the material type Deliverable: Prepare Table 1 comparing the old layer thicknesses with the newly designed thicknesses, and make concluding remarks.

Solution

Module 1 :

Procedure

The steps in the 1993 AASHTO flexible pavement design

Flexible pavement design procedure high volume urban condition the analysis period is 30-50 years, let us take 30-year design life.

       2. Evaluate the design traffic:

               W₁₈   = D_D x D_LW₁₈

D_D - directional distribution factor for heavy vehiclea 03-07, let us take 05

D_l - Lane distribution factor, here 2 lane road , it ranges from 80 -100% let us take 80

W₁₈ = 11.6 million ESALs.

3. Determine the design reliability factors: Reliability for urban roads ranges from 85-99% let us take 90%,

Hence from the table we adopt

Z_{R ,}standard normal deviate (function of the design reliability level) = -1.282,

S₀ ,overall standard deviation (function of overall design uncertainty) = 0.45.

4. Determine the allowable serviceability loss due to traffic:

              ?PSI= P_o – P_t

The initial serviceability Po for flexible pavements is 4.2.

A terminal serviceability index of 2.5 or higher is recommended for design of major highways.

?PSI = 4.2 – 2.5 = 1.7

5. Evaluate the seasonally averaged subgrade resilient modulus M_R

M_R = 1500 x CBR or A+B x R value

Where A ranges from 772 – 115

            B ranges from 369 - 555

Here given 20000, 20000, 6000, 6000, 6000,9000,9000,9000,9500,9500,8000 and 20000psi from jan to dec respectively

6. Determine the layer properties:

Table 5-44 in Section 5.4.5. Values are a₁ = 0.44, a₂ = 0.17.

To find the required overall structural number: SN

log₁₀ ( W₁₈ ) = Z_R S₀ + 9.36 log₁₀ ( SN + 1 ) - 0.20 + log ₁₀?PSI/4.2-1.5                

                                                                                              0.40 + 1094 / SN +1

SN = 5.07

7. Determine the design layer thicknesses for the pavement section: