QUESTION

Perform a mechanistic-empirical design on the roadway shown in the attached boring log. The roadway
consists of 2-lane flexible pavement system with asphalt, base, and subbase layers, underlain by soft soils.
The thicknesses of various layers were originally designed using the AASHTO method. New traffic
information shows significant growth estimated to be 2 million ESALS in a 20-year design life, which can
be uniformly distributed on an annual basis. The M-E design uses failure criteria and transfer functions,
along with design constraints as outlined below. Therefore, the M-E design would likely result in different
thicknesses compared to what currently exists. Assume that the asphalt concrete has a constant resilient
modulus of 500,000 psi which means it is linear elastic. The foundation layers are considered non-linear.
(1) Assume new thicknesses and approximate seed values of resilient moduli for various layers of a 3-
layer pavement (asphalt, base and subbase) and calculate the average stresses in the individual layers.
Use non-linear analysis for the foundation layers. Show all initial assumptions and average stresses in
Table 1.
(2) Using the stresses determined in (1), calculate a “refined” set of resilient moduli using
appropriate constitutive relationships. Present the stresses and the equations in Table 2.
(3) Use these final moduli values to calculate the refined final stresses and deflections in the
pavement layers using a linear-elastic analysis. Show these values in Table 3
(4) Design must satisfy the following design constrains by using the transfer functions (a through d) and
ensuring that (i) there is no fatigue failure, (ii) there is no permanent deformation in the subgrade, and
(iii) the total rut depth does not exceed 1 inch. Refine the design if all conditions are not met. Show
the final calculations and the total calculated rut depth in Table 4
a. Asphalt concrete fatigue relationships: Nf=a(εt)-b(E)-c, where a, b, and c are experimentally
determined constants whose suggested values are 0.0796, 3.291, and 0.854 respectively. Nf is the
number of cycles to failure, E is the elastic or resilient modulus, and εt is the asphalt concrete tensile
strain.
b. The permanent strain in the asphalt: εp = I Ns
, where I and S are experimentally determined constants
whose suggested values are 0.000053 and 0.263 respectively. εp is the permanent strain in 0.00001
in/in. N is the number of cycles or load applications.
c. The permanent deformation response of the granular base material:
δp = 2.111 x 10-6 (N)O.1792, where, δp is the permanent deformation in meters, and N is the number of
cycles or load applications.
d. The criterion for limiting permanent deformation in the subbase is: Nd = 1.365×10-9 (εc)
-4.477, where, Nd
is the allowable number of repetitions to restrict permanent deformation, and εc is the compressive
strains on the top of the subgrade.