Transcript Transportation Engineering

Transportation Engineering
Approaches to Pavement Design
General Approaches to
Pavement Design
• In its simplest form, the design approach
consists of the following steps:
– Define the loading and environmental conditions;
– Select materials with appropriate properties for
use in the pavement;
– Select the thickness of pavement, based on
empirical rules or on a stress – strain analysis of
the pavement structure; and
– Adjust the initial design using different materials
until a satisfactory design emerges.
Approaches to Pavement
Design
• There are two approaches to the
pavement design.
–
Empirical Approach
–
Mechanistic-Empirical Approach
Empirical Approach
“An empirical approach is one
which is based on the results of
experiments or experience.”
Generally, it requires a number of
observations to be made in order to
ascertain the relationships between input
variables and outcomes.
It is not necessary to firmly establish the scientific
basis for the relationships between variables and
outcomes as long as the limitations with such
approach are reorganized.
Benefits
– It uses material properties that relates
better to actual pavement performance
– It provides more reliable performance
predictions
– It better defines the role of
construction
– It accommodates environmental and
aging effects on materials
Empirical Approach
• An empirical approach is one which is
based on the results of experiments or
experience.
• Generally, it requires a number of
observations to be made in order to
ascertain the relationships between input
variables and outcomes.
• It is not necessary to firmly establish the
scientific basis for the relationships
between variables and outcomes as long
as the limitations with such approach are
reorganized.
• Many pavement design procedures use an
empirical approach. This means that the
relationship between design inputs and
pavement failure were arrived at through
experience, experimentation or
combination of both.
• Empirical design methods can range from
extremely simple to quite complex.
• Empirical Equations used in design of
pavement.
• Empirical equations are used to relate
observed or measurable phenomena
(pavement characteristics) with outcomes
(pavement performance). There are many
different types of empirical equations
available today but this section will present
the 1993 AASHTO Guide basic design
equation for flexible pavements as an
example.
• 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.)
ESALs
– ZR=standard normal deviation
– 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 =a1D1 + a2D2m2 + a3D3m3+...
ai =ith layer coefficient
di =ith layer thickness (inches)
mi
=ith layer drainage coefficient
Δ PSI=difference between the initial design
serviceability index, po, and the design
terminal serviceability index, pt
MR=sub-grade resilient modulus (in psi)
Empirical Methods in
Practice
• Empirical methods in use are;
– Group Index Method
– CBR Method
Mechanistic-Empirical
Approach
• Mechanics is the science of motion and
the action of forces on bodies. Thus, a
mechanistic approach seeks to explain
phenomena only by reference to physical
causes.
• In pavement design, the phenomena are
the stresses, strains and deflections within
a pavement structure, and the physical
causes are the loads and material
properties of the pavement structure.
• The
relationship
between
these
phenomena and their physical causes is
typically described using a mathematical
model. Various mathematical models can
be (and are) used; the most common is a
layered elastic model.
• Along with this mechanistic approach,
empirical elements are used when defining
what value of the calculated stresses,
strains and deflections result in pavement
failure.
Benefits
• The basic advantages of a mechanisticempirical pavement design method over a
purely empirical one are:
– It can be used for both existing pavement rehabilitation
and new pavement construction
– It accommodates changing load types
– It can better characterize materials allowing for:
• Better utilization of available materials
• Accommodation of new materials
• An improved definition of existing layer properties
– It uses material properties that relates better to actual
pavement performance
– It provides more reliable performance predictions
– It better defines the role of construction
– It accommodates environmental and aging effects on
materials
Mechanistic-Empirical
Methods
• Mechanistic-Empirical methods in
use are
–
AASHTO
–
Portland Cement Association Method
–
British Method