Equivalent Granular Thickness
6.04 Apply the concept of Equivalent
Granular Thickness to convert a layered
system of different materials into an
equivalent thickness of a given reference
material.
The following presentation contains references to Figure
6.04b, Table 6.02 , “GBE Example 1”, all of which are
posted under subsection 2.6 of the course notes on the
instructor’s website. Viewer discretion is advised as some
scenes contain material of a mathematical nature.
Why is the Equivalent
Granular Thickness concept
necessary?
a pavement design consists of the
specification of thicknesses of layers of
different materials
 an infinite number of combinations of
these parameters are possible
 what is needed is one unit or parameter
that characterizes the pavement’s strength

Basic Theory
Layered Elastic Theory is used to derive
layer equivalency factors for each layer of a
pavement
 knowing the thickness and Modulus of
Elasticity of each pavement layer, an
equivalent thickness of a standard reference
material is calculated
 this equivalent thickness is used to
characterize the pavement’s strength

And now…
Some Mental High-jinks
H1
M1
3
h1
M ref
and
therefore
H1
Mref
H2
h2 h1
M3

Mref
Mref
H3
M2
h3
M1
A layered system of a
reference material
He
A layered system of
different materials is
converted to…
M1
H1  3
 h1
M ref
M3
M1
M2
He  3
 h1  3
 h2  3
 h3
M ref
M ref
M ref
In Ontario…
The elastic modulus for new hot mix
asphalt is assumed to be 400,000 psi
(2.758 GPa)
The elastic modulus for new granular ‘A’
is assumed to be 50,000 psi (0.345 GPa)
The elastic modulus for new granular
subbase is assumed to be 15,000 psi
(0.103 GPa)
The Standard, Reference
Material is NEW GRANULAR ‘A’
Therefore, in Ontario, the layer equivalency
factor for new asphalt hot mix is…
3
400000 3
 8  2.0
50000
This means that 100 mm of new hot mix
asphalt has equivalent strength to 200 mm
of new Granular ‘A’
In Ontario, granular ‘A’ is used for the
granular base layer in conventional
pavement design
Therefore the layer equivalency factor for
granular base is…?
right, 1.0!
What would the layer equivalency factor for
new granular subbase in Ontario be?
3
15000 3
 0.3  0.67
50000
i.e., 300 mm of new granular subbase
would be equivalent in strength to 200 mm
of new granular ‘A’
MTO Tabulated GBE’s
(After: "Pavement Design and Rehabilitation Manual", 1990)
The MTO has published Granular Base
Equivalencies (GBE’s) for three design
situations:
New Projects
Resurfacing Projects
Reconstruction Projects
These are listed in Table 6.02 in the course
notes.
New Projects
 these are the maximum values to be assumed
for new pavement designs
Resurfacing Projects
 these are the values to be assumed for existing
pavement layers when an overlay is being
considered
 these are based on average test values on
pavements overlaid by the MTO
Reconstruction Projects
 These are the lowest values for the materials in
a pavement structure (at their weakest)
GBE’s can be…
 estimated during coring and boring surveys
based on the operator’s descriptions of each
layer material or based on an estimated
percent loss in strength
 calculated from measured elastic modulus
values for samples of each layer of the
pavement
 used to evaluate the strength of an existing
pavement, determine overlay requirements or
design a new pavement
Example: Given the bore hole log
TABLE 6.02
GRANULAR BASE EQUIVALENCY FACTORS
data below, evaluate the GBE of this
pavement.
(After: "Pavement Design and Rehabilitation Manual", 1990)
NEW PROJECTS
Material
Layer
1
New or Recycled HL
Granular A in Base
Granular B in Subbase
Cement Treated Material in Subbase (with Gr. A base)
Cement Treated Material in Base (no subbase)
Bituminous Treated Material in Base (with Gr. A subbase)
Cold Mix
OGDL (Open Graded Drainage Layer)
Material
M
(psi)
Old HL (30%
loss
RESURFACING PROJECTS
in GBE) Material
Crushed RBM* (in base)
Bituminous Treated Crushed RBM*
Crushed RBM* Blended with New Granular A
Pulverized Bituminous Surface Mixed with Existing Base
Old HL
Old Granular Base
Old Granular Subbase
2
Old HL
(crumbles)
3
Old Granular
30,000
RECONSTRUCTION PROJECTS
Base
Material
4
Old Granular
Subbase
Granular A Layer
Old HL
Old Granular Base
Old Granular Subbase
8,000
Equivalency
Factor
2.00
1.00
0.67
1.40
1.80
1.50
1.80
1.00
Thickness
(mm)
Equiv.
Factor
45
1.7
76.5
90
1.0
90
170
0.84
142.8
340
0.54
183.6
-
-
Equivalency
Factor
1.00
1.50
1.00
1.00
1.25
0.75
0.50
Equivalency
Factor
1.00
1.00
0.60
0.40
Note: For design purposes OFC and Surface Treatment, because of their thickness, are assumed to have no
structural value and
5
Silty Sand
Subgrade
act only as a surface wearing course.
*RBM - Recovered Bituminous Material
-
-
He
(mm)
The total GBE for the pavement is therefore:
H e  76.5  90  142.8  183.6  492.9mm
What was the original GBE for the pavement ?
He0  45 2.0  90 2.0  1701.0  340 0.67
He0  90  180 170 227.8  667.8mm
What % of its original strength has been lost?
%GBE lost

667 .8  492 .9 

 26%
667 .8