Page 308 - Planning and Design of Airports

266 Airp o r t D e sign

Young’s Modulus (E Value)
The most recent accepted method for determining the strength of
subgrade by the FAA is based upon the elastic modulus of the sub-
grade, or E, also known as Young’s modulus. In general, a structure’s
E value is its measure of “stiffness” or “elasticity.” The greater the E value,
the more stiff the material, and the less the material is susceptible to
deformation under a given stress load. A pavement’s E value may be
empirically estimated by evaluating its stress to strain ratio, accord-
ing to Eq. (7-1)

/
E ≡ Tensile stress = σ = FA 0 = FL 0 (7-1)
/
Tensile strain ε Δ LL A ΔL
0 0 0
where E = Young’s modulus (modulus of elasticity)
F = force applied to the pavement
DA = original cross-sectional area through which the force is
0
applied
L = amount by which the shape of the pavement changes
L = original shape of the object
0
Alternatively, the subgrade modulus E value, in pounds per
square inch, may be found using the following conversion formulas:
To find E based on CBR value the equation is
E = 1500 * CBR (7-2)
To find E based on the modulus of subgrade of reaction, k, the equa-
tion is

E = 26k 1.284 (7-3)
This conversation is provided by the FAA in its advisory circular pri-
marily to facilitate the transition from the more traditional pavement
design methods to the most current software-based method of pave-
ment design and evaluation.

Effect of Frost on Soil Strength
While there are a variety of soil types, the behavioral properties of
any given type are relatively similar regardless of other climatic char-
acteristics, such as the average ambient temperature and amount of
precipitation. One factor that does significantly impact the strength
of soil, however, is the presence of frost on the surface of or within the
soil, either on a seasonal or a permanent basis.
Frost action, if severe, results in nonuniform heave of pavements
during the winter because of the formation of ice lenses within the
subgrade, known as ice segregation, and in loss of supporting capac-
ity of the subgrade during periods of thaw. Figure 7-3 illustrates the
process of ice segregation.