Although originally developed to study stresses in complex airframe structures, it has since been extended and applied to the broad field of continuum mechanics (Huebner et al., 2001 ). The finite element method (FEM) is a numerical analysis technique for obtaining approximate solutions to a wide variety of engineering problems. Critical analysis locations in a pavement structure. Used to predict rutting failure in the subgradeįigure 2. Used to predict rutting failure in the base or subbase Top of Intermediate Layer (Base or Subbase) Used to predict fatigue failure in the HMA Used in imposing load restrictions during spring thaw and overlay design (for example) Critical Analysis Locations in a Pavement Structure However, there are a few critical locations that are often used in pavement analysis (Table 1 and Figure 2). The use of a layered elastic analysis computer program will allow one to calculate the theoretical stresses, strains, and deflections anywhere in a pavement structure.
Deflection is expressed in units of length (mm or µm or inches or mils). Since the strains in pavements are very small, they are normally expressed in terms of microstrain (10 -6). The unit displacement due to stress, usually expressed as a ratio of the change in dimension to the original dimension (mm/mm or in/in). Stress has units of force per unit area (N/m 2, Pa or psi). The intensity of internally distributed forces experienced within the pavement structure at various points. The outputs of a layered elastic model are the stresses, strains, and deflections in the pavement: This can be done because we are assuming that the materials are not being stressed beyond their elastic ranges.įigure 1 shows how these inputs relate to a layered elastic model of a pavement system. Multiple loads on a pavement surface can be accommodated by summing the effects of individual loads.
Although most actual loads more closely represent an ellipse, the effect of the differences in geometry become negligible at a very shallow depth in the pavement. Usually specified as being a circle of a given radius (r or a), or the radius computed knowing the contact pressure of the load (p) and the magnitude of the load (P).
The layered elastic approach works with relatively simple mathematical models and thus, requires some basic assumptions. This section covers the basic assumptions, inputs and outputs from a typical layered elastic model. Today, Boussinesq influence charts are still widely used in soil mechanics and foundation design. Boussinesq who published his classic work in 1885. The origin of layered elastic theory is credited to V.J. In other words, it is the same everywhere and will rebound to its original form once the load is removed. Layered elastic models assume that each pavement structural layer is homogeneous, isotropic, and linearly elastic. Layered Elastic ModelĪ layered elastic model can compute stresses, strains and deflections at any point in a pavement structure resulting from the application of a surface load. Both of these models can easily be run on personal computers and only require data that can be realistically obtained. There are a number of different types of models available today (e.g., dynamic, viscoelastic models) but this section will present two, the layered elastic model and the finite elements model (FEM), as examples of the types of models typically used. Mechanistic models are used to mathematically model pavement physics.
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