Equivalent single wheel load (ESWL)

Dual wheel load assembly.

In order to limit the maximum load on single wheel within the specified limit and to carry greater load it is necessary to provide dual wheel assembly on the rear axles of heavy road vehicles. Equivalent Single Wheel Load (ESWL) of the dual wheel load assembly at the depth, ‘z’ may be defined as the single wheel load replacement of the dual wheel load assembly which will cause the same magnitude of vertical deflection or same value of compressive stress at that depth, z. thus ESWL at any selected depth, z may be determined based on either equivalent deflection or equivalent stress criterion.

Westerguaard’s THEORY.

• Westerguaard’s analysis or theory is used for rigid pavement which is made of cement concrete with reinforcement or without reinforcement.

• High tensile strength of rigid pavement develops the slab action. Because of the load spreading capacity, the stresses in the sub-grade below a rigid pavement are greatly decreased.

• Over-stressing of the concrete usually causes the failure of rigid pavement, but failure of rigid pavement is not caused by the over stressing of the sub-grade.

• Hence the thickness is generally determined by considering the calculation of stresses developed in the concrete.

• Westerguaard’s theory is used to determine the stresses developed in the concrete slab because of wheel loads.

$\rightarrow$ Assumption made by Westerguaard’s Theory.

• Following are the important assumptions considered by Westerguaard’s Theory.

1. The rigid pavement is assumed as a thin elastic plate resting on soil sub-grade.

2. The upward reaction at any point is considered to be proportional to the deflection at that point.

   $\therefore$ upward reaction at any point $\alpha$ deflection at that point.

3. Vertical subgrade reaction is assumed to be proportional to the deflection.

4. Cement concrete slab is homogeneous and isotropic (i.e. having uniform elastic properties).

• The slab deflection depends upon the flexural strength of the slab and the stiffness of the subgrade concluding that the pressure deformation properties of a rigid pavement depends upon the relative stiffness and the subgrade below the rigid pavement.