Three or more layer system

A three or more layer system (multi-layer system) simulates better the real situation of flexible pavements, since there are more than two distinct layers in a typical pavement. 

In a three-layer system, all asphalt layers are considered as one layer. The same applies for the unbound layers of base and sub-base. The third layer is the subgrade. 

The increase of the number of layers to more than three represents the real situation of the pavement but requires more complicated calculations for the determination of stresses and strains developed at critical interfaces. When also considering variations of the properties of the materials that constitute the layer owing to the duration of loading or temperature, variation of loading area and applied load and non-truly elastic behaviour of the materials, the use of nomographs or tables for the determination of stresses and strains is almost impossible. Today, such calculations are carried out by software programs. 

1 Determination of stresses/strains from tables or nomographs 

Despite the complexity of three or more layer systems, several researchers tried to resolve the problem without the aid of computers. Burmister (1943) was the first to extend his theory to a three-layer system. A few years later, Acum and Fox (1951) presented the first tables for determining the vertical and horizontal stresses developed on the axis of symmetry of the circular loading area and at the interface of the layers. 

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It should be mentioned that the above assumptions were also used for theories/analyses for determining the stresses in three or more layer systems. 

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