The exposed perimeter of a pile element and the properties of the coupling springs should be chosen to represent the behavior of the pile/medium interface commensurate with the problem being analyzed. For piles in soil, the pile/soil interaction can be expressed in terms of a shear response along the length of the pile shaft as a result of axial loading (e.g., a friction pile), or in terms of a normal response when the direction of loading is perpendicular to the pile axis (e.g., piles used to stabilize a slope).

Pile/soil interaction will depend on whether the pile was driven or cast-in-place. The interaction is expressed in terms of the shear resistance that can develop along the length of the pile. For example, driven friction piles receive most of their support by friction or adhesion from the soil along the pile shaft. A cast-in-place end-bearing pile, on the other hand, receives the majority of its support from soil near the tip of the pile.

If the failure associated with the pile/soil response is assumed to occur in the soil, then the lower limits for coupling-friction-shear and coupling-cohesion-shear can be related to the angle of internal friction of the soil (for coupling-friction-shear) and the soil cohesion times the perimeter of the pile (for coupling-cohesion-shear). If failure is assumed to occur at the pile/soil interface, the values for coupling-friction-shear and coupling-cohesion-shear may be reduced to reflect the smoothness of the pile surface.