Design of Laterally Loaded Piles - Limits of Limit State Design?

Design methods for laterally loaded piles assume a specific bearing behavior depending on the pile-soil stiffness. Short piles are designed for rigid body deformations mobilizing passive and active earth pressure along the complete length of the pile. This represents a typical ultimate limit state problem based on a plastic failure mechanism for the earth pressure approach. For typical applications the deformations of such piles are of minor importance. Long or flexible piles are modelled as a beam on elastic springs, where the spring characteristics depend on the pile-soil stiffness. This model provides the pile deformations for a serviceability limit state analysis, but cannot directly account for failure of the ground. Nevertheless, limit state design requires the proof of both limit states for one design problem. Application of this concept leads to inconsistencies if available design methods only cover a specific behavior of the structure in question.