Evaluation of soil-pipeline longitudinal interaction forces

This work is part of an extensive research program concerning the design of buried pipelines crossing unstable slopes. In these cases the most common choice is that of the pipe axis running parallel to the slope, so that longitudinal interaction is of foremost significance. Soil deformations will influence the pipe, to an extent which depends on pipe-soil interaction; it is desirable that this should be low, so that after exceeding the maximum interface shear strength the soil will deform without further stressing the pipeline. Current methods in use for quantifying shear longitudinal interaction bear some similarity to calculation methods for driven piles. Even though the majority of existing pipelines has been thus designed, these methods are in fact not satisfactory, and some recent research is dedicated to finding new more appropriate methods by means of extensive use of in-situ tests in order to quantify shear strength in soil-structure interaction. In this paper the traditional methods deriving from pile analogies are examined and their inappropriateness in the case of pipelines is shown. The results of some in-situ tests are examined, and the information that can be drawn is analysed. In addition, the results of some laboratory interface tests are presented. In the aim to create reference values valid for general design rather than for some specific case, these are examined in order to highlight the physical mechanisms responsible for the development of shear strength. Various material combinations are considered, and conclusions are drawn regarding appropriateness of these. Comparison between in situ and laboratory results in part confirms the qualitative analysis of the shear mechanisms; on the other hand, attention is drawn to some physical phenomena, occurring in situ and non reproduced in the laboratory tests.