In two exploratory studies, the influence of soil stress history effects was examined on single piles that were laterally loaded in sand (Lin et al. 2010) and in soft clay (Lin et al. 2014). Lin et al. (2010) found that ignoring stress history effects in sand subjected to a scour depth of 5D (D represents pile diameter) resulted in pile head displacements up to 33% greater than when stress history effects were considered. The study on single piles laterally loaded in soft clay demonstrated that neglecting stress history effects re- sulted in pile head displacements that were 13–15% less than when

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stress history effects were considered for scour depths ranging from 5 to 10 times the pile diameter. These two initial studies strongly suggested that soil stress history effects were important to consider during a scour analysis and that neglecting stress history effects was not necessarily a conservative assumption.

Other research studies have been performed regarding the stress history effects related to piles (Brown and Castelli 2010; Delphia, unpublished data, 2009); however, none of these have examined the effect of soil stress history on scour susceptibility by examining a full bridge system. Given the introductory findings from Lin et al. (2010, 2014), further investigation into the implications of stress history effects on scour susceptibility analyses is warranted and needed.

The objective of this study was to examine the influence of soil stress history on a scour susceptibility analysis that directly inte- grated both substructure and superstructure components. The intent of the research was to determine whether an increased level of re- finement in the analysis approach would increase the accuracy and usefulness of scour susceptibility assessments. The scope of this study included (1) the development and verification of software [termed the integrated analysis program (IAP)] that would accom- modate the inclusion of soil stress history effects in a model that included soil resistance, substructure elements, and superstructure elements, and (2) an examination of the influence of stress history effects by analyzing an example bridge under scoured conditions in two different soil types (sand and soft clay). It was hoped that an additional goal could be achieved during the course of this study in that a greater level of understanding of how bridges respond to scour could be realized.

Stress History of Soils

The effect of scour on structures in water is more evident in the lateral direction than axial direction. Therefore, behavior of structures under lateral loads has received more attention. Lin et al. (2010, 2014) evaluated laterally loaded piles under scour conditions using the modified p-y curves that were capable of considering the changes of stress history of cohesive and cohesion- less soils.

For cohesive soils, responses of laterally loaded piles evaluated by the p-y methods (e.g., Matlock 1970) depend on the soil effec- tive unit weight and undrained shear strength. Lin et al. (2014) in- vestigated these two parameters and established relationships of each parameter before and after scour through the OCR and scour depths. Based on the relationships, a new set of p-y curves for soft clays was established to consider the stress history effects under different scour conditions. More details of the stress history of soft clays due to scour can be found in Lin et al. (2014).

For cohesionless soils, lateral behavior of piles depends on the soil effective unit weight, the p-y modulus, and friction angles based on the p-y method for sands (e.g., Reese et al. 1974). Lin et al. (2010) developed a framework for estimating these important parameters in different scour conditions. Based on this framework, the Reese p-y curves were modified to consider the changes of stress history of sands under scour conditions. The development of the modified p-y curves for sands was detailed by Lin et al.  (2010).