Abstract: Scour is a major cause of bridge failures, and the Federal Highway Administration (FHWA) now mandates that states assess the susceptibility of their bridge inventory to scour. The authors have developed and described an integrated analysis program (IAP) that allows the user to accurately approximate nonlinear soil behavior around pile foundations within a structural analysis model. This paper describes a study performed to examine the influence of soil stress history on the outcome of a scour susceptibility analysis. Twenty-two models including the superstructure, substructure, and soil reactions were examined in which scour depth, soil type, and inclusion of soil stress history effects were varied. It was found that the stress history of soils had a limited effect on the performance of the bridge analyzed. This result was found to be in contrast to prior findings from studies performed on isolated piles, indicating that performing an integrated-type analysis is key to obtaining an accurate assessment of bridge scour susceptibility. DOI: 10.1061/(ASCE)CF.1943-5509.0000681. © 2014 American Society of Civil Engineers.

Author keywords: Scour; Bridge; Pile foundation; Integrated analysis program (IAP); Soil stress history; Soft clay; Sand.

Introduction：Scouring of soil from around bridge foundations is known to be a major cause of bridge failures, producing significant financial losses and threats to public safety (Richardson and Davis 2001; Wardhana and Hadipriono 2003). The Federal Highway Administration (FHWA) has recognized this risk and now requires that all state highway agencies evaluate whether bridges in their inventory are scour-susceptible.

When analyzing a bridge for scour susceptibility, many analysis options are available, each having advantages and disadvantages. The most common approach taken by bridge engineers for evalu- ating a bridge’s scour susceptibility utilizes a very basic assumption that some point along the depth of a pile or pile group can be assumed to act as a fixity, and adjusting the point of fixity allows the engineer to neglect the presence of soils altogether. This assumption makes the analysis quite straightforward; however, it is known to be less accurate than a model that directly includes the soils surrounding the piles. Additionally, while many ap- proaches to this analysis problem examine only the interaction be- tween piles and soil, it is clear that a bridge structure is a system comprised of a substructure and superstructure, and the behavior of that system is likely to be closely affected by the characteristics of both of these major  components.

Recognizing this, Lin et al. (2012) developed an iterative analysis procedure that accommodated both superstructure and substructure characteristics and included an idealized soil model surrounding the piles. The iteration procedure utilized the widely used structural analysis software STAAD.Pro 2007 and the pile foundation software FB-MultiPier. The advantage of this iterative technique was that it provided a rational method for performing a holistic analysis of a bridge under scour. The disadvantage was that the method required a significant level of effort on the part of the user to perform the iterations between STAAD.Pro and FB-MultiPier. Additionally, the iterative approach introduced by Lin et al. (2012) did not allow for consideration of soil stress history effects due to limitations within the software packages   used.

The stress history of soils refers to the fact that soils experience loading and unloading over time, and thus soils experience different states of stress and properties depending on their history. The au- thors hypothesized that the stress history of soils was an important factor when conducting a scour analysis because the process of scour by definition implies that soil is removed from around a pile or pile group, having the effect of unloading the remaining soil. Therefore, the remaining soils that have not been removed by scour are unloaded and thus develop a new state of stress. Soil stress his- tory can be characterized by the overconsolidation ratio (OCR), which is defined as the ratio of the maximum historical soil stress to the present soil stress. Therefore, as scour depth increases, the present state of stress in the soil would be expected to reduce, in- creasing the OCR value for the soil. In a conventional scour analy- sis, this effect would be ignored. It was an objective of this study to determine the influence of stress history effects on the results of a scour analysis.