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8.5 COMBINATIONS OF EXTREME EVENTS AASHTO (2014) provides two Extreme Event combinations. Extreme Event I is a combination of scour with earthquake loading, and Extreme Event II is a combination of scour with ice, vehicle
and vessel collision loads, and hydraulic loads. As noted in Arneson et al. (2012), the Extreme Event I combination has a low occurrence probability for both the check flood and earthquake loading. Therefore, scour for the mean discharge or normal non-flood flow maybe applied to this event combination. For the Extreme Event II combination, research is ongoing to assess the probability of joint loading conditions
during the check flood, and judgement should be used based on site-specific factors. If ice or debris jams near the structure dictates the use of a more extreme flood event than the check flood, this maybe used to assess the extreme event limit (Hannigan et al. 2016).
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9 DESIGN FOR EARTH RETENTION STRUCTURES 9.1 OVERVIEW The subject of this chapter is limited to non-gravity cantilevered walls with discrete
vertical wall elements, such as drilled shafts, driven piles,
or drilled-in piles, embedded below finish grade to provide passive resistance to the retained earth. Often referred to as top-down walls, soldier pile walls,
or beam and lagging walls, the supporting drilled shafts or piles are spaced center-to-center generally at least three effective diameters apart, with fascia lagging,
precast concrete panels, or temporary lagging with a cast- in-place concrete facing, positioned within the interval between the above grade portion of the vertical elements, as shown by Figure 9-1. Center-to-center spacing of vertical elements less than three diameters may result in overlapping passive stress influence between the supporting elements. Methods of analysis for continuous vertical wall elements (secant pile or tangent pile walls) are discussed in Brown et al. (2010).
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