Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
Figure 6-16: Effects of cyclic loading (a) schematic static and cyclic p-y curves and (b) loss of soil
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| Figure 6-16: Effects of cyclic loading (a) schematic static and cyclic p-y curves and (b) loss of soil contact around deep foundation element (after Reese et al. 1975). The influence of cyclic loading on p-y curves has been also studied by Wang (1982) and Long (1984), who observed that cyclic loading results in larger deflection and bending moments than with short-term static loading. For load levels mobilizing less than 50 percent of the static lateral resistance, deflections increase with cyclic load levels. For cyclic load levels, greater than 50 percent of the static resistance, deflections tend to increase only with the cycle numbers. This tendency was attributed to repeated cycles of relative large strains in the clay followed by scour of the soil around the pile if water is above the ground surface. Confirming other researchers observations, Welch and Reese (1972) noticed that a gap between the soil and the pile may develop under cyclic loading. This mechanism was confirmed by Reese et al. (1975) and O’Neill and Dunnavant (1984). With the soil being washed out, the gap tends to increase and cause additional water to penetrate the gap. p y a b c d Reduction in Epy Static Curve Cyclic Curve (a) (b) 6.7.8.3 Considerations for Transient Loads, Temporary Loads, and Permanent Loads Transient loads are of short duration such as wind, wave, impact, or seismic loads. These loads are applied at a higher rate of loading than is typically accounted for in analysis or testing. Most load tests are performed under static loading conditions, and the analysis methods have been developed for static loads and therefore do not consider higher rates of loading. In general, rapid rates of loading would be expected to produce a higher lateral load resistance from the soil and a stiffer overall response. The use of methods based on static loads is therefore anticipated to be somewhat conservative for more rapid lateral load applications. 95 Loads can be also be of short, temporary duration or permanent. In this case, temporary loads refer to short term loads often associated with construction rather than rapid transient loads. Short term temporary loads are analyzed in the same manner as permanent design loads. Where there are different considerations for temporary vs. permanent loads, these considerations are usually addressed through the level of reliability considered (different load or resistance factors) or through different performance requirements. For example, a lower load factor or higher resistance factor maybe used for design or a temporary structure, or higher deflections maybe allowed fora temporary structure. In some cases, such as performance of a pile and lagging support of excavation system adjacent to a critical structure, the reliability maybe the same as a permanent structure and the performance requirements maybe more stringent than maybe used fora permanent structure. For permanent loads, long term sustained lateral loads may result in soil creep or reduced lateral resistance overtime. This is especially true for clay soils, where permanent loads may result in soil creep that may not need to be considered for temporary or transient loads. The resulting soil stiffness as a result of soil creep maybe less than that predicted from p-y curves based on static loading conditions and undrained strengths. Download 6.03 Mb. Share with your friends:
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