Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
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| 6.7.8 Loading Considerations 6.7.8.1 Axial Loads The presence of axial loads can significantly influence the response of a deep foundation element to lateral loads and/or moments, and therefore axial loads must be included in the lateral load analysis when present. In general, for fully embedded piles (piles completely below the ground surface) with other considerations being equal, axial loads and the confining pressures of the surrounding earth create an increase in stiffness of the foundation element, and therefore reduce the amount of deflection that occurs under the same lateral load. For partially embedded piles with an unsupported length above the ground surface, axial loads will increase the amount of lateral deflection, assuming all other considerations being equal (Davisson and Robinson 1985). If possible, lateral load proof tests should include conditions comparable to the actual design condition. This should include axial loads comparable to the design loads and/or unsupported pile lengths similar to the design condition. However, in many cases it is not practical to include axial loads within a lateral load testing program. Similarly, it is often not possible to include an unsupported pile length in a lateral load test program. An analysis should be performed to design the lateral load test so that the test load conditions produce a result that is as equivalent as possible to the design condition, or a condition that is more conservative compared to the design condition. For example, lateral proof load tests for fully embedded piles will tend to be conservative from the standpoint of deflection because the actual condition with axial loads will produce a stiffer response (i.e., less lateral deformation. 94 6.7.8.2 Cyclical Loads The influence of cyclic loading can be significant. This effect has been assessed for deep foundations in clays and sands. Most p-y curves in the literature were developed for sustained loading. However, it was found that cyclic loading (caused by wind and ocean waves, among other factors, can cause a substantial loss in the lateral resistance of soils, especially at sites with stiff clay as a result of the progressive loss of contact between the soil and the foundation element, as shown in Figure 6-16. Figure a) shows the general shape of a p-y curve for static loads versus a p-y curve for cyclic loads. The figure shows the general degradation of resistance over applied cycles of load to a lower final resistance compared to the static p-y curve. This strength degradation was observed by Matlock (1970) and Reese et al. (1975) in full-scale experiments. These researchers observed that after a number of loading cycles a loss of soil-pile contact was produced and some of the soil around the pile was suctioned into suspension in the water around the piles (Figure b, as evidenced by clouds of suspension around the front and back faces of the pile. Download 6.03 Mb. Share with your friends:
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