Cyclic axial loading of drilled shaft foundations in cohesive soil for transmission line structures

Foundations for transmission line structures differ from foundations for buildings or bridges because their live-load-to-dead-load ratio is quite high. As a consequence, live loads, such as cyclic wind loads, can be the dominant consideration in foundation design. Because little was known about the design of drilled shaft foundations under cyclic loadings, design had to be conservative to address the problem. Researchers constructed model drilled shaft foundations in three diameters and two depth-to-diameter ratios in uniform cohesive soils of both low- and high-overconsolidation ratios. On the basis of wind-loading evaluations, they developed a test loading program to simulate significant wind storms. They loaded the model shafts cyclically and then to failure. These results were compared with test results obtained by monotonic axial loading. Test results showed that side resistance of drilled shaft foundations can be influenced significantly by cyclic loading. One-way cyclic load levels less than about 2/3 of the static shaft side resistance cause minor displacement or capacity reduction. Higher load levels will cause large permanent displacements and capacity reductions. Usual levels of two-way cyclic loading for transmission line structure foundations cause little, if any, reduction in capacity. On the basis of these results, design guidelines were developed to addressmore » various loading combinations.« less