Comportamento à tração de estacas helicoidais em areia submetidas a carregamentos cíclicos

Abstract

Helical piles are foundation systems used for supporting compression, tension, and lateral loads. Ad-vantages of helical piles over other foundation systems include fast installation and possibility to apply load immediately after installation. Despite the significant growth of the use of helical piles around the world in the last 25 years, studies on the behavior of this type of foundation remain scarce. Field test results in pure sand deposits are particularly needed. In order to fulfil this demand, a field testing pro-gram involving helical piles subjected to vertical uplift loads has been conceived. A full-scale prototype of a helical pile with three bearing plates has been designed and built. Seven axial tensile load tests with cyclic loading have been performed in a pure sand deposit. Three tests were carried out with static cyclic loading, and included up to 15 load-unload cycles. Two other tests were conducted with quasi-static cyclic loading. The results of the static load tests revealed that soil-pile system stiffness increases after completion of each static loading cycle. Theoretical and empirical methods were used to predict pile ultimate static tensile load capacity, and resulted in conservative predictions as compared to the experi-mental field test results. The two tests with quasi-static cyclic loading were carried out with distinct mean cyclic loads and load amplitudes. The results of these tests revealed that increasing mean cyclic load and load amplitude tend to reduce pile stability and soil-pile system stiffness. The static uplift capacity of the pile was found to improve after application of quasi-static cyclic loads. Two dimensional numerical models using the finite element method were developed to evaluate how the behavior of a pile under static loading and embedded in a sandy soil profile is affected by variations of important design parameters, such as depth of installation, plate diameter, spacing between helices, and number of plates. Parametric analysis results gave insight into the distribution of stresses and strains in the soil mass around the pile, and clarified some aspects of soil-pile failure mechanisms. Uplift capacity in-creases linearly with depth of installation, and the pile transitions from shallow to deep failure mode when the ratio between depth and the diameter of the first helix is around four. Capacity was found to increase linearly with plate area. The failure mechanism of the pile transitions from cylindrical shear to individual capacity with a spacing ratio equal to three. It was also observed that multi-helix configura-tions are less efficient than single-helix piles, in the investigated conditions.