A new approach for the evaluation of shaft friction of bored piles in sandy soils

In recent decades much research efforts have been dedicated to understand piles and piled foundations performance, but still a big amount of uncertainties remains on single pile behaviour. Although improvements have been made in identifying the processes that occur within the critical zone of soil immediately surrounding the pile, in literature are not available theories which allow to quantify the changes in state of soil during the load (stress, void index). This assessment is particularly true for piles embedded in sandy soils, on which the paper will focus on. Granular soils at shallow depth from surface are subjected to low confining stresses, hence potentially dilating when sheared at failure. This is what systematically happens in the upper portion of axially loaded piles, as demonstrated by several experimental data collected by a number of researcher: it is well known from literature that measured unit shaft resistances are usually greater than theoretical values (e.g. O’Neill e Hassan, 1994; Chen e Kulhawy, 2002; Rollins et al., 2005). In the thesis a conceptual framework for estimating pile shaft capacity has been presented, in which the dilatant behaviour of sand in the shear band, partially forbidden from the surrounding soil, is explicitly considered: shear band develops in a small portion of soil close to pile shaft and its thickness is controlled by grain size, since it is of the order of 5 to 20 times d50. Theoretical model led numerical analyses (which have been performed with FLAC 2D, a two-dimensional explicit finite difference code), which focused on shaft friction evolution at pile-soil interface for “wished in place” axially loaded single piles, embedded in sandy soils. In detail, parametric analyses have been performed by changing relative density, stress history (OCR), friction angle and grain size distribution. Numerical results highlighted those critical aspects which cannot be ignored if shaft friction evolution must be predict. Dilatancy of sand in shear bands at the pile-soil interface plays a major role in increasing normal stress (thus unit shaft friction) during loading, but its effect is strictly modulated by its initial thickness, or rather by grain size distribution. In others words, irrespective of its dilatants potentiality, fine sands does not allow for large values of qs as a consequence of plastic volumetric strains confined in a very thin shear band. Obviously all above strongly influences the pile load-settlement curve for a given soil condition, but with different grading. From these results, a new designing approach is suggested in which shaft resistance prediction moves from a physical and mechanical behaviour description of soils surrounding pile shaft. The applicability of the proposed approach is showed for some real case study of vertical load tests on instrumented piles