Bearing capacity of vertically loaded FRP and PVC single piles

Heavy structures, high-rise buildings, and marine structures increase the tendency to use pile foundations. Since conventional pile foundations are mainly made of timber, steel, and concrete, and these piles have a limited life cycle and high maintenance costs, especially where they are used in marine applications, industrial environments and corrosive soils. In order to extend the service life of these piles and reduce the impact of deterioration on the pile bearing capacity a larger pile cross section is often recommended, as well as some chemical treatments are also applied, while both of these options have led to spending an extra budget to build a required pile foundation. Despite the fact that chemical treatments have a negative impact on environmental pollution and the health of those people who do it. Recently, because of their high corrosion resistance, Fiber Reinforced Polymer (FRP) and recycled plastic piles have been developed for use as an alternative to these piles in the aforementioned environments. Since piles carry and transfer axial loads through the pile point capacity and/or the skin friction resistance (i.e., pile shaft capacity or pile shaft friction resistance), and despite the uncertainties associated with the pile behavior, the mechanical, physical and structural behavior of conventional piles were well documented. But due to the novelty of composite piles, the database of these piles still does not contain sufficient data. One of the concerns in composite piles and although it is not yet quite clear in the conventional piles is the pile shaft behavior (i.e., the pile-soil interface resistance). The friction force at the soil-pile interface is estimated primarily using the interface shear box test. Some uncertainties regarding this test method, especially for sandy soils, leads to the measurement of high values of the interface parameters (i.e., c and δ) in relatively low-to-high relative density. Because of the uncertainties associated with the pile axial behavior and in particular the pile shaft behavior, the designers often use the approximation values to estimate the interface friction angle (δ) during their design. For example, Terzaghi and Peck (1948) reported that many designers take into account that the interface friction angle (δ) is equal to two of three (2/3) of the measured value of the internal friction angle (φ), Coyle and Castello (1981) considered 0.8φ, and according to various studies, the δ seems to be in the range between 0.5 to 0.8φ (Das, 2015). These approximations in the evaluation of shear interface parameters prevent making economical designs and sometimes lead to stability problems. However, the actual bearing capacity of the pile is measured by performing field load tests, but it is impossible to determine the independent contribution of the pile shaft capacity to the ultimate pile bearing capacity. Therefore, in this study, the ultimate bearing capacity and pile base resistance of two composite piles (i.e., concrete-filled FRP and PVC pipe piles), as well as the independent contribution of pile shaft capacity were determined by carrying out a number of load-displacement model experiments at different relative densities (i.e., Dr = 10%, 40%, 65% and 90%) of model sand. The experimental results demonstrated the fact that the interface friction angle increases with increasing relative density considerably whereas this behavior was not observed in the interface shear box test, and that the interface friction angle at low relative density (i.e., Dr = 10%) can be ignored while in the interface shear box test, this relative density has high interface friction angle and cannot be easily ignored in the calculation of piles bearing capacity. In addition to model experiments, piles were modeled in the finite element environment to calculate the ultimate pile bearing capacity, as well as compare them to theoretical and experimental results. Finite element analysis has provided roughly the same settlement values at failures as that model experiments have. However, in particular, for high relative densities (i.e., Dr = 40%, 65%, and 90%), finite element analysis has not yielded a good agreement in terms of load-displacement behavior (p-y behavior) under certain conditions of this study. In model experiments, p-y behavior is a nonlinear relationship in all relative densities, whereas in finite element analysis it has become a nonlinear relationship where Dr = 10 and has become a linear relation where Dr=40 %, 65%, and 90%. In fact, this behavior is far from being expected, because, at high relative densities, the pile shaft capacity must be increased; and therefore, the degree of curvature of the p-y behavior must also be increased (i.e., it should be a non-linear relationship).