Performance of Battered Driven Minipile Groups under Uplift Loading in Sand

The performance of a new battered driven minipile group system under uplift loading in a noncohesive material was investigated by field testing and physical and three-dimensional (3D) numerical modeling. A series of uplift tests were conducted on footings with four and six minipiles and on a single minipile for comparison at field and laboratory scales to assess the impact of system efficiency and installation angle, and minipile configuration. A 3D numerical model was also developed and validated based on the field results for further investigation. The results indicate that a group of minipiles carry higher loads, compared with the same number of single minipiles. This is attributed to the system efficiency and the fixity of the minipile heads. Numerical and physical modeling showed that the configuration of the minipiles significantly impacts the uplift capacity. Of three different configurations examined in this study, it was understood that when minipiles are installed diagonally, Case 2, they engage a larger volume of soil with minimal negative group effect, achieving a higher uplift capacity. The installation angle is found to play a critical role when it comes to the uplift capacity. The uplift capacity of this new minipile group increased with installation angle from 0° (vertical minipiles) up to 25° but decreased with higher installation angles, in agreement with findings reported in the literature. However, all battered minipile groups showed higher uplift capacities, compared with footings with vertical minipiles. Two types of minipile groups were also tested using a 1g physical modeling facility, subjected to uplift, to investigate the observations further. Two minipiles of each group were instrumented with optic fibers to record the strain profile, along with the minipile shaft corresponding to each loading stage. The load-carrying capacity obtained from the physical model corroborated the findings from the numerical simulations.