Piled Raft Floats On Moraine

Instrumented load testing of high capacity micropiles, followed by detailed analysis and numerical modeling, provided an opportunity for real foundation performance to be incorporated into the design of a complex urban high-rise structure. This paper discusses a design-based load testing program and subsequent analysis for micropiles incorporated into a piled raft foundation beneath a portion of a high-rise residential structure in Brooklyn, New York. Preliminary numerical analysis, performed using the commercial finite element package PLAXIS 3D Foundation, indicated that soil stiffening and transferring heavy core loads to greater depths was required to minimize the total and differential settlements. High capacity micropiles were modeled below the tower core mat to create a piled raft effect, reducing the settlements to acceptable levels. Final design of the foundation and micropiles, and final numerical analysis of the proposed foundation system required an instrumented load testing program to provide the Owner and design team with a level of comfort. The instrumented load test provided data on the evolution of load transfer, including the mobilization of significant tip resistance, frequently ignored in design, and micropile axial stiffness. Final numerical analysis of the foundation system involved refining the PLAXIS model based on the micropile behavior and load transfer mechanism observed during the load test program. The authors believe that the combination of design-based load testing and numerical analysis in this project yielded a more refined foundation system with realistic expectations of performance.