Bearing Capacity and Deformation of the Tandem Compound Piles Improved Foundation: A Parametric Study.

The tandem compound piles are a combination of a granular column in the deep section and a concrete pile in the shallow section. This method effectively utilizes the consolidation and densification effects of the granular column, as well as the cementation strength of the concrete material. The granular column acts as a consolidation path, aiding in the densification of the surrounding soil. On the other hand, the concrete pile prevents the bulging deformation that commonly happens in granular columns during field construction. To study the bearing capacity and deformation of the improved foundation with tandem compound piles, a coupled continuum-discrete numerical model was developed in this study. The accuracy of the model was confirmed by comparing its results with experimental measurements. Additionally, a parametric study was conducted, considering three influential factors: (1) cushion thickness and modulus, (2) length, modulus, diameter, and spacing of the tandem compound pile, and (3) soil modulus. The results indicated that reducing the cushion thickness and increasing the cushion modulus allowed the pile to bear more loads. Moreover, increasing the length and modulus of the deep section of the pile reduced deformation and improved the bearing capacity. The pile modulus, however, had a limited effect on enhancing the bearing capacity. It is important to maintain a critical pile spacing of at least twice the pile diameter. Finally, a high modulus of the underlying stratum led to higher vertical and radial stresses in the pile.