The effects and vertical bearing capacity of two jacked piles in sand are still not well understood, and the mechanism of the adjacent pile’s uplift caused by the jacking pile in a double pile system is especially unclear, but these facets are important to the stability of the jacked pile. In this paper, a series of tests is performed on jacked model piles in sand, where in the influences of the pile length and the driving pile’s speed on the effects and vertical bearing capacity of two jacked piles were studied. The results revealed that the effects and vertical bearing capacity of the two jacked piles were mainly in relation to pile length and influenced by the driving speed. The horizontal displacement of the top of the first jacking pile during the installation of the post-jacking pile was caused by the difference in the stress state of the first jacking pile between the side of the pile’s face and its back side, in which the uplift displacement of the first jacking pile was also involved. The radial stress of the pile increased nonlinearly with the depth under different pile lengths and gradually converged to the passive earth pressure. The ultimate capacity of the double pile is approximately twice that of a single pile, and the ratio of the ultimate capacity of a single pile to the final jacking pressure was approximately 1.04.

This study aims to clarify the relationship between surface hardness and the peak interfacial frictional coefficient under different particle sizes. The relationship between particle size and the peak interfacial frictional coefficient with different surface hardness values has not been addressed in previous literature. A pile-soil interfacial shear test was conducted using a multifunctional interfacial shear instrument was developed in the laboratory. The influences of surface hardness and particle size on the shear characteristics of pile-soil interfaces and the peak interface friction coefficient were studied. The test results revealed that the interfacial shear stress-shear displacement curves showed typical softening behavior when the surface of the model pile was smooth. The difference in curve characteristics for different particle sizes gradually cleared as the surface hardness decreased. The initial shear stiffness of the interface increased with increasing surface hardness and decreased with increasing particle size. The peak interfacial friction coefficient decreased with increasing surface hardness for different particle sizes. Furthermore, the peak interface friction coefficient increased with increasing particle size when the surface was hard. However, when the surface was soft, the peak interfacial friction coefficient decreased with increasing particle size. The linear relationship between the peak interfacial friction coefficient and particle size for different surface hardness values was established.

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