Seismic Behavior of Unconnected Piles–Caisson Foundation Using Slow Cyclic Tests

The unconnected piles–caisson foundation (UPCF) consists of gravity caisson, mattress, and pile group. This foundation has been an attractive design for the mattress as it can reduce the seismic loads applied on the superstructure. However, a lack of understanding regarding its seismic behavior has hindered the application of this type of foundation. This paper reports on a series of slow cyclic tests (SCTs) conducted to investigate the seismic behavior of the UPCF. Six tests were conducted on model foundations on dry sand with the variables of mattress material, mattress thickness, vertical safety factor, and ground improvement. The experimental schemes, including the model design, the instrumentation, and the loading scheme, are illustrated in detail. The test results show that a large amount of energy was dissipated through the mattress during dynamic loading. With increasing loading displacement, the shear stiffness of these models decreased to a stable value. An empirical equation for the stiffness and loading displacement is developed. Below a drift ratio of 0.6%, there is no evident difference in the dynamic behavior of the foundation. After that, the aseismic behavior of the foundation depends on the mattress material, the mattress thickness, and the vertical pressure and is affected by the earthquake intensity and frequency.