In this study, the effect of near-field and far-field ground motions on the seismic response of the soil pile system is investigated. The forward directivity effect, which includes a large velocity pulse at the beginning of the velocity time history of the ground motion is the most damaging phenomenon observed in near-field ground motions. To investigate the effect of near-field and far-field ground motions on the seismic response of a soil-pile system, a three-dimensional model consisting of the two-layer soil, liquefiable sand layer over dense sand, and the pile is utilized. Modeling is conducted in FLAC 3D software. The P2P Sand constitutive model is selected for sandy soil. Three fault-normal near-field and three far-field ground motion records were applied to the model. The numerical results show that near field velocity pulses have a considerable effect on the system behavior and sudden huge displacement demands were observed. Also, during the near-field ground motions, the exceeded pore water pressure coefficient (Ru) increases so that liquefaction occurs in the upper loose sand layer. Due to the pulse-like ground motions, a pulse-like relative displacement is created in response to the pile. Meanwhile the relative displacement response of the pile is entirely different due to the energy distribution during the far-field ground motions.

Screw piles are uniquely-shaped concrete piles with screw threads that have been widely used in various fields, including construction, structural design, and geotechnical engineering. Research on the dynamic characteristics of screw piles under vertical loads is limited compared with that investigating traditional circular piles. This report describes an analytical solution that has been developed to investigate the dynamic features of a screw pile under a longitudinal load while considering the cushion cap effect. The Laplace transform and Potential functions are applied to decouple the three-dimensional wave equations of the soil. The dynamic response of the screw pile is deduced using a modified impedance transfer function method. Finally, the cushion cap displacement and velocity in the frequency domain are determined by combining the initial conditions. The analytical solutions are compared with field-measured curves to validate the developed method. The results indicate that the soil around the pile can be regarded as a threedimensional continuous medium to simulate the radiation-damping effect as the wave propagates outward. The cushion cap reduces the screw pile damage caused by resonance, particularly in the low-frequency range. Considering the effects of vibrational loads, a screw pile should employ a large lightweight cushion cap, i.e., with the largest reasonable dimensions and with concrete materials that are as light as possible. The results of this study provide a theoretical basis for designing a dynamic foundation of a screw pile.

To reuse industrial solid wastes and waste clay with low liquid limit, a kind of soil solidification material by using cement, quicklime and industrial solid wastes such as ground granulated blast -furnace slag (GGBS), silica fume (SF) was developed in this study. Response surface methodology (RSM) based on central composite design (CCD) was used to design the experiment and optimize the mix ratio of GGBS, quicklime and SF under certain cement content conditions (i.e., the content ratio of cement, GGBS, quicklime, and SF was 5: 9.14: 1.7: 2.13). A soil solidification agent named O-QGS was developed to solidify waste clay with low liquid limit. To clarify the solidification mechanism of solidified soil, a series of laboratory experiments such as UCS test, water stability test, and scanning electron microscopy (SEM) test were carried out to capture the mechanical properties, water stability, and microstructure of O-QGS solidified soil and cement solidified soil. For practical purpose of O-QGS, a method for forming prefabricated pile by using O-QGS solidified soil was developed, and a method for strengthening soft foundations with prefabricated O-QGS solidified soil pile was proposed. Based on the results of load tests, the bearing capacity of prefabricated O-QGS solidified soil pile and cement high-pressure rotary jet grouting pile, as well as the composite foundations bearing capacity of prefabricated O-QGS solidified soil pile and cement high-pressure rotary jet grouting pile used for strengthening soft foundations, were analyzed. The feasibility of prefabricated O-QGS solidified soil pile used for strengthening soft foundations was verified in practice. The present study shows that the UCS of O-QGS solidified soil is 7.25 MPa at 28 days, and the water stability coefficient of O-QGS solidified soil is larger than 0.8. Compared with the method of cement highpressure rotary jet grouting pile to reinforce soft foundation, the bearing capacity of prefabricated O-QGS solidified soil pile to reinforce soft foundation is higher, and the cost can be saved by 22.4 %.