In-situ investigation on installation mechanisms of pre-bored grouted planted piles in deep and thick sand layers

Due to merits of low noise and vibration, the pre-bored planted method has been widely used in engineering practice to assist the installation of the prestressed high-strength concrete (PHC) piles in area where deep and thick sand layers are commonly encountered. However, the research on the influence mechanism of pile driving methods in hard soil layers mainly focuses on the static pressing method and the hammering method, while the penetration characteristics under pre-bored assistance remains unclear. Therefore, a full-scale field test was conducted to investigate the installation effects. The penetration mechanisms of the piles with pre-bored assistance in areas with a deep and thick sand layer was discussed. Besides, an analytical method was proposed to predict the energy (hammer blow counts) necessary to drive pile to design depth. The results showed that the pre-bored grouted planted method significantly reduced the energy required for pile driving, compared to both the hammering method and the pre-bored planted method. This was because pre-drilling reduced displaced soil volume and relieved lateral stress. The pile load transfer mechanism was optimized by pre-drilling, which effectively leveraged the high compressive strength of the deep and thick sand layers. Meanwhile, the shaft friction within the grouted depth was significantly enhanced by the permeation, cementation and reinforcement effects of the concrete slurry, thereby improving the pile bearing capacity. Furthermore, the analytical method proposed based on the cavity expansion theory was capable of predicting the energy required for pile driving, which could guide the selection of pile driving equipment with minimum energy consumption and prevent pile damage caused by excessive hammering.