Analytical approach for lateral dynamic behaviours of pipe piles enhanced by cement-improved soil in unsaturated ground

Pipe piles enhanced by cement-improved soil (hereinafter referred to as enhanced pipe piles) have excellent bearing capacity compared with traditional piles and are often used as the foundation of offshore wind turbines and coastal soft-soil embankment. This study aims to clarify and gain an insight into the lateral dynamic response of enhanced pipe piles in unsaturated soil by developing an analytical approach. In the proposed approach, the enhanced pipe pile divides into two parts: The first part is considered as a composite pile formed by a concrete pipe pile and a cement-soil mixing pile through high-strength bonding, and the second part formed by a concrete pipe pile and an unsaturated soil column. The lateral vibration behaviours of the enhanced pipe pile and the unsaturated soil resistance are deduced by the Euler–Bernoulli beam theory and the porous viscoelastic theory of three-phase mixture, respectively. The closed-form solutions for the horizontal, rocking and horizontal-rocking dynamic impedances at the pile head of enhanced pipe pile under horizontal dynamic loads have been determined and then validated by comparing with the existing results. Numerical discussions are finally conducted to analysis the influence of physical parameters of enhanced pipe pile and unsaturated soil on the three types of dynamic impedance at the pile head. The main findings can be summarized as: (a) For the cement-soil mixing pile, its length should not exceed half of the concrete pipe pile, its radius size should be moderate and its elastic modulus can be as large as possible; (b) the wall thickness and elastic modulus of the concrete pipe pile can be appropriately increased to make the enhanced pipe pile achieve better vibration resistance and (c) the increase of the soil saturation will reduce the anti-vibration ability of enhanced pipe piles.