Torsional Vibration of Partially Exposed Pipe Piles Considered Soil-plug Effect in Viscoelastic Saturated Soil Expressed by Fractional Derivatives

Based on the viscoelastic theory expressed by fractional derivatives and the theory of porous media, the torsional dynamic control equations in the form of displacement for viscoelastic saturated soil expressed by fractional derivatives were obtained by considering the soil-plug effect of pipe piles, the torsionally dynamic interaction model to describe viscoelastic saturated soil expressed by fractional derivatives and partially exposed pipe piles was constructed. The equivalent stiffness and damping coefficients of the equivalent Winkler torsional spring-damper model were obtained by considering properties of fractional derivatives and Bessel Function, and boundary conditions of soil. Considering the effect of saturated soil around and in pipe piles, the torsional vibration of partially exposed pipe piles in saturated soil was solved by using the transfer matrix method, and the torsional stiffness factor and equivalent damping coefficient at tops pipe piles were obtained. The influences of mechanical and geometric parameters of pipe piles and soil on torsional vibration of pipe piles were discussed by numerical examples. The results showed that the effect of the geometric parameters for pipe piles on torsional vibration of partially exposed pipe piles in viscoelastic saturated soil expressed by fractional derivative was greater than that of mechanical properties for materials. When considering the soil-plug effect, the influence of soil viscosity could not be ignored on torsional vibration of partially exposed pipe piles in viscoelastic saturated soil, but the difference of properties between saturated soil around and in pipe piles could be ignored. The wall thickness of pipe piles was sensitive to the torsional vibration.