Based on the multiphase poroelasticity theory, the reflection characteristics of an obliquely incident acoustic wave upon a plane interface between overlying water and a gassy marine sediment layer with underlying elastic solid seabed are investigated. The sandwiched gassy layer is modelled as a porous material with finite thickness, which is saturated by two compressible and viscous fluids (liquid and gas). The closed-form expression for the amplitude ratio of the reflected wave, called reflection coefficient, is derived theoretically according to the boundary conditions at the upper and lower interfaces in our proposed model. Using numerical calculation, the influences of layer thickness, incident angle, wave frequency and liquid saturation of sandwiched porous layer on the reflection coefficient are analysed, respectively. It is revealed that the reflection coefficient is closely associated with incident angle and sandwiched layer thickness. Moreover, in different frequency ranges, the dependence of the wave reflection characteristics on moisture (or gas) variations in the intermediate marine sediment layer is distinguishing.

This study investigates the influence of subsequent tunnel boring machine (TBM)-driven processes on the responses of the first tunnel in twin-tunnel construction using the ultra-rapid underground pass (URUP) method. A comprehensive finite element analysis (FEA) is performed to simulate the URUP TBM tunneling, considering the non-uniform convergence caused by the TBM geometry, the tunnel face supporting pressure, and the tail-grouting pressure. The FEA model is validated by the monitoring results of the bending element of the first tunnel lining. The FEA results reveal that the grouting pressure of the second tunnel has significant influence on lining deformation of the first tunnel, while the face supporting pressure shows little effect. The relationship between the grouting pressure and the maximum bending moment of adjacent first tunnel can be fitted by linear function. A grouting pressure equals to the lateral earth pressure is able the reduce the variation of the bending element of the first tunnel during the TBM-driven process of the second tunnel. The bending element of the first tunnel shows a typical lognormal relationship with the face supporting pressure during the TBM advance of the second tunnel. A critical cover-to-depth ratio, under which the horizontal and vertical soil arching effect vanishes, can be deduced to be within the range of 0.55–0.60.

Earthquake-induced hydrodynamic pressure on the ocean floor is a key factor in evaluating the surface disturbance and dynamic response of the seabed under the action of a submarine earthquake. An analytical study considering the compressibility of seawater was carried out to obtain the closed-form solution for the hydrodynamic pressure, which depends on water depth, excitation frequency, and seabed characteristics. The commonly used approximate expression for the hydrodynamic pressure, which neglects the compressibility of seawater and is associated with the dynamic displacement of the seabed surface, is presented and compared with the analytical solution. The results reveal that the formulation neglecting the compressibility of overlying seawater could underestimate the hydrodynamic pressure at the ocean bottom and the induced dynamic responses in a poroelastic seabed. Finally, a modified formula for the seismic-induced hydrodynamic pressure at the ocean bottom interface, which is able to consider the compressibility of seawater, is proposed by incorporating the nondimensionalized frequency.