An integrated three-dimensional model of wave-induced pore pressure and effective stresses in a porous seabed: II. Breaking waves

The evaluation of the wave-induced liquefaction potential is particularly important for coastal engineers involved in the design of marine structures. Most previous investigations of the wave-induced liquefaction have been limited to two-dimensional non-breaking waves. In this paper, the integrated three-dimensional poro-elastic model for the wave-seabed interaction proposed by [Zhang, H., Jeng, D.-S., 2005. An integrated three-dimensional model of wave-induced pore pressure and effective stresses in a porous seabed: I. A sloping seabed. Ocean Engineering 32(5/6), 701–729.] is further extended to simulate the seabed liquefaction potential with breaking wave loading. Based on the parametric study, we conclude: (1) the liquefaction depth due to breaking waves is smaller than that of due to non-breaking waves; (2) the degree of saturation significantly affects the wave-induced liquefaction depth, and no liquefaction occurs in full saturated seabed, and (3) soil permeability does not only significantly affect the pore pressure, but also the shear stresses distribution.     

Note on the propagation of a shallow water wave in water of variable depth

In this technical note, the phenomena of non-linear water-wave propagation above a seabed with variable depth is re-examined. The conventional Korteweg-de Vries (KdV) equation is re-derived for the general case of variable water depth. In the new form of KdV equation, the seabed bottom function is included. Two different bottom profiles are considered in this study; case 1: $b^{\prime }\left(x^{\prime }\right)=c\epsilon \sin \lambda x^{\prime }$ and case 2: $b^{\prime }\left(x^{\prime }\right)=c\epsilon {\mathrm{e}}^{-\lambda {\left(x^{\prime }-x_0^{\prime }\right)}^2}$. The effects of three bottom profile parameters, c, λ and ε on the wave profile are examined. Numerical results indicate that both ε and λ affect the wave profile significantly in case 1, while ε significantly affects the wave profile in case 2.     

NON-LINEAR WAVE-INDUCED RESPONSE OF POROUS SEABED: A FINITE ELEMENT ANALYSIS

Conventional investigations of waves–seabed interaction problems have been only concerned with the soil response due to two-dimensional linear progressive waves over a uniform seabed. However, the effects of non-linear waves which have been reported in the literature may be significantly different. In this paper, a finite element model is developed to investigate the non-linear wave-induced seabed response with variable permeability and shear modulus in a three-dimensional domain. The finite element formulations are fully presented in this paper. The numerical model is verified with the previous investigations through the reduced form of the present solution. The numerical results indicate that the influence of non-linear wave components cannot always be ignored without substantial error. Furthermore, the wave-induced seabed response is affected significantly by variable permeability in coarser seabeds and variable shear modulus in finer seabeds. © 1997 by John Wiley & Sons, Ltd.     

The deformation of overburden soil induced by thrust faulting and its impact on underground tunnels

Overburden soil beds situated above a fault are often deformed by propagation of bedrock thrusting from the fault during large earthquake. The deformed beds formed a triangular shear zone. This coseismic faulting often causes damage to underground tunnels located in the shear zone. The present research studies the deformation behavior of the overburden soil beds and the tunnel, the associated mechanism and the impact on the safety of tunnel linings induced by a large blind thrust slip. Based on sandbox experimental and numerical studies, it is found that results from numerical analysis are in agreement with the sandbox model tests with regard to growths of the shear zones within the soil beds, location of the tunnel in this shear zone and deformations of the tunnel. The potential major shear zone may be bent or bifurcated into two sub-shear zones owing to existence of a tunnel inside the shear zone. Furthermore, the occurrence of back-thrust faulting will threaten the safety of nearby structures. It was also identified that stiffness of the soil and the fault dip angles are among the major factors controlling the configuration of shear zones, the stresses within the soil, and the loads on tunnel linings. Based on the identified mechanisms, the strategies for hazard prevention are accordingly suggested and discussed.     

Aliphatic hydrocarbon concentrations in short sediment cores from the southern Okinawa Trough: Implications for lipid deposition in a complex environment

Seven short sediment cores from the southern Okinawa Trough were collected and analyzed for the aliphatic hydrocarbon concentrations by capillary gas chromatography to explore the deposition of hydrocarbons to this area. For all cores studied, ratios of Σhydrocarbons/TOC, (nC₂₇+nC₂₉+nC₃₁)/TOC, terrigenous/aquatic, and diploptene/ΣC_25–33n-alkanes fluctuated around a mean value with coefficients of variation ranging from 9.0% to 19.7%, 4.9% to 20.0%, 27.3% to 129%, and 3.8% to 163%, respectively. For the nC₃₁/(nC₂₇+nC₂₉+nC₃₁) ratio, only station 21 showed fluctuation. Moreover, the carbon preference indexes in the C₂₅–C₃₃n-alkane range also exhibited fluctuating values with coefficients of variation of 1.9–14.4%. These results indicate that concentrations of hydrocarbon inputs to the sampling sites vary with time; this may result from complex current flow and sediment transport, leading to variable lipid deposition. In addition, significant correlation between diploptene (hop-22(29)-ene) and higher plant n-alkanes was found for cores 21, 42 and 46, indicating that diploptene was predominantly from higher plant sources. However, no correlation between diploptene and higher plant n-alkanes was found for cores 20, 36, 43 and 44; autochthonous sources of diploptene in these cores were quite probable.     

Dynamic response of a piecewise circular tunnel embedded in a poroelastic medium

Recently, considerable efforts have been devoted to evaluation of seismic dynamic response of a circular tunnel. Conventional approaches have considered integral liners embedded in an elastic medium. In this study, we re-examine the problem with piecewise liners embedded in a porous medium. Surrounding saturated porous medium of tunnels is described by Biot's poroelastic theory, while the liner pieces and the connecting joints are treated as curved beams and characterized by curved beam theories. The scattered wave field in the porous medium is obtained by the wave function expansion method. The differential equations governing the vibration of a curved beam is discretized by the General Differential Quadrature (GDQ) method. The domain decomposition method is used to establish the global discrete dynamic equations for the piecewise tunnel. The surrounding soil and the tunnel are coupled together via the stress and the displacement continuation conditions which are implemented by the boundary collocation method. Numerical results demonstrate that the stiffness difference between the liner piece and the connecting joints has a considerable influence on the internal forces of the liner piece.     

Monitoring active fault creep as a tool in seismic hazard mitigation. Insights from creepmeter study at Chihshang,Taiwan

In 1998 we installed five creepmeters across the Chihshang Fault, the active plate suture in eastern Taiwan. Daily creepmeter data indicated decreasing creeping rate from 1999 to 2003, suggesting increasing seismic hazard. The fault was ruptured by the Chengkung earthquake ($\mathit{Mw}=6.6$) on 10 December 2003. Through extrapolation of our earlier creep data of 1986–1991 and 1992–1997, we evaluate the minimum deficit in aseismic creep shortening as 106 or 46 mm (respectively) before this earthquake. The near-surface co-seismic shortening was limited, but the total shortening resulting from the earthquake, including post-seismic creep, was about 97 mm. This suggests that near the surface most of the detectable deficit has been absorbed by this earthquake and subsequent creep. We thus point out that creepmeter installation and monitoring bring a powerful tool in seismic hazard mitigation. To cite this article: J.-C. Lee et al., C. R. Geoscience 337 (2005).     

Beach water table fluctuations due to spring–neap tides: moving boundary effects

Tidal water table fluctuations in a coastal aquifer are driven by tides on a moving boundary that varies with the beach slope. One-dimensional models based on the Boussinesq equation are often used to analyse tidal signals in coastal aquifers. The moving boundary condition hinders analytical solutions to even the linearised Boussinesq equation. This paper presents a new perturbation approach to the problem that maintains the simplicity of the linearised one-dimensional Boussinesq model. Our method involves transforming the Boussinesq equation to an ADE (advection–diffusion equation) with an oscillating velocity. The perturbation method is applied to the propagation of spring–neap tides (a bichromatic tidal system with the fundamental frequencies ω₁andω₂) in the aquifer. The results demonstrate analytically, for the first time, that the moving boundary induces interactions between the two primary tidal oscillations, generating a slowly damped water table fluctuation of frequency ω₁−ω₂, i.e., the spring–neap tidal water table fluctuation. The analytical predictions are found to be consistent with recently published field observations.     

Dynamic response of a porous seabed around pipeline under three-dimensional wave loading

The evaluation of the wave-induced pore pressure around a buried pipeline is particularly important for pipeline engineers involved in the design of offshore pipelines. Most previous investigations of the wave-induced dynamic response around an offshore pipeline have limited to two-dimensional cases. In this paper, a three-dimensional model including buried pipeline is established, based on the existing DYNE3WAC models. Based on the proposed numerical model and poro-elastic soil material assumption, the effects of wave and soil characteristics, such as wave period, water depth, shear modulus and permeability, and configuration of pipelines, such as pipeline radius and pipeline buried depth, on the wave-induced excess pore pressure will be examined. Numerical results indicated that the normalized excess pore pressures versus z/h near the pipeline increase as the obliquity angle, wave period and water depth increase, and they decrease as the burial depth and radius of pipeline increase above the pipeline. Soil permeability has obvious influence on the wave-induced normalized excess pore pressure, and different soil material will result in distinct computation results.     

Application of artificial neural networks in tide-forecasting

An accurate tidal forecast is an important task in determining constructions and human activities in ocean environments. Conventional tidal forecasting has been based on harmonic analysis using the least squares method to determine harmonic parameters. However, a large number of parameters are required for the prediction of a long-term tidal level with harmonic analysis. Unlike conventional harmonic analysis, this paper presents an artificial neural network (ANN) model for forecasting the tidal-level using the short term measuring data. The ANN model can easily decide the unknown parameters by learning the input–output interrelation of the short-term tidal records. Three field data with three types of tides will be used to test the performance of the proposed ANN model. The numerical results indicate that the hourly tidal levels over a long duration can be predicted using a short-term hourly tidal record.