Advances in Marine Engineering: Geological Environment and Hazards

1. Introduction

With the continuous advancement of coastal, offshore, and deep-sea engineering construction (e.g., marine oil, gas, and mineral resource development, offshore wind power projects), associated studies on marine geological environments and hazards have also advanced in parallel. This Special Issue (SI) was organized by Prof. Xiaolei Liu from the Ocean University of China, Prof. Thorsten Stoesser from University College London, and Dr. Xingsen Guo from University College London to document research advances in specific topics of marine engineering. The objective of this SI “Advances in Marine Engineering: Geological Environment and Hazards” is to collect high-quality papers in this field, including marine geological environments, marine geological hazards, marine engineering geology, marine hydrodynamics, marine fluid mechanics, and marine geotechnical engineering.

The SI includes 1 review paper [1] and 13 research papers [2,3,4,5,6,7,8,9,10,11,12,13,14] that cover different aspects related to the subject. A total of 95 scholars from universities, research institutions, and industries have contributed to this SI. The high-quality papers collected in this SI involve many classic and innovative methodologies in this research field, including, but not limited to, analytical and statistical analyses, numerical simulations, laboratory testing, and marine surveys. Specifically, some researchers carried out in situ marine surveys and monitoring [2,3], some implemented discrete element and finite element numerical simulations [4,5,6], some conducted indoor unit tests [7,8,9,10,11,12], some developed laboratory model tests [13,14], and others applied analytical and statistical methods [1,6,7,8]. These papers represent the state-of-the-art in the latest research concepts, advanced methods, and data that will contribute greatly to the development of the field of marine geological environments and hazards. The highlights of each paper in this SI are synthesized in the following section.

2. Papers Details

Shan et al. [1] systematically reviewed the past literature related to submarine landslides and proposed three stages of submarine landslide disaster evolution, namely, the submarine slope instability evolution stage, the large deformation landslide movement stage, and the stage of submarine landslide deposition. Based on these, they revealed the conversion mechanism of the submarine landslide disaster chain from the perspectives of in situ investigation techniques, physical simulation methods, and numerical simulation methods, respectively. Their study provides a useful reference for future research in the field of submarine landslides.

Liu et al. [2] developed a high-resolution three-dimensional seismic detection system for detecting gas hydrates. This system consists of a 50 kJ ultrahigh-energy plasma source, two high-resolution digital seismic streamers with a group interval of 3.125 m, and a seismic acquisition system, which can detect anomalies such as bottom simulating reflection, the blanking zone, velocity anomaly, and the polarity reversal of gas hydrates. Their acquisition tests carried out in the northeastern part of the South China Sea show that the system has obvious advantages in terms of the frequency band range and minimum acquisition binning, which can provide technical support for highly targeted gas hydrate exploration and development and a fine description of the ore bodies.

Hua et al. [3] found a sediment wave field on the western slope of the Chukchi Rise in the Arctic Ocean using multibeam bathymetric data and high-resolution shallow sub-bottom profiles acquired during China’s 10th Arctic Scientific Expedition Cruise in 2019. They conclude that these sediment waves are formed by contour currents and result from the interaction between continental slope bottom currents and the seabed; the sediment waves are still active, and their formation seems to date back to the Middle Pleistocene; and the sediment waves are formed by a bottom current, which may be genetically related to the Arctic Circumpolar Boundary Current flowing through the study area, aiding our understanding of the characteristics of the bottom current, thereby providing useful information on the Arctic Ocean environment.

Pang et al. [4] proposed nonlinear prediction methods for the slope effect on the initial stiffness of p-y curves of the undrained laterally loaded pile. They analyzed the slope effect on the initial stiffness of a laterally loaded pile based on the pile–slope position relationship, conducted a series of finite-element analysis simulations to calculate the slope effect, and calibrated parameters in the models using the results of the finite-element analysis. The results show that the methods they proposed are in a reasonable range and can predict more cases.

Jiang et al. [5] studied the mechanical properties of natural gas hydrate sediments under dynamic loading. They carried out triaxial compression tests on the discrete element specimens of hydrate sediments under static loading by using a particle flow code, verified the numerical method by some comparison, and determined the reasonable simulation parameters. Subsequently, multiple triaxial compression tests on hydrate sediment specimens under cyclic loading with different frequencies and amplitudes were carried out, revealing the influence of dynamic loading on their macro–meso mechanical properties. The results of their study are useful for the understanding of the dynamics of hydrate sediments and the prevention and control of submarine geohazards.

Chen et al. [6] derived the analytical solutions of a steady seepage field for a deep-buried tunnel with grouting ring considering anisotropic flow, and the analytical solutions are validated by numerical simulations. Subsequently, they carried out parameter analysis and the results show that (1) the seepage field of surrounding rocks around the deeply buried circular tunnel is no longer uniformly distributed and presented an elliptical distribution, (2) the difference between the permeability coefficient of the lining structure and the surrounding rock is small, (3) the change in the permeability coefficient of the lining structure has a significant influence on the hydraulic head, and 4) the size of the grouting ring has a more significant influence on the grouting effect.

Chen et al. [7] used the extreme gradient boosting (XGBoost) algorithm to predict the sound speed of the seafloor sediments based on the acoustic and physical data of the typical seafloor sediment samples from the East China Sea. They conducted the super parameter selection and contribution of the characteristic factors and established an XGBoost model based on five physical parameters. Moreover, they compared the machine learning prediction method with the traditional prediction method, demonstrated that the XGBoost prediction method for the sound speed of seafloor sediments was superior to the traditional single- and two-parameter regressional equation prediction methods in the study area, and provided the accuracy, R2, mean absolute error, and mean absolute percent error under the optimal model.

Li et al. [8] proposed a novel approach to hydrochemical analysis combined with the health risk model and the water quality index to explain the hydrochemistry characteristics and risks to human health of groundwater in the Guangdong coastal areas, and determined the average concentration of the total dissolved solids, the quality of water, the dominant hydrochemical types, and the main influencing factor of the hydrochemical composition. Using principal component analysis, they found that seawater intrusion and anthropogenic inputs also have an effect on the water quality. They evaluated the drinking quality of the water in the Guangdong coastal cities, and the result shows that most of the groundwater samples were acceptable for drinking except for samples of Maoming, Zhanjiang, and Shantou, and also found that children in the study area are at more danger to health risks than adults.

Wang and Zhang [9] used a dynamic triaxial apparatus to conduct long-term drainage cyclic loading tests for the unsaturated calcareous sand subgrade, and studied the cumulative strain development law of calcareous sand under different confining pressures, dynamic stress amplitudes, consolidation stress ratios, subgrade soil compactness, load frequencies, and moisture contents. They proposed a prediction formula combined with a hyperbolic model for the permanent deformation of calcareous sand under aircraft loading, providing a theoretical basis for the construction of a calcareous sand subgrade in the Nanhai island and reef project.

Gu et al. [10] investigated the low-temperature effect on the undrained shear strength of undisturbed marine clay samples from the South China Sea using a full-flow penetrometer. They found that the undrained shear strength at an in situ temperature (4 °C) is bigger than at room temperature (20 °C), which is valuable for offshore geotechnical engineers. In addition, they explained the effect of a low temperature on the undrained shear strength of marine clay samples under the framework of microscopic soil mechanics, which also provides a good insight for explaining some phenomena that microscopically changes the effect of the macroscopic physical and mechanical properties of marine sediments.

Wang et al. [11] investigated the acoustic properties of three fine-grained sediment types in the high-frequency range of 27–247 kHz in the South China Sea and established the relationship between the sound speed and attenuation dispersion of fine-grained sediment. Their study showed that the sound speed dispersion is a positive linear relation at the measured frequency range, and the attenuation follows nonlinear frequency dependence. In addition, they demonstrated that the Biot–Stoll model has a good applicability to different types of sediments in a high-frequency range.

Yang et al. [12] investigated the physical and mechanical properties of samples taken from the Shenhu sea area in the South China Sea (SCS), particularly normalized stress–strain behavior and microstructural and mineralogical characterization, using a series of basic geotechnical tests, triaxial compression tests, scanning electron microscopy (SEM), and X-ray diffraction (XRD). They discussed the factors and conditions for normalizing the stress–strain relationship in soft soil based on the hyperbolic equation of the stress–strain relationship proposed by Konder, applied the normalization factors to the consolidation undrained test of soft soil in the SCS, and established normalized models to predict the stress–strain relationship.

Yu et al. [13] investigated the cyclic shear stress–strain mechanical properties of the silt–steel interface in the Yellow River Delta, China. Their study carried out the constant normal load cyclic shear test (CNL) with a large interface shear apparatus and focused on the effects of normal stress, shear amplitude, roughness, and water content on the profiles of the shear–strain relationship and volumetric deformation characteristics. Based on the modified hyperbolic model, they also built a mathematical model that can describe the stress–strain relationship of the silt–steel interface in the Yellow River Delta.

Fu et al. [14] developed a novel experiment flow loop and presented the rheological experiments of CO₂ hydrate slurries. Their experiments verified that CO₂ hydrate slurry is a power-law fluid that exhibits a strong shear-thickening behavior and revealed the effects of a hydrate concentration and shear rate on the rheological properties of CO₂ hydrate slurry. By correlating with the experimental data, they developed a new Herschel–Bulkley-type model to describe the rheological properties of CO₂ hydrate slurry with a high accuracy.