Promoting Sustainable Marine Development: Geotechnical Engineering Problems and Environmental Guarantee Technology in Marine Space, Energy, and Resource Development

To coordinate the conflict between economic development and climate change caused by energy consumption, countries worldwide are actively developing renewable energy, including solar energy, hydropower, and wind energy. Among all renewable energy sources, solar energy, hydropower, and onshore wind energy are limited by region and environment. The ocean is important in the development of renewable energy, including marine wind energy, wave energy, and tidal energy [1]. At the same time, various resources, including biological resources, oil, and gas, are rich in the marine environment [2].

In the development of marine resources, various offshore structures, including platforms, foundations, and seabed systems, are generally used [3]. The long-term supporting structures and foundations of these systems generally face geotechnical problems. For example, monopiles are widely adopted as the foundations for bottom-fixed wind turbines, and predicting their cyclic accumulation deformation is a challenge in design [4]. For the jacket foundation, the load distributions of different piles are important to calculate the structure inclination angle, among which the t–z curve, considering the stiffness degradation, plays an important role. For the suction caisson, the assessment of installation and bearing capacity is the focus of the design. In addition, the whole-life concept should be considered in the anchor design when analyzing their long-term service [5]. All the above issues are related to the seabed–structure interaction. For example, interface shear strength influences the resistance in foundation installation, foundation capacity, pipeline–soil interaction, and mooring line–seabed friction [6].

From another perspective, the characteristics of strata influence the construction of offshore engineering and the sustainability of the marine environment. Carbonate sands with high void rates, irregular particle shapes, and fragile characteristics are often encountered in practice [7], and their stability under dynamic loads, e.g., earthquakes and waves, is of significance in engineering [8]. In addition, other geohazards can appear in the subsea environment, such as landslides, which destroy all kinds of seabed structures, e.g., pipelines, mooring lines, anchors, etc. [9]. The marine environment is variable, and the wave as a periodic load acting on the seabed and structures increases the loads on the structures and liquefies the shallow sand, which is completely different from the onshore loads.

Therefore, a full understanding of these geotechnical problems is necessary to solve these related issues. To promote the sustainable development of marine space, energy, and resources, this Special Issue focuses on the geotechnical problems in marine technology development. A total of 5 peer-reviewed articles have been published in this Special Issue out of 12 submitted manuscripts.

It is known that the wave-breaking process is significantly affected by a current, but little attention has been paid to the effect of wave–current interaction on the breaking wave forces acting on a monopile. A total of 88 flume tests, among which solitary and regular breaking waves were generated with a following current, are presented. The waves propagated over an impermeable slope and induced impulsive loads on a vertical monopile. The moments on the monopile were measured utilizing a high-precision load cell, and the effect of current velocities on the peak moment was analyzed. Test results indicate that there was an obvious nonlinear effect between breaking waves and a following current. For solitary waves, a following current accelerated the breaking process, leading to an increase at maximum in breaking wave forces. However, for regular waves, both the wave heights and the reversing flow were restricted with the increasing velocity of a following current, delaying the wave-breaking process (Contribution 1). The wave loads also cause the cyclic motion of floating structures, which need a mooring system, including anchors and mooring lines, to be positioned. The mooring line cyclic motion induced by the wave-structure interaction repeatedly cuts the seabed, which leads to seabed trenches in front of the anchors [10]. Seabed trenches reduce anchor capacity, but the adverse influence is not well considered in the current design. A framework for mooring and anchor design in sand considering seabed trenches based on floater hydrodynamics was developed. First, a hydrodynamic calculation of the floater coupled with the mooring system was conducted. Then, the potential trench profile was assessed using a mooring line–seabed dynamic model. Third, after assessing the suction anchor performance based on its installation and capacity, a refined anchor, a caisson–plate gravity anchor (CPGA), was proposed, and the capacity mechanisms were analyzed (Contribution 2). Offshore slope stability is of significance to the structures above, but it is influenced by slope erosion. Sandy slope erosion leads to coast degradation and exacerbates coastal zone instability and failure. As an eco-friendly engineering technology, microbial-induced calcium carbonate precipitation (MICP) can provide a protection method against sandy slope erosion. In this study, a series of flume tests were conducted to investigate the wave erosion resistance of the MICP-treated sandy slope. The penetration tests were conducted to measure the slope surface strength, and the calcium carbonate content was evaluated by the acid-washing method. The scanning electron microscope (SEM) was employed to study the microstructures of MICP-treated sand particles. In addition, the influence of MICP treatment on the wave shape and the excess pore water pressure was also analyzed (Contribution 3). Near the offshore region, the infrastructure construction related to tunnels used for the subway metro developed rapidly [11]. The impact of soil heterogeneity on the bridge piles beneath a nearby tunnel excavation was assessed using Monte-Carlo stochastic analysis. A sensitivity analysis was specifically used for the variation of stratum range, variation coefficient (COV), and fluctuation distance of Young’s modulus of the soil. Meanwhile, the reliability evaluation approach was also applied to systematically examine the impact of the COV on the likelihood of a pile failing. The findings suggest that more consideration should be given to the degree and range of geological parameter variations in the strata surrounding the tunnel. The fluctuation range influences the frequency of low-stiffness zones in the soil. Additionally, the variation coefficient has a significant effect on pile deformation, presenting a positive association. The pile deformation exhibits an increasing tendency in the wake of the growing variation coefficient. More significantly, the increase in the COV will directly lead to a rising failure probability of the pile settlement. It is quite necessary to attach importance to the soil heterogeneity effects in the pile foundation stability under construction disturbance (Contribution 4). The effect of freeze–thaw cycling on a cement-based early strength anchor material, the compressive strength of which at 24 h is 14 times that of ordinary cement, was found through indoor freeze–thaw cycling tests. The appearance changes, quality loss, and strength loss reveal the evolution of the mechanical properties and micro-mechanisms of the cement-based early strength anchor material. The sample freeze–thaw failure criteria were determined, evaluation indicators reflecting the degree of damage were defined, and their relationships with the number of freeze–thaw cycles were fitted to assess the durability of the cement-based early strength anchor material. This provides a theoretical reference for further improvements in material properties in different environments (Contribution 5).

In summary, though some specific problems have been partly solved, the geotechnical engineering problems and environmental guarantee technology in marine space, energy, and resource development still need more attention and investigation. Solving the problems related to offshore geotechnics can help establish a more sustainable world.