The generation of excess pore water pressure (EPWP) and liquefaction characteristic of soils under seismic loading have long been topics of interest and ongoing discussion. Based on the structural state exhibited in the liquefaction process, the mechanical property of saturated coral sand is divided into solid, pseudo-fluid, and liquid phases. New indices, zeta q (generalized deviator strain evolution) and zeta(y)q (generalized deviator strain evolution rate), are proposed to evaluate the evolution and evolution rate of complex deformation. In the solid phase, the saturated coral sand primarily exhibits the properties of a continuous solid medium, the peak EPWP ratio (rup) shows a power correlation with generalized deviator strain evolution amplitude (zeta qa). While in the pseudo-fluid phase, the saturated coral sand primarily exhibits mechanical behavior characteristic similar to that of a fluid, and the rup shows a significant arctangent function relationship with generalized deviator strain evolution rate amplitude (zeta(y)qa). The correlation of rup with zeta qa and zeta' qaduring liquefaction is significantly affected by loading conditions (cyclic stress ratio, CSR, loading direction angle, alpha sigma, and loading frequency, f). To quantify the impact of these loading conditions on the generation of rup in different phases, unified indicators delta S (for the solid phase) and delta L (for the pseudo-fluid phase) are defined. Eventually, An EPWP model based on mechanical property exhibited in different phases is developed, which has normalized the effects of loading conditions. It provides a comprehensive framework to predict the rup of saturated coral sand under complex geological activities, and this model facilitates the understanding and simulation of the mechanical properties and behavior of saturated coral sand during the liquefaction process.

Evaluating the stability of coral islands and reefs in dynamic marine environments, such as waves, tsunamis, storm surges, and earthquakes, is a critical scientific issue in the field of marine geotechnical engineering. Nansha coral sand was used as the study object, and stress-controlled drained and undrained cyclic-loading tests were conducted. The undrained excess pore-water pressure and the drained cumulative volumetric strain of saturated coral sand were determined at various non-plastic fine contents (FC), relative density (D-r), and cyclic stress ratio (CSR). The results indicated that cumulative volumetric strain (epsilon(vp)) developed in coral sand via two modes: cyclic stabilisation and cyclic creep. Analyses revealed that when the potential damage coefficient (DP) x CSR 0.05, epsilon(vp) transitioned into the cyclic creep mode. Utilising cumulative dissipation energy as a linking factor showed an arctangent function relationship between the excess pore water pressure ratio (R-u) and epsilon(vp) values of saturated coral sand with different FC, D-r, and CSR. This relationship was applicable to both stress- and strain-controlled cyclic-loading tests. Parameters m and n of the R-u-epsilon(vp) function model increased with an increasing CSR. Additionally, an increase in the D-r or FC resulted in a decrease in m and an increase in n. Multiple regression analysis further revealed that model parameters corrected for compactness and cyclic stress levels exhibited distinct trends as the void ratio (e) increased. Specifically, CSR alpha x m(D)(R) decreased, and CSR1-alpha x n(D)(R) increased. Both parameters displayed a single power function relationship with e. Based on these findings, a coupled incremental model for the cyclic pore pressure and volumetric strain of saturated coral sand, based on energy conversion, was developed.

The biocemented coral sand pile composite foundation represents an innovative foundation improvement technology, utilizing Microbially Induced Carbonate Precipitation (MICP) to consolidate a specific volume of coral sand within the foundation into piles with defined strength, thereby enabling them to collaboratively bear external loads with the surrounding unconsolidated coral sand. In this study, a series of shaking table model tests were conducted to explore the dynamic response of the biocemented coral sand pile composite foundation under varying seismic wave types and peak accelerations. The surface macroscopic phenomena, excess pore water pressure ratio, acceleration response, and vertical settlement were measured and analysed in detail. Test results show that seismic wave types play a decisive role in the macroscopic surface phenomena and the response of the excess pore water pressure ratio. The cumulative settlement of the upper structure under the action of Taft waves was about 1.5 times that of El Centro waves and Kobe waves. The most pronounced liquefaction phenomena were recorded under the Taft wave, followed by the El Centro wave, and subsequently the Kobe wave. An observed positive correlation was established between the liquefaction phenomenon and the Aristotelian intensity of the seismic waves. However, variations in seismic wave types exerted minimal influence on the acceleration amplification factor of the coral sand foundation. Analysis of the acceleration amplification factor revealed a triphasic pattern-initially increasing, subsequently decreasing, and finally increasing again-as burial depth increased, in relation to the peak value of the input acceleration. This study confirms that the biocemented coral sand pile composite foundation can effectively enhance the liquefaction resistance of coral sand foundations.

Revetment breakwaters on reclaimed coral sand have demonstrated vulnerability to seismic damage during strong earthquakes, wherein soil liquefaction has been identified as a substantial contributor. Based on the results of three centrifuge shaking table tests, this study investigates the characteristic seismic response of revetment breakwater on reclaimed coral sand and the influence of soil liquefaction. The basic mechanical properties of reclaimed coral sand were measured using undrained triaxial and hollow cylinder torsional shear tests. The centrifuge test results indicate that liquefaction of coral sand can result in significant displacement and even failure of revetment breakwaters, encompassing: (a) tilting, horizontal displacement, and settlement of the crest wall; (b) seismic subsidence in the foundation and backfill. The liquefaction consequence of the reclaimed coral sand increased with a decrease in soil density and rise in sea water level (SWL). Post-earthquake rapid reinforcement measure via sandbags is found to be effective in limiting excess pore pressure buildup in foundation soil and structure deformation under a second shaking event. Based on the test results, the effectiveness of current simplified design procedures in evaluating the stability and deformation of breakwaters in coral sand is assessed. When substantial excess pore pressure generation and liquefaction occur within the backfill and foundation coral sand, the pseudo-static and simplified dynamic methods are inadequate in assessing the stability and deformation of the breakwater.

Coral sand is characterized by low cohesion and high porosity, posing a potential liquefaction risk. Thus, coral sand stabilization is necessary in coastal construction projects. Polyurethane, with its excellent toughness, rapid reaction speed, and strong adhesive properties, is an ideal choice for reinforcing coral sand. However, the diffusion range of non-water reacting foamed polyurethane in coral sand is limited. This study explored the use of water-reacting polyurethane (PRP) to solidify coral sand. PRP is known for its high permeability and bonding strength. Despite its potential, the dynamic mechanical properties and reinforcing mechanism of PRP-solidified coral sand, which are crucial for site seismic analysis and seismic design, have not yet been fully understood. Thus, the resonance column and uniaxial compression tests were conducted to investigate the variations in dynamic shear strain, dynamic shear modulus, damping ratio, and uniaxial compressive strength of the solidified material under different confining pressures, PRP incorporation ratios, and mass moisture contents of coral sand. To further investigate the underlying mechanisms of the variations in its mechanical properties, scanning electron microscopy (SEM) and mercury intrusion tests were conducted to analyze the morphology and pore characteristics of the PRP-solidified material. The results show that, at a constant moisture content, increasing the PRP proportion enhanced the dynamic shear modulus, damping ratio, and uniaxial compressive strength of the coral sand. However, excess moisture content reduced these properties. The pore ratio decreased with the increase of PRP and moisture content, with a larger reduction before drying and a smaller one after drying. The tortuosity of the specimens was mainly affected by the incorporation ratio of PRP, which increased with the increase of the incorporation ratio. However, the moisture content of coral sand had a fewer effect on the tortuosity. The permeability gradually decreased with the increase of the PRP incorporation ratio and the moisture content of coral sand. PRP strengthened the coral sand, primarily through its covering, filling, and bonding effects, enhancing the friction and mechanical occlusion. These findings are significant for the applications of PRP in future coastal engineering projects.

Vibroflotation has proven to be an effective method for treating loose and unevenly graded coral sand foundations formed through hydraulic filling. In this study, a series of model tests were conducted to investigate the effects of particle gradations on the response of coral sand foundation reinforced by vibroflotation. The main focus was on analyzing the changes in excess pore water pressure (EPWP) and horizontal earth pressure. Cone penetration tests (CPTs) were then used to evaluate the effectiveness of vibroflotation. The results indicate that the maximum settlement occurs after the first vibroflotation, with surface settlement significantly increasing as the distance to the vibro-point decreases. The reinforcement range expands radially, and the foundation can achieve a medium or dense state after vibroflotation. During the penetration stage, the EPWP rapidly peaks and increases with depth. Shallow foundations exhibit a higher excess pore pressure ratio compared to deep foundations. Foundations with lower coarse particle content show higher EPWPs compared to those with higher coarse particle content. Lower vibration frequency results in diminished reinforcement effects in foundations with high coarse particle content and increases the difficulty of penetration. Additionally, the residual soil pressure in foundations with high coarse particle content significantly rises after three vibroflotation reinforcements. The increase in strength after reinforcement is more pronounced because the foundation has a greater coarse particle content. The reinforcement effect diminishes with increasing distance from the vibrator.

Liquefaction and earthquake damage to coral sand sites can cause engineering structure failure. Both testing and analyzing the seismic response characteristics of pile groups on coral sand sites are highly important for the seismic design of engineering structures. To address the lack of research on the seismic dynamic response of group pile foundations in coral sand sites, this study analyzes the characteristics of the seismic dynamic response of vertical and batter pile foundations for bridges in coral sand liquefaction foundations via the shaking table model test and investigates the variation patterns of acceleration, excess pore water pressure (EPWP), and the bending moment and displacement of foundations, soil, and superstructures under different vibration intensities. Results show that the excitation wave type significantly affects liquefaction: at 0.1 g of peak acceleration, only high-frequency sine wave tests liquefied, with small EPWP ratios, while at 0.2 g, all tests liquefied. Vertical pile foundations had lower soil acceleration than batter piles due to differences in bearing mechanisms. Before liquefaction, batter piles had smaller EPWP ratios but experienced greater bending moments under the same horizontal force. Overall, batter piles showed higher dynamic stability and anti-tilt capabilities but endured larger bending moments compared to vertical piles in coral sand foundations. In conclusion, batter pile foundations demonstrate superior seismic performance in coral sand sites, offering enhanced stability and resistance to liquefaction-induced failures.

To date, numerous coral sand revetment breakwaters have been constructed in oceanic regions to resist wave impact and scour. However, frequent earthquakes significantly threaten their stability, especially during mainshock-aftershock sequences, where aftershocks can further exacerbate the risk of damage or collapse. This study proposes a reinforcing countermeasure, i.e., geosynthetics reinforced soil technique, to mitigate seismic deformation and enhance the resilience of revetment breakwaters against earthquakes. A series of shaking table tests were conducted on coral sand revetment breakwaters to examine the effect of geogrid reinforcement on their seismic performance under mainshock-aftershock sequences. Additionally, the reinforcement mechanism of geogrid was elucidated through supplementary cyclic triaxial tests. The results indicate that acceleration amplification intensifies during aftershocks, while geogrid reinforcement mitigates this detrimental effect. The inclusion of geogrid also decreases the buildup of excess pore water pressure (EPWP) under mainshockaftershock sequences. Coral sand shear dilation results in the generation of notable negative EPWP within revetment breakwaters, and more significant negative EPWP oscillation, compared to the aftershocks, is observed in the mainshock. Additionally, geogrid decreases the maximum cumulative settlement in reinforced revetment breakwaters by over 54 % compared to unreinforced structures. The cumulative damage induced by aftershocks exacerbates the damage to coral sand revetment breakwaters, leading to the emergence and rapid progression of lateral displacements. Nevertheless, geogrid reinforcement mitigates this adverse effect and prevents the formation of plastic slip planes, thereby altering the deformation pattern of the revetment breakwater subjected to mainshock-aftershock sequences. Overall, geogrid reinforcement is found to be highly effective in enhancing the stability of coral sand revetment breakwaters against mainshock-aftershock sequences and holds promising applications in infrastructure construction in coral sand island and reef areas.

Coral sand, as a geological material for foundation filling, is widely used for reclamation projects in coral reef areas. The coral sand is characterized by a wide grain size distribution. A series of centrifuge shaking table tests were conducted to explore the seismic response of a shallow buried underground structure in saturated coral sand and coral gravelly sand. The emphasis was placed on comparing the similarities and differences in the dynamic behavior of the underground structure at the two sites. The responses of excess pore pressure, acceleration, displacement, and dynamic soil pressure of the structure were analyzed in detail. The results indicated that the underground structure in coral sand had a significant influence on the development of excess pore pressure in the surrounding soil, but this effect was not evident in coral gravelly sand due to well-drained channels. Liquefaction was observed in the soil layer around the structure in coral sand, but it did not occur in coral gravelly sand. In coral sand, the liquefaction of the soil layer at the bottom of the structure caused a significant attenuation in the acceleration of the structure. Compared to coral gravelly sand, the acceleration response of the soil layer near the bottom of the underground structure was higher in coral sand. During the shaking, the displacement pattern of the structure in coral gravelly sand was slight subsidence-slight upliftsignificant subsidence, while it exhibited a significant uplift in coral sand. The maximum dynamic soil pressure distribution on the structural sidewalls presented a trapezoidal distribution, and the dynamic soil pressure had a strong connection with the development of excess pore pressure in the surrounding soil.

In marine environments, cyclic loads induced by earthquakes can lead to complex soil responses in marine coral sand. Waves and storms, often at different frequencies, can also contribute to these responses. These factors can finally contribute to instability or failure of offshore structures. To better understand the effect of loading frequency on the dynamic properties of marine coral sand, a series of cyclic triaxial tests on saturated coral sands were carried out. These tests were performed with different gradations at different loading frequencies and loading modes. A GDS dynamic triaxial instrument was used for the tests. The experimental results demonstrate that loading frequency has a significant effect on the cyclic response of coral sand. The maximum shear modulus of saturated coral sand rises with increasing loading frequency. The cyclic strength of saturated coral sand also increases with loading frequency. A strong linear relationship exists between the maximum shear modulus and cyclic strength. This suggests the existence of a cyclic yield strain that is relatively insensitive to loading frequency. Loading frequency significantly affects the axial strain development of saturated coral sand under diverse loading modes. Three stages of axial strain development were identified employing incremental strain analysis. Based on these findings, a new model for axial strain development is proposed, the accuracy of this model is verified by fitting it to the data from this study and existing literature.