Search Results (4)

Mega land reclamation projects have been carried out on the coral reefs in the South China Sea. Coral sand was used as a backfill material through hydraulic filling, with fill heights ranging from 6 to 10 m. To enhance foundation stability, vibro-flotation and impact rolling have been employed. However, the uneven distribution of coral sand, irregular particle shape, lower single-particle strength, and paucity of engineering cases for reference have posed challenges in evaluating the effectiveness of these foundation treatments. In this study, the effectiveness of vibro-flotation and impact rolling on the densification and bearing capacity of coral sand foundations has been investigated. In situ tests, including the plate load test, California Bearing Ratio (CBR) test, density measurements, dynamic penetration test (DPT), and settlement monitoring, were conducted at four distinct zones: an untreated zone, a vibro-flotation zone at a 5 m depth, a vibro-flotation zone at a 10 m depth, and an impact rolling zone. The findings suggest that coral sand exhibits promising characteristics for foundation construction. Seepage and self-weight consolidation following land reclamation formation significantly enhance the compaction degree of the coral sand foundation, thereby meeting the requirements for areas with lower bearing capacity demands. Both vibro-flotation and impact rolling techniques could significantly enhance the foundation-bearing capacity, with marginal differences between them. Since the machinery is simple and construction speed is quick, the impact rolling method is considered to be the most efficient for the treatment of coral sand foundation. The DPT results suggest that the reinforcement effect of both vibro-flotation and impact rolling on the deep foundation is not as substantial as the surface layers. This study provides valuable insights into optimizing foundation treatments for land reclamation projects on the coral reefs. Full article

There are currently at least five aircraft runways built on the reclaimed coral sand foundations in the South China Sea (SCS). The seismic dynamics and stability of the revetment breakwaters and nearby aircraft runways built on reclaimed lands with coral sand in the SCS deserve attention. Taking the reclamation engineering in the SCS as the background, this study performed several centrifuge shaking table tests (N = 50 g) to explore the seismic dynamics and stability of a revetment breakwater and a nearby aircraft runway built on a reclaimed coral sand foundation. It is revealed that the revetment breakwater, aircraft runway, and their coral sand foundation have intensively responded to the excitation of seismic waves. The pore water pressure accumulates in the coral sand foundation; however, the accumulation amplitude is not significant. As a result, only soil softening, rather than liquefaction, has occurred in the coral sand foundation. The final residual subsidence of the revetment breakwater and aircraft runways are both about 0.5 mm, indicating that they are finally in a stable state. Through comparative study, it is found that the pore water pressure in the loose coral sand foundation is much easier to accumulate, and the corresponding acceleration amplification effect is also more significant. It means that a loose coral sand foundation is more detrimental to the seismic stability of the revetment breakwaters and airport runways. Full article

Coral sand is the main filling material for the island–reef foundation. Under tidal actions, the saturation (Sr) of coral sand layers varies with the specific depths in the reclaimed foundation. Studying the Sr effect of coral sand’s mechanical behaviors is crucial for the stability of the reclaimed foundation of island–reefs. In this study, a “quantitative injection method” was designed to prepare coral sand with saturation ranging from 90% to 100%, and unconsolidated–undrained (UU) triaxial shear tests were conducted on coral sand under different effective confining pressures (${\sigma }_3^{\prime }$). The results indicated that the stress–strain curves of coral sand under various conditions were of the strain-softening type. When ${\sigma }_3^{\prime }$ = 200, 400, 600, and 800 kPa, the shear strength of coral sand decreased exponentially by 13.1, 9.1, 16.8, and 15.2%, respectively, with the increase in Sr from 90% to 100%. As Sr rose, the internal friction angle (φ) dropped by 3.77°. The cohesion (c) was not significantly affected by Sr compared to φ. In consideration of the physical susceptibility of coral sand to breakage, relative breakage ratio (B_r) and modified relative breakage index ($B_r^{*}$) were introduced to evaluate the particle breakage behaviors of coral sand samples with different Sr levels in the triaxial shear process. It was found that B_r and $B_r^{*}$ increase linearly with increasing Sr; the effect of Sr on the particle breakage of coral sand weakens significantly when ${\sigma }_3^{\prime }$ is sufficiently large. The median particle size (d₅₀) of coral sand decreases with increasing Sr, and presents a negative linear correlation with both B_r and $B_r^{*}$. Based on comparing the strength and particle breakage characteristics of coral sand samples with varying Sr levels, this study suggests that 92.5% should be considered as the Sr value of coral sand available for testing. Full article

In the past several years, a series of artificial islands have been constructed on the top of coral reefs by China in the South China Sea by way of reclamation. A large number of revetment breakwater also has been built along the margin of these artificial islands. The stability of these revetment breakwater is the precondition for the normal service performance of these reclaimed coral sand islands. In this study, taking the reclamation engineering in the South China Sea as the background, a series of wave flume physical model tests (geometrical similarity scale is set to 1:10) are performed to investigate the dynamics and the stability of the revetment breakwater and its reclaimed coral sand foundation under the impact of regular wave. Experimental results show that the revetment breakwater has a maximum final settlement of 6 mm if built on loose coral sand foundation. Furthermore, there is indeed excess pore pressure generated in the reclaimed coral foundation with a maximum magnitude of 1.5 kPa. It is found that the excess pore pressure has not caused liquefaction in the coral sand foundation due to the fact that the accumulation of excess pore pressure only occurred in the first 10 cycles of wave loading. Finally, it is concluded that the revetment breakwater and its reclaimed coral sand foundation basically are stable under regular wave impacting. However, excessive water overtopping would be a potential threat for the vegetation behind the breakwater, as well as for the desalinated underground water of the reclaimed lands. Full article