Study On Seepage Self Compacting Characteristics Of Hydraulic Fill Coral Sand Foundation

The construction of coral sand islands and reefs in the South China Sea is an important measure to implement China's strategy of becoming a maritime power.It is also the basic guarantee for the development,utilization,protection and control of the ocean.In order to reduce the cost and shorten the construction period,coral sand and coral debris in the lagoon are usually used as reclamation materials in the construction of islands and reefs in the South China Sea.In the construction process,it is found that although the density of the dredged coral sand foundation can reach a higher level without treatment,the self compacting mechanism and the basic law of seepage compaction of the dredged coral sand foundation are not clear,so it is impossible to determine the subsequent impact on the reef engineering.In this paper,through in-situ test,physical simulation test and simulation,the self compacting mechanism and the basic law of seepage compaction of hydraulic fill coral sand foundation are studied:1.the indoor tests and tests of coral sand show that the coral sand in the research area is well graded;The shape of the particles mainly includes sheet,branch,spindle and block.Among them,the ratio of length to width of branched and spindle particles is the largest,and the shape of particles is very irregular,and the content of branch and spindle is the highest in coral sand particles,which results in irregular shape and excessive angular angle of coral sand.2.The compression deformation of coral sand is mainly plastic deformation.Due to the influence of high calcium carbonate content,the coral sand will have particle breakage under stress,and it will produce dilatancy effect under low confining pressure,while it will produce particle breakage under high confining pressure,resulting in shear shrinkage.3.The field standard penetration test shows that the compaction degree of untreated coral sand foundation within 1-7m depth has reached the design requirements;Compared with the typical SPT curves,it is found that the compaction quality of area B(flight area)is better than that of area a(construction area),and the number of SPT hammers in area a and B increases with the increase of depth.4.According to the theory of equal diameter spherical compaction,three basic factors of self compaction of coral sand can be determined: 1)the boundary conditions should change;2)The particles will slide or rotate;3)There is an external force to make the particles move.According to Talbot method,the n value(Talbot linear slope)of natural graded coral sand is about 0.25,which indicates that natural graded coral sand is easy to achieve good natural compaction state,and it is easier to develop to the trend of increasing compactness when the external force and boundary conditions change;In the process of coral sand foundation seepage,the downward seepage hydrodynamic force has a special effect on the skeleton particles and fine particles of coral sand,which promotes the increase of the density of coral sand layer.5.The indoor simulated Hydraulic Reclamation test of coral sand shows that with the progress of drainage process,the water content of coral sand layer decreases,and the density and compactness increase.It shows that the drainage process promotes the density and compactness of coral sand;The results show that the compaction degree of coral sand layer is74.3%-76.6% without hydraulic reclamation,and 87.3%-89.2% after hydraulic reclamation,which indicates that hydraulic reclamation is beneficial to achieve a more dense coral sand layer;According to the orthogonal test of coral sand permeability,it can be seen that among the three factors of coral sand porosity ratio,non-uniformity coefficient and curvature coefficient,the porosity ratio has the greatest influence on the permeability.6.Using ABAQUS software to simulate the settlement of coral sand foundation model,the results show that the settlement of coral sand foundation is 11.5-17.2mm when the upper load of coral sand foundation is 200kpa-300kpa;With the increase of coral sand base thickness,the settlement of coral sand foundation will gradually increase;Through the calculation of uneven settlement,the uneven settlement of 7m thick coral sand foundation is only 0.019%;According to the nephogram of settlement displacement and the curve of non-uniform settlement,it can be seen that the non-uniform settlement of coral sand mostly occurs in the contact part of coral sand and reef limestone.