Study On Static Characteristics Of Polyurethane Foam Adhesive Improved Calcareous Sand And Liquefaction Mechanism Of Calcareous Sand

With the proposal of the strategy of "maritime power" and the promotion of the construction of the "21st century Maritime Silk Road" economic belt,the construction of the South China sea reef project has gradually become the focus of attention.Different from the land engineering construction,the South China Sea is short of land resources and geotechnical engineering materials.It is a trend to fill the sea and land with the method of filling calcareous sand.However,the calcareous sand has a high content of CaCO3 and its particles are easy to break,which is quite different from the ordinary terrigenous sand in mechanical properties.If the calcareous sand is directly used as the foundation soil,the uneven settlement of the foundation and the road surface fracture will occur,which will seriously affect the safety and service life of the buildings or structures.In addition,the South China Sea area is an earthquake prone area,and the island and reef engineering is subject to the objective impact of the ground motion load,which is easy to cause the liquefaction of the calcareous sand engineering site and the destruction of the island and reef engineering.Therefore,it is necessary to study the solidification characteristics and liquefaction mechanism of calcareous sand to serve the construction of South China Sea project.The main research contents and conclusions are as follows:(1)Based on unconfined compressive strength test and triaxial shear test,the strength characteristics and compression deformation characteristics of solidified calcareous sand are analyzed.The results show that the compressive strength and shear strength of calcareous sand are obviously improved,and the compression deformation characteristics are also significantly improved,reducing the settlement of foundation under the same stress condition.In addition,the Duncan Chang hyperbolic model is used to describe the stress-strain curve of the specimen,and the main parameters of the model are obtained.(2)Through dynamic triaxial test,the development process of dynamic pore pressure,dynamic strain accumulation characteristics and hysteresis curve evolution law of undisturbed calcareous sand are analyzed.It is found that under the action of dynamic load,the increase of confining pressure makes the cyclic activity strength of calcareous sand increase gradually.When the ratio of confining pressure to dynamic stress increases,the cumulative plastic strain increases.When the calcareous sand is liquefied and destroyed,there is still a certain residual strength in the particle skeleton structure.The law of hysteresis curve evolution is that the area increases first,then decreases,and finally tends to stable shape.(3)The anti liquefaction ability of calcium sand solidified by polymer is obviously enhanced.By filling the pores and cemented particles,the connection strength and ductility between the sand particles are improved,and the deformation characteristics are effectively improved.The hysteresis curve of solidified calcareous sand is mainly a series of approximately parallel curves,which are slightly different from that of undisturbed calcareous sand,but the evolution rule of the shape of the hysteresis loop is the same.(4)Under the actual earthquake load,the pore pressure of calcareous sand develops in a"stepped" way,with the threshold cyclic stress ratio of liquefaction failure.Under the same equivalent working condition,the liquefaction time of calcareous sand under the actual earthquake load is longer than that under the sinusoidal load,and the final strain is smaller.The final strain is affected by the peak load.In the study of Dynamic Liquefaction Characteristics of calcareous sand,pore pressure failure criterion should be selected under the actual earthquake load,and strain failure criterion should be selected under the sinusoidal load.(5)Two components of Wolong earthquake wave in Wenchuan were selected for CM test and EM test respectively.The results show that the amplitude and the final strain value of CM test are larger than those of EM test,which indicates that the change and asymmetry of stress direction of seismic wave make the liquefaction characteristics of samples different greatly.By modifying the reduction coefficient of seed equivalent method,the equivalent reduction coefficient of calcareous sand is obtained,so that the equivalent sinusoidal load can produce the same pore pressure development law as the actual seismic waveform.