Experimental Study On Static And Dynamic Characteristics Of Saturated Calcareous Sand

Calcareous sand is a special geotechnical distributed in tropical oceans.Due to the particularity of its genesis,its engineering mechanical properties are significantly different from ordinary terrestrial sands.The seabed of calcareous sands is often subjected to wave loads.So it is very important to investigate the dynamic behavior of calcareous sand for promoting the mechanical properties research level,guiding the analysis of sea bed stabilization,directing the design and construction of practical projects.At present,scholars at home and abroad have conducted some research on the dynamic characteristics of calcareous sand,but there are few reports that comprehensively consider the effects of continuous rotation of the principal axis of stress and the change in the depth of the sea floor,so it is impossible to understand and grasp wave loads Mechanical properties of calcareous sand.In this paper,a series of static and dynamic characteristics experimental researches are performed by a hollow cylindrical torsional shear apparatus.The specific research contents are as follows:(1)A series of undrained directional shear tests were carried out using a hollow cylinder torsional shear instrument,and the effective stress paths,generalized stress-strain relationships,relationship between super-pore water pressure and generalized shear under different confining pressures and different principal stress direction angle undrained tests were detailed analyzed.The research shows that the difference in the direction angle of the large principal stress axis causes the sand sample to show different stress-strain development modes.Under different directions of the large principal stress axis direction,the sand sample exhibits the phenomenon of first shear shrinkage and then dilatation,but the principal angle the larger the specimen,the stronger the initial shear shrinkage.When the direction angle of the high principal stress axis is 90°,calcareous sand shows instability under low confining pressure,but this phenomenon does not occur under high confining pressure.The orientation of the principal stress has no effect on the phase transition line(PTL)and failure line(FL)of calcareous sand samples(2)A series of cyclic undrained tests on saturated calcareous sand were performed using a hollow cylindrical torsional shear to simulate wave loads.In the past,the wave load was simplified to a circular stress path in the eccentric stress space(assuming that the depth of the sea floor is infinitely thick).Since the depth of the sea floor is not infinitely thick in practice,the stress path is elliptical in the eccentric stress space.Therefore,in this test,in order to comprehensively study the dynamic characteristics of saturated calcareous sand under the interaction of waves,stress path tests were performed,which were circular,vertical oval,and horizontal oval.cyclic stress ratio(CSR)and cyclic loading amplitude ratio(CTSR)are implemented.The study found that when the CSR is equal,the CTSR has a great effect on the development rate of the cumulative pore pressure ratio(r_u).Saturated calcareous sand is easier to liquefy when the cyclic circular stress path is loaded than when the cyclic elliptic stress path is loaded.Saturated calcareous sand is easier to liquefy when the cyclic horizontal elliptical stress path of CTSR=2 is loaded than the cyclic vertical elliptical stress path of CTSR=0.5,which indicates that the cyclic vertical stress has a greater effect on the liquefaction of calcareous sand than the cyclic torsional shear stress in the cyclic loading.The size of CSR and CTSR have important influence on the liquefaction sensitivity of saturated calcareous sand.When the CTSR is identical,as the CSR increases,the liquefaction vibration(N_L)gradually decreases;for the same CSR,when CTSR=1(circular stress path)the calcareous sand sample has the lowest resistance to liquefaction.Under cyclic circular stress path and cyclic oval stress path loads,the N_Lrequired for saturated calcareous sand to reach liquefaction decreases with the increase of the equivalent cyclic stress ratio(ESR)value applied;And the unique relationship between N_L and ESR required to reach the liquefaction standardγ_g=5%,ESR decreases with increasing N_L.