Stability Mechanism Analysis Of Steel Cylinder Structure In Calcareous Sand Foundation Under Lateral Horizontal Load

The islands and reefs in the South China Sea are mainly composed of calcareous sand with irregular grain shape,porous and easily broken,and most of the enclosure structures used in development and construction are steel cylinder structures.At present,there are few studies on the influence of progressive instability of steel cylinder structure and calcareous sand particle breakage on structural stability.Therefore,it is of great theoretical significance and engineering application value to analyze the stability mechanism of steel cylinder structure in calcareous sand foundation under lateral horizontal load.The triaxial tests of calcareous sand under different confining pressures was carried out in this paper,and the NHRI double yield surface model is modified by combining the test data to established the constitutive model of calcium sand considering particle breakage;The steel cylinder structure is regarded as a rigid body,and the theoretical analysis model is established according to the plane strain problem;Theoretical calculation and finite element numerical simulation are carried out based on engineering examples and experimental data.The main conclusions are as follows:(1)Based on the fractal theory and Mohr Coulomb theory,the mathematical relationship between the particle breakage index and the peak internal friction angle is established,and the tangent deformation modulus and tangent volume ratio in the NHRI double yield surface are modified by hump curve and piecewise function,which can better describe the strain softening and dilatancy of calcareous sand under the influence of particle breakage with the established constitutive model.(2)There are equilibrium states in three stages of progressive instability of steel cylinder structure in calcareous sand foundation under lateral horizontal load,and the equilibrium stability is mainly determined by the internal and external soil pressure and friction.With the increase of lateral horizontal load,the main displacement of the cylinder is translational movement.The calcareous sand particles change from rearrangement to fragmentation,which leads to the redistribution of soil pressure.The active area of soil pressure is formed on the offshore side of the cylinder,and the passive area of soil pressure is formed on the onshore side of the cylinder.Based on the above process,a theoretical analysis model for different stages of progressive instability of steel cylinder structure in calcareous sand foundation considering particle breakage is established.(3)Based on the structural parameters of steel cylinder used in the construction of artificial island of Hong Kong-Zhuhai-Macao Bridge and the constitutive model of calcareous sand in the South China Sea,the stability mechanism of steel cylinder structure in calcareous sand foundation under lateral horizontal load is analyzed by using ABAQUS finite element software.The results of numerical simulation are compared with those of theoretical analysis,which are in good agreement.At the same time,the simulation results show that the calcareous sand inside and at the bottom of the cylinder has almost no obvious breakage,which can be analyzed and calculated according to the static equilibrium state;The particle breakage of calcareous sand on both sides outside the cylinder should be considered,and the breakage of calcareous sand on the onshore side of the cylinder is more serious at the limit stage.In addition,the influence range of the soil outside the cylinder is larger than that of the soil at the bottom of the cylinder under the lateral horizontal load,and does not change with the stress state of the soil and the degree of particle breakage.