A Bounding-surface Plasticity Model For Cyclic Loading Analysis Of Sand

In the marine engineering mooring system,the bearing performance of the anchor is concerned.Because the anchor is directly or indirectly affected by the cyclic load caused by wind,wave and flow,its cyclic bearing capacity becomes more important and will be very different from its static bearing capacity.At present,the determination of the cyclic bearing capacity of the anchor is mainly based on the model test,but the numerical calculation is not satisfactory.The key to the numerical calculation is to establish a constitutive model which can reflect the characteristics of the soil under cyclic loading and is much easier to realize.This paper is devoted to establish a boundary surface plasticity model of saturated sand under drained cyclic loading.Firstly,within the critical-state theory framework,a bounding surface plasticity model for describing the stress-strain behavior of saturated sand subjected to drained cyclic loading is presented in the p-q plane.A hardening rule depending on incremental deviatoric strain is adopted.During the virgin loading,the stress point always locates on the bounding surface.During the unloading and reloading,the bounding surface is just the historical maximum yielding surface.This hardening rule can describe the softening phenomena of dense sands and makes the bounding surface remember the stress history.The shape of the bounding surface is a modified ellipse,which makes the model be able to describe the plastic strain during constant-stress-ratio loading.This model incorporates state-dependent dilatancy and adopts the non-associated flow rule,so it can reasonably describe the volume change regularity of sandy soil.A mapping rule which passes through stress reversal points is adopted.A single set of 10 model constants which can be calibrated by conventional triaxial tests is needed for one type of sand under different initial void ratios and different confining pressures.Secondly,triaxial tests of saturated sand under drained monotonic and cyclic loading are simulated.A number of conditions for Hostun sand,Nevada sand,Toyoura sand and Fuji river sand with different confining pressures and different densities were simulated.Comparing the simulation results and test results,the model can reasonably describe the basic mechanical properties of saturated sand.Finally,the bounding surface model in p-q plane is realized in three-dimensional space.Based on the modified Von-Mises criterion,the interpolation function g(?)considering soil anisotropy is used to modify the critical state stress ratio M and the bounding surface plasticity model in three-dimensional is realized.