| Under the reciprocating action of traffic moving load,it is easy to cause the deformation of saturated soft soil subgrade,resulting in a large settlement problem,which is gradually reflected in the pavement surface,causing road diseases such as cracking,holding,rutting and so on.To ensure the stability of pavement structure under long-term traffic load through shakedown analysis is a reasonable and effective method to obtain the ultimate bearing capacity of pavement structure.Exploring the dynamic stress response of pavement structure under moving load is the premise to reveal the shakedown mechanism of pavement structure.On the one hand,the dynamic stress response of saturated subgrade under traffic load is affected by both the load moving speed and water-soil coupling.The assumption of quasi-static load and single-phase material will lead to different stress fields compared with the real condition,and then lead to the change of shakedown limit value.On the other hand,the interaction between vehicle and pavement surface belongs to the problem of rolling and sliding contact.The real traffic load should be a vertical-horizontal coupling load(that is,the rolling-sliding contact between the wheel load and pavement).Especially when the vehicle braking at high speed,the effect of horizontal load induced by sliding cannot be ignored.Therefore,when the high-speed rolling and sliding traffic load acts on saturated soft soil,the induced traffic problems will be more serious and complex.In this paper,an easy-toimplement semi-analytical method of lower-bound dynamic shakedown analysis was proposed.Based on the numerical and analytical solutions of dynamic stress response of pavement under rolling-sliding moving load,the dynamic shakedown behavior of saturated pavement structure under high-speed moving load was studied in depth.The main research contents and achievements of this paper are as follows:1.The numerical analysis of the dynamic stress response of pavement under a rolling-sliding moving load is studied.Based on a user-defined subroutine in ABAQUS three-dimensional finite element code,an infinite element artificial boundary is set,and the body-force thin-layer element technology is customized to realize the numerical simulation of wheel-pavement sliding friction.The dynamic stress response of uniform and double layered pavement under high-speed braking wheel load is revealed.The numerical analysis shows that the sliding friction caused by wheel load speed and braking has an important influence on the dynamic stress response of the foundation:the dynamic stress level increases nonlinearly with the increase of moving load speed,and reaches the peak value when the load speed approaching Rayleigh wave speed,and the dynamic magnification factor(amplification effect)increases with the increase of depth.When the sliding friction effect caused by wheel load braking is taken into account,the shear stress level of the foundation is significantly increased,but the sliding friction also reduces the dynamic effect of load speed.With the increase of modulus ratio of upper layer(pavement surface + base)and lower layer(subgrade),the amplifying effect of load moving speed on dynamic stress decreases gradually,and the dynamic stress will decay faster with depth,resulting in the concentration of dynamic stress in the upper layer.2.The analytical analysis of the dynamic stress response of saturated subgrade under rolling-sliding moving load is studied.Based on Biot dynamic consolidation theory,Fourier transform and the conversion relationship between plane wave and cylindrical wave,the foundation is regarded as a semi-infinite space of saturated porous media,and the analytical solution of dynamic stress response under rolling-sliding moving load is calculated.The effects of soil permeability coefficient,load moving speed and horizontal friction coefficient on the dynamic stress response of saturated soil foundation are studied.Based on transmission matrix method,the analytical solution is extended to layered saturated soil foundation,and the effects of modulus ratio and friction coefficient on the dynamic stress response of double-layer road pavement structure are studied.It is found that with the increase of the load moving speed,the influence of the permeability coefficient on the dynamic stress of saturated subgrade is gradually significant.When the load moving speed is large,the dynamic stress level of the saturated foundation increases with the decrease of the soil permeability coefficient,and the increasing amplitude of shear stress is much larger than that of normal stress.The influence degree of permeability coefficient on dynamic stress firstly increases at first and then decreases with depth.Compared with the singlephase medium foundation,the amplifying effect of moving speed and friction on dynamic stress of saturated foundation is slightly weakened.3.The dynamic shakedown analysis of single-phase medium pavement under rolling-sliding moving load is studied.Based on Melan’s lower-bound shakedown theorem and combined with dynamic finite element technology,a method for calculating the dynamic lower-bound shakedown limit of geotechnical materials under high-speed moving load is proposed.Considering the dynamic effect of rolling-sliding moving load,the influence of different moving speed and friction coefficient on the dynamic shakedown limit of the pavement is calculated and analyzed.Additionally,the distribution of critical residual stress is revealed.It is found that the dynamic shakedown limit decreases with the increasing speed or friction when the moving speed is less than the Rayleigh wave speed.The influence of sliding friction on dynamic shakedown limit decreases with the increasing speed.The increasing friction coefficient will also weaken the dynamic effect of load moving speed on the dynamic shakedown limit.Moreover,the moving speed and sliding friction coefficient also have significant effects on the location of critical shakedown failure of pavement structure.The critical shakedown failure is prone to occur in the lower layer of the double-layer pavement structure when the speed is large,while it is more likely to occur in the upper layer when the friction coefficient is large.4.The dynamic shakedown analysis of saturated pavement under rolling-sliding moving load is studied.The dynamic stress solution of saturated subgrade is introduced into the dynamic shakedown theory,and a semi analytical dynamic shakedown analysis method for saturated subgrade is established.The evolution of dynamic shakedown limit of saturated subgrade is discussed and is compared with the analysis results of single-phase medium.It is found that the effects of load moving speed,friction coefficient and material properties on the dynamic shakedown limits of the two types of pavement structures are basically the same by comparing the viscoelastic singlephase medium subgrade with the porous medium saturated subgrade.Furthermore,the dynamic shakedown limit of saturated porous medium subgrade is larger compared with single-phase medium subgrade,and decreases with the decreasing permeability coefficient of saturated subgrade.The influence of permeability coefficient on the lower layer is more significant than that on the upper layer for double-layer saturated pavement.The smaller the modulus ratio or the cohesion ratio of upper and lower layers,the more significant the influence of permeability ratio on the dynamic shakedown limit at high speed.5.The approach to determine shakedown strength based on laboratory soil tests is proposed,and the assessment analysis of shakedown state is carried out with respect to engineering cases.Based on the cumulative deformation response observed from cyclic shear tests,the shakedown state of sand and saturated clay is classified and evaluated respectively.Based on the evaluation of the existing criteria for the determination of shakedown strength,the criteria for the determination of shakedown strength of saturated clay are proposed.Furthermore,the shakedown strength criterion and dynamic shakedown theory are applied to the stability evaluation of pavement structure of Shanghai Hongqiao Airport East Runway.It is found that the cyclic shakedown strength is far less than the traditional static failure strength.The current European code EN13,286-7(2004)is not applicable to saturated clay.The dynamic shakedown limit of Hongqiao Airport runway subgrade system is evaluated.The results show that the shallow layer of the subgrade may still evolve into an unshakedown state,and the full cycle shakedown state of the subgrade system can be achieved by appropriately increasing the thickness of the surface(or base)layer. |
PDF Full Text Request