Study On Subgrade Settlement Calculation Method Under Vehicle Loads

Additional permanent deformation induced by traffic load mainly post-construction settlement and differential settlement, will greatly influence the pavements in the forms of cracking and rut. The stress in the ground induced by traffic load is associated with the cyclic rotation of principal stress axes and dynamic behaviour, however, the settlement caused by these two factors is not included in practical engineering. highlighted the degradation of soil modulus and dynamic stress by using cyclic rotation of principal stress axes test and numerical calculation. Finally, the layed summation method a settlement calculation method was proposed, which considered the impact of vehicle loads. The main work and achievements are as follows:1. In order to consider the distribution and magnitude of the additional stress, normally first calculated the anti-force distribution on the top of embankment caused by train load, then by assuming this reaction force act on the foundation soil surface as a moving load the dynamic stress in the foundation can be obtained. Here the difference of the moving load and static load was analyzed and the influence of train speed and different load distribution were also focused. The purpose of this research was to study the characteristics of dynamic stress in the foundation caused by train loads and provide a sound base for numerical simulation in practical use. Studies have shown that:(1) dynamic-static stress ratio increases highly with the velocity, in low speed range the increment of dynamic stress is linear with the velocity. Considering the train load is a type of moving load, velocity coefficient method is more approriate to calculate the dynamic stress on the surface of the foundation. The dynamic stress distribured in the subgrade soil increases with the load velocity. And it increases slowly with the velocity at low velocity stage, however the dynamic stress increases fast if the vehicle loads moving very fast and approaching the shear wave velocity of the ground.(2) Compared with the dynamic stress under moving square load, the dynamic stress under moving strip load is bigger in same depth, and the rate of increase with velocity is lower than that under square load. The dynamic stress caused by strip load can be ignored, when the load velocity slow less than0.6times of shear wave velocity. According to the above analyse and combined with the train loads characteristics, it is reasonable to suggest that the dynamic stress in ground soil caused by vechicle loads can be ignored.2. This paper simulated the cyclic rotation of principal stress axes caused by traffic loads through ZJU-5Hz hollow cylinder apparatus, and the results that:(1) cyclic rotation of principal stress axes will lead to the deformation of the soil, and the deformation gradually increases with the number of cycles.(2) the modulus of the soil decreases with the cyclic rotation of principal stress axes, but after cycles, the modulus of soil will become stable, and not change with the rotation of principal stress axes any more.(3) An equation for the modulus degradation with the number of cycles was proposed by the regression analysis method, and a modified coefficients Φ to correct the additional settlement caused by the cyclic rotation of principal stress axes was obtained, then a settlement calculation method to consider the impact of cyclic rotation of principal stress axes was proposed. Finally, compared with other research results, it shows that the method proposed in this paper can well predict the settlement. The range of the modified coefficients Φ is also recommended for practicle use.3. reason of the differential settlement was analysed. Meanwhile the practical methods of settlement calculation were summarized together with the advantages and disadvantages. Based on layer-wise summation formulacombined with the characteristics of vehicle loads, the cyclic rotation of principal stress axes and the effects of dynamic stress on the settlement, a settlement calculation method which included the impact of vehicle loads was proposed.This method will provide better predictions for long term settlement of road bed.