Application Of New Raft Subgrade In High Latitude Island Frozen Soil

The permafrost in the island region of high latitude has the characteristics of discontinuous distribution,small thickness of permafrost and extremely sensitive to temperature and moisture,as a result,the roads in this area are suffering from frost heave,melt sinking and mush,which is not conducive to driving comfort and safety.This paper is based on the collection of previous research data,analyzing the application effect of existing treatment measures for permafrost roadbed;Combined with the application of raft foundation in construction and considering the engineering characteristics of high latitude island frozen soil,a new type of raft subgrade is proposed;Laboratory test and numerical simulation were used to systematically studying the bearing mechanism and deformation characteristics of the new raft subgrade.By collecting the existing data,the treatment measures of three typical tundra sections of Bei-Hei expressway,Bo-Ya expressway and Qian-Nen expressway in the island tundra of high latitude were summarized,considering the thickness of frozen soil and thawing and settling characteristics of frozen soil,this paper analyzed the working principle,treatment effect and engineering cost of various treatment measures,and gets the limitations of various treatment measures.Considering the effect of raft in reducing the uneven settlement of foundation,combined with the application of raft foundation in the field of housing construction,a new type of raft composite subgrade structure is proposed,which mainly includes stress diffusion layer and new type raft reinforcement layer.In order to analyze the influence of the new raft on the bearing characteristics of soil subgrade,an indoor bearing characteristics test device of the new raft subgrade was developed to test the bearing and deformation characteristics of the soil mass with and without raft model.The results show that the new raft model can share the load transferred from the upper soil and coordinate the uneven deformation;under the action of high stress,the settlement of soil column in the raft model decreased by 2.5mm compared with that in the no-raft model,and it was 17% lower than that in the no-raft model;the raft model has little effect on the frost heave deformation during soil freezing.The frost heave and thaw settlement test of the soil with raft model was carried out,the effect of frost heave and thawing on the deformation characteristics of raft model is analyzed,the results show that frost heave and thaw settlement will make the soil softer and reduce the overall bearing capacity of the soil.Although the new raft model can share more load,the settlement of the soil body that has experienced frost heave and thaw settlement is still about 8mm larger than that of the soil body that has not experienced frost heave and thaw settlement.The finite difference software was used to simulate the indoor raft model test,and it was found that the simulated settlement and frost heave deformation were similar to the test results.The analysis of the cloud chart of soil pressure and settlement shows that the raft model has a remarkable effect in coordinating the uneven deformation of soil mass.Rely on S308 road,the construction and operation process of embankment with new raft foundation and ordinary embankment is simulated,and the settlement deformation of embankment is calculated,which shows that the new raft can significantly reduce the subgrade settlement during construction and operation;Comparing the settlement deformation of subgrade under different raft foundation sizes,the results show that the settlement deformation of subgrade decreases with the increase of raft height under the same cross section area.