Improvement Method Of Properties Of Backfill Soil For Rapid Construction Of Bad Subgrade With High Water Content In Urban Area

With the accelerating process of urbanization in China,a large number of newly constructed urban roads are constantly emerging,which often faces with outstanding contradictions of short construction period,strict construction quality control,and soil moisture content of the backfill backfill that is far higher than the optimum moisture content.It can be seen that studying the effect of high water content on the mechanical properties of the backfill of roadbeds and establishing a set of improved methods and evaluation systems for backfilling soils with high moisture content have become key technical problems in solving the problem of rapid construction of bad roadbed with high water cut in the city.Based on this,this paper relies on the PPP project of the Smart Driving Test Area in Xiangjiang New District of Changsha City,and carries out a series of indoor tests on the backfill soil with high moisture content and the influence of the change of water content on the road performance of the backfill was studied.Finally,through using the finite element analysis,the influence of the depth of the backfill soil on the CBR value of the subgrade was discussed,and the suggestions for the corresponding improvements were given.The main research contents of this paper are as follows:1)Through a series of indoor geotechnical tests,the basic physical and mechanical properties of the project fill were determined.Among them,the compression index is 0.1628,the rebound index is 0.0400,the compression modulus is 1.721 MPa,the internal friction angle is about 25°,the cohesion force is 10 kPa,and the optimum water content for compaction is 16.3%.2)Based on the optimum water content of compaction,the road performance tests of four different types of subgrade fillings with soil moisture content of 16.3%,20%,25%and 28%were compared and analyzed,namely,compaction test,triaxial compression test,unconfined compressive strength test,rebound test and California bearing test.The tests show that under the best conditions(maximum dry density,optimum moisture content),the pavement performance of the subgrade filling of this project is not ideal,and the California bearing ratio of the soil sample is 0.58,which does not meet the regulatory requirements.When its moisture content rose to 28%,its road performance decreased significantly.Compared with the property when the water content was 16.3%,the triaxial compression strength decreased by more than 50%,the unconfined compressive strength decreased by 72.6%,and the elastic modulus decreased by 80.1%.3)The project will be filled with several water contents of 16.3%,20%,25%and 30%.It has improved the soil quality by four methods like adding compaction work,adding lime,adding cement,and adding gravel.A comparative test study was conducted on the road performance of different soil improvement methods to obtain an improved method and evaluation system for backfill soil with high moisture content.The test results show that the effect of a single increase in compaction work and a single sandy soil improvement is not obvious and is not recommended.In a comprehensive comparison,when the moisture content is lower than 30%,adding 5%lime,the roadbed filling has the best road performance improvement effect,and its rebound modulus and CBR value can meet the regulatory requirements,and the price is the most economical.When the moisture content is 30%,soil improvement using 4%cement can be considered.4)Combined with the high-water-filled soil improvement test before and after the indoor soil test,the improved soil adopts a visco-plastic constitutive model based on the lower load surface.And an ABAQUS finite element numerical simulation of the on-site CBR performance test was adopted to analyze the effect of different modified depth on the subgrade CBR.Thus,the improved depth range of the backfill for the bad embankment were obtained.The numerical simulation results show that when the water content of the poor subgrade soil is 30%and the modulus of resilience is 6.62MPa,the corresponding CBR value can meet the specification requirements when the improvement depth is 0.6m through the incorporation of 7%lime;When the water content is 25%,other conditions remain unchanged,and the improvement depth is reduced to 0.4m.