Study On Collapsible Loess Improvement And Reinforcement Materials For Existing Railway Lines

The Baozhong Railway is an existing transportation railway that built in 1990.The loess roadbed of the Guyuan section of the railway affected by the flood irrigation of the local farming and there was a large area of collapse.Cause uneven subgrade settlement,seriously affecting traffic safety.In order to cooperate with the Northwest Research Institute of China Railways,the“Research on Complete Sets of Technology for Treatment of Existing Railway Subgrade Lesions”,it proposed to use phosphogypsum-cement to improve and reinforce the subgrade collapsible loess in this section to ensure the effectiveness,reliability and environmental protection of the project management.The way of reinforce collapsible loess can be divided into physical methods and chemical methods.The physical method is to eliminate the collapsibility of loess and increase its bearing capacity by increasing the compactness of the soil.Common methods include dynamic compaction method,compaction pile method,and impact compaction method.The chemical method is to add solidified material to the loess layer,and through these materials such as gelation,ion exchange,etc.the loess cemented into a whole,thereby increasing the strength of the loess,eliminating the collapsibility,and reducing the permeability to achieve improvement.The purpose of the loess nature.At present,the types of solidified materials are very abundant,and common ones are sodium silicate,calcium chloride,sodium hydroxide,ammonium acrylamide,chromium lignin pulp liquid,cement and organic polymer materials.The use of solidified materials to cure collapsible loess had been used by more and more companies because of its advantages of wide application range,good reinforcement effect,and wide selection of solidified materials.A large number of engineering practices have proved that cement is the softest solidified material with the largest amount of cement.Compared with other solidified materials,cement as a solidified material has the advantages of wide source,large amount,good effect and low cost.However,cement also has poor permeabilities,high water-cements ratio and long cement consolidation time,poor strength and other shortcomings.Phosphogypsum is a kind of solid waste whose main component is CaSO₄·2H₂O.The waste of phosphogypsum in China is very large,but the utilization rate is very low.Long-term accumulation will cause a series of environmental problems,such as groundwater pollution and space occupation.The main component of phosphogypsum,CaSO₄·2H₂O,is used rationally,waste can be utilized and waste can be turned into treasure.Theory and practice show that if the cement and phosphogypsum are combined can improve collapsible loess,the improvement effect is better than single cement,and it can greatly reduce the cost and reduce the environmental pollution caused by the accumulation of phosphogypsum.For this purpose,this paper studies the phosphogypsum-cement modified cementitious loess loess.The research content of this article is mainly divided into two aspects.The first is the requirement of long-term soaking under combined load on the high strength and low permeability of roadbed loess.The phosphogypsum-cement is selected as the reinforcement material,and the phosphogypsum-cement modified loess and cement modified loess under different ages and different dosages are selected.And phosphogypsum modified loess was tested.The effect of improving loess by cement,improving phosphogypsum and improving phosphogypsum-cement to improve loess was compared.The second is to study the mechanism of phosphogypsum-cement modified loess.According to the XRD diffraction test results and electron microscopy scan results.The mineral composition of undisturbed loess,phosphogypsum-modified cement loess,and cement modified loess was quantitatively analyzed.The microstructure of modified loess and undisturbed loess was observed and analyzed.The mechanism of improving loess by phosphogypsum-cement was analyzed.The research content of this article is divided into two aspects.The first is the requirement for high strength and low permeability of subgrade under long-term soaking conditions under combined loads.Phosphogypsum-cement was selected as the reinforcement material,and phosphogypsum-cement modified loess,cement modified loess and phosphogypsum modified loess under different ages and different dosages were tested.The effects of modified loess cement,modified phosphogypsum,and phosphogypsum-cement modified loess were compared.The second is to study the mechanism of phosphogypsum-cement modified loess.Based on XRD diffraction test results and scanning electron microscopy?SEM?.The mineral composition of undisturbed loess,phosphogypsum-cement modified loess,and cement-modified loess was quantitatively analyzed.The microstructure of modified loess and undisturbed loess was observed and analyzed.The mechanism of improving loess by phosphogypsum-cement was analyzed.The research results obtained in this paper are as follows:The results of tests on modified loess shows that.Cement,phosphogypsum and phosphogypsum-cement can all improve the performance of collapsible loess.The compressive strength and impermeability of phosphogypsum-cement modified loess are superior to those of cement modified loess and phosphogypsum modified loess.Phosphogypsum-cement modified loess is more suitable for the environment in the study area where long-term water collection and heavy rain concentration occur during the irrigation season.The frost resistance and water resistance of phosphogypsum-cement modified loess is better than that of cement modified loess and phosphogypsum modified loess.Under the freeze-thaw cycles,the strength loss of phosphogypsum-cement modified loess is smaller.The water absorption of phosphogypsum-cement modified loess was lower than that of cement-improved loess and phosphogypsum-improved loess.The softening coefficient and water stability coefficient were better than those of cement modified loess and phosphogypsum modified loess.Phosphogypsum-cement modified loess is more suitable for freeze-thaw cycle conditions under flooding conditions.The results of XRD diffraction and SEM scanning experiments showed that the mineral composition of modified phosphogypsum-cement all have changed.Phosphogypsum-cemented cement improved the content of easily-hydratable clay minerals such as illite and chlorite in loess,and increased the content of hard and water-insoluble minerals as calcite.The microstructure of phosphogypsum-cement modified loess has changed.The porosity of the improved loess decreases,the large pores are filled,and the aerial structure is improved.There are more ettringites and stronger cementation between the particles,and the connections between the soil particles are tight.The mechanism of phosphogypsum-cement to improve collapsible loess was studied,the role of phosphogypsum in improving phosphogypsum-cement to improve collapsible loess was clarified.In this paper,the effect of phosphogypsum-cement to improve collapsible loess was evaluated and compared with cement modified loess,and phosphogypsum modified loess.The results showed that the effect of phosphogypsum-cement on the improvement of collapsible loess loess was better than that of cement,and the lower cost,more conducive to reduce the accumulation of phosphogypsum,reduce environmental pollution,has a certain practical value.This provides a new idea for improving the collapsible loess and improving the utilization rate of phosphogypsum.