Study On Preparation And Properties Of Lime Soil From Shield Muck Of Subway Tunnel In Hefei

In recent years,with the rapid development of urban modernization,various cities have continuously increased investment in the construction of rail transit,and the subway is experiencing an unprecedented construction climax.Shield method has become the first choice for subway construction because of its advantages of economy,efficiency and safety.However,a large amount of shield muck is inevitably produced.In the process of shield excavation,a certain amount of modifiers are generally added to the shield muck to improve the flow plasticity,such as foaming agent,bentonite,polymer,etc.,and these modifiers will cause certain pollution to the environment.At present,after some treatment measures,the subway shield muck can be used to prepare green building materials such as unburned bricks,heaping hills for landscaping,ceramsite production,etc.,and can also be used as mine fillers and high-flow filling materials.However,due to the regional,dispersive and complex composition of the subway shield muck,the utilization of muck is extremely low.Based on the actual situation of Hefei rail transit shield muck,this paper studies the physical and mechanical properties of shield muck lime soil after adding lime,and determines the optimum lime content of shield muck lime soil for engineering application.Finally,the solidification mechanism and microstructure of shield muck lime soil are studied to judge the feasibility of preparing lime soil from shield muck.The main achievements are as follows :(1)The physical and chemical analysis of the shield muck of Hefei Metro Line 8was carried out.The free expansion rate,boundary water content,specific gravity,optimum water content and maximum dry density of the shield muck were analyzed.Compared with the performance of ordinary plain soil in Hefei in the lime improvement test of expansive soil in Hefei area,the expansibility,plasticity and compactness of shield slag soil are not as good as those of ordinary plain soil in Hefei.The mineral composition was analyzed by XRD technique.(2)When lime is added to the shield muck,the lime content will affect the physical properties of the shield muck.When the lime content is small,the liquid plastic limit of the shield muck lime soil increases,the plasticity index decreases,the optimum water content increases,the maximum dry density decreases,and the specific gravity increases.When the lime content is 5 %-7 %.The physical properties of shield muck lime soil are better.(3)The variation trend of unconfined compressive strength of shield muck lime soil under different lime content,different curing age and different soaking time was revealed by means of test.With the increase of lime content and curing age,the unconfined compressive strength of shield slag lime soil increases gradually.The unconfined compressive strength of shield muck lime soil will gradually decrease with the increase of immersion time.When the lime content is 5 %-7 %,the unconfined compressive performance of the shield slag lime soil increases the most.(4)In the CBR test,when the lime content is different,with the increase of lime content,the expansion of lime soil decreases gradually,while the CBR value of lime soil increases first and then decreases gradually.According to the analysis of CBR test results,when the lime content is 7 %,the CBR value of the shield slag lime soil is the largest.(5)The microstructure of shield muck lime soil was observed by scanning electron microscope.It can be seen that the microstructure of shield muck lime soil was different under different lime content.When the shield muck was just mixed with lime,the structure of shield muck lime soil became dense after compaction and maintenance for a period of time.With the increase of lime content,the compactness of shield muck lime soil increases first and then decreases gradually.With the increase of curing age,the structure of shield muck lime soil becomes more and more dense.When the lime content is 5 % or 7 %,the compactness of shield muck lime soil is the largest.The feasibility of using shield muck to prepare lime soil is further verified.