Collapse Test And Mechanism Study Of Loess Under Unloading

The "One Belt and One Road" policy has driven the rapid development of urban construction in the Northwest region.The urban underground space project has received special attention and development.A large number of projects are built on the loess foundation,and the problem of loess collapsibility must be considered.At present,loess collapsible research has been very common,but for underground space projects,especially the actual foundation stress such as comprehensive pipe corridors and subway pipelines is less than the self-weight stress of the overlying soil,the foundation is in an unloading state,and the foundation collapsing occurs in this state.Research on the situation is extremely scarce.This article takes loess in a site of Tongchuan New District as the research object,and uses the method of indoor collapsible test,electron microscope scanning test and field test to explore the law and mechanism of loess collapsible under unloading.The contents are as follows:(1)Through the indoor unloading collapsible test,the characteristics of loess collapsible deformation under unloading were studied,and it was obtained that the change of the collapsible coefficient of the loess under different unloading levels showed a decreasing trend;comparing the deformation of loess collapsible under unloading and constant pressure It is obtained that the initial pressure of collapsible loess is increased under the influence of unloading.The increase range of 5m?20m soil is 1.5%?9.8%,and the increase ratio increases with the increase of burial depth;it is expressed by the unloading collapsible water sensitivity coefficient The degree of influence of water on unloading collapsibility,the results show that:as the unloading amount increases,the water sensitivity gradually weakens,and the smaller-the buried depth,the stronger the unloading loess collapsibility;the loess is studied from the perspective of the change of void ratio The internal structure features are the same when unloading and constant pressure collapsing.(2)Based on the scanning test of electron microscope to compare the mesostructure change process of loess collapsing under unloading and constant p ressure;using PCAS software to quantitatively analyze the changes of pore,particle size and fractal dimension during unloading collapsing process of loess law.Combining the two methods,the unloading collapsible mechanism of loess under unloading is as follows:after unloading,the upper pressure can not reach the pressure failure conditions required by the stable structure of the soil,and some large and medium particles are not damaged,and a large amount of particle slip filling is not generated.The phenomenon of porosity,so the amount of collapsing is reduced or no collapsing occurs.(3)Defining the unloading stress ratio,unloading collapsing ratio and unloading collapsing attenuation coefficient to describe the amount of collapsing and unloading,combining the multi-factor orthogonal test and the unloading collapsing test under different pressures to determine the unloading The load-stress ratio K is a key factor in unloading collapsible.The expression of unloading collapsible coefficient is established,which shows that unloading collapsing is a reduction of constant pressure collapsing to some extent;the unloading collapsing influencing factor reflects the influence of unloading on the deformation of collapsing,with the unloading stress ratio K When it is increased,the unloading collapsible factor j gradually increases.When K reaches about 0.6,j increases rapidly,that is,the smaller the unloading amount,the faster the loess collapsing attenuation rate,and the unloading causes a larger change in the amount of collapsing deformation..(4)The field test actually measured the loess dead weight collapsible test value under unloading,and the comparison between the collapsible test value and the calculated value shows that the actual site collapsible amount is much smaller than the standard calculated collapsible amount,reflecting the current site collapsibility The evaluation has certain limitations and verifies the applicability of the unloading collapsible coefficient expression.(5)The field test actually measured the experimental value of the loess's self-weight collapsible amount under unloading.Comparing the collapsible amount test value with the calculated value shows that the actual collapsible amount on site is much smaller than the standard calculated collapsible amount,reflecting the current site collapsibility The evaluation has certain limitations and verifies the applicability of the unloading collapsible coefficient expression.