Study On Unloading Expansion Law And Mechanism Of Xi’an Loess

Because loess is widely distributed in many areas of China,the study of loess properties has always been a hot topic.In the test,it is found that loess will expand after unloading in the compression test.At present,the researches mainly focus on loess’ s collapsibility,and few scholars have explored the unloading expansion phenomenon of loess.The mechanism and its influencing factors of the unloading expansion phenomenon of loess are worthy of further study.In this thesis,the remolded loess and undisturbed loess in Xi’an of Shaanxi Province are tested by laboratory compression test,unloading expansion test and scanning electron microscope.Based on these test results,the law and mechanism of loess swelling under unloading are analyzed,and the collapsibility mechanism of loess is further explored.The main research work of this thesis are as follows:(1)The experimental study on the swelling potential of Xi’an loess shows that the loess in the test site is non-expansive soil,but after the unloading of a compression test,the loess will expand under certain conditions.Compression and unloading tests are carried out on undisturbed loess samples and remolded loess samples.The unloading tests of remolded soil samples under different pressure,dry density and water content are compared.The unloading expansion law of loess is studied by using the newly defined parameter unloading deformation coefficient.It is found that the external pressure,dry density and water content of loess have an influence on the unloading expansibility of loess.(2)A new method to determine the self-weight collapsibility of loess by the test of unloading expansion is explored.The test results show that the loess sample with no unloading expansibility has self-weight collapsibility,and the loess sample with unloading expansibility has no self-weight collapsibility.The verification results show that the method proposed in this thesis is consistent with the field test results.(3)On the basis of previous studies,the microstructure of the loess site in this experiment is classified.The undisturbed soil samples at a depth of 5m before and after collapsibility and undisturbed and remolded soil samples at a depth of 22 m before and after unloading expansion are tested by SEM.Combined with the results of macroscopic and microscopic tests,the microstructure of loess samples is analyzed,and the expansion mechanism of loess is explained.The results show that the overhead porosity,point contact and porosity of loess decrease after collapsibility.After expansion,the vertical overhead pores and point contacts of loess increase,and the pores become larger.(4)Combined with the macro and micro test results,the unloading expansion and collapse mechanism of loess are analyzed.It is found that the special connection mode of loess cement is the cause of loess expansion.It is also found that the unloading expansibility of loess and the failure of the cement connection is the important reason for loess collapsibility.