Research On The Influence Of Single And Double-Oedometer Methods Estimating The Collapsibility Of Loess

The geological disasters mainly caused by collapsibility are widespread in loess areas,which have great influence on the safety and stability of construction activities;therefore,it is critical to evaluate the collapsibility of loess and study on its collapsibility mechanism.The collapsibility of loess is a significant subsidence caused by the rapid deformation and breach of loess' s structure under a certain pressure and soaking.In the research and evaluation of loess collapsibility,quantitative study respectively to the external variable factors including soaking and pressure were conducted to summarize the laws of collapsibility and evaluation methods,the microstructure was observed and analyzed by scanning electron microscope(SEM)along with analysis software,different between test methods is further analyzed and summarized.Combines macroscopic characters and microscopic phenomena,summarizes the differences of test methods on the base of loess collapsibility character research,thus conducts further study on the collapsibility of loess,and therefore provides better service for scientific research and engineering activities.Representative undisturbed collapsible loess samples from Lanzhou area were selected in this paper,single and double-oedometer tests under different pressure conditions were conducted on samples with natural gradient water content and samples which are artificial gradiently humidified.Loess collapsibility character laws resulted from the experiment are analyzed,the differences between the single and double-oedometer test methods in evaluating the collapsibility of loess are also analyzed according to the test results.Through studying on the micro image of undisturbed loess and calculating fractal dimension of pore size,the characteristics of collapsibility concluded from single and double-oedometer test methods are explained,comparatively analyses vary laws of microstructure and pore structure of undisturbed loess,loess before and after compress and collapsed loess samples,the influence of soaking and pressure on the microstructure of collapsible loess is obtained.The results show that the collapsibility of loess increases with the increase of pressure then decreases after a certain pressure.As it comes to single and double-oedometer test methods of both artificially humidified samples and natural test samples with gradient water content,collapsibility measured from single-oedometer method is greater than it from double-oedometer method when water content is low,with the increase of water content,the curves of two test methods cross,then the collapsibility measured from double-oedometer method is greater when the water content increases to a certain extent.This phenomenon is due to the different order of water immersion and pressure in collapsible process,and is related to initial water content and pressure degree.The microstructure of collapsible loess is analyzed before and after the experiment,result shows that undisturbed collapsible loess is characterized by loose internal structure and obvious porous structure.After compression,the structure system is more compact,the number and size of pores are reduced,and the fractal dimension of pore size increases accordingly.The original pore structure of samples collapsed by single and double-oedometer tests are destroyed in different degrees,and the number and area of pores are relatively reduced,double-oedometer test method changed loess structure more thoroughly than single-oedometer test method and has greater fractal dimension of pore size.