The Study Of A Field Water Immersion Test For Underground Rail Transit Project In Xi'an

Loess collapsing plays a causal role in various adverse engineering geologic phenomena,such as building structure instability and track baseplate settlement,which could eventually contribute to great potential hazards on underground rail traffic construction and operation.There is a large area of collapsible loess distributing along the present project that might lead to the occurrence of undesirable accident.This fact strongly alerts us to the elimination of the whole collapsibility existing in soil foundation.Accordingly,with regard to the rail traffic construction,performing a large-scale field water immersion test and obtaining the complete and reliable test data are of great significance for us to determine the degree of loess collapsibility in project area,explore effective methods for hazards elimination and ensure the environment safety of the project.Firstly,with the combination of regional geological conditions,data of preliminary reconnaissance and laboratory experiment,test position has been selected when we considering the engineering geological situation,special requirements of rail traffic and site conditions.After that,final experimental project has been fully demonstrated by means of laboratory test,numerical simulation and data analysis.Basing on our premilitary results,this work was smoothly carried out and relevant factors,including water consumption,surface settlement,deep settlement and infiltrating range,were detected in this study.Furthermore,development processes of surface and deep settlement were deeply analyzed.Deviation between calculated and observed value on loess collapsing and the leading reasons have been carefully discussed.Several conclusions of this study are listed as follows:(1)One-dimensional and two-dimensional unsaturated seepage models have been established with the implementation of excavation,sampling and laboratory unsaturated tests.Meanwhile,the infiltration effects with or without infiltrative hole have been compared and the acquired experimental data of water immersion test have been arranged.We finally came to the conclusion that infiltration holes should be considered in order to accelerate the infiltration when the field water immersion test is conducted in this area.(2)The results of loess moisture sensor,pore pressure transducer and the samples of drilling hole indicated a steep,zigzag saturation line of the test position,suggesting the obvious effects of infiltration holes.(3)The monitor results of surface deformation showed differences existed on the collapsibility of shallow surveyor's pole among different areas at an average of 5.1mm.Therefore,this study area could be identified as a position without self-weight collapsibility.(4)The observed results of deep collapsibility uncovered that the collapse settlement sum of deep strata did not match that of surface,which mainly results from the existence of multilayer paleosol in this area.Paleosol layer is characterized by low perviousness and mechanical strength and thereby the self-gravity pressure of the upper soil layer is supported and the deformation of collapse is restricted.(5)With the combination of delay and mutation effect of collapse theory effect theory,?-p curve method has been proposed and applied to calculate the total self-weight collapsibility and adjust deep collapsibility calculation as those results are close to the actual value.