Study On Site Stability Of Sandstone Gas Well Station In Lvliang Area With Collapsible Loess

Lvliang area is rich in sandstone gas reserves and will be an important energy supply area for the central and western regions in the future.The gas well station will be put into operation for a long period,and the stability of the site has an important impact on ensuring the normal operation and maintenance of the gas well station.The site stability of the gas well station in the Lvliang area is inseparable from the microscopic unsaturation of the soil in the area,the special collapsibility of loess,and the stability of the slope formed by excavation and filling.This paper mainly studies the site stability of sandstone gas well stations in the Lvliang collapsible loess area.In this paper,the pore size distribution curve of loess was measured by mercury injection apparatus and nano-pore measuring instrument,and the soil-water characteristic curve and unsaturated shear strength of loess were deduced.The fractal fitting of the curves before and after immersion and subsidence was carried out,and the method and formula for solving the soil-water characteristic curves of the loess after an immersion collapse were given.The pore distribution curve,soil-water characteristic curve,and unsaturated shear strength curve of loess obtained by unsaturated microscopic analysis before and after immersion collapsibility were compared,based on which the mechanism of loess collapsibility that affects site stability is explained.The basic physical properties and the collapsibility coefficient of the unmodified loess of the site were measured by experiments,and the multiple regression equation for the rapid evaluation of the collapsibility coefficient by using the porosity ratio,saturation,and water content was given through factor analysis.The collapsibility mechanism and evaluation of the loess of the site were deeply analyzed.Aiming at the slope generated by the excavation and filling of loess,numerical software was applied to solve the stability coefficient of slope and analyze the effects of slope type,soil parameters,rainfall and seepage coupling on slope stability.The multiple regression equation for the rapid evaluation of the stability of intact loess slope by slope ratio and water content was figured out.According to the remolded soil samples with different compaction degrees,water content,and collapsibility coefficient,the expression of stability coefficient of remolded loess slope in this area was solved.Research indicates:1)Compared with the undisturbed loess,the micro-unsaturated structure,pore distribution,soil-water characteristic curve,and unsaturated strength curve of loess after immersion and collapsing have changed.Loess that has been immersed in water to eliminate its collapsibility has greater unsaturated shear strength under the same matric suction.As the immersion time increases,the small pores inside the soil increase,and the average pore diameter decreases with time.According to the changes of pores before and after immersion,a method for solving the soil-water characteristic curve of loess after collapsibility was given.Compared with the direct or indirect methods measuring the soil-water characteristics of the loess,this method is more concise,and the predicted results are in good agreement with the measured values.2)Considering the microscopic changes of loess before and after soaking and collapsing,the analysis shows that the main reason for the sudden drop in the strength of the loess during the collapse is the failure of the capillary suction caused by the immersion.After the suction fails,as the immersion time increases,the internal support of the aggregate clay particles disappears,the easily soluble chemical cement in the loess,such as calcium oxide and organic matter,also dissolves with the increase of water immersion time.The factor analysis method is used to eliminate the correlation between the physical properties of loess,and based on the cumulative variance contribution rate,a multiple regression equation is given to quickly evaluate the macroscopic site collapsibility coefficient using void ratio,saturation,and water content.The fitting result is closer to the measured data.3)Taking the regional differences of loess into account,the measured data of the project site is substituted into the slope model,and a multiple regression equation is given to quickly evaluate the stability of the loess slope of the undisturbed site by using slope ratio and water content.Several groups of data of collapsibility coefficient,compaction degree,and water content were obtained through the test sample preparation,and the expression of each influencing factor and the stability coefficient of remolded loess slope was fitted.4)The soil-water characteristic curve data of the site and the annual rainfall data were substituted into the model to simulate the coupling of seepage and slope stability,and the influence of rainfall intensity and rainfall duration on slope stability was obtained.When the total amount of rainfall is constant,the impact of continuous rainfall under small rainfall intensity on loess slopes is greater than that of high-intensity rainstorms.The same coupling model is used to analyze the influence of the parameter ‘a' and ‘n' in the soil-water characteristic curve formula on the slope stability coefficient.The research-based on the micro and macro perspective uses experimental comparison,theoretical derivation,and simulation to provide the theoretical and empirical basis for the changes and causes of the unsaturated soil-water characteristics of loess,analyses the collapsibility mechanism,and evaluation method of site loess with obvious regional characteristics in-depth.The research has obtained the influence of site unsaturated characteristics and collapsibility characteristics on the digging and filling slope,which provides a new method for theoretically calculating the slope stability coefficient.