Study On The Stress And Deformation Law Of The Loess Deep Foundation Pit Of The Zhongchuan Tunnel (Open Cut) On The Third And Fourth Lanzhou-Zhangye Line

There are a large number of loess deep foundation pit projects in the construction of loess area.Because loess has the characteristics of vertical joint development,large pores,loose and collapsible particles,the loess foundation pit is prone to collapse,support structure instability and other major foundation pit safety accidents.The stress and deformation characteristics of the foundation pit are different due to the different surrounding environment,the different engineering geological properties of the loess layer,the different supporting structure and excavation method.Therefore,it is necessary to study the stress and deformation law of loess deep foundation pit in combination with specific engineering practice.Based on the deep foundation pit project of Zhongchuan loess tunnel(open excavation)of Lanzhou-Zhangye three-four line railway,this paper studies the horizontal displacement and settlement of deep soil on the left side of foundation pit(soil nailing wall support),and the horizontal displacement of pile top and pile body on the right side of foundation pit(pile anchor support),the settlement of soil behind pile and the axial force of anchor cable by means of field investigation,engineering measurement and finite element analysis.Through monitoring,the deformation of the foundation pit is grasped in time,and the stability of the foundation pit is evaluated to provide technical support for the smooth and safe construction of the foundation pit.The main work results are as follows:(1)The construction and engineering geological characteristics around the study area are analyzed from the aspects of the surrounding environment,soil structure,stratum lithology and hydrogeology of the foundation pit.Combined with the actual situation of the foundation pit,the design scheme of the supporting structure is analyzed.(2)The measured results show that the horizontal displacement of the deep soil of the soil nailing wall on the left side of the foundation pit increases with the increase of the excavation depth of the foundation pit,and the maximum value is 7.78 mm,which appears at a depth of about 12.5m from the ground.The soil settlement value increases slowly when the foundation pit excavation is shallow,and increases rapidly in the middle and late stages of excavation,and the maximum value is 4.25 mm at the end of excavation.(3)With the increase of the excavation depth of the foundation pit,the horizontal displacement value of the pile top on the right side of the foundation pit(pile anchor support)gradually increases,and the measured maximum value is 9.14mm;the horizontal displacement of the pile body shows that the upper deformation is large and the downward deformation gradually decreases.After the excavation of the foundation pit,the measured maximum horizontal displacement of the pile body is 9.83mm;the measured maximum value of soil settlement behind the pile appears at about 10 m from the edge of the foundation pit,and its value gradually increases with the excavation depth of the foundation pit.The measured maximum value is 8.45 mm.(4)At the end of excavation,the axial force of the first to fifth anchor cables is 136.57 k N,149.71 k N,275.16 k N,221.34 k N and 103.28 k N respectively.Compared with the initial prestress,the axial force growth rates of the first to fifth anchor cables are 36.57%,49.71%,37.58%,10.67% and 3.28% respectively.It can be seen that the axial force of the first three anchor cables increases more.The reason is that the horizontal displacement on the right side of the foundation pit is large in the middle and upper part,and the deformation of the foundation pit causes friction between the anchor cable and the soil around the anchor cable,so that the stress transfers to the anchor cable.Therefore,the axial force of the anchor cable will also increase with the increase of the excavation depth of the foundation pit,so the axial force of the first three anchor cables increases more.(5)The stress and deformation law of foundation pit under different excavation conditions are analyzed by numerical simulation,and compared with the field measurement results.The two are close to each other,which verifies the rationality of the model and parameter selection.The numerical simulation results can better reflect the actual stress and deformation of the foundation pit.(6)The single factor test method is used to optimize the diameter and embedded depth of the supporting pile.The maximum horizontal displacement of the pile decreases with the increase of the pile diameter.When the pile diameter exceeds a certain value,the effect of restraining the horizontal displacement of the pile is no longer significant.Therefore,the pile with reasonable diameter should be selected under the premise of satisfying the safety of the foundation pit supporting structure.With the increase of the embedded depth of the supporting pile,the maximum horizontal displacement of the pile gradually decreases,indicating that increasing the embedded depth can inhibit the horizontal displacement of the pile.When the embedded depth is too large,its inhibitory effect no longer increases;Therefore,piles with smaller embedded depth should be selected under the conditions of safety and economy.The analysis results can be used to guide the engineering design.