Study On Supporting Structure And Formation Deformation Law Of Loess Deep Foundation Pit In Dalinggou Open-Cut Tunnel Of Lan-Zhang Three-Four Line

In the process of railway construction,there are often some cross-mountain shallow buried tunnels.There are the possibility of collapse,roof fall and other engineering accidents in the construction process of such tunnels,and the construction process is complex.In order to reduce the safety risk of the project,the open excavation method can be selected after the comparison of technology,economy and construction period.The safety of foundation pit engineering and the stress and deformation law of enclosure structure are affected by geotechnical characteristics and surrounding environmental conditions,design parameters of foundation pit enclosure structure and construction technology.Due to the special engineering properties of loess and the complexity of the surrounding environmental conditions of the foundation pit,how to reasonably select the supporting method and construction method of the foundation pit and carry out the construction monitoring of the deep foundation pit in the design and construction process of the deep foundation pit of loess is particularly important for studying the deformation law of the foundation pit during the construction of the deep foundation pit and ensuring the safety and smooth construction of the foundation pit project.Based on the deep foundation pit project of loess in Dalinggou open-cut tunnel of Lanzhou-Zhangjiakou three-four line,this paper studies the deformation law of foundation pit and mountain slope around foundation pit in the construction process of loess deep foundation pit by combining Midas GTS finite element software with field measured data of foundation pit.The main research contents and conclusions are as follows:(1)Through on-site investigation and access to relevant information,the scope of application,support characteristics and mechanism of the commonly used spray anchor support,row pile support,underground continuous wall support and internal support loess deep foundation pit support scheme are compared and analyzed.On this basis,by analyzing the topographic and geomorphological conditions,engineering geological conditions and hydrogeological conditions of the construction site,the deep foundation pit construction is divided into four construction sections for research,and the support scheme,excavation method and specific construction process of each construction section are proposed.The foundation pits of Section I and Section IV are directly excavated.The foundation pit of Section II adopts the supporting structure of ’ bored pile + reinforced concrete internal support ’.The left line of the foundation pit of Section III adopts row pile support,and the right line adopts slope excavation and spray anchor support.(2)The finite element software Midas GTS is used to simulate the deformation of the foundation pit and the mountain slope around the foundation pit during the excavation of the foundation pit.The results show that in the direct excavation section,when the relative height difference between the left and right sides of the foundation pit is large,the settlement of the mountain slope around the foundation pit increases first and then decreases in the direction of the higher side of the mountain away from the foundation pit,and finally shows a ’ leakage spoon type ’.The other side shows the uplift deformation that gradually decreases from the edge of the foundation pit along the direction away from the foundation pit.In the pile-brace support section,the horizontal displacement of the pile top and the pile body gradually increases with the excavation of the foundation pit,and the pile body shows the deformation characteristics of’ large in the middle and small at both ends ’.The axial force of the internal support increases with the increase of the excavation depth of the foundation pit.The settlement of the mountain slope around the foundation pit increases first and then decreases from the edge of the foundation pit along the direction away from the foundation pit.In the pile + shotcrete support section,the horizontal displacement of the top of the left line retaining pile gradually increases with the excavation of the foundation pit,and the horizontal displacement of the pile body gradually decreases from the top of the pile to the bottom of the pile,and the bottom of the pile also has obvious horizontal displacement.The settlement of the slope around the left line foundation pit increases first and then decreases along the direction away from the foundation pit,and the uplift deformation opposite to the left line appears at the edge of the right line foundation pit.(3)By analyzing the field monitoring data of each construction section,the results show that in the direct excavation section,with the excavation of the foundation pit,in the two construction sections of the I-1 section and the IV section,the settlement of the mountain slopes on both sides of the foundation pit shows vertical settlement deformation on the higher side of the mountain,and the other side shows uplift deformation.In the I-2 section,the mountain slopes around the foundation pit of the left and right lines are symmetrically distributed and show obvious vertical settlement deformation.After the excavation of the foundation pit is completed,the settlement deformation of the mountain slopes around the foundation pit tends to be gentle.In the pile-brace support section,the horizontal displacement of the pile top and the pile body increases with the increase of the excavation depth of the foundation pit,and gradually tends to be stable after the excavation is completed.The deformation of the pile body finally shows that the horizontal displacement at about 1/3 position above the bottom line of the foundation pit changes greatly,and the horizontal displacement at both ends changes little.The reinforced concrete internal support has a certain horizontal internal support axial force before the excavation of the foundation pit,and then increases rapidly with the excavation of the foundation pit,then increases slowly,and tends to be gentle after the excavation is completed.The settlement of the mountain slope around the foundation pit increases slowly with the excavation of the foundation pit,and then increases rapidly.Finally,the change trend tends to be gentle and the settlement of the mountain slope on the left line is always greater than that on the right line;in the section of row piles + shotcrete-anchor support,the horizontal displacement of the pile top and the settlement around the foundation pit increase with the increase of the excavation depth of the foundation pit,and tend to be stable after the excavation is completed.However,there is obvious uplift deformation on the right surface of the foundation pit,and obvious vertical settlement deformation on the left side of the mountain slope.(4)The numerical simulation results of each construction section are compared with the field monitoring data.The results show that the numerical simulation data is basically consistent with the field measured data.The deformation trend of the two in the process of foundation pit excavation is consistent and the deformation of the foundation pit is within the warning value.It shows that the Midas GTS finite element calculation model is reasonable and reliable,and the parameters are selected appropriately.It can better reflect the deformation of the foundation pit and the settlement of the slope around the foundation pit during the excavation of the foundation pit,and further illustrate the reliability of the foundation pit support scheme.In similar deep foundation pit projects,before the excavation of the foundation pit.The numerical simulation method can provide reference for the design and construction of foundation pit engineering.