Study On Numerical Simulation Of Bolt Composite Soil Nailing Support Based On Deformation Control

With the rapid development of urban construction,the exploiting of underground space is more and more.The excavation depth and excavation area of the foundation pit are increasing,and the higher requirements of the foundation pit support technology are put forward.Composite soil nailing wall support because of its low cost,shorter duration,flexible and so on,has been widely used in the project.Pre-tensioned bolt composite soil nailing support as a way of composite soil nailing wall support,actively restrain the deformation of the soil to meet the requirements of deformation control through the application of prestressing force on the anchor,it's application in the foundation pit support is more and more widely.However,the theoretical research of this form of support is lagging behind in engineering practice,the interaction mechanism of soil nail and bolt and other issues is to be resolved.In this paper,the finite element analysis software MIDAS / GTS NX is used to establish the numerical model of soil nailing and bolt composite soil nailing by summarizing the research results of bolt composite soil nailing support.The influence of bolt arrangement,anchor length and prestressing force on the interaction between bolt and soil nailing,deformation of foundation pit is analyzed based on the premise of deformation control,and compared with pure nail support.The correctness of the conclusion is verified by an engineering case,which provides an empirical basis for the design and construction of similar foundation pit.This paper draws the following conclusions:(1)The deformation mode of the side wall of the foundation pit of the soil nailing is approximately "inverted triangle",and the deformation mode of the bolt composite soil nailing is "concave and convex".Partial soil nails can be replaced with pre-tensioned bolt when the deformation of the soil nail can not meet the deformation requirements.The deformation control effect is the best when the bolt is arranged in where the deformation is relatively large.In this numerical model,the maximum horizontal deformation of the top of the slope with pre-tensioned bolt composite soil nailing support is about 17% less than that of the soil nail support which effectively controls the deformation of the foundation pit.(2)The deformation of the foundation side wall will be different when the arrangement of the bolt is different.The deformation control effect by replacing first,three soil nails with bolts is the better than the effect by replacing first,four soil nails with bolts.Compared with the soil nail support,the deformation of foundation pit side wall where set bolt has a clear turning point with pre-tensioned bolt composite soil nailing support,it shows that the active force bolt changes the deformation mode of the soil.The force of the soil nail near the pre-tensioned bolt is much smaller than that of the soil nail in the same position in the soil nailing,the maximum reduction rate is 20%.The maximum principal stress of the soil is increasing from top to bottom.The maximum principal stress of soil near the foundation pit is smaller than that of the unoccupied soil at the same horizontal position.(3)The longer the length of the bolt,the smaller the deformation of the foundation pit.Based on the model of pre-tensioned bolt composite soil nailing support,the critical length of the model is studied theoretically,and the deformation of the foundation wall is not affected by the length of the bolt when its length is more than 30 m.The length of the free section of the bolt is changed under the same length of the bolt,and it is found that the deformation of the foundation pit is the smallest when the free section of the bolt is 5.5m.The axial force of the soil is gradually increased with the decrease of the length of the free section of the bolt.The maximum principal stress of the soil is increasing when the length of the free section of the bolt increases,The smaller the free section of the bolt,the greater the stress that the bolt will share.In the case of construction conditions permit,the anchor section of the bolt can be appropriately increased.(4)The horizontal deformation of the foundation pit decreases with the increase of the prestressing force of the bolt.The deformation effect of the foundation pit is ideal when the prestressing force is 75% axial bearing capacity design value.The axial force of the nail decreases with the increase of the prestressing force of the bolt.(5)Comparing the simulation data with the measured data,it is found that the deformation trend is basically consistent.It proves the correctness of this model.Optimize the project plan,get the best solution.