Application Research And Numerical Simulation Of Micro-Pile Composite Soil Nailing In Deep Foundation Pit Support

As one of the important supporting forms of deep foundation pits,micro-pile composite soil nailing walls have been applied in actual engineering,but there are few studies on their mechanism of action,and the design calculation theory has not been perfected.The research on the mechanism of micro-pile composite soil nail supporting structure,and the reasonable design calculation through numerical analysis to ensure the construction safety has important research significance.Based on the foundation pit engineering under a complex geological condition in Liuzhou City,this article uses the ABAQUS finite element analysis software to establish a three-dimensional model of foundation pit construction,and analyzes the whole process of foundation pit excavation under the condition of micro-pile composite soil nail support.And compared with the on-site monitoring data;at the same time,the mechanism analysis of soil nails,micro-pile and prestressed anchor cable was carried out to find out the optimal design value and optimize the foundation pit support scheme.The main research results achieved in this article are:(1)Considering the deep soil thickness of the backfill,the hydrogeological conditions and the complicated surrounding environment relying on the foundation pit project,after the comparison and optimization of the schemes,the "grouting mini steel pipe pile +grouting mini tree root pile" + Hanging net sprayed concrete + anchor rods + prestressed anchor cable ",a micro-pile composite soil nail comprehensive structural support design scheme;and the specific design of soil nail length,inclination angle,pile diameter,etc.are optimized through numerical simulation,The effect is good.(2)Through the finite element simulation of the excavation of the foundation pit supported by the composite soil nail wall,it is found that as the excavation progresses,the top horizontal displacement increases first and then decreases and then increases,and its vertical displacement continues to increase.After the excavation is completed,the two tend to be stable;the calculated maximum horizontal displacement of the top of the supporting structure is 4.95 mm,which is consistent with the monitoring value of 4mm;the maximum vertical displacement of the top is 4.54 mm,although it is greater than the monitoring value of 3.39 mm,but The resulting simulated calculated values and measured values are in line with the overall trend line and meet the requirements of the specifications.(3)With the increase of the excavation depth,the general trend of horizontal displacement at different depths of the foundation pit is to increase first and then decrease and then increase and then decrease.The maximum horizontal displacement occurs at the depth of 6m near the top of the miniature steel pipe pile The location indicates that this part is more prone to problems and should be paid more attention to in the construction of the foundation pit;the vertical displacement of the parts of the foundation pit at different depths will gradually increase,and there will be uplift of about 19 mm in the soil body at the bottom of the pit.(4)The equal-length arrangement of soil nails can limit the horizontal displacement of the foundation pit slope compared with other arrangements.The length is preferably about10m;the smaller the soil nail inclination angle,the smaller the horizontal displacement of the foundation pit slope.The inclination angle can be taken as about 10 °;the more the number of prestressed anchor cables,the more obvious the effect of limiting the deformation of the foundation pit.A certain amount of prestressed anchor cables can be added to the foundation pit with large deformation;The excavation platform is set with a reasonable length of grouting micro steel pipe piles,which can effectively restrain the horizontal displacement of the foundation pit slope.Its length is preferably 14m;the diameter of the micro pile should not be too small,and its value is recommended to be greater than 250mm;Before the diameter of the pulp tree root is 450 mm,the effect of diameter change on the slope of the foundation pit is not obvious.Therefore,when the diameter of the root pile is less than 450 mm,the diameter of the root pile can be appropriately reduced.