Study On Optimization Design Of Soil Nailing Wall Combined Pile Anchor Supporting Structure In Xi’an Area

In the soil nailing wall combined pile anchor supporting structure,soil nailing wall is used to reinforce the upper soil mass,and the pile anchor structure is used to support the lower soil mass of the foundation pit.,the safety is much higher than single soil nailing wall and the cost is lower than pile anchor support,therefore,it is widely used in building,municipal,subway and other deep foundation pit support engineering in Xi’an area.But there are some problems in the application process,outstanding performance as design theory lags behind engineering practice,at present,there are no calculation functions for soil nailing wall combined pile anchor supporting structure used in domestic foundation pit design software,most of the design uses traditional method which separate the upper soil nailing wall and the lower pile anchor to design individually,ignoring the interaction between the soil nailing wall and the pile anchor support,it is inconsistent with the practice.In this paper,first,the design parameters of typical soil nailing wall combined pile anchor supporting foundation pit in Xi’an area were analyzed statistically.Then,the numerical simulation method was used to study the performance of soil nailing wall combined the pile anchor support structure in each construction phase of the foundation pit in Xi’an area,the virtual design and sensitivity analysis of the design parameters of the foundation pit supporting structure were carried out as well.Finally,based on the above work,optimized design of soil nailing wall combined pile anchor supporting structure was carried out.The main conclusions are as follows:1.The main design parameters of the soil nailing wall combined pile anchor supporting structures in Xi’an area are as follows:(1)The ratio of the upper soil nail supporting height to the excavation depth of the foundation pit is 0.22~0.39;(2)In the lower pile anchor support structure,the ratio of the embedded depth to the total length of the pile is 0.36~0.59,the diameter of the pile is 0.7m~1.0m,and the pile spacing is 1.3m~1.8m;(3)The average vertical anchor spacing is 1.8m~3.9m,the average length of anchorage segments is 13.0m~19.7m,and the prestress is 180kN~500kN.2.Numerical simulation results of the soil nailing wall combined pile anchor supporting structure:(1)In the various stages of foundation pit construction,the deformation of the pile is large in the middle and small in both ends.When the excavation is completed,the maximum positive bending moment of the pile is 1m above the excavation surface of the foundation pit,and the maximum negative bending moment is 4m below the excavation surface;(2)The ground surface settlement outside the excavation is "funnel" shaped and the maximum settlement was about 23 m outside the excavation.(3)Soil excavation will cause a sudden axial force increase of a row of anchor rods that has just been completed.With the subsequent excavation of soil and construction of anchor rods,the axial force of anchor rods that have already been completed will gradually decrease.The axial force of the row of anchors gradually increases along the depth of the foundation pit.3.Sensitivity analysis results:(1)The maximum horizontal displacement of the supporting pile is most sensitive to the relative height of the soil nail wall,the sensitivity to the platform width of the pile top is the lowest,and the sensitivity to the design parameters such as pile diameter and pile spacing is at the middle level;(2)The maximum bending moment of the pile is the most sensitive to the diameter of the supporting pile,followed by the sensitivity of the pile spacing and the relative height of the soil nail wall,and the sensitivity of the bolt prestress,the depth of the pile embedded and the width of t he pile top platform is smaller(3)The maximum settlement on the ground surface outside the pit is the most sensitive to the relative height of the soil nailing wall,and the sensitivity to the anchor prestress is the lowest.The sensitivity of other design parameters is at the middle level.4.Based on the results of virtual design and the conclusion of sensitivity analysis,an optimized design scheme for the soil nailing wall combined pile anchor supporting structure is proposed.After optimized design,the horizontal displacement of the supporting pile,the bending moment of the pile and the ground surface settlement outside the pit were significantly reduced;The amount of earth excavation and backfilling was significantly reduced.The amount of reinforced concrete used in the support piles was reduced by 20.98%,and the length of the anchor rods was slightly increased by 10%.