The Field Test Study On Pile-anchor Supporting Structure Of Deep Foundation Pit In Northwest Loess Region

In the supporting project of deep foundation pit in Northwest area of our country,the influence of factors,such as the depth of the foundation pit and the surrounding environment,has made the pile-anchor retaining structure to be the most important and economical form of support.The advantages of pile-anchor retaining structure include the simple and mature construction technology,good control effect to the distortion of foundation pits,and the low cost of supporting structures.At present,the theoretical knowledge of foundation pits lags behind its practice.Therefore,there are still many existing deficiencies that need to be studied and analyzed continually.In this paper,an experimental study of a deep foundation pit project in Chengguan District,Lanzhou is taken as an example.Selecting three support piles unit sections,the internal force of supporting structure in different operating conditions and horizontal displacement of pile bodies are monitored and analyzed by embedding transducer and inclinometer.As the main of site soil is backfill and the depth is great,the detailed and accurate soil parameters are obtained by civil experiment in an indoor laboratory after soil sample collecting.The parameters can provide a reliable basis for parameter settings of finite numerical simulation.Finally,finite element numerical simulations were carried out to simulate the foundation pit engineering.The specific work is as follows:(1)Combining with the present internal and international development situation of the pile-anchor support structure,researcher analyzes the working mechanism of the pile-anchor support,earth pressure at the side of the pile,the calculation of the internal force of the supporting pile and bolt(cable)and so on in the paper;studies the interaction and coordination of pile-anchor support;analyzes the interaction between piles and soils and the characteristics of earth pressure on different soil layers;also analyzes and studies the internal forces of the support piles and anchors(cables).In addition,the free end of the bolt(cable)and the length of anchoring section are calculated and analyzed in the paper.(2)Combining with actual engineering,laboratory tests and field tests are carried out.The civil experiments mainly test the density,moisture content,internal friction angle,cohesion and osmotic coefficient of soil body.In field tests,the concrete strain meter and steel transducer are buried in support piles to monitor the internal force of piles;the internal force of anchor cable is monitored by rope meter;and the horizontal displacement of pile-top is monitored by buried inclinometer.But the analysis of subsequent experimental data is affected because of the local damage of sensor caused by various irresistible factors,such as construction or weather.(3)According to the analysis of the on-site monitoring data,the paper summarizes changing rules of the force of support piles and prestressing jet grouting anchor cable and piles' horizontal displacement in different construction process.Based on the above,the paper draws conclusions: the axial force of prestressing jet grouting anchor cable increases steadily with the excavation of foundation pits,and reaches the maximum at the central of foundation pits.The value of prestress applied during field construction is much smaller than the design value,and the prestress loss at the initial stage of anchor cable stretching and locking is serious,so it is necessary to improve the construction quality and follow the graded stretching requirements strictly;the bending moment value is introduced according to the pile sensor.But due to the complex environment and the existence of saturated soil,the bending moments' rules of the two test piles are different,and the maximum values appear after the foundation pit was completely opened;as the excavation depth of the foundation pit increases,the horizontal displacement of the pile body shows a steady growth.The speed of displacement growth accelerated in the later stage of excavation of the foundation pit.But the horizontal displacement of the pile body gradually stabilizes after the bottom plate is poured.(4)Conclusions are obtained through the simulation of finite element software as follow: the horizontal displacement of the pile body increases with the increasing of excavation depth,and its overall trend is basically in correspondence with the field measurement,but the value is larger,moreover the uplift phenomenon occurs at the bottom of the foundation pit and the surrounding soil sinks slightly;The shearing force of the supporting piles is generally small,and the extreme value of shearing force occurs at the inside corner of the foundation pit.The bending moment of the foundation pit is positive at the inside corner,and is negative at the other part,and the force condition is great.The minimum axial force of anchor cable is at the inside corner of the foundation pit.The axial force of the bottom row of anchor cables is smaller,and that in the middle position is larger.