Research On Deformation Characteristics Of Deep Foundation Pit In Soil And Rock Strata And Optimal Deformation Prediction Period

With the rapid development of infrastructure projects in China and the continuous improvement of the level of science and technology in the construction industry,the scale and excavation depth of deep foundation pit are increasing,especially the frequent occurrence of deep foundation pit disasters in recent years,which has aroused widespread attention to the safety of deep foundation pit construction.Excavation of the foundation pit will cause the change of soil stress,which will affect the surrounding buildings,pipelines and the support of the foundation pit,and cause the deformation of the foundation pit or even the collapse of the foundation pit.Therefore,it is of great significance for the safety of the construction of deep foundation pit to study the deformation characteristics and accurately predict the deformation in the process of deep foundation pit excavation.In this paper,based on a deep foundation pit,the deformation characteristics under the condition of soil and rock strata was studied.Combined with the measured data,the deformation was predicted and the optimal prediction period was given.The deformation characteristics and the optimal prediction period were verified by MIDAS modeling.On this basis,numerical tests under soil and rock strata were carried out by changing the parameters of supporting structure of deep foundation pit,aiming to obtain the deformation characteristics of deep foundation pit excavation under different supporting structure parameters under this stratum condition,optimize the support scheme,and provide reference for the design and construction of deep foundation pit under similar conditions.The main research contents are as follows:(1)Based on the research status at home and abroad,the deformation laws of deep foundation pit were summarized;(2)Combined with a deep foundation pit engineering case,the vertical displacement of the slope top of the deep foundation pit,the horizontal displacement of the slope top,and the deformation characteristics of the surrounding ground settlement displacement along with the construction process were analyzed in detail;(3)The original gray theory is modified by the method of combining polynomial fitting and optimizing initial value,and the prediction accuracy was improved.The comparison between GM-BP neural networks model,which is based on the combination of the improved grey theory,BP neural networks and wavelet neural networks and an improved GM-wavelet neural networks model.The improved grey theory model,BP neural networks model,improved GM-BP neural networks model and improved GM-wavelet neural networks model were used to predict the deformation of deep foundation pit respectively,and the prediction accuracy of deep foundation pit was analyzed,among which the improved GM-wavelet neural networks model was the most accurate.On this basis,taking into account the nonlinearity and volatility of deep foundation pit deformation,the optimal prediction period was given to provide accurate dynamic prediction methods for actual projects.(4)By matching Midas back analysis with the existing monitoring data,the parameters of the deep foundation pit soil and rock formation were determined.Based on the soil and rock parameters,the numerical tests of deep foundation pit excavation under the soil and rock strata were carried out by changing the supporting structural parameters of deep foundation pit,aiming to obtain the deformation characteristics of deep foundation pit excavation under different supporting structural parameters of deep foundation pit under this stratum condition,so as to provide reference for the design and construction of deep foundation pit under similar conditions.