Research On Optical Fiber Sensing Monitoring Technology For Uplift At The Bottom Of Foundation Pit

During the excavation of the foundation pit,the excavation and unloading of the soil,the buoyancy and seepage effect generated by the groundwater,and the squeezing of the soil into the inside of the foundation pit by the maintenance structure below the base will all cause the foundation pit to uplift.Foundation pit uplift monitoring is one of the important contents of foundation pit safety monitoring.Effective and accurate monitoring of it can ensure the safety of construction.This article attempts to use the quasi-distributed optical fiber sensing technology based on Bragg grating to monitor the uplift of the foundation pit.Optical fiber sensing technology has the advantages of corrosion resistance,electronic interference resistance,long-distance signal transmission,etc.,which is very suitable for monitoring in geotechnical engineering.Through theoretical analysis and related indoor and outdoor experiments,a quasi-distributed optical fiber monitoring system for the uplift of foundation pits is established and applied to the monitoring of actual foundation pit projects.The main research work of this paper is as follows:1.Research on a foundation pit uplift monitoring device based on optical fiber sensing technology.The various components of the monitoring device were studied,a set of practical monitoring device for the excavation and uplift of the bottom of the foundation pit was designed,and an indoor calibration test and an outdoor feasibility test were carried out.The experimental results show that when the distributed strain optical cable combined with the fiber grating displacement meter body is stretched to 25 mm,the 2mm strain sensing optical cable breaks,which is not suitable for the combination of displacement meters to monitor deformation;simple foundation pit uplift monitoring device The error percentage between the measured soil uplift value at the bottom of the foundation pit and the true value obtained by the traditional foundation pit rebound monitoring method does not exceed 10%.2.Perform on-site monitoring in conjunction with the project,and apply quasidistributed optical fiber sensing technology to the displacement measurement of foundation pit uplift.The designed quasi-distributed foundation pit uplift monitoring equipment is buried in the subway deep foundation pit to be excavated,and two different demodulation technologies are used,wireless fiber grating demodulator and cabinet dense distributed fiber demodulator.FBG displacement device and dense distributed sensing optical cable strain for demodulation measurement.The results show that the foundation pit uplift monitoring device based on the FBG displacement meter and the dense distributed strain sensing optical cable can well adapt to the harsh environment of the foundation pit excavation site,meet the requirements of long-term monitoring and long-distance transmission of monitoring signals,indicating that the quasi-distributed The optical fiber sensing technology is suitable for the uplift monitoring of foundation pits in geotechnical engineering,and can realize automatic and intelligent monitoring.3.Combine the monitoring results of the excavation site of the foundation pit,as well as the design plan,construction plan and geological survey report.According to the calculation method of the foundation pit uplift and deformation of the deadweight stress offset method,the uplift of the actual engineering foundation pit uplift is calculated.Midas numerical simulation software is used to simulate the actual excavation uplift of the foundation pit,and a curve diagram of the uplift of a section of the foundation pit with the excavation depth is obtained.Comparing the calculation results,finite element analysis results and the results of the foundation pit uplift monitoring equipment based on the FBG displacement meter,the results show that the formula calculation results and the finite element simulation results are similar to the uplift measured by the monitoring equipment,indicating that the designed Quasi-distributed foundation pit monitoring devices can accurately and effectively monitor,and optical fiber sensing technology can be used for foundation pit uplift monitoring.