Research On Uplift Deformation Of Foundation Pit Bottom Based On OFDR Technology

When the foundation pit is excavated,the lower soil mass will produce unloading stress and will be squeezed by the retaining structure,resulting in the upward uplift of the soil mass.Foundation pit heave monitoring is an indispensable part of foundation pit monitoring.The monitoring of foundation pit heave can judge the safe state of foundation pit and ensure that the foundation pit is excavated in a safe state.In this paper,the distributed optical fiber sensing technology is used to monitor the uplift of foundation pit.It has the advantages of high precision,high degree of automation and continuous measurement.At present,it has been widely used in foundation pit monitoring.Through the design of monitoring device and indoor calibration test,a set of foundation pit heave monitoring process based on OFDR is established and applied to practical engineering monitoring.The main research work of this paper is as follows:1.Study the deformation characteristics of foundation pit uplift.The heave deformation of foundation pit is mainly composed of two parts: one is the first type of elastic deformation caused by unloading,the other is the second type of elastic deformation caused by the extrusion of retaining structure,and is also affected by factors such as pore water,soil,seepage,buoyancy and whether engineering piles are set.The existing foundation pit heave monitoring technology mostly adopts leveling method,which has low precision,low degree of automation,and the monitoring device is easy to be affected,resulting in inaccurate monitoring results.By combining with traditional monitoring equipment,the coupling between optical fiber and soil is transformed into the coupling between sedimentation meter and soil,which makes the monitoring equipment easier and easier to protect the optical fiber in the measurement section.2.The monitoring device of foundation pit heave based on OFDR distributed optical fiber sensing technology is studied and designed.The structural composition of the device is designed and studied.Finally,the foundation pit uplift monitoring device based on OFDR is designed and calibrated indoors.The test results show that the strain measurement effects of 0.9mm strain sensing optical cable and 2mm strain sensing optical cable are basically the same.Both strain sensing optical cables break when stretched to 30 mm,indicating that the maximum stretching amount of 1.4m optical cable is 25 mm.3.Combined with the stretching uplift monitoring device based on OFDR,a monitoring process based on the device is designed.Firstly,the possible maximum value of foundation pit heave needs to be obtained through theoretical calculation,and the tensile heave monitoring device is made according to the maximum value of possible heave.The deformation of the optical fiber in the measuring section is greater than the possible maximum heave.Then,the monitoring device is buried according to the steps of point distribution,positioning,drilling,hole cleaning,placing in the monitoring device and backfilling.Finally,the data is obtained by OFDR demodulator and processed to obtain the uplift deformation of foundation pit.4.The foundation pit heave monitoring device based on OFDR is applied to the foundation pit of Suzhou Metro.The monitoring points are arranged in combination with the excavation characteristics,deformation characteristics and dangerous section position of the foundation pit.According to the size of the foundation pit,geological conditions and construction scheme,the possible maximum uplift is calculated by using the residual stress method and gravity stress offset method.Finally,the measuring range of the device is determined as 6cm,and the engineering measurement is carried out.The results show that the device is suitable for uplift monitoring in practical projects,and the monitoring data are similar to that of traditional monitoring devices.ABAQUS finite element simulation software is used to simulate the foundation pit,and the uplift deformation nephogram of the monitoring section where the measuring point is located is obtained.The theoretical calculation results,the measured values of the uplift monitoring device based on OFDR and the finite element simulation results are compared and analyzed.The results show that the theoretical calculation results and the finite element simulation results are similar to the measured values,indicating that the foundation pit uplift monitoring device based on OFDR can accurately and effectively monitor the uplift deformation of the foundation pit,distributed optical fiber sensing technology can be applied to the monitoring of foundation pit uplift.For the second type of rebound deformation,the main influencing factor is the buried depth of diaphragm wall.Through numerical simulation analysis,the results show that with the increase of the depth of diaphragm wall,the deformation characteristics of pit bottom change from inverted "U" type to double peak or three peak type.