The Fiber Optic Sensing Technique And Stability Research On Deep Foundation Pit

Stability monitoring and evaluation of deep foundation pit are necessaries to ensure the safety of infrastructures.Starting with the importance of the deep foundation pit monitoring,the author reviewed the current status and the imperfections of engineering monitoring technologies in deep foundation pits.A monitoring system on deep foundation pit stability using Distributed fiber optic sensing(DFOS)technology was developed.Based on the key issues of the monitoring system,several fiber optic sensors were also developed and the assessment methods on the sensor performance were provided.This thesis provided the scheme and the installation methods of DFOS concerning the monitoring contents and the characteristics in deep foundation pit monitoring.Additionally,the evaluation method and the advantages of stability monitoring in deep foundation pit using DFOS were discussed.An example in Nanjing Olympic Sports Center illustrated the feasibility and advantages of DFOS technology in deep foundation pit stability monitoring.The main statements of this thesis are as follows:(1)The author introduced two main DFOS technology namely,FBG and BOTDR which are applicable in deep foundation pit monitoring.From the view of the measuring principle and characteristics,the feasibility and the advantages of these two technologies in deep foundation pit monitoring were introduced as well.(2)According to the characteristics and the monitoring contents in the deep foundation pit,the author introduced three kinds of deep foundation pit monitoring sensor cables,which are metal-reinforced cable,belt-shape cable and fixed-point cable.Theoretical analysis and laboratory tests have been carried out with regard to the aspects of sensing performance of the developed sensor cables.The results show that the metal-reinforced cable has high tensile strength and shear strength,the belt-shape cable has very good coupling with structures and the fixed-point cable can detect big deformation of structures.These three new-developed cables all meet the criterions on accuracy,durability and sensing property of cables for engineering practice and they are applicable for deep foundation pit monitoring.(3)The author designed a monitoring scheme for deep foundation pit,which includes the measurement methods,the measurement contents and the implementation routes of the internal force and deformation of vertical anti-slide structure of deep foundation pit,the internal force and deformation of the internal support system,the internal force and deformation of anchor reinforcement system,and the deformation of soil and surrounding facilities.The author also introduced the installation methods of different cables and standardized the monitoring scheme for deep foundation pit.(4)Stability evaluation method based on DFOS technology for deep foundation pit was proposed.This method consists the selection of indicators strain data,the transitions of parameters and the stability evaluation model for foundation pit.The advantages of stability evaluation using DFOS technology were illustrated.(5)The author introduced the details of monitoring scheme and installation methods by giving an example in Nanjing Olympic Sports Center,where continuous monitoring using DFOS technology was implemented.Additionally,a numerical simulation model based on discrete element method for foundation pit was developed for the guidance of monitoring scheme and better stability evaluation.The stability of a deep foundation pit was evaluated with the data acquired from PCMW method piles and two bolting structures.The result indicated that the monitoring scheme and the stability evaluation method proposed in this thesis is feasible and has a very broad prospect for futher development.