Study On Security Monitoring System Of The Railway Bridge With Deep Foundation Pit Excavation In Soft Soil

As a new means to monitor safety of a bridge, bridge security monitoring system involves multi-disciplinary and multi-field theory and technology, including modern sensor technology, network communication technology, signal analysis and computer technology. In recent years, bridge security monitoring has gradually become one of the hotspots at home and abroad.The Ningbo south station railway bridge (NSSRB) is located in the busy traffic Shanghai-Hangzhou-Ningbo railway line. The bridge is supported by many light steel lattice structures. Beneath the bridge soft soil deep foundation pit are excavated at the same time on both sides of the bridge. Considering the above reasons, it has become a top priority event to guarantee service security of the NSSRB and construction safety of the soft soil deep foundation pit excavation under the railway bridge. In this paper, based on the new structure characteristics of the NSSRB and complex condition of soft soil foundation and deep excavation under the railway bridge, a series of key problems on establishing an automatic security monitoring system to monitor service conditions of the bridge are investigated. The main work and conclusions are as follows:1. In response to phased foundation pit excavation, the changing foundation and boundary conditions of the NSSRB, sensors of the security monitoring system will be installed step-by-step. The modular monitoring software is developed to meet the needs of multiple batches of sensor installation based on platforms of MATLAB, Visual Basic and SQL server.2. The bridge security monitoring system, in which automatic acquisition, automatic analysis and processing, automatic report generation, automatic alarm and other functions are realized, is established.3. Based on time domain modal parameter identification theory, for instance empirical mode decomposition, random decrement technique and ITD method, applying the digital filtering technique and the least squares method in MATLAB signal analysis toolbox to eliminate vibration signal trends and to analyze and calculate the measured signals of the NSSRB, the first5order modal natural frequencies and damping ratios of the bridge for each excavation stage are obtained.4. The influence of deep foundation pit excavation on trends of displacements and accelerations of the NSSRB is studied by long-term monitoring. The dynamic response characteristics of different model of trains passing through the NSSRB are summarized. The monitoring results indicate there are no displacement, acceleration and dynamic stress value beyond the default security alert value during monitoring period. The bridge is safe, controllable and in good condition.