Study On Mechanical Properties And Safety Monitoring Of Railway Bridge With Deep Foundation Pit Excavation In Soft Soil

Abstract:The construction of large-scale comprehensive transport hub, lead the contradiction between large and deep excavation and existing railway line operation to further intensified. To ease the contradiction, shorten the construction period and reduce costs, the "one step" mode that the deep pit of all regions include both sides and down of the existing railway excavated synchronously emerge as the times require. This mode requires the design of special bridges to realize the deep excavation under the existing railway line. The boundary conditions of this special bridge are constantly changable during the process of excavation, which leads out the static and dynamic interaction problems of the Train-Track-Bridge-Pit large system. To ensure the security of the "one step" mode in implementation, it is an urgent and important subject to study the mechanical properties of this time-varying system that has strong time-varying characteristics. Currently there is still no related research response to this strong time-varying system at home and abroad. In view of this, taking the construction of Ningbo south station hub transformation as the background, the static and dynamic performance of the new bridge used in construction of Ningbo South Station Hub transformation (Ningbo Bridge) under deep excavation in soft soil and the safety monitoring techniques during the process of deep excavation are studied in this paper. The main contents and results obtained are as follows:1. The advantages and disadvantages of the "one step" and the traditional "two-step" construction mode are summarized, and the research status of interactions of Train-Track-Bridge system and safety monitoring, signal processing and other aspects are reviewed.2. A finite element model of Track-Bridge is established by using solid elements to simulate the ballast, finite element simulation method of the interaction between soil and structure and the determination of its parameters is discussed. The vertical stiffness formula of soil spring is proposed. Taking the Ningbo Bridge as the object, the static performance under dead load and seasonal temperature variation are analyzed. The allowed difference between two sides of the bridge is determined when the pit is excavated unsynchronized, which provides evidence for the construction and operation safety.3. Using contact element to simulate the wheel-rail contact relation, a finite element model of Train-Track-Bridge coupling system by considering deep excavation is established based on ANSYS. The nonlinear dynamic response analysis procedures "displacement coupling method" and nonlinear dynamic stability analysis procedures "dynamic eigenvalue method" combined with "displacement coupling method" are programmed by using APDL. Taking the Ningbo Bridge as the object, its dynamic characteristics, train-bridge responses and dynamic stabilities under deep excavation are analyzed. The effectiveness of the program is verified.4. The concept of "strong time-varying system" is proposed, the basic characteristics of safety monitoring system for strong time-varying railway bridge are made clear, and the guidelines and principles in design of the monitoring system is established. The construction scheme of the monitoring system installation in batch and implement step by step with dynamic thresholds is determined. The basic components, monitoring content and selection of sensors and instruments are investigated in detail.5. The automated safety monitoring system for the Ningbo Bridge with functions of signal acquisition, processing, report, early warning and alarm is built. The dynamic control software for monitoring system and signal analysis software are developed independently to achieve the monitoring signals processing online and efficiently.6. The measured signals by the Ningbo Bridge safety monitoring system are analyzed and compared with the finite element results. The rationality of the finite element model of Train-Rail-Bridges large system by considering deep excavation is verified. All the monitoring indicators are safe and controllable, and the operation is in good condition during whole monitoring period. The study achievements of this paper provide important measured data and strong technical support for the successful use of "one step" mode in construction project of hub transformation.