Seismic Behavior Of Grouted Sand Gravel Soil-Abumtent-CFST Arch Bridge System

With the rapid development of traffic infrastructure in China,the demand for long-span concrete-filled steel tube(CFST)bridge engineering under complex geological conditions is increasing.Sand gravel soil is a kind of complex loose coarse accumulation,which has a special characteristic with good compactness,high bearing capacity,and anti-liquefaction.The foundation type with a sand gravel layer as a loading bearing layer are concerns by engineers.However,the horizontal resistance of natural sand gravel soils may be insufficient to meet the seismic requirements despite having a high vertical bearing capacity.Furthermore,arch bridges built on sand gravel soil sites have been seriously structural damage or failure in previous earthquake disasters.As a result,the seismic performance of a sand gravel soil-CFST arch bridge system in terms of the soil-structure interaction(SSI)problem has become a key scientific issue that cannot be ignored.To study the seismic performance of long-span CFST arch bridges built on sand gravel soils,a new method of grouting to strengthen sand gravel foundation was proposed to enhance the seismic capacity of sand gravel foundations to achieve the semi-active seismic reduction and isolation of foundations based on the self-isolation energy dissipation of the soils by controlling the grouting depth of sand gravel soil.On this basis,in this study,shaking table model tests and numerical simulation are combined to systematically study the seismic response,soil-structure dynamic interaction,and seismic energy dissipation mechanism of the sand gravel soil and arch abutment and CFST bridge system.Furthermore,the seismic response of the soil and abutment and CFST bridge system under the sand gravel foundation,different soil types,and sand gravel soil with various grouting depths are investigated.The main research work and achievements are as follows:(1)To clarify the seismic response of CFST arch bridge abutments under a natural sand gravel foundation,a CFST arch bridge engineering project was adopted as the research engineering prototype.A scaled physical model soil and simplified model abutment were established.A series of shaking table tests of the sand gravel soil and bridge abutment system were carried out.Based on the acceleration response,displacement response,and dynamic shear stress-strain relationship of model soil and abutment,the seismic response of sand gravel soil and bridge abutment system with considering SSI effects were explored.Hence,the seismic safety of the natural sand gravel soil and arch bridge was evaluated.The results show that the amplification factor of ground acceleration may be less than 1.0 under strong earthquake excitation,showing a natural geotechnical seismic isolation(GSI)capacity of sand gravel soil.(2)To investigate the difference in the dynamic response of the bridge abutment under different soil conditions,shaking table tests on the seismic response of the CFST arch bridge system on sand gravel,clay,and clay-sand gravel layered soil were carried out.A special particle sliding friction energy dissipation mechanism in sand gravel soils was verified by comparing and analyzing the difference in dynamic characteristics of three soils.A new technical strategy for the seismic isolation method of the sand gravel soils was proposed by grouting-treated soil utilizing and optimizing the dynamic characteristics of sand gravel soil.(3)Shaking table tests of sand gravel soils in the state of no-grouting and grouting reinforcement were carried out,and the seismic behavior of the two cases was compared and analyzed.The feasibility of the method of grouting reinforcement technology to control the seismic reduction and isolation performance of the system was verified.Furthermore,the critical design and optimization methods of the seismic reduction and isolation control technology were also clarified.The results show that the grouting behavior improves the lateral resistance and seismic capacity of sand gravel soils.As a result,improvements in the seismic performance of sand gravel soil and arch bridge systems reinforced by grouting methods are suggested in relevant engineering projects.(4)The model tests of grouted sand gravel soils and CFST arch bridge systems with different grouting deaths were carried out to verify the impact of grouting depth on improving the seismic performance of the soil-bridge system.The results show that the mechanism of earthquake resistance or isolation of sand gravel soils with two grouting depths is significantly different.Finally,the optimal control strategy of grouting depth for different depth foundation was improved.(5)Based on ABAQUS software,a numerical model of the in-situ soils and CFST arch bridge system was established,and numerical simulation analysis of the engineering prototype of the soil-CFST arch bridge system under different grouting conditions was carried out.The effectiveness and reliability of the above research on the seismic isolation mechanism of sand gravel foundation in the actual project were evaluated considering the main factors of the CFST bridge.