Study On Type Selection For Bridge Foundation Under Complex Geological Conditions

The development of the highway and high-speed railway construction has been greatly promoted by national improvements and progresses.The large scale and rapid development of the construction have shock the people all over the world.Bridges,as an effective form for crossing the river,bad geological areas and even channels also catch the pace.Recently,the domestic bridge technologies have developed into the cutting edge in the world from the early days when the forms were single and the technologies and appliances lagged behind.Because of the wide river surface,deep water,rapid flow and complicated geological conditions in the districts located at the lower reaches of the Yangtze River,foundation construction is more and more challenging under these complex natural geography conditions and engineering geological conditions.In addition,the bridge foundation is a concealed project and it is hard to find and repair breakages.As a part of connecting the structures with rocks and soil,the foundation is the key part of a bridge relating to the structure safely.The bridge stability is principally controlled by engineering geological conditions.Consequently,the research in this paper focuses on the basic type selection of long-span bridges and the solutions to the technology problems in foundation construction from the perspective of engineering geology.The main research findings are as follows.1.The development of recent bridge technologies in our country is summarized systematically and four aspects are summed up.a).The study for super long-span bridges is broken through constantly.b).The new bridge forms and structures show up continuously.c).Breakthroughs are made in the experiences and technologies of bridge design and construction,as well as in the new materials and advanced technologies.d).The practicability of bridges is guaranteed and the aesthetic concepts are enhanced.2.Systematic analysis of the primary controlling factors in the election of bridge foundation forms is made.The stability of bridge foundations are mainly controlled by the engineering properties and hydrogeological conditions of the foundation soil,load characteristics,the application and structure of the bridge,the difficulty of obtaining materials and constructing.Combining the principle of reliable operation,simple and feasible technique and economical budget,different schemes are compared and analyzed,and then a better scheme of bridge foundations will be selected.3.An engineering geological evaluation is made for the foundation of Hutong Changjiang River Bridge and the election for bearing strata.With regard to the newly-built Hutong Changjiang River Bridge(including the railway and highway)which is located at the lower reaches of Yangtze River and connects Shanghai and Nantong,Jiangsu Province,an engineering geological evaluation is made for the complicated geological conditions.These conditions include the field characteristics of this engineering(geographical position,topographic features,weather conditions,etc.),field geological conditions(strata,geological structures,main fractures and their activity,neo-tectonic movements,earthquakes and characteristics of rocks and soil),basic physical and mechanical properties of the strata in the field,hydrogeological conditions,bad geological phenomena and special soils.Based on the field investigation and engineering geological evaluations and considering the existing foundation forms,the bearing strata are elected for different kinds of foundations.4.An optimization analysis of the selection for deep water foundations is made.According to the comparison of over one hundred of deep water foundations of constructed and under-constructed bridges at home and abroad,the available deep water foundations are picked out and a library of deep water foundations is established for the bridge.Then,from the perspective of engineering geology,mechanics and economics,the election for the bridge foundations is optimized and the final deep water foundation form is decided with the method of analytic hierarchy process(AHP)and fuzzy comprehensive evaluation.In this paper,the newly-built long-span bridge project was taken as an example to make an analysis.According to the geological investigation,stratigraphic analysis and local bridge construction experiences,the bearing stratum is decided combining the local climatic conditions and navigation requirements.In addition,available foundation forms are selected based on the design intention and operating requirements.At last,the most reasonable foundation form is decided and the geotechnical engineering evaluation is performed using AHP and fuzzy evaluation methods.