Study On Stress Deformation And Seepage Control Effect Of Double Row Steel Sheet Pile Cofferdam With Large Spacing In Gravel Layer

The issue of "Japanese chemical weapons" is a major historical issue between China and Japan,which still poses a serious threat and harm to the safety of people’s lives,property and ecological environment in relevant areas of China.The cofferdam project of"Japanese chemical weapons" treatment in the Songhua River of Jiamusi is the first attempt in China to excavate abandoned chemical weapons buried underwater by cofferdam method,and there are few similar reports in the world.The Songhua River has deep water level and large flow.Due to the impact of flood scouring and artificial sand mining in flood season,the riverbed terrain is very complex.How to ensure the stability of stress and deformation of the super large area cofferdam project under complex hydrogeological conditions has become the primary engineering problem faced by the project.The cofferdam project is located in the area where shells are buried,and the sand layer and sand gravel layer below the river bed alternate with each other or transition relationship.The complex stratum conditions will not only increase the difficulty of pile foundation construction,but also bring great challenges to seepage theoretical modeling and precipitation scheme selection.The design scheme of suspended anti-seepage with super buried depth and large spacing double row steel sheet piles,pumping and discharging the river water in the cofferdam adopted in this project is the known engineering case with the longest sheet pile,the widest spacing and the most difficult soil conditions.There is no precedent for reference.The existing research lacks the deformation calculation theory of large spacing double row steel sheet piles.The above key technical problems and scientific problems not only need to establish a rigorous theoretical analysis model,but also need to verify and revise the theoretical model in combination with in-situ tests and indoor geotechnical tests.In order to solve the above engineering problems and provide technical support for the subsequent large-scale excavation project of "Japanese chemical weapons",this paper carries out the following main studies based on the engineering background of large-distance double row steel sheet pile cofferdam in the gravel layer of the main channel of the Songhua River in Jiamusi:(1)In order to obtain the physical,mechanical and hydraulic properties of the sand gravel layer,the systematic heavy dynamic penetration test,borehole pumping water injection test,indoor particle size test and permeability test were carried out based on the engineering geological drilling.This work provides valuable basic data for the structural design of the steel sheet pile cofferdam in the sand gravel layer of the Songhua River main channel,the cofferdam water level prediction,and the dewatering design for the first time.(2)The stress deformation and anti-seepage effect of the double row steel sheet pile coffer-dam rely heavily on the variation of water level in Songhua River.Therefore,an artificial intelligence prediction model of Songhua River water level has been established based on the principle of SVM support vector machine.The hydrological data collected in the past 70 years are standardized and normalized.The artificial intelligence model is trained and tested,and the appropriate kernel function has been cross verified.The optimal penalty coefficient c and kernel function g have been found.Continuously optimizing the key parameters of the artificial intelligence prediction model and comparing the SVM prediction results under different kernel functions,the accurate medium and short-term water level prediction model has been established,to provide precise hydrological data for subsequent cofferdam structure stability design and precipitation scheme design.The theoretical model has been proven to have sufficient application accuracy to guarantee engineering safety.(3)To explore and investigate the structural design method of steel sheet pile cofferdam in sand gravel layer,the finite element numerical model of "double row steel sheet pile,tie rod,sand filling between piles" in the gravel stratum has been established.The ideal elastic-plastic constitutive model based on Mohr Coulomb failure criterion is used to analyze the force characteristics of single and double steel sheet piles under horizontal load.The calculation method of effective buried depth of steel sheet piles has been presented.This paper expounds the exertion mechanism of the pull rod between double piles and the influence of sand filling between piles on the working properties of double row steel sheet piles.The rationality and calculation accuracy of the theoretical model have been verified by using the field test data.The important findings can provide valuable theoretical basis for the structural optimization design of large spacing double row steel sheet piles.The analyses indicate that sand filling between piles can effectively enhance the deformation coordination ability of the inner and outer row steel sheet piles,and fully invoke the bearing capacity of the inner row piles.However,when the horizontal load is large enough,the filling sandwill increase the displacement and internal force of the inner row steel sheet piles,which is not conducive to the stability of the inner row steel sheet piles.Due to the relatively small horizontal load of the cofferdam project,the comprehensive consideration of the influence of sand filling between piles still has more advantages than disadvantages.(4)The Songhua River channel is composed of silt layer,sand layer and gravel layer with great difference in formation age,formation thickness and permeability coefficient.At present,the exsting study still lack efficient calculation method for the seepage of double row steel sheet pile cofferdam with large spacing and over buried depth in complex formation.Systematical studies have been carried out by using the theoretical analysis,laboratory test,field test and numerical simulation.The adaptability of common permeability calculation theory under this special working condition has been analyzed,and a reasonable indoor model test method of equivalent permeability coefficient has been advised.The seepage finite element analysis model of large spacing and over buried steel sheet pile maintenance structure in gravel layer has been established.The flow field distribution models under different design conditions have been established to provide theoretical basis for the design of water stop curtain and precipitation.The research shows that although the permeability coefficient of sand gravel layer is high,the silty sand layer plays a key role in the equivalent permeability coefficient of cofferdam structure.When a continuous and uniform silty sand layer is distributed on the foundation surface,it is equivalent to laying "geotextile" on the riverbed surface of the cofferdam,and the equivalent permeability coefficient of the foundation can be reduced by more than 50%.The improved resistance coefficient method will significantly underestimate the equivalent permeability coefficient of the double row steel sheet pile cofferdam structure with large spacing and over buried depth in the gravel layer,leading to the theoretical water seepage far less than the measured value,giving rise to great safety risk.However,the seepage theory of homogeneous earth dam has achieved good calculation results in the cofferdam maintenance pumping period and cofferdam demolition period.(5)The succrss of the actual cofferdam construction has proven that the artificial intelligence water level prediction model,structural optimization design method,seepage path,precipitation design and numerical model developed in this study are reasonable and efficient.The optimal scheme of "double row to single row,constant pile length,two cloth and one membrane" is put forward.The corresponding construction early warning system has been proposed.The findings in this study can not only provide theoretical basis and technical support for the subsequent large-scale cofferdam construction,but also lay a theoretical design foundation for the popularization and application of cofferdam technology of large-distance double row steel sheet piles under complex stratum conditions.