Research On Optimization Method And Engineering Application Of Frozen Wall Thickness For Cross-Passage In Urban Rail Transit

The artificial ground freezing(AGF)method originated from the field of mine shaft construction.Since it has the characteristics of "seal and reinforcement",it can effectively overcome the engineering problems that cannot be easily solved by the conventional means,such as grouting.As the main construction method for vertical shaft construction to pass through the water-rich and weak stratum,it has been widely used in mine shaft construction all over the world.Since then,this special ground reinforcement technology has been further introduced into the field of civil engineering,such as tunneling,and has also achieved very good construction resultsDuring the recent 30 years,with the rapid development of urbanization in China,in the field of underground engineering construction such as urban rail transit,similar or even more serious engineering problems to those in the mining construction field have also been encountered.The artificial ground freezing method has now become an inevitable technical choice in the construction process of certain specific municipal projects.At the same time,a series of new technical problems and engineering challenges have also arisen.Among them,the optimal selection of horizontal frozen wall thickness for cross-passage is one of the most prominent problems need to be solved urgently.This thesis takes the AGF construction of cross-passages between the Financial Island Station and the No.3 Working Well of the Zhuji Intercity Rail Transit Project in Zhuhai,China,as the engineering background.The in-depth researches focused on the optimal design of frozen wall thickness and the response relationship between frozen wall thickness and the ground surface deformation induced by frost heave and thaw settlement during the AGF construction process have been carried out.Based on that,a complete set of procedures suitable for determining the frozen wall thickness of cross-passage in the similar urban rail transit projects are further proposed and constructed.The main research conclusions and innovative results obtained in this paper are as follows:(1)In view of the actual force characteristics of the straight-arched frozen wall in the deep-buried clay layer,the shear failure theory of clay layer is introduced into the frozen wall thickness design of cross-passages for the first time.Based on the reasonable calculation of real supporting pressure of frozen wall,an optimization method that is different from the current traditional method and is more suitable for the preliminary thickness selection of the straight-arched frozen wall in the deep-buried clay layer is proposed.Compared with the current preliminary selection method,this proposed method can quickly determine the primary thickness of frozen wall,and the result obtained is more reasonable and optimized.The relevant conclusions and results have been verified by applying to the actual engineering,which can lay a good foundation for the further correction and optimization of frozen wall thickness.(2)Based on the fluid-solid coupling theory,the mechanical response and deformation stability of frozen wall in the real water-rich construction environment are investigated via numerical calculation.By comparing the mechanical response of frozen wall models with same thickness under the action of a separate stress field and a fluid-solid coupling stress field,respectively.It is proved that the presence of water puts forward higher requirements on the overall mechanical properties of frozen wall.Thus,the effect of water cannot be ignored,when the force analysis and thickness design of the cross-passage frozen wall in water-rich stratum are carried out.Based on that,by comparing the mechanical response,deformation law and failure trend of the frozen wall models with different thicknesses under the same construction environment,the verification of preliminary frozen wall thickness calculated by the pre-sequence step is realized.Furthermore,by carrying out the comparison and selection study,the necessary corrections can even be performed on the obtained thickness schemes.(3)In view of fact that,it is difficult for the existing prediction methods to balance the prediction accuracy and the engineering practicability.By combining the indoor experiment method and the numerical calculation method,a practical method that can effectively simulate and predict the surface deformation induced by frost heave and thaw settlement during the whole process of AGF construction is created and established.Based on this,it further researches and reveals the response relationship between the frozen wall thickness,the freezing temperature of soil and the characteristics of frozen-thawed soil,and the laws of surface deformation induced by frost heave and thaw settlement.It provides an effective evaluation method and necessary reference basis for the verification and control of surface deformation during the subsequent AGF construction.(4)Based on the general idea of the current design process of frozen wall thickness,by further considering the characteristics of soil layers,the water-rich environment where the frozen wall is located,and the effect on the surrounding environment may induced by the actual AGF construction,a complete set of procedures suitable for determining the frozen wall thickness of cross-passage in urban rail transit projects is expansively proposed and constructed in this thesis.Compared with the existing traditional design process,the result obtained by the process proposed in this paper is no longer just a certain frozen wall thickness value that only satisfies the carrying capacity needs,but a frozen wall thickness range that meets both the structural strength requirements and the surrounding environment stability requirements.The on-site engineers can further reasonably optimize the design thickness within this range,according to the actual conditions and specific needs of the construction site.It is verified by engineering examples that the obtained frozen wall thickness schemes are more optimal and reasonable,and both the safety and economy of construction can be taken into consideration.The above-mentioned research results have been successfully applied in the AGF construction of the No.4 cross-passage between the Financial Island Station and No.3 Working Well of the Zhuji Intercity Rail Transit Project.According to the results of on-site monitoring and inspection,the overall construction effect is good.The relevant conclusions and results can provide necessary supplements to the existing design theory of the frozen wall thickness of cross-passage,and can also provide a useful reference for the design and construction of similar projects in the future,which is expected to be promoted and applied.