velopment of shieldtunneling, the geologic and hydrologic conditions are usually very complex, thestability of excavation face is very difficult to forecast and control duringconstruction stage. It is easily causing disasters, such as surface subsidence, waterinrush accidents. According to incomplete statistics, in the construction of subwayshield tunnel the instability of shield-tunneling faces have sometimes taken placefor recent years to make heavy losses of economy and calamities of injuries anddeaths in Beijing, Shanghai, guangzhou, nanjing, shenzhen and other major citiesof China. For the complicated environmental conditions, formation sensitivity andthe strict control standards, the stability of excavation face is a key problem toshield tunnel construction technology.
In this paper, earth pressure balanced （EPB） shield tunneling projects as themain object of study. Studied the instability mechanism of shield一tunneling faces,disaster evolution characteristics of instability of shield一tunneling faces underseepage action and its control technology system of instability in depth bylaboratory experiment, theoretical analysis, numerical calculation, model test andfield test. The result has theoretical significance and practical value in engineering project, mainly includes:
（1）Summarized and proposed the construction method and equipmentselecting. The equipment selecting suitability obtains from the permeabilitycoefficient of formation, grain composition, characteristics of geotechnical andgroundwater state. Developed the software system which had a good man一machineinterface with shield construction method. The rules to select the shield methodunder complex geological environment especially the water一rich sand stratum isconducted in一depth study.
（2） By sorting and analyzing the cases of surface subsidence and water inrush,caused by excavation surface instability, the disaster environment and evokedfactor are analyzed. The basic characteristics of disaster sources were introducedin the paper. And set up a system of evaluation indexes of excavation surfaceinstability. The attribute recognition model is established. It provided a dynamicassessment of the high risk period of tunnel based on the software system. Putforward the prevention and control measures of excavation surface instability byearth pressure balance shield construction.
（3） Based on the limit analysis upper limit theorem, the 3D revolving bodydamage model is built. The formula of ultimate supporting force of excavationface is derived by the critical failure power of doing work by external forces isequal to the internal dissipation of soil.The critical failure supporting force ofexcavation face in homogeneous formation and complex strata is solved by thecomputer program. The computer program is compiled by MATLAB.For complexstrata, this paper respectively analyzed the influence factors such as tunnel depth,characteristics of soil, groundwater level under two working cases. They are thecasing soil and crossing layer. Compared with the calculated results with thepredecessor's results, it verify that the validity of the theoretical model.
（4） Based on the 3D revolving body damage model, introduced thegroundwater seepage action to th
