| ABSTRACT:In recent years, as shield tunneling is applied widely in the tunnel constructions all over the world, higher quality demands on methods of shield tunnel constructions are required. How to ensure the safety of construction and to reduce the influence on the environment has arisen wide attention of scholars both at home and abroad. China started later than Japan and the West on the application of shield methods. There consists in many problems in the theoretical research on large slurry shield tunnel constructions, and the stability problem of soil body is particular obvious. But now it is still in the stage of experience as to the instability mechanism of the slurry shield tunneling excavation and its stability analysis, especially to the research on instability mechanism, judgment methods and calculation methods for the complex stratum cases. Therefore, it is emergent to carry out comprehensive research such as. test research, theoretical analysis and numerical calculation systematically.Based on the project Nanjing Yangtze river tunneling, in this paper, regarding the excavation face in slurry balanced shield of the high permeable sand layers as research object, static triaxial tests and large-diameter modeling test, together with a three-dimensional Fast Lagrangian finite difference program are adopted to do systematical research on the excavation face instability features when influence factors, such as buried depths, the ratio of slurry, properties of sand body and waterhead heights etc are different. On the basis of amounts of modeling tests, the redefinition of judging criteria of face instability of the high permeable sand layers is suggested, and considering of all influence factors, the excavation surface supporting force calculation formula which is suitable for sandy soil strata is established. The major works and results of this dissertation are as follows:(1) Research about the influence of mud on strength of soil body of excavation face. Mud plays an important role in the slurry shield tunneling constructions. The mud membrane made up in the excavation face is the supporting force transfer medium to maintain the stability of excavation face and it acts on the soil body of excavation face directly. However, it is still in the stage of exploration at present for the influence of mud on properties and strength of the soil of excavation face. Therefore, the indoor triaxial tests under different confining pressure,100kPa,200kPa,300kPa, were done to study the changes of properties and strength of the soil body before and after grouting. And the result is that the cohesive force of sandy soil rises and its angle of internal friction decreases. And with the increase of confining pressure, the strength of soil body increases after grouting.(2) Research about the influence of different factors on active failure of excavation face. In the modeling tests, Different influence factors, such as depths of overburden, the ratio of slurry, properties of sand body and waterhead heights are researched to study conditions of the active failure on excavation face. The larger the depths of overburden are, the more the supporting pressure is. Because of the existence of water, it can form transfusion and excess pore water pressure in tunneling constructions and it would lower the stability of excavation face. The test confirms that there exists arching effect in the process of excavation face instability, but it should not be used as the basis of reducing the supporting force of excavation face. And by analysis of numerical stimulation, angle of internal friction, depths of overburden and waterhead heights respond obviously when the maximal displacement deformation of excavation face takes place.(3) Research on slurry fracturing. In the process of shield tunneling constructions, slurry fracturing phenomenon will occur when the slurry pressure is too high. The slurry is not restrained by ground reaction force after slurry fracturing, which leads to the decrease of slurry cabin pressure dramatically and instability of the excavation face. The modeling tests stimulate different slurry fracturing phenomenon when the influencing factors, such as depths of overburden, the ratio of slurry, properties of sand body and waterhead heights, are different. And the results show that the ratio of slurry, waterhead heights and properties of sand body all have great influence on the slurry fracturing pressure:the slurry fracturing pressure occurring in the test using the second kind of slurry is higher than that in the test using the first kind of slurry; in the test with no water, the slurry pressure is higher and the time is later when the slurry fracturing occurs compared with the test with certain waterhead height, that is to say, the slurry fracturing is more likely to occur when the soil body is saturated; properties of soil body, such as soil gradation, cohesive force, angle of internal friction etc have also much influence on the occurrence of slurry fracturing, and the slurry fracturing pressure of silty-fine sand is higher than medium coarse sand's.(4) Judgment about stability of excavation face. The failure of excavation face has two aspects:active and passive failure which are both sliding failures. However, in the modeling test, it is found that the sliding failure is more likely to occur when the slurry pressure is higher as for sand layers. Therefore, the passive failure criteria is redefined as that the upper limit value of supporting force when the passive failure occurs should base slurry fracturing pressure on. Besides, the main judgment method of excavation face instability is stability coefficient method come up with by Broms. But, it is believed that stability coefficient method is not suitable to be applied in the instability judgment of sand layers when lots of researches have been carried out. This paper is trying to adopt mutation theory to do research on stability of excavation face and it can meet demands after calculation on the background of the project Nanjing Yangtze river tunneling.(5) Calculation of supporting force. Based on the analysis of common calculation of supporting force, it shows that the result of lower limit value calculation which makes use of such method:soli pressure plus water pressure plus20kPa is within safe range. And the results of else methods of calculation are beyond that range. Therefore, Permeable sand layer was studied by numerical simulation in the mud in the tunneling excavation surface supporting force value problems, combining with the experimental phenomenon and influence factors, making use of the strain control mode and by introducing the concept of cover-span ratio (K) and supporting force ratio (lambda), empirical formula of upper and lower limit value solution for calculating the supporting force is established. |
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