Research On Ground Cavity Mechanism Of Shield Tunnelling In Saturated Sandy Pebble Stratum And Corresponding Measures

For the past few years, with the heavy investment to the national fundamental facility construction, a great number of the big cities set off a phenomenon of" subway-hot", and with the increasing number of subways coming into service,the problem exiting in its own is drawing the public’s attention.In the subway construction, including the wrong way of the shield mechine’s excavation and the insufficient back grouting pressure etc, that easily cause the cavities behind the lining.The cavity makes the lining bearing the irregularly distributed load,and makes the produced ground reaction force be not enough,so the lining segment which under the bending moment which should have suffered the pressure,which greatly affect the lining segment’s carrying capacity and its durability.The cavities’exitance can lead to structural cracks, which has great threatens to the heavy traffic supporter such as the subway.During the construction of Chengdu Metro line1and line2, some ground cavities in the process of shield-bored tunnel construction have taken place,part of which were caused by the weakness zone from the ground itself.such as the karst cave,the other parts of which are caused by the loss of the soil during the shield machine driving.The advantage,such as efficient,rapid and economical,has been balanced out by the cavity-sequelae after the construction,including ground collapse,building cracking etc.It has been a urgent problem for the relevant professionals that how to effectively judge cavities and reliably deal with them after the shield mechine’s work.Nowadays,there are much more research literature about the cavity which is caused by the karst cavity and mined-out areas,whose research contents are more detailed.However,there are less studies about the randomly earth layer cavity resulted from the shield driving,the cognition and research to the strata cavity still in its infancy,and the research depth ramains in the level of experience analogy and language description.Base on the background of the Chengdu Metro line2,the thesis mainly make a research to the formation and evolution mechanism of earth layer cavity caused by the shield construction in the sandy gravel layer,and find out the solution to the earth layer cavity.All in all,it completes the following work-parts:1. Through the construction practice and related Geological exploration report, the main soil character of saturated sandy cobble ground in the Chengdu area was put forward.Basing on the sandy cobble ground’s character and combining with the metro construction practice,this thesis makes a study of the mechanism of layer cavity which is caused by the shield driving,and works out its main reason. 2. On the base of some related articles, the force mechanism of the layer cavity under different geological conditions is put forward,and denpending on this, a new way to caculate the critical depth of the shield driving in the sandy cobble ground is obtained.3. A system analysis of the evolution and deformation rules of ground cavity in the different sizes and burial depth in the the sandy cobble condition has been made in the thesis.4. The principle ’prevention first’ was put forward in the treatment of ground cavity,and during the driving,governance works play as assistance.And the preventive measures before the driving,during the driving, and after the driving are obtained base the shield excavation practices.5. Analysis of shortcomings of several governance measures of the ground cavity at present has been made in the thesis, and parts of reason why the ground cavity is hard to handle are found,and a improved measure by improving the existent measures which have shortcomings is put forward.6. Through the analysis of monitoring data of the Chengdu Metro Line2, the application condition of the Peck formula to the sandy cobble ground of Chengdu Metro is proposed,at the same time distribution of the settlement parameter in the sandy cobble ground of Chengdu Metro is reached by inverse analysis.7. Base on the analysis on the shortages of the existent ways to evaluate the ground cavity safety,a new method which base on the force calculation of the sandy cobble ground is put forward.