Risk Evaluation Of Tunnel Engineering Under Complicated Geological Conditions In The Yangtze River Estuary

In recent years, the number of tunnel and underground engineering construction in China is rising, the difficulty of the engineering construction continues to increase, and the scale is becoming larger as well. But the geological conditions of underground engineering are always not clear, so there will be many problems we should be faced are a lot of tunnel construction risk, which will not only bring about the loss of lives and property, also can trigger social instability. In the phase of project planning, survey and design, evaluating tunnel and underground engineering by using a suitable method of risk analysis, not only can estimate degree of risk, can also be have a knowledge for the accidents. Finally, we will know that what risk control measures should be taken, in order to reduce the losses caused by risk accident effectively.This thesis will be based on the tunnel engineering under complicated geological conditions closed to the Yangtze River estuary, and taking the getting water tunnel of Changshu power plant as an example. According to analysis its complex geological and hydrological conditions, summarize risk factors, and put forward a method for risk evaluation, which in accordance with the engineering practice. The main research contents and results were as follows:(1) To analysis the risk factors, starting from the tunnel, segment, and foundation characteristics of tunnel of Changshu power plant. According to the existing theory, the stability of various aspects in the process of tunnel construction has been analyzed. And we get some conclusions, such as confined aquifer may cause quicksand, and biogas can also be a major hidden dangers.(2) Engineering area is located at the junction of the Yangtze river and the Xuliuujing River, and The river regime will affect the tunnel buried in the bottom of the river. Under combination working of tide and runoff river bed form is manly controlled by ebb tide. But ebb in the Yangtze river main location is in the Yangtze river deep groove, which is about 800 meters apart from the tunnel and the slope is only 10%; Combined with recent years, the middle and lower reaches of the Yangtze river area reservoir and construction of revetment engineering, which make the segments of Xuliujing in a relatively stable state, so in the period of tunnel using the change of river regime will not make an impact on tunnel stability.(3) Tunnel excavation will lead to the stability of soil around, by computing of numerical simulation. Finally, we found that the increase of tunnel embedment depth is conducive to stability, but when the tunnel is in ④ and ⑤ layer soil, the excavation of tunneling will produce large displacement of surrounding soils of tunnel. Another model has also been established for estimating the volume of water gushing. The result has suggested that the relationship between the tunnel water gushing and confined aquifer is closed, and the confined aquifer has control action on the water gushing. So the necessary measures must to be taken to prevent it happen.(4) According to the different geological conditions, this thesis divide the tunnel into 5 sections, combined with the actual engineering practice, and the new risk evaluation system is established. The method of comprehensive risk index method of risk evaluation is used on the tunnel, and the importance of the risk factors of each section is compared. Calculated by the proposed risk evaluation system, finally it suggest that section D and section E is in the risk level of medium, especially section E of risk index is close to high level of risk. In addition section B and section C is on a lower risk level. We will put forward some control actions, on the basis of the index of risk that has been calculated, in order to reduce the loss.