From the point of view of survival needs, human beings have chosen the principle of building on water, and the history of human civilization for thousands of years is also the history of human conquest of rivers, lakes and seas. On the one hand, rivers, lakes and seas provide various conveniences for human beings, on the other hand, they also greatly limit the space for human activities. Especially in today's society, how to eliminate the obstacles of rivers, lakes and seas has become an inevitable demand of society, so the application of underwater tunnels has gradually become an important development direction in the field of underground engineering. The high risk of underwater tunnel has also become the most concerned issue, so good risk management has become the key factor for the success of underwater tunnel.
1. Development of underwater tunnels
The development of underwater tunnels can be traced back to Babylon from 2 180 BC to 2 160 BC, when Babylon built a tunnel across the Euphrates River. Modern underwater tunnels were built in Britain. 1807, Britain began to build a pedestrian tunnel connecting the banks of the Thames in London. Since 1960, many tunnels have been built in China, especially since entering the 2 1 century, the underwater tunnel construction in China has shown a rapid development trend.
2. Risk analysis
In the field of engineering, it is an acknowledged fact that the safety risk of tunnels is higher than that of other projects, while in tunnel engineering, underwater tunnel construction is considered to be extremely high. Generally speaking, there are the following risks:
① Cognitive risk. At present, the understanding and analysis of engineering risks by technical level and engineering theory are still in the primary stage, and many theories only stay on the basis of assumptions, with many uncertainties and inaccuracies. However, the uncertainty and inaccuracy of the objective risk cognition of the project will have a great impact on the risk management of the project. In the decision-making and design stage, overestimating the risk will make the design too conservative and increase the investment risk, and complex engineering measures will also increase the construction risk. Underestimating the risk will artificially increase the engineering risk by reducing the necessary engineering measures; In the construction stage, the uncertainty and inaccuracy of the objective risk cognition of the project will confuse the engineering technicians and managers on the construction site, and then they will distrust some engineering theories or detection methods and management models, and pay more attention to the first feeling of the site, relying on feelings to manage risks and increase management risks.
② Decision risk. In the current construction environment, although the decision-making risk mainly comes from the construction unit, it does not mean that other participants do not need to bear the responsibility of decision-making risk. Every contractor has decision-making problems. Even if it is impossible to reverse the decision-making risk of the contractor from the source, it is necessary to make a good decision on the risk management of the contractor with a positive attitude within its ability to reduce the risk of underwater tunnel engineering. With good risk management and control, we will gradually change the concept of the construction unit or other units and create conditions for the smooth development of the project.
③ Managing risks. Management risk exists not only in the construction unit, but also in the construction, design and supervision units. If there is a problem in the risk management of one party, it may increase the risk of underwater tunnel engineering. An objective phenomenon we are facing is that many people put all the responsibilities on the construction unit when it comes to safety risk management, and unilaterally require the construction unit to take engineering measures other than contract and design at all costs to ensure safety. From the management point of view, this behavior greatly dampened the enthusiasm of the construction unit to prevent risks, apparently reduced the risks of other contractors, and actually increased the overall risks of the project. From the legal and moral point of view, in the event of a major engineering accident in an underwater tunnel, neither party will reduce or even aggravate its due responsibility by adopting the means of shirking and evading responsibility. "Stealing the clock" and "ostrich policy" are both negative ways, which are not conducive to safety risk management, and are meaningless to themselves, people and units, and should be resolutely abandoned.
④ Geological risk. The geological risk of underwater tunnel mainly comes from the complexity and uncertainty of underwater geological conditions. Different underwater tunnel construction methods have great differences in engineering geology and hydrogeological environment requirements, and a little carelessness may lead to great risks in the construction process. As an underwater tunnel project, the geological conditions related to water are the most concerned, and we must pay close attention to them at all stages of the project progress. They can be simply divided into two categories: one is directly related to the safety of tunnel construction stage, such as water permeability, geological structure, surrounding rock integrity, water pressure and so on. The other is related to the safety of the tunnel after operation, such as earthquake, underground corrosive environment and water level change. Detailed and accurate geological exploration is the premise of successfully resolving geological risks.
⑤ Operational risk. Due to the limitation of current engineering construction technology, the main engineering state of underwater tunnel may change after it is put into operation, which may lead to safety risks.
3. Risk characteristics of different construction methods
There are various construction methods for underwater tunnels. Taking the tunnels currently under construction in China as an example, Shiziyang Tunnel in Guangzhou-Shenzhen-Hong Kong is constructed by shield method, and general subway tunnels crossing the river are also constructed by shield method. Xiamen Xiang 'an subsea tunnel and Qingdao Jiaozhou Bay subsea tunnel are constructed by drilling and blasting method. Two municipal tunnels in Guangzhou Bio-Island are constructed by immersed tube method. In some cities, when crossing some small rivers or lakes, cofferdam excavation method is used.
There is a debate in the engineering field about which construction method is safer for underwater tunnels. The author always thinks that this argument has no practical significance. Personally, I think the most suitable construction method is the safest construction method! Therefore, the key to the problem is how to choose the most suitable construction method. On the basis of accurately grasping the geological conditions of the area where the underwater tunnel is located, according to the engineering characteristics of different construction methods, the construction method with less safety risk and less investment is selected. The following are the risks faced by various construction methods.
① Drilling and blasting method: The main risks of drilling and blasting method are water inrush, water inrush, collapse of surrounding rock and leakage of main structure. (2) Shield method: The main risks of shield method are high water pressure, the shield is not suitable for surrounding rock conditions, the equipment is damaged and trapped, and the main structure leaks. ③ Immersed pipe method: The main risks of immersed pipe method include wading operation risk, failure of waterproof structure and maintenance of structure on land. ④ Cofferdam cover digging: The main risks of this method include foundation treatment, maintenance structure and waterproof structure failure. ⑤ Freezing method: The biggest risk of freezing method is freezing failure or local failure. ⑥ Milling and excavation method: This method has very high requirements for surrounding rock conditions, poor water permeability of surrounding rock, rock hardness suitable for mechanical crushing and cutting, and is rarely used. Generally, tunnels that can be constructed by milling and excavation can be constructed by drilling and blasting in principle. ⑦ Suspension method: Suspension method is a new theory in the theoretical exploration stage. Its core is to divide the tunnel into several pipe sections (similar to immersed pipes), connect them into a tunnel main body, suspend in a certain depth of water, and anchor the tunnel on underwater rocks by anchoring means such as anchor cables. The biggest engineering risk of suspension method lies in whether it can solve the problems of engineering materials and tunnel attitude control.
4. Comprehensive risk management
Once an accident occurs in the process of conventional tunnel construction, which causes suspension, it can generally be reopened at the original site through certain engineering treatment measures, which will not have a fundamental impact on the project construction. However, underwater tunnels are different. Once the waterproof measures fail and water gushes in, it will often cause extremely bad consequences, seriously affect the construction period, cause huge economic losses, and even lead to the abandonment of the whole project until it is relocated and rebuilt.
① Decision-making stage. This stage has a great impact on the safety risk of underwater tunnels, and once a decision-making error occurs in a certain link, it will have a serious impact on engineering construction. Risk management in the decision-making stage should pay attention to the following issues: avoiding the idea of "preconceived ideas"; Do a good job in geological exploration; Do a good job of expert evaluation and avoid geological risks as much as possible; Reasonable investment control; Necessary redundancy design concept.
② Construction stage. When entering the construction stage, the construction unit naturally becomes the main body responsible for the safety risk management of underwater tunnels. Everyone talked a lot about how the construction unit carries out risk management. This paper only briefly talks about two issues, which are not limited to construction units.
Geological survey. Considering that there are some differences in the construction capacity and level of each construction unit, before the construction, geological survey or supplementary geological survey should be arranged according to the specific situation of the construction unit and the situation of engineering geological exploration. If the geological survey can be included in the project budget, it will help to reduce the project risk, especially for complex underwater tunnel projects, the construction unit should encourage the construction unit to carry out geological survey.
Dynamic management of quick response. Because of the strong space-time effect in underground engineering, especially in underwater environment, it is necessary to deal with it quickly after special working conditions appear, otherwise delaying the treatment opportunity may lead to serious consequences.
③ Surgical stage. Risk management at this stage can be carried out from three aspects.
Engineering condition monitoring. It mainly includes three aspects: condition monitoring of the project itself, external environment monitoring and project operation monitoring. The monitoring of ontology state should adopt necessary means to monitor water leakage, entity durability, working environment and so on. External environmental monitoring is the process of monitoring the environmental conditions in the area where the tunnel body is located; The monitoring of operating conditions should be carried out in the real-time monitoring of traffic conditions and air quality in tunnels.
Establish corresponding emergency mechanism. Set up special emergency agencies and formulate various targeted emergency plans. Prepare corresponding emergency personnel, equipment and materials. Prepare personnel, equipment and materials according to the emergency plan, especially conduct regular emergency training for relevant emergency personnel to ensure that emergencies can be put into use quickly, control the events in the initial stage, and minimize disaster losses.
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