## 2 Coupling Mechanism of Risk Factors in Tunnel Construction
### Analysis on the Coupling Mechanism of Risk Factors  
We use risk flow to express the interaction between risk factors and the effect on the results. The coupling chain of tunnel construction risk factors includes risk factors, transmission media and risk carriers. The risk generated by a single risk factor flows through the transmission medium and acts on the risk carrier. On the other hand, there are two ways to couple the risk factors with them: one is to influence the risk causing factors to increase the risk flow nonlinearly and act on the risk carrier through the transmission medium; for the second, by changing the physical properties of the transmission medium, the transmission effect of the transmission medium increases nonlinearly.  
In tunnel engineering, surrounding rock, as the transmission medium, plays the role of transmitting risk flow. Groundwater is the most common risk factor. Groundwater amplifies the risk of tunnel construction by reducing the engineering characteristics of surrounding rock. Groundwater will reduce the strength of the stratum by reducing mechanical parameters such as cohesion and friction angle. It also increases the overall density of the stratum, increasing the load acting on the structure. It will also change the properties of some strata. For example, dry natural loess has good performance, but its performance will be sharply reduced in case of water collapse, or sandy soil will be liquefied under dynamic action when its water content is high, etc.  
From the perspective of risk, tunnel excavation is the risk factor of all tunnel construction risks, which causes the deformation of surrounding rock. The intrusion of groundwater reduces the mechanical parameters of surrounding rock, makes the self-stability ability of surrounding rock worse, indirectly increases the disturbance and deformation of surrounding rock caused by tunnel excavation, and finally causes engineering accidents such as surface collapse, tunnel collapse, mud inrush and water inrush at tunnel face.  
For example, the Humaling tunnel of Lanzhou Chongqing railway encountered water-rich silty fine sand stratum in the excavation process. The existence of groundwater further reduces the self-stabilization ability of surrounding rock after tunnel excavation. The tunnel face is unstable, which is particularly prone to collapse. There are frequent phenomena such as water gushing from the bottom plate, sand gushing from the tunnel face, water gushing and special water pockets, which greatly increases the construction difficulty and seriously restricts the construction progress. The tunnel finally took 8 years to complete the connection, resulting in huge construction safety risks and construction delay risks. Reviewing the project, it can be found that the core factor causing a series of problems in the construction process was the water-bearing silty fine sand stratum. When the silty fine sand is in the wet state, the internal friction angle is about 38\({}^{\circ}\), and it is slightly viscous, with certain uprightness and shear strength. However, similar to most sandy soils, the properties of silty fine sand are extremely sensitive to the change of water content. When the silty fine sand changes from wet to dry, with the disappearance of matrix suction, the cohesion of silty fine sand will decrease, and when the silty fine sand enters the saturated state, it will soften and muddy rapidly, which is extremely unfavorable to the construction of underground engineering.