## 2 Project Overview
### Analysis on the Causes of the Tunnel's Collapse  
(1) Geological factors  
The overburden above the tunnel contains Malan loess, Lishi loess, and paleosol. The soil is relatively loose and characterized by the development of vertical joints, high porosity and strong water sensitivity [26; 27]. After the tunnel was excavated, the stability of the loess was poor. As a result, the kind of soil in the tunnel could not resist the additional stress caused by secondary stress adjustment during the tunnel excavation, and the arch effect of the tunnel excavation contour could not be formed [28]. The weakness of the loess was the internal cause of this collapse.  
(2) Impact of groundwater  
Tunnel excavation in the water-rich loess stratum destroyed the balance of groundwater and caused water to collect in the rock surrounding the tunnel, which scoured the soil around the tunnel and caused soil particle loss. In addition, the dynamic water pressure during the process of groundwater seepage significantly affected the stability of the soil. The unloading and rebound effect of the tunnel excavation caused a certain degree of relaxation of the rock mass around the tunnel, improved the seepage effect of the groundwater, weakened the self-supporting capacity of the loess and led to the tunnel's collapse [29; 30].  
(3) Influence of atmospheric precipitation  
According to the geological field survey, the collapse section is a good groundwater enrichment zone. A few days before the collapse, Xi'an experienced heavy rain for several days and the collapse area received a long-term replenishment of atmospheric precipitation (Figure 4). In the case of insufficient drainage equipment and insufficient drainage time, rainwater seeped along the vertical joints and cracks of the loess. The infiltration of rainwater made the loess collapsible, increased the weight of the loess and increased the underground water level and pores of the tunnel. According to the Mohr-Coulomb strength criterion, under pore water pressure, the expression of the shear strength of saturated rock is as follows [31; 32; 33]:  
\[\tau_{f}=(\sigma-p_{w})\tan\theta^{\prime}+c^{\prime} \tag{1}\]  
Figure 2: Tunnel section and support parameters.  
where \(\sigma\) is the total stress, \(p_{w}\) is the pore water pressure, \(\tau_{f}\) is the shear strength of the rock, \(\varphi^{\prime}\) is the internal friction angle, and \(c^{\prime}\) is the cohesion of the rock.  
Equation (1) shows that pore water pressure in the rock reduces its shear strength, resulting in large deformation of the surrounding rock and collapse of the tunnel. During severe rainfall, the large-scale and centralized rainstorm increases the instability of the tunnel surrounding rock, which is the external factor causing tunnel collapse accidents.  
(4) Design and Construction Factors  
The burrow excavation width of this project exceeds 18 m, the height is 12 m and the excavation area exceeds 195 m\({}^{2}\). It is currently the largest three-lane loess tunnel in China, with the largest cross-section. When the deep-buried section was excavated, there was no awareness of the presence of unfavorable loess. When the tunnel collapsed, the tunnel face was constructed to YK58 + 550, and the second lining was constructed to YK58 + 613. The distance between the secondary lining and the tunnel face was 63 m, which did not meet the specification requirements for class V surrounding rock. During the measurement process, the vault settlement measurement was not timely and did not attract sufficient attention. Therefore, human factors, such as the untimely application of secondary lining, low initial support parameters, and untimely measurements, are primary causes of accidents [34,35].  
Figure 3: Tunnel collapse scenario: (**a**) surface house collapse; (**b**) slab staggering fissure; (**c**) collapse and roof fall in the tunnel; (**d**) concrete cracking and peeling.