## 1 Introduction  
Tunnel construction in China is gradually developing towards the direction of large tunnel burial depths, long distances, and complex and diverse geological conditions [1]. Tunnels built in Southwestern China are generally buried deep, resulting in high geo-stress [2]. Moreover, the surrounding rock is weak, coupled with strong geological structures, which often cause major deformation disasters in tunnels [3]. In large deformation tunnels, the support structure is severely distorted and deformed [4], with a deformation rate of up to 50 cm/d in severe cases [5], and even the secondary lining is crushed [6]. Therefore, studying the characteristics of tunnel deformation and loose zones and analyzing the causes of large deformation is crucial. The complexity of studying large deformation tunnels is reflected in the diverse reasons and influencing factors that cause large deformation [7]. High geo-stress, rock mass structure [8], tectonic movement, and water-rock interaction are the leading causes of surrounding rock deformation [9]. During the excavation process of Changing Tunnel, there was significant sliding and peeling of the surrounding rock, which mainly occurred in the shear sliding part of the stratum. The influencing factors of large deformation in tunnels mainly include in situ stress conditions [10], strata occurrence [11], rock strength, rock integrity [12], and groundwater conditions [13]. The large deformation characteristics of tunnels are mainly reflected in the large deformation amount, fast deformation rate, and the inability to converge for a long time. After the excavation of the Yulinzi Tunnel, there is a large amount of deformation, with a deformation of 400-600 mm in the shallow buried area and 800-1000 mm in the deep buried area. The arch deformation of the Yulinzi Tunnel is relatively large, and the overall deformation distribution of the tunnel is related to the excavation sequence [14]. The average deformation rate during the excavation process of the Jiangluling Tunnel is 10-30 mm/d, and the cumulative maximum deformation is 40-80 mm. The horizontal convergence of the tunnel is greater than the settlement of the arch crown, and the degree of deformation increases approximately linearly with the burial depth [15]. Affected by the geological suture zone, groundwater, and the mechanical characteristics and occurrence of Luang Prabang rock mass, the deformation of the China Laos Railway surrounding rock presents obvious asymmetric characteristics. The tunnel arch crown and right arch waist deformation are significant [16]. [PERSON] et al. [17] also studied the mechanical deformation characteristics of the surrounding rock caused by the unloading effect of tunnel excavation under different section types. They believed that the plastic zone and deformation value of the surrounding rock of circular tunnels are the smallest. In contrast, the loose zone of the surrounding rock in a horseshoe-shaped tunnel is larger. The large deformation of the tunnel has caused severe damage to the support structure. For example, in the early stage of the Dapingshan Tunnel, continuous longitudinal cracks appeared on the support side wall, with a width of about 5-13 mm and a length of 0.5-2.0 m [18], and the steel arch frame was severely twisted and bent [19]. The maximum horizontal convergence of the Maoxian Tunnel reached 1.8 m, and the large deformation caused by compression was relatively severe [20]. There were phenomena such as the concrete cracking and peeling, the steel frame twisting and breaking, inverted arch uplift, and bolt fracture failure [21]. It is found that the causes of large deformation in tunnels are diverse, and the deformation characteristics are also different under different types of large deformation. There is limited research on the characteristics of large deformation and loose zones in high-geo-stress soft rock large cross-section tunnels.