## 1 Introduction  
The IV- and V-grade surrounding rock sections of super large cross-section tunnel construction mainly adopt the partial excavation method used in China. Three-step methods, single-side heading methods, CD methods, CRD methods, and double-sided heading methods are widely used. The support system formed by a small pilot tunnel and temporary support [1, 2, 3, 4] has the problems of a narrow working space, complex process, and slow construction speed. When this support system is used in karst cave strata, because the influence of karst cave development on construction is not considered, there are a series of problems, such as difficulty in implementing karst cave treatment measures, difficulty escaping construction personnel during disasters, and inability to use large-scale construction machinery during disaster treatment. The two-step method can provide a large construction space and is conducive to the use [5; 6] of large-scale construction machinery due to the small number of blocks. However, under the condition of weak surrounding rock, there are also problems such as poor stability of the tunnel face and large deformation of the supporting structure. In the past, this method was mostly used in the II-grade and III-grade surrounding rock of the 3-lane tunnel. In recent years, with the development of large construction machinery and surrounding rock reinforcement technology, the micro-step method has been widely used in the IV-grade surrounding rock of 3-lane tunnels (the relevant provisions of the _Technical Specifications for Construction of Highway Tunnels_ (JTG/T 3660-2020)) [7] and has been explored and applied in the V-grade surrounding rock of 3-lane tunnels. By means of numerical simulation, [PERSON] [8] and [PERSON] [9] studied the effects of geometric parameters such as step length, height, and excavation footage on the deformation control ability of the surrounding rock and supporting structure under the geological conditions of the weakly broken surrounding rock. [PERSON] [10], [PERSON] [11], [PERSON] [12], [PERSON] [13], and [PERSON] [14] studied the measures of advanced support, initial support, and locking anchor rods in step method construction. Some of the research results have been verified by engineering applications. This shows that the two-step method and reasonable engineering measures can improve the tunnel deformation control ability and reduce the risk of collapse and large deformation. From the above analysis, the two-step method has advantages in terms of the stability of karst caves, karst cave treatment, and mechanized construction (construction speed). Based on this excavation method, targeted technical measures can be taken to solve the problems of construction speed, cost, and safety of super large cross-section tunnels crossing karst cave areas. However, at present, research on the stability of tunnel construction in karst cave areas has focused on analyzing the effect of karst caves on the stability of surrounding rock [15] and the deformation and stress of supporting structures via numerical calculations [16; 17]. The research on the systematic theoretical analysis and construction technology system of the two-step method for the construction of super large cross-section tunnels passing through karst areas is insufficient. The Lianhuashan Tunnel in Liuzhou City is a 3-lane super large cross-section tunnel located in an area where karst caves are strongly developed. Many intrusions into karst caves are detected in the additional construction survey stage. Due to the construction period, the original designs of the double-sided heading method and CD method are optimized to the two-step method, but many disasters, such as collapse, large deformation, and karst cave inrush, have occurred during the construction process.