## 1 Introduction  
Karst landscape is a typical topographical features which is widely distributed in the world [1; 2; 3], according to the statistics, there are 15% land area of the world is covered with karst landscape [4; 5]. The area of karst in China is approximately 3.44 million km\({}^{2}\), and is mainly dis-triributed in the Yunnan province, Guizhou Province, Guangdong Province and Guangxi Province [1; 6; 7]. The karst cave is the result of long-term interaction between soluble rock and groundwater [8], tunnel-ing in the karst cave involves great challenge [9; 10; 11; 12], such as water and mud inrush, tunnel infra-structure collapse and surface subsidence [12; 13; 14]. Sealing the through hydraulic crack and improving the carrying capacity by grouting is an effective and widely used method to ensure the safety of tunnel [15; 16; 17]. Therefore, scholars have carried out extensive and deeply researches on grouting for water plug and reinforcement. For the Maanshan Park station project of Guangzhou metro Line 9, [PERSON] et al. (2015) reduced the risk of water inrush and improved the safety of excavation by the comprehensive methods containing backfilling the karst cave, building diaphragm wall and grouting sealing the hydraulic crack, and proposed the eight-step treatment method of karst tunnel [18]. [PERSON] et al. (2020) studied the mechanism and method of grouting sealing cracks through numerical simulation and experiment [19]. The results showed that the key to improve the sealing performance of karst tunnels is to reduce the groundwater flow rate. The influence degree of different factors on the sealing effect of grouting is obtained, the grouting quantity is considered to be the most important factor. [PERSON] et al. (2021) used numerical method to analyze the change law of groundwater seepage velocity and the pore pressure under the different groundwater flow rate [20]. The results indicated that the pore pressure and the retention rate of cement slurry increase with the decrease of seepage velocity. By incorporating the interval analytical hierarchy process into the technique for order preference by similarity to an ideal solution, a new risk identification model based on the fuzzy mathematics was proposed by [PERSON] et al. (2021). The identification result was verified with the project of Maanshan Park station of Guangzhou metro Line 9. The proposed model can effectively identify the excavation risk and reduce accident occurrence [21]. By using two parameters of tunnel water inflow rate and quantized lining stability, [PERSON] et al. (2020) proposed a probabilistic model of karst cavity distribution to evaluate the risk of tunnel excavation [22]. A numerical model was used for trial calculation, and good evaluation results are obtained. By using model test, [PERSON] et al. (2020) studied the proper-ties including the compressive strength, bending strength, gel time and viscosity of a new grouting material of potassium phosphate magnesium cement-based slurry, and the new grouting material is used to solve the problem of low grout retention rate in karst tunnel grouting [23]. However, the aforementioned research are based on the small karst cave, and the stability of rock strata between tunnel and small karst cave is the main topic. When the tunnel passes through a giant karst cave, the influence of engineering economy and size effect are quite significant [8]. The use of engineering spoil to backfill giant karst caves become a choice that taking count of the engineering economy and construction convenience. In this case, the governing factor of tunnel stability are transformed into the bearing capacity and sealing property of surrounding rock. Existing research [24; 25] indicates that the engineering spoil is essentially a mixture which rock blocks is embedded in soil matrix, the mechanical property is extremely related to its microstructure and size [26; 27; 28].