## 1 Introduction  
With the continuous development and utilization of underground space, more and more hydropower projects are being constructed at increasing depths, which means high in situ stress appears easily. The effect of a high in situ stress on the excavation stability of hydropower underground caverns is being obvious, especially under high hollow ratio, multi-intersection and large section. Underground caverns, such as Ertan, Jinping I, Houziyan, etc., are usually faced with failure phenomena, including spalling, slabbing, rock-burst and so on [1; 2; 3; 4; 5]. The failure phenomena are not only related to internal factors such as geological conditions, in situ stress and rock mechanical properties, but also have connections with external factors, including cavern arrangement, excavation procedure and support timing [6; 7; 8]. To control and reduce the influence of high in situ stress, the optimization design of excavation procedure for hydropower caverns is often selected as one of the most important solutions. For large-scale underground cavern construction, the main excavation method is the drill-blasting method. The selection of excavation procedures and specific construction parameters, such as middle-drift method, bench method and specific stratification scheme, is directly related to the excavation quality and construction efficiency. Therefore, especially under high in situ stress, the reasonable selection of excavation procedure and construction method is of particular importance. Figure 1 presents the arrangement and typical excavation section of hydropower caverns, including the main powerhouse, main transformer room and tailrace gate chamber. Three caverns are of large scale and the excavation methods of them are basically the same. Due to the characteristic of the large size, numerous intersections and high hollow ratio, the main powerhouse has become a typical representative of them and deserves the most attention. With the technology development, a set of corresponding excavation methods has been formed for hydropower caverns in China on the basis of construction experience, theoretical and experimental research. An excavation procedure design of underground powerhouse is mainly to divide the large cavern into various little parts and excavate them from top to bottom, following the principle of multi-sequences and cross-sequences [10]. For the main powerhouse, the excavation procedure design today is mainly divided into four parts (see Figure 2): top arch zone, rock anchor beam zone, high side-wall zone and machine pit zone. In the recent decades, more and more experts have realized that the blasting excavation procedure design can influence the excavation stability, especially under high in situ stress. From the change of stress state, different excavation procedures correspond to different stress paths or stress histories. It not only affects the stress distribution, damaged depth and deformation range during the construction period, but also influences these of surrounding rock after the final forming [11]. When in situ stress level becomes relatively higher, in situ stress will become the dominant factor of rock failure [12], such as 'V-shaped' failure in tunnels [13]. Today, it is generally believed that there is a coupling effect between stress transient unloading and blasting excavation [14; 15; 16]. For a blasting excavation  
Figure 1: The arrangement and typical excavation section of hydropower caverns. (**a**) The arrangement of hydropower caverns [9]. (**b**) The typical excavation section of hydropower caverns. disturbance, the dynamic blasting disturbance is one of the most important rock-failure mechanisms [17; 18] that trigger the rock-burst around underground excavation, exactly as the two largest rock-bursts (8 September 1995 and 30 April 1996) occurred during the blasting excavation process at the Ertan underground powerhouse [19].