A study of excavation sequence and contour blasting method for underground powerhouses of hydropower stations

The choice of a reasonable excavation sequence and contour blasting method is one of the key techniques for the construction of underground powerhouses. Firstly, the excavation sequences and contour blasting methods commonly adopted for main large-scale underground powerhouse under construction or completed in PR China is introduced and analyzed. Secondly, on the basis of an analysis of the propagation of cracks driven by explosion gases during contour blasting, and taking into account the combined effects of initial in situ stress of surrounding rock mass and the explosion pressure of adjacent blast boreholes, the process of propagation of crack formed along excavation contour under action of laterally high in situ stress is analyzed and demonstrated. The results show that the in situ stress of surrounding rock mass is one of the main factors affecting the crack propagation for contour blasting, and when the in situ stress is higher than 10–12 MPa, it is improper to adopt an excavation sequence of pre-split first followed by the main rock mass excavation, alternatively, the excavation sequence with middle cut blasting carried out first, followed by pre-split or smoothing blasting is recommended.

► Excavation sequence and blasting method for underground powerhouse are studied.
► High in situ stress affects forming expected crack during pre-split blasting.
► Pre-split blasting is improper if in situ stress is higher than 10–12 MPa.
► Middle cut followed by contour blasting is proposed for high stress condition.