A Rock Mass Classification Method for Tuff Tunnel Based on the High-Pressure Gas Expansion Method

Table 5

Description and recommendations of tunnel rock mass classification for HPGEM.

Rock grades	Description	Excavation recommendations
	At this point, the strength and integrity of the surrounding rock are moderate, neither because the strength is too high to increase the GGUC nor because the integrity is poor to cause rock-breaking failure	HPGEM rock breaking is highly recommended when dynamite blast is not applicable
	At this point, the strength and integrity of the surrounding rock are slightly lower, the GGUC required for lower strength of the surrounding rock decreases, while the GGUC required for poorer integrity needs to increase	When dynamite blast is not applicable, it is recommended to use HPGEM rock breaking, which can effectively improve the rock-breaking efficiency and reduce the cost compared with other alternative methods
	At this point, the strength and integrity of the surrounding rock is low although the strength is low, but the integrity is poor and will still lead to a poor rock-breaking effect	When dynamite blast is not applicable, HPGEM rock breaking can be considered and mechanical methods of rock breaking can be used as well
	At this point, the strength and integrity of the surrounding rock are too high, and the rock mass is difficult to be fractured by the high-pressure gas, and even if the rock breaking is successful, the GGUC required for this process is significantly higher	When dynamite blast is not applicable, HPGEM can only be used as a reference considering the cost and duration, but it is still recommended to use as the method for rock breaking
	At this point, the strength and integrity of the surrounding rock are extremely poor and the large number of structural surfaces present in the rock mass can cause gas to escape, making it often difficult for HPGEM to be effective	When dynamite blasts are not applicable, mechanical methods may be a better method of rock breaking