## 6 Conclusions  
This paper analyses the construction time and advance rate of a 3 km long D&B tunnel under various geological conditions using an upgraded NTNU drill and blast prediction model. The analysis was carried out for the five types of Korean tunnel supports according to rock mass quality, ranging from Type 1 (very good rock mass quality) through to Type 5 (very poor rock mass quality), and four kinds of rock properties, as well as the rock mass quality for each tunnel support type, were applied to simulate different geological conditions based on the previous studies and the NTNU model. The construction time was classified into five categories: basic, standard, gross, tunnel and total, according to the characteristics of the operations for a more effectively analysis of the construction times; the standard construction time covers not only the operations for excavation, such as drilling, charging, loading and hauling, etc., but also usual the tunnel supports such as shotcrete and rock bolts, etc., In addition, to properly consider the actual geological conditions in tunnelling, the construction times for the three mixed geological cases were analysed. The primary conclusions of the study are summarised below:  
(1) The ratio of the basic construction time to total construction time increases from 34% to 40% with the increasing tunnel support type. Despite the greater number of drill holes in Type 2 (96 holes) relative to Type 1 (88 holes), the reason for a more basic construction time in Type 1 is mainly that the drilling rate in Type 1 (177 cm/min) is slower than that in Type 2 (198 cm/min) due to the very hard rock mass quality, even though the drill length is the same for the two types. (2) The ratio of the standard operation time for the operations of the usual tunnel supports to total construction time increases from 10% to 31% with a worsening rock mass quality, which is attributed to the increase in the kinds and amounts of tunnel supports. In addition, the standard advance rate that is generally used as the usual advance rate of a D&B tunnel decreases from 40.8 m/week to 11.6 m/week with the variation of support types from Type 1 to Type 5, which correspond to 6.8 m/day for Type 1 and 1.9 m/day for Type 5; it is assumed that there are six working days per week in this study. This means that 1.9 and 1.6 blasting rounds can be performed during one day for Types 1 and 5, respectively. (3) The gross construction time covers the operation time for the final scaling, water protection and concrete lining, in addition to standard construction time. The gross operation time accounts for between 30% and 14%, decreasing with the increase in the tunnel support types since little variation occurs in the gross operation time (50 weeks for all the types) but the total construction time increases. (4) The total construction time covering all the operations, site preparations, and unforeseen rock conditions, etc., is between 167 and 365 weeks, which corresponds to between 3.8 and 8.2 years, provided that one working year is considered to be 44 weeks. This means that the total construction time of a tunnel in a very poor rock mass quality is more than twice that of a tunnel in very good rock mass quality for the same tunnel length. (5) Three cases representing mixed geological conditions, i.e., Case A (good geological conditions; 20%, 30%, 30%, 10%, and 10% of a 3 km tunnel length for Tunnel Support Types 1, 2, 3, 4, and 5, respectively); Case B (medium geological conditions; 10%, 25%, 30%, 25%, and 10%, respectively) and Case C (bad geological conditions; 10%, 10%, 30%, 30%, and 20%, respectively) were also analysed. The results show that the standard advance rate is 26, 23 and 20 m/week for Cases A, B and C, respectively, and the total construction time for Case A is 212 weeks and increases to 8% or 19% compared to Case A with worse geological conditions. (6) A quantitative term, equivalent rock mass quality (ERQ), is suggested in this study to intuitively express the overall rock mass condition of a tunnel.