Evaluating D&B and TBM tunnelling using NTNU prediction models S. Zare a,⇑ , A. Bruland b , J. Rostami c a Department of Mining, Petroleum and Geophysics Engineering, Shahrood University of Technology, Shahrood, Iran b Department of Civil and Transport Engineering, Norwegian University of Science and Technology, NTNU, Trondheim, Norway c Department of Mining Engineering, Earth Mechanics Institute, Colorado School of Mines, Golden, CO, USA article info Article history: Received 10 December 2015 Received in revised form 9 June 2016 Accepted 17 June 2016 Keywords: TBM tunnelling Drill and blast tunnelling Excavation time and costs abstract Drill and Blast (D&B) and Tunnel Boring Machine (TBM) are the two dominating excavation methods in hard rock tunnelling. Selection of the cost effective excavation method for a tunnel is a function of tunnel cross section area, rock conditions, tunnel length, availability of skilled labour and proper equipment, and project schedule. Over the past few decades, major technological development and technical advances have been achieved in both methods. Yet, in many tunnelling projects, choosing the excavation method is still a challenge and requires considering pros and cons of each method and estimating construction time, costs, as well as post construction and operation & maintenance, and related risk in the planning phase. In this study, the productivity and efficiency of the D&B and TBM options for excavating certain size tunnels have been examined. The analysis is based on recent NTNU prediction models for advance rate and unit excavation cost for given ground conditions and tunnel geometry. For excavation of large size tunnels in very hard rock, the D&B method seems to be the cost effective choice irrespective to tunnel geometry. This is compared to smaller long tunnels with good boreability were the TBM has higher advance rate. The tunnel size and rock conditions have higher impact on the TBM performance and costs than for D&B. This refers to lower risk of using D&B where the use of this method is otherwise justified. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction Tunnel and underground construction industry has experienced many technological advances, yielding more economic, productive and safer excavation technologies in recent years. In hard rock con- ditions, D&B and TBM excavation method are the two main choices for tunnelling. Selection of the excavation method is an important issue that will arise during planning of a tunnel project, depending chiefly on tunnel geometry (tunnel shape, size, length, slope and turning radius), geology and rock mass conditions, labour skills/ex- perience/costs, contract settings, project schedule, operational con- strains and available area for portal or launch shafts, muck transportation, access to the site, availability of power, and finally local regulations. Both the TBM and D&B methods have pros and cons related to excavation time and costs, safety and environmen- tal aspects, risks, and flexibility, which all should be clearly pointed out and analysed prior to the design and construction phase. The matter has been discussed in the literature where Nord and Stille (1988), Holen (1998), Kaiser and McCreath (1994), Tarkoy (1995), Barton (2000, 2012, 2013), Nord (2006), Stewart et al. (2006), Suorineni et al. (2008), Zare and Bruland (2013) and Macias and Bruland (2014), have compared the methods and stud- ied the influencing parameters. Nord and Stille (1988) reviewed the factors that determine the choice of D&B or TBM and highlighted them by examples from two case histories where both methods had been applied. Later on, Nord (2006) discussed some specific differences of the two meth- ods focusing on excavation time, risk and costs and presented a simple cost indicator on when a TBM solution might be suitable. Nord (2006) concluded that it is impossible to give precise guide- lines for the choice of excavation method. Excavation time and cost and required rock support and related risk evaluation will be guid- ing to choose the most appropriate method for a given project with certain ground conditions and performance specifications. Suorineni et al. (2008) showed the progress of the D&B methods in civil and mining drifts and cost comparison of the two methods as a function of advance rate. They concluded that for rates of advance less than 23 m/day, TBMs are more expensive compared to D&B. The source of the data used for the comparison was not mentioned, nor the tunnel geometry and rock conditions. Barton (2000) proposes that choice between two methods is clearly Q-value dependent (and in general dependent on rock mass characteristic), with adverse effects for TBM at extremely low and high rock mass qualities. In a follow up work, Barton (2012) http://dx.doi.org/10.1016/j.tust.2016.06.012 0886-7798/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: zare@shahroodut.ac.ir (S. Zare). Tunnelling and Underground Space Technology 59 (2016) 55–64 Contents lists available at ScienceDirect Tunnelling and Underground Space Technology journal homepage: www.elsevier.com/locate/tust