ORIGINAL PAPER Analysis of Unit Supporting Time and Support Installation Time for Open TBMs Ebrahim Farrokh • Jamal Rostami • Chris Laughton Received: 12 November 2010 / Accepted: 14 February 2011 / Published online: 9 March 2011 Ó Springer-Verlag 2011 Abstract Estimation of advance rate and utilization of tunnel boring machines (TBM) are some of the important steps in planning a TBM tunneling project. Estimation of the utilization factor depends on realistic analysis of downtime components. Among the different parameters influencing TBM downtime, tunnel support is the most influential factor, which can take up to 50% of the total excavation time in some cases. Although, there are some rock mass classification systems specifically developed to link ground conditions with the type and amount of support installed in TBM tunneling, the related downtime for support installation has not been studied in detail. Unit supporting time (UST) is the time required for the instal- lation of ground support per unit length of tunnel. Support installation time (SIT) is the time required for installation of a single ground support element. In this paper, approx- imate ranges of UST and SIT are discussed and analyzed on the basis of recorded ground SIT from a number of TBM tunneling projects. The primary goal of this paper is to link UST with rock mass classifications that have been specifically developed to assess ground support require- ments for different tunnel sections using open-type TBM. An accurate estimate of UST and SIT allows for realistic determination of the related downtime and TBM utilization rate. Keywords Hard rock TBM  Tunneling  Open-type TBM  Ground supporting  Utilization  Rock mass classification 1 Introduction Installation of ground support is one of the essential components of tunneling. Ground support is installed to stabilize the rock mass surrounding the tunnel and provide a safe environment for workers and equipment. In some rare cases the rock mass is competent and stable and the tunnel does not require support. However, in most cases, roof and wall support is necessary owing to the limited stand-up time of the rock mass at the heading, as well as to ensure the safety of the work environment and containment of possible roof falls. In TBM applications, installation of ground support has a strong impact on overall TBM performance. When applying open-type TBMs, systematic support installation is necessary in weak ground. As rock mass strength is reduced, more time is required for support installation and the TBM advance rate decreases. Figure 1 illustrates the influence of support installation on the advance rate of a gripper TBM (open TBM), compared to the advance rate of a shielded TBM. A gripper TBM can achieve higher advance rates than a shielded TBM only if a small amount of ground support is required. In a highly jointed rock mass, which requires the installation of extensive supports, the risk of experiencing a longer construction period and increased cost for open-type TBMs is higher; thus, shielded machines are favored in such conditions. As can be seen in this graph, the tunnel advance rate is almost constant for shielded TBMs over a range of rock conditions (classes). This is because the installation of the E. Farrokh (&)  J. Rostami EME Department, The Pennsylvania State University, University Park, PA, USA e-mail: exf170@psu.edu C. Laughton Laughton Associates, Austin, USA 123 Rock Mech Rock Eng (2011) 44:431–445 DOI 10.1007/s00603-011-0135-8