This study has two aims: firstly, to develop an instrumentation and monitoring method for the excavation of a tunnel in a highly-fractured quartzite-slate mass that is subject to large stresses; secondly, to use a back-analysis with computer-based models and real instrumentation measurements to identify the properties that best model the behaviour of the mass and hence determine the best characterisation method. The results show that the monitoring procedure defined is highly effective, thanks to a number of innovations, such as the measurement of the complete convergence of the section of the tunnel and continuous, real-time data collection using GPRS. Comparing the analysis of the results with the computer-based models allows us to conclude that neither geomechanical classifications nor on-site and classic laboratory testing offer a high degree of precision when it comes to characterising masses of this nature and that the new on-site penetration test using a hydraulic cylinder (designed by the authors of this study) provides the best means of characterising highly fractured rock masses.

1 Introduction

At an international level, the current state of the art for recognising and determining general actions for operations in poor-quality underground rock masses often uses the same working method used for excavations in competent rock masses. This often results in serious errors in analysing and defining the stress-deformational properties of the soil; as such, in order to guarantee the safety of the excavation, it is necessary to establish high levels of safety using oversized supports, which increases the cost of operations and delays execution.

Furthermore, it is important to note that there is still a lack of suitable research into poor-quality or "soft" rock masses (Kurzweil 2004).The answer to this problem appears to lie in two fundamental factors:

  • In recent times, a number of authors, including the authors of this study, have raised doubts as to the application of the Hoek-Brown classification to "soft" masses, since empirical evidence shows it is unable to define their behaviour.

  • There is a lack of suitable laboratory and on-site testing for such materials.

This has led to different methods being used to characterise rock masses (geomechanic classifications, on-site and classic laboratory testing, and a new onsite penetration test using a hydraulic cylinder) (Isik et al. 2006; ÁlvarezVigil et al. 2011). Specific excavation zones have subsequently been implemented using various devices (Chung et al. 2006).

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