ABSTRACT
The stand-up time is a main topic in the field of rock mechanics. It represents the time that an unsupported underground excavation with a given dimension remain stable. In tunneling, it leads to assess the available time to install rock support, and thus, the length of the advance step, which implies a high economic impact.
Complexity of rock masses makes the derivation of predictive equations difficult. Some authors have analyzed the main factors involved, and have proposed analytical approaches to assess the stand-up time. Nevertheless, until now, the empirical approximatios are the methods currently used to solve this issue. Probably, the most widespread nowadays is the 1989 Bieniawski′s chart. This approach allows one to relate the stand-up-time to the dimensions of the excavation (active span) and to the rock mass quality, estimated through the RMR (Rock Mass Rating).
An analytical solution based on this approach is proposed here. The aim of this work is to extract the underlying mathematical law that govern the phenomenon and, furthermore, to permit to assess the stand-up time in an easier manner, keeping the reliability of the original chart.
