Estimation of stand-up time soon after excavation in underground is necessary to gain time to clean up the debris and to erect at least a temporary support. The stand-up time is a function not only of rock mass parameters but also of the geo-environment and excavation technique. Over the years charts have been evolved between rock mass classifications, width of unsupported excavation and stand-up time by Lauffer, Bieniawski and Barton et al. An expression is now proposed to estimate directly the stand-up time taking care of, most of the factors affecting it and it has been verified with the existing data to justify its adoption in practice.
The concept of stand-up time as the bridging action period in underground excavation was first introduced by Lauffer (1958). It is the time taken by the rock mass above the crown to transmit the overburden pressure to the side walls without undergoing excessive noticeable deformation or collapse in the absence of any temporary support. The stand- up time for maximum un-supported span was suggested for seven levels of descriptive rock mass classification varying from category A to category G. Class-A is a very good intact rock similar to the first category proposed by Terzaghi (1946) and Class-G is a very poor rock corresponding to Terzaghi's squeezing rock. This concept was perused further, modified and linked to rock mass classifications like Q-System by Barton et al (1975) and toRMRby Bieniawski (1976).An updated version of the chart linking stand-up time, un-supported excavation andRMRwas proposed by Bieniawski (1993). The limiting boundaries of total collapse and no-support requirements have been some what enlarged. Initially Barton et al. (1974) related the maximum unsupported span (S u) to the excavation support ratio (ESR) and rock mass quality, Q-value as Later on Barton et al. (1975) suggested the limits of unsupported span of tunnel and stand-up time by linking Qvalues in a chart form as indicated in Fig. 1. The limits as suggested by Lauffer (1958) are also incorporated in this figure, (Biemiawski 1984). From this figure it is observed that the ratio of the maximum to the minimum un-supported spans is about 6 for the same class of rock masswhen the ratio of maximum to minimum stand-up times for the corresponding spans and rock mass is about 5. Bieniawski (1993) presented the updated version of 1976 chart as indicated in Fig. 2. Recent information from Lauffer (1988) has also been included by Bieniawski (1993) and shown to full within the modified limits suggested by him. This chart suggests that the ratio of the maximum to the minimum un-supported spans is about 6 and the corresponding ratio of stand-up times is about 12 for the same class of rock.
The un-supported span, also called the effective span, is the span between the advancing face and the nearest support provided or the width of the excavation between the side walls, which ever is larger.