###### Abstract  
Throughout all the countries in the world, including Vietnam, nations with well-established mining industries have undertaken extensive research on the stability of rock masses when constructing underground tunnels in varied geological conditions. The present study aims to provide a comprehensive overview of the risk assessment related to rock masses during the construction of pit lines in mining operations. Consequently, the standing time of unsupported tunnels is assessed based on different values of the strength index and deformation characteristics of the rock mass. The objective was to perform both experimental and theoretical investigations to analyse how the stand-up time of rock masses surrounding a tunnel affects the unsupported span. The analyses were based on considering the rock parameters, including strain modulus; geological strength index; and allowable displacement values, and consideration of hereditary creep properties. By examining tunnels excavated in rock strata, it was concluded that varying geological strength index values resulted in distinct creep behaviour in the surrounding rock masses. Thus, it was reasonable to compute the unsupported span and stand-up time of tunnels. The research revealed that permissible displacements are significantly influenced by the types of rock materials surrounding the tunnel structure. Recognising the significance of time, the authors introduce a more practical interpretation and evaluation of the stability of rock masses, thus enhancing the precision of commonly available models. risk; rock mass; stand-up time; underground mines; GSI; modulus +
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addressing a critical factor that has usually been neglected in available published research. By considering the time factor, the authors provide a more realistic scenario for evaluating rock mass stability, thus contributing to the enhancement of models established in the past. The term \"stand-up time\" was first proposed in [PERSON]'s rock mass classification [1]. The purpose of the classification was to generate the relationship between the time that it took to apply supporting systems and rock mass quality after the excavation of unsupported tunnels. The unsupported tunnel span is defined as the distance between the tunnel face and the nearest support if it is greater than the width of the cross-section of the structure. In addition, [PERSON] [2] defines stand-up time as the amount of time a tunnel is capable of withstanding stresses without any supporting systems. In other words, it is the time that a rock mass remains stable and no collapsing event is observed. The magnitude of stand-up time plays a key role in tunnelling since it significantly affects excavation works [3]. Rock masses are typically classified into different grades, which vary from A to G corresponding to the relationship between the stand-up time and the supporting structures. Thus, A stands for highly stable rock masses, and G refers to poorly stable ones. The analyses of the research outcomes proposed by [PERSON] [1] confirmed that values of stand-up time decrease with an increase in unsupported tunnel spans. This proposal given by [PERSON] was modified and linked to the rock masses classification suggested in several other studies. Improved classifications refer to the Q system (a concrete grade of rock quality), the RMR system, and the NATM (New Austria Tunnelling Method). [PERSON] and [PERSON] [4] introduced a new relationship between stand-up time and maximum unsupported tunnel span (L). In addition, based on the Rock Mass Rating (RMR), [5] also proposed a method of determining average values of stand-up time corresponding to the unsupported tunnel span (L). Stand-up time is greatly influenced by the mechanical parameters of the rock mass, which comprise a modulus of rock mass; compressive strength; in-situ stress state; water pressure; seepage; and joint systems.