【正文】 nel closure may be associated with inward displacements of 100 to 300 mm. If the damage mechanism is associated with rock ejection, the support systems will be called upon to absorb substantial amounts of energy in order to decelerate these blocks. If the damage mechanism is associated with seismicallyinduced rockfalls, it will be necessary to strengthen the support systems such that the factor of safety against failure under static conditions is significantly increased.Observations and research indicate that support systems bee largely ineffective if the fractured rock zone exceeds m in thickness, as the Maximum Practical Support Limit will be reached or exceeded (Section ). Estimating damage severity The expected rockburst damage severity can be based on observations and empirical evidence (Kaiser et al. 1992。 Kaiser 1993), or it can be estimated from stresstostrength ratios and geometric considerations.Observations from previous rockbursts Past observations of damage from rockbursts often provide a good indication of future damage. The documentation of rockburst damage should include descriptions of the type of rockburst damage mechanism (bulking, ejection, or rockfall) and the volume or mass of rock that failed. In addition, the depth of the rock failure and the performance of existing ground support should be noted. In particular, if rehabilitation is required, the depth of loose or fractured rock that is removed should be carefully noted. This information can provide a basis for estimating the extent of future rockburst damage under similar conditions. In addition, the amount of bulking should be estimated for each support type based on preand postrockbust opening dimensions.Rock stresstostrength ratio It is well recognized that the rockbursts can occur in highly stressed, massive or jointed rock. The rock fails because the stresses overe the rockmass strength around a drift. Therefore, the ratio of rockmass strength to rock stress around the drift boundary provides a good estimate of how deep the rock fracturing may extend. It is useful to have a quick method to roughly assess the available strength of the rockmass and the approximate depth of the fractured rock zone. Based on practical experience, a good first approximation of the rockmass unconfined pressive strength is given by:=( to )Where is the unconfined pressive strength of intact rock (typically based on NX core). The upper end of the range is applicable to massive rock with few, nonpersistent joints and the lower end for well clamped, good quality rock (., for a Q=100 rockmass, Hoek and Brown (1988) give: s= or = ).Rock failure will initiate in the zone of highest stress around the drift. For drifts with a roughly circular crosssection, the maximum stress at the boundary is given by:=3Where is the major farfield principal stress and is the minor farfield principal stress. For noncircular solutions (Hoek and Brown 1980), or approximated by circumscribing an equivalent circular tunnel (Chapter 6).The ratio can then be used to estimate the depth of failure (=ra) in massive or good quality rock (4). The result can be expressed in terms of the unconfined pressive strength of intact rock:Where a is the equivalent opening radius. Equation is valid for details on estimating the depth of failure and the resulting rock dilation or bulking are presented in Chapter 6.Peak particle velocity Seismic events can trigger rockbursts because they create a dynamic increment in the stresses around nearby excavations. The peak stress caused by the seismic event is directly proportional to the peak particle velocity ppv of the seismic wave and can be calculated for planar waves from:ppvWhere c is the propagation velocity of either a pression or a shear wave and is the rockmass density.The effect of dynamic stresses on an underground opening can be estimated by performing a pseudostatic analysis whereby the static stresses are replaced by a superposition of the dynamic and static stresses around an opening. This approach is applicable when the dominant wavelength of the seismic waves is about 10 times longer than the excavation width. For shear waves, the dynamic increment in stress can be simultaneously added to the major principal stress and subtracted from the minor principal stress to determine the most critical stress conditions(). The anticipated depth of damage can then be estimated using the approach described above (Eqn. 。 for details refer to Chapter 6). SummaryFor each rockburst damage mechanism (Section ) and for each level of anticipated damage severity (Section ), Table summarizes the underlying cause of the rockburst damage, the thickness of the rock zone involved, the weight of rock involved per square metre of excavation surface, the approximate wall deformation that may result, typical block ejection velocities that may occur, and the amount of kinetic energy that may be transferred to the support system. These values have been obtained from analyses presented in Chapters 6 to 8, and are given here as a general summary of the factors that are relevant for support design.Table Rockburst damage mechanisms and nature of the anticipated damageDamagemechanismDamageseverityCause of rockburstdamageThickness[m]Weight[KN/㎡]Closure*[mm][m/s]Energy[KJ/㎡]BulkingMinorHighly stressed rock715Not criticalwithoutModerateWith little excess2030Not criticalejectionMajorStored strain energy5060Not critical BulkingMinorHighly stressed rock750 to 3Not criticalcausingModerateWith significant20150 to 32 to 10ejectionMajorExcess strain energy50300 to 35 to 25Ejection byMinorSeismic energy715033 to 10 remoteModerateTransfer to20300310 to 20SeismiceventMajorJointed or broken rock50300320 to 50RockfallMinor