This paper describes case examples where blasting damage to excavated rock faces was observed. One of the examples describes a major slide which commenced immediately following a blast.
Cet article decrit des exemples de degats causes par des explosions aux fronts de taille en galeries. Un des exemples concerne en glissement majeur qui a commence immediatement après un tir.
Dieser Bericht gibt einige typische Falle der Wirkung von Sprengen auf Felsböschungen. Eines der Beispiele beschreibt eine Felsrutschung, welche sich als Folge einer Sprengung entwickelt hat.
When blasting work has to be performed in the neighborhood of man-made structures the ground vibration is often the factor which finally decides how the blasting is to be performed. Comprehensive data has been collected and published during the last two decades on seismic monitoring, however, there is still some difficulty in establishing limit values for varying degrees of damage. There have been relatively few cases where damage could be proven to be associated to any man made structure. Less attention is directed during excavation to blast damage of unexcavated rock mass and excavated slopes and rock faces. In practice, if the damage is small, scaling will usually rectify the problem. However, if the damage is extensive, remedial measures such as secondary blasting or rock bolting in combination with shotcrete treatment is often considered. Damage to rock slopes can be caused by blasts set off next to or near an excavated face. This paper presents - case histories of the damage to rock slopes which is considered caused by blasting.
Knowledge of the mechanisms of rock fracturing necessary part of all blasting operations to potential damage to a nearby rock surface that remain in place. Three major zones, namely crushed, fractured and seismic zones develop around an explosion. Concerning rock damage and slope stability, the fractured and seismic zones are the more important. Fracturing is a function of the energy release and the rock the elastic limit and no fragmentation occurs, except where compressive stresses are transformed on reflection into tensile stresses which may cause the rock to fracture or spall.
Approximately 5 to 20%of the total energy released in detonation of explosives is transmitted directly into the surrounding rock mass in the form of stress waves. The amount and the percentage of total energy which goes into stress waves depends on (a) type of explosive (impedance), (b) weight of explosives, (c) burden and (d) length of delay interval between explosive groups. It is possible to obtain some idea of the comparative effects of different explosives since the impedance of the explosives is equal to its mass density multiplied by the detonation velocity. As a comparison, a 90% strength gelatin dynamite has an impedance ratio of around 1000 gs/cm3 and an ammonium nitrate (ANFO) blasting agent only about half of this. Selecting the proper explosive is therefore an important factor in reducing vibration levels. The stress wave is distributed all around a charge, a part of its energy will be distributed within the angle of breakage and the remainder of its energy will travel through the rock at a velocity of 2000 - 5000 m/s. If it is possible for the burden in front of a blast hole to move forward freely and, the ignition, of the next hole in the row occurs with an adequate delay, a smaller part of the energy will go into the rock. Consequently, if the burden approaches infinity, as for example around a presplit hole, a larger part of the energy will be transferred to the rock. It is, therefore, quite possible that at sites where presplitting is used the greatest vibration problem may be associated with the presplit shots as suggested by Devine et al., 1965. The stress waves associated with ground motions observed at a given point are dependent upon (a) the energy transmitted by the stress waves, (b) the distance between the detonation and observation point; and (c) the transmission characteristics of the rock mass. From a bedding or joint plane reflected waves can cause fracturing far behind the blast. This is due to the tensile strength of the rocks being much less than the compressive strength. Transmission characteristics of the rock mass can vary from site to site. From three different sites particle velocity observation data were plotted against scaled distance on log-log co-ordinates, as shown in Figure 1. The effect of differences, in charge weight, was eliminated from the data by dividing the scaled distance by the cubic root of the total or per delay charge weight, as suggested by Hendron (1970). The charge weight W used in Figures la and lb is the maximum weight per delay if the delay interval was more than 1/4 of the transit time.