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Occurrences like rockfall, loosening of blocks of rock, cave-in, rockburst and spalling during the excavation of underground structures pose a threat to the workforce, installations and equipment. It is also not unusual for them to cause delays in construction. Such phenomena are extremely varied in their form, place and point in time of occurrence and require appropriate safety measures. Mostly they result from the unfavourable intersection of the excavated area with joints, bedding planes and foliation. They can be favoured by failure process, especially in deep tunnels and in the case of low rock strength.
The aim of this work is to further our knowledge of the phenomena of rock loosening as well as to study in depth suitable measures to be taken. The results of the study should be put in a suitable form for planning and execution. It was quickly realised that in the relevant literature numerous individual aspects of the problem have been thoroughly investigated, but a comprehensive treatment from the rock mechanics modelling to the structural analysis of support elements together with considerations of how to increase safety are missing. As a result, in tunnelling practice often well-reasoned criteria for the measures adopted are missing and subjective arguments play a role in the choice of the means of support. The frequent changes in the geological conditions in the course of the tunnel advance and the uncertainties in identifying them demand a clear methodology in dealing with the corresponding risks. In addition, it is necessary to properly understand the possible occurrences with regard to their nature and origin as well as the effectiveness of safety measures.
By looking at cases of damage due to rockfall that have taken place in the last 30 years at tunnel construction sites the question could be studied in its diversity and the most important triggering mechanisms could be identified. The basic data was provided by damage records compiled by the Swiss Accident Insurance Institute (SUVA), questioning experienced tunnel engineers as well as from the literature. Detachment of rock with similar causes and of similar shape was summarised, whereby besides rockfall, also loosening in disturbed zones (cave-in, collapse), spalling and rockburst were distinguished.
For the loosening of rock blocks it is shown that the various phenomena can be traced back to just a few failure mechanisms. A procedure is described, with which the potential failure body can be determined at the excavation line with respect to shape and size. The procedure is based on information on the position and orientation in space, the frequency and extent of the cleavage areas in the rock. The actual separation of the block from the surrounding rock is generally extremely complex and as a rule it is accompanied by failure processes in the rock mass. Whether failure occurs here is often decided by small changes of the properties of the rock fabric. That the information on these properties is hardly accessible for those responsible at the construction site, often makes an exact assessment difficult. This uncertainty plays an important part in deciding upon appropriate safety measures.
Rock mechanics models with the assumption of limiting equilibrium permit, for characteristic cases of rock failure, the investigation of the decisive influence factors for the stability of the failure body. The method of action of the support measures can be introduced into the analysis, independently of the support elements employed, by means of a resulting force and the required support for the different model cases is determined. It is shown, however, that the interaction of the rock mass with the support elements is in general rather complex. Therefore, for the main load-bearing elements of rock supports (roof bolts, sprayed concrete and steel ribs) the method of action was treated separately and each time the resulting force at the limit state for typical arrangements was quantified with the help of statical models:
Rock bolts are usually installed as non-prestressed free or fully bonded bolts and allow a rapid acting, point-wise support. To secure potential failure bodies they are effective if thereby the anchors reach beyond the areas separating the body from the surrounding rock mass and the anchorage zone is sufficient. The forces in the bolts are governed by the relative movement of the opposite sides of the rupture or sliding surfaces.
By applying a layer of sprayed concrete to the rock surface, a surface (i.e. shell) structure, which acts depending on the thickness of the sprayed concrete, the bonding and shear strength between the concrete and the rock surface. Thus, on the one hand, the sprayed concrete layer fixed to the surface of the rock can be considered as a structure, whose bearing capacity is exceeded on reaching the shear strength of the concrete or failure of the bonding. On the other hand, the sprayed concrete shell does not presuppose bonding, but requires, however, a uniform curvature and an adequate positioning as bearing surface. Its bearing capacity is achieved though the interaction of bending moment and normal force and is influenced greatly by the shear connection between the rock and the concrete. The shear connection results for example by interlocking of the concrete with an uneven rock surface. This necessitates that for this support element one has to pay special attention when driving by means of blasting or use of a TBM.
Steel ribs, after all, are very quick acting, line-wise support elements. Bonded, embedded arches exhibit a large structural capacity as a defence against collapse. This, however, has to be ensured by means of constructional measures (bonding, longitudinal stiffening and providing the right shape).
For these support elements, by means of suitable diagrams the bearing capacity to safeguard against collapse are presented clearly allowing comparisons to be made. It is shown, with which arrangement, combination or time staggering of the measures a failure can best be prevented in a particular situation. The uncertainties in recognising and evaluating the risks do in many cases not permit the application of the safety measures at particular points. One possibility of ensuring safety, nevertheless, is by covering such risks, which cannot be recognised in situ or do not permit an exact assessment, by means of a systematic minimum safety measure applied over long stretches of tunnel. Such a measure has to be defined already at the planning stage on the basis of a detailed analysis of possible undesirable occurrences. It is definitively not an option in the competition.
rock; TunnelingGroup; Schneider, Alex