Optimizing blast fragmentation and reducing the damage from it are two important research subjects in this field. Detonation and explosive charge induces three sets of tension cracks in the monolith rock. Radial tension cracks are formed under the influence of the pressure wave whose cylindrical propagation induces tension. Along with the explanation of how radial cracks are formed, formulation is given as to how their length can be calculated using laboratory and drill and blast parameters. Cracks subparallel with the free surface (face) are related with amounts of absorbed and recoverable strain energies. The distance between subsequent cracks can be calculated using the results of simple load-unload laboratory tests. Third sets of tension cracks are formed as a result of excessive deformation of the beam or cantilever formed by radial cracks. Once the length of the cracks and the distance between them are known, it is possible to apply these results for estimation of fragment sizes and blasting pattern design.

A common technique in hard rock tunneling, and underground excavation in general, is drilling and blasting. This method of excavation assumes that damage will be done to the surrounding rock mass depending on its quality. Herein is a proposal for how to estimate blast-damaged zone extent and shape, and how to quantify rock mass properties in this zone. Comparison was made, using FEM, between models with and without blast-damaged zone consideration through analyzing its impact on the support load. Results showed greater support loads in the case of blast-damaged zone modeling. Also, comparison was made between the proposed method for blastdamaged zone quantification and quantification using blast damage factor D. Results showed that in both cases, the support loads are in same magnitude order. In other words, compared to the blast damage factor D method, the proposed is more objective and makes it possible to describe the blasting process before it is actually done.

Abstract:Metal grade dilution is the main production disadvantage of the sublevel caving method, and overcoming this problem has been investigated over the years using different methodologies. Herein, numerical simulation using the discrete element method is used to analyze the influence of friction and fragmentation change in caved rock mass on ore dilution and recovery. The individual and mutual change of each parameter is analyzed. It is considered that at the beginning, the friction angle can be lower or higher than the basic friction angle, and after a certain moment, it will come close to the basic friction value, while fragmentation always decreases. The results showed that both friction and fragmentation, when decreasing, are influencing the higher dilution due to smaller kinematic resistance in the caved mass. If lower friction than the basic one is considered, with the drop of fragmentation, the decrease of dilution occurs. Once the basic friction angle is reached, the fragmentation of the caved mass becomes the dominant influencing factor, and its decrease will continuously increase the dilution until the end of production. However, identifying periods when these changes occur, the possibility for better production planning opens at the design stage, as well as the application of different sublevel designs.

Abstract:Stress field or in-situ stress, beside rock mass strength, determines stability or instability of underground openings. It plays key role of every rock engineering project, and has been subject of many researches. There are many theoretical models of the stress field of the Earth's crust which have more or less limited domain of applicability. However, those models usually express horizontal stresses as ratio with vertical stress component. On the other side, it is well known that deformation modulus of the rock mass is dependent on the lateral stress it is subjected to. This dependence is, herein, coupled with practical findings of the stress field behavior and incorporated into the stress filed model. As the result new formulation for the stress filed components is obtained.Key words: field stress, rock mass, underground excavation Apstrakt: Pored čvrstoće stenskog masiva, stabilnost podzemnih objekata zavisi od napona u stenskom masivu. Naponsko stanje stenskog masiva je jedan od glavnih faktora za procenu stabilnosti kod podzemnog otkopavanja i kao takvo je predmet mnogih istraživanja. Danas je dostupno više modela za procenu naponskog stanja, a njihova primena je manje ili više ograničena. Većina modela horizontalne komponente napona izražavaju kroz odnos sa vertikalnom komponentom napona. Sa druge strane, poznata je činjenica da modul elastičnosti stenskog masiva zavisi od intenziteta bočnih (horizontalnih) komponenti napona. Ova zavisnost je uz pračktično poznate činjenice inkorporirana u model za procenu naponskog stanja. Na ovaj način dobijena je formulacija modela za procenu komoponenti napona u stenskom masivu.Ključne reči: naponsko stanje, stenski masiv, podzemno otkopavanje

Important parameters that describe successful sublevel stoping operation are optimal fragmentation and low damage of rock mass around stopes. In current state of operation fragmentation of blasted material is not optimal since high percentage of fragments are oversized which requires additional sizing. Also, excessive damage of surrounding rock mass is found. By proposing new blasting pattern with slightly increased amount of drilling, but with decreased hole diameter decrease of explosive usage is obtained for more than 100kg. Along with better charge distribution it is expected that rock mass damage is to be decreased as well.