Drill and blast in gypsum quarries: Optimization of technical, economic and safety aspects in Monte Tondo midmountain case study

Drilling and blasting operations are one of the common operations in openpit mining. Recently, mathematical rigorous patterns are attracting attention, and digitalization and optimization of mining operations are highly suggested. In this work, drilling and blasting operations are modeled and various optimization scenarios are simulated. The aim is to improve the blasting efficiency in quarrying through proper understanding of the fracture mechanisms and optimization of blasting alternatives. The analysis was developed over the Monte Tondo gypsum quarry, owned by Saint-Gobain Italia. The case study was modeled by the openpit blasting software O-Pitsurface. Background Studies over fragmentation prediction account for managing both controllable and uncontrollable parameters. Kuznetsov developed a mathematical equation based on the relationship between the mean fragment size and the specific charge [1]. Cunningham used the Rosin and Rammler distribution function and the Kuznetsov model to develop a new model (Kuz-Ram) to predict rock fragmentation curve by blasting [2], later improved in 1987 [3]. The current challenge is how to adapt the modified Kuz-Ram model to the topographical, geological and blast design setting of each mine and quarry operations. In recent years, blasting softwares such as O-Pitsurface have undertaken this task, and the fragmentation can now be predicted. Many parameters are considered, such as the different free faces, the blasting pattern design, the imposed delays and the overall mutual interactions among blasts of single blastholes [4]. This work presents blasting optimization strategy using the modified Kuz-Ram model implemented in O-Pitsurface, conducted over the gypsum openpit Monte Tondo quarry in Italy. The stable geological conditions allowed the use of the Kuz-Ram approximation. The quarry occupies around 374,500 m2, with a mountain peak of 436.5 m. The current gypsum exploitation extends from 200 to 320 m above sea level. The quarry is located beside the Gypsum Vein of Romagna Regional Park, an area classified as a UNESCO natural World Heritage site in 2023, thus it is subjected to strict environmental rules. The gypsum grade highly varies in the area, according to the geological strata. The highest grades (up to 90 percent) can be found in the layers III, IV, V and VI (Fig. 1). Fig. 1 Monte Tondo gypsum quarry and geological stratawith highest grades. The target blasting fragmentation sizes for the Monte Tondo gypsum exploitation are: (1) 600 mm: maximum dimension for the fragmented material entering the crusher and (2) 1,000 mm: maximum dimension for guaranteeing the safety of operations. All fragmented materials exceeding the thresholds must be treated with a jackhammer, and this affects the excavation procedures and materials managements. Moreover, such a situation poses a serious threat to operators during the moving, loading and hauling phases. The quarry process, with the potential achievable time savings if blasting was optimized, is presented in Fig. 2. Validation of model with current blasts The blasting pattern model in O-Pitsurface was created based on three real blasts conducted between March and June 2023. Table 1 reports the features of each blast. Fig. 2 Current process of the Monte Tondo gypsum quarry,with potential time savings due to blasting optimization. The topography was reproduced by inserting the digital terrain model (DTM) relating to 2023. Afterwards, the three blasthole schemes were inserted on the model, with proper charges, detonating types and delays. The geomechanical information was obtained by both historical data and new measurements on blast fragments made in the Geoengineering and Natural Resources Laboratory of Bologna University (LAGIRN), while specific on-site surveys allowed to the obtaining of an indication of the fractures and discontinuities of the in situ rocks. Parameters for optimization scenarios After the model calibration over the quarry real operation, the optimization phase began, with the targets of: time saving, economic saving, and improvement of safety conditions. All optimization scenarios considered do not increase the drilling time or the costs of explosives. The main constraint of such optimization is geometrical as the quarry geometry and scheme, and the width of the benches, cannot be changed. Three indexes were used to evaluate the quality of the optimization, by respecting the constraints. The time saving index, TSI (h/t), is related to the time necessary for exploiting 1 t of material; the cost saving index, TCI (€/t), is related to the cost of 1 t of blasted material; and the safety upgrade index, SUI (percent/t), is related to the percentage of retained material having dimensions greater than 1,000 mm, as larger blocks put the operators at risk. The higher the index value, the worse blasting results are expected. Because the three indexes have different units, a quality matrix was introduced to compare the optimization scenarios. The best hypothesis was finally selected, based on time saving of 145 h/y of work related to drilling and breaking; economic saving of €24,150/y, due to the reduced number of blastholes and diminished use of jackhammers; and improvement of safety conditions by 12 percent thanks to lower dimension of fragments, which leads to easier and safer management. This scenario concerns the use of a different type of explosive and a slightly improved blasthole pattern. The quarry process was then modified accordingly (Fig. 3). Fig. 3 Prospected modified process of the Monte Tondo gypsum quarry,due to the blasting optimization results. Discussion and conclusions The obtained results are inevitably influenced by the geological uncertainty of the gypsum body, which affects the rock parameters and then the rock factor used in the Kuz-Ram model. Due to this, a sensitivity analysis was conducted in order to understand how an error on the rock parameters measurement and estimation could affect the model result. The software O-Pitsurface was used as well to perform such analyses. As expected, an increase of rock factor causes an increase of retained material, due to the reduced effect of the explosives. However, it is worth noticing that the increase of retained materials greater than 1,000 mm is much higher, in percentage variation, than those greater than 600 mm. With the proposed optimization scenario then, an error in the definition of the rock factor provokes notable variations in blasting results, while the current scenario gives more margins for geological investigation errors. This aspect must be carefully considered in the next decisional steps for the prosecution of the gypsum exploitation. Selected references Kuznetsov VM (1973) The mean diameter of the fragments formed by blasting rock. Sov Min Sci 9(2):144–148 Cunningham CVB (1983) The Kuz–Ram model for prediction of fragmentation from blasting. In: Proceedings of First International Symposium on Rock Fragmentation by Blasting, Lulea, pp 439–454 Cunningham CVB (1987) Fragmentation estimations and the Kuz-Ram model - four years on. In: Fourney W (ed.) Proceedings of Second International Symposium on Rock Fragmentation by Blasting, Keystone, Colorado, pp 475–487 O-Pitblast (2021) O-Pitblast User Manual, 3rd ed., Porto: O-Pitblast LDA

Drill and blast in gypsum quarries: Optimization of technical, economic and safety aspects in Monte Tondo midmountain case study

Authored by: Daniele Casertano, Francesco Tinti, Sara Kasmaeeyazdi, Vanessa Cellini and Roberto Bruno

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