Abstract Among several works, constructions in rock masses are the most complex in engineering, due to many uncertainties of the environment. Tunnel construction is a case in point. A common problem found in tunnel excavations with the use of explosives is the occurrence of overbreak beyond the boundary line of the tunnel. Many researchers worldwide have proposed forecasting techniques based on the use of regression models or machine learning, however, these require many samples and variables, such as the explosive rate and rock mass. Predicting overbreak before blasting is essential in project management, as it can modify parameters of the blasting plan, design, schedule and employee safety. Therefore, this study sought to employ an exponential smoothing model, which could predict the percentage (%) of overbreak in a tunnel section, based solely on the previous overbreak values. This time series methodology has never been used before in tunnel excavations, although it is used in the financial market. The technique was tested in one Brazilian tunnel. The model proved to be very efficient in predicting overbreak in tunnel construction, since it can be adjusted at each advance. The model can be adjusted as a function of α: where α is close to zero, the model prioritizes past events, while for α close to 1, it prioritizes recent events. The best fit occurred with α = 0.9; 0<α<1.

Abstract In underground mines, stability problems can cause unplanned dilution, increasing the costs of mining operations, such as loading, transport, crushing and grinding. This is usually dealt with by simply increasing investments with support. However, the design geometry of stopes and drifts also has a great effect on a stope’s stability. The use of empirical stability methods is a very common practice, however because of their few input parameters, it cannot predict the actual performance related to each stope and drift design. This study has as main goals a back analysis of the actual dilution, using the Equivalent Linear Overbreak Slough (ELOS) method and analysis in the drift developments at the hanging wall contact, using numerical models. The case study is an underground hard rock gold mine, with a pillar-less Transversal Stope method. The hanging wall failure was quantified using a database from 19 stopes measured by the Cavity Monitoring System (CMS). Numerical finite elements models (FEM) were used, and 5 primary stopes were selected to calibrate models. The volumes of actual dilution showed a good correlation with the volumes of the relaxation zones produced by the models. Then, new optimized drift geometry layouts at the hanging wall contact were proposed, showing a decrease in 2.35 times in the relaxation zone of the numerical models, therefore justifying the use of these models as a dilution optimization tool. This methodology also proved to be useful in the definition of cable bolt length.

Abstract The creation of tridimensional models that represent the quality and geomechanical characteristics of the rock mass to be excavated can be made by using spatial interpolation techniques. The aim of this research is to improve the safety in underground activities, as well as to spread and popularize the use of spatial interpolation methods to represent both the spatial continuity of the quality of the rock mass and the geomechanical variables through geostatistical methods. This article presents and demonstrates the viability of using Q-System block models estimated from the borehole data for previous classification of the rock mass surrounding the underground excavation and rock support design. The block model for the support design of galleries is justified because it allows to previously forecast the kind and quantity of support necessary for the stability of the excavation, as well as the costs with these devices, besides increasing the safety in the operations. Techniques of indicator kriging and ordinary kriging were used to build the Q-System index block model. The model was validated through the techniques of visual inspection and cross validation. The data used in this study were collected from the geotechnical description of 39 borehole core samples with a total measurement of 4,015 meters in Fazenda Brasileiro Mine, located in the town of Teofilândia, Bahia State, Brazil.

Abstract Knowledge of the physical properties of rock masses is fundamental for the economics and safety of mining projects. The determination of these properties in rock samples in the laboratory requires time, expensive equipment and qualified personnel, which considerably increases the information's cost. Indirect methods were developed to obtain properties related to rock masses, which have been shown to be a viable alternative to traditional procedures. The determination of the compressional mechanical wave velocity (Vp) and subsequent correlation with lithological mechanical properties are indirectly obtained. This study’s objective was to obtain correlations between Vp and the resistance to uniaxial compression, UCS (Unconfined Compressive Strength), as well as the density and porosity of the siltstone and sandstone lithologies present in the coalfield of Candiota, located in the southern region of Rio Grande do Sul, Brazil. The Vp records were obtained in laboratory samples, using ultrasonic velocity sensors, and in-situ by geophysical well logging (directly in boreholes). The results indicate the possibility of using Vp to determine the physical parameters of the investigated lithologies. In the specific case of the correlations between Vp and Unconfined Compressive Strength, determination coefficients R2 above 0.70 were obtained, indicating sufficiently high reliability for using this information (e.g. in roof support projects). The correlation between Vp and density was also high.

Abstract An existing cut slope on Highway RS-471/BR-153 is studied. The slope is composed of the Serra Geral Formation's material: a highly altered volcanic breccia and above it a slightly altered and very fractured rhyolite. A numerical analysis of the slope is carried out in two dimensions. Fracturing of the rhyolite necessitates the use of a constitutive model for materials with families of discontinuities (Ubiquitous-joint). To simplify the analysis of the problem and to examine some of the most sensitive variables, two more analyses are made, replacing the initial model by a Mohr's constitutive model, which considers the material as continuous. The results obtained from the Ubiquitous-joint model provide an acceptable representation of the field conditions, showing a failure mechanism type of rockfall from the face of the slope in the rhyolite, and the rupture surface of the breccia, without propagation of surface failure. In the analysis, when considering both materials as continuous, it is not possible to take account the fault conditions simultaneously, and this analysis also requires values of the mechanical parameters different from those recommended in literature.

A função de um pilar-barreira é proteger o painel de lavra em operação contra a transferência de pressões imposta pelos painéis de lavra adjacentes nos quais os pilares foram recuperados. No caso das minas de carvão em Santa Catarina, o pilar-barreira tem função de proteger o eixo da mina e os painéis em atividade contra a ruptura de pilares inesperada dos painéis de lavra antigos que já encerraram sua operação. O objetivo desse trabalho é verificar a validade do método empírico para o dimensionamento de pilar-barreira apresentado em Peng (1986) por meio da simulação numérica de uma geometria de lavra-padrão, que é aplicada para minas de carvão. Modelos numéricos em duas dimensões foram construídos obedecendo à geometria dos pilares do eixo e do painel de lavra para diferentes profundidades da camada Bonito. Foram consideradas as propriedades das materiais que formam o sistema piso-pilar-teto. Os resultados mostram que o método empírico é válido para a geometria projetada, onde os pilares do eixo não sofrem sobrecarga devido à lavra do painel e nem o teto sofre ruptura devido à redistribuição das tensões.

The function of the barrier pillar is to protect the mining panel in activity from the abutment load of adjacent mining panels that were mined. In the case of underground mines in Santa Catarina State, the barrier pillar has functioned to protect the main entries of the mine against pillar failure from old mining panels. The objective of this paper is to verify the application of the empirical method to design barrier pillars as proposed by Peng (1986), using numerical simulation following the mining geometry of the coal mines in Santa Catarina State. Two-dimensional numerical models were built taking into account the geometry of the main entries and mining panels for different overburden thickness, and considering the geomechanical properties for the rock mass that forms the roof-pillar-floor system for the Bonito coal vein. The results of the simulations showed that the empirical method to determine the barrier pillar width is valid for the studied coal vein and considered mine geometry. Neither did the pillar at the main entry become overstressed due to adjacent mine panels, nor did the roof present any failure due to stress redistribution.

O dimensionamento dos pilares e do reforço do teto em mineração subterrânea de carvão deve levar em consideração parâmetros como o comportamento geomecânico do maciço rochoso e os equipamentos de lavra empregados. Esse artigo demonstra o método de classificação geomecânica baseado no Coal Mine Roof Rating (CMRR), uma ferramenta de engenharia que descreve qualitativamente as condições geológicas e geomecânicas das rochas em minas de carvão para seleção do tipo de suporte de teto. Diferentes índices de CMRR foram identificados e então considerados para definir a largura máxima nos cruzamentos, o comprimento e a capacidade de carregamento dos parafusos de teto. Essa metodologia provou ser adequada, minimizando os riscos de ruptura de teto e predizendo o tipo de suporte mais apropriado a ser empregado às várias zonas dentro do depósito, conforme ilustrado no estudo de caso.

The pillar and roof support design should take into account the rock mass mechanical behavior and excavation method (blasting or mechanical excavation). This work aims to improve the approach for roof support design in underground coal mines to reduce the risks of roof failure and determine the adequate roof support for each kind of roof typologies found in the mine area. This design approach is based on geological mapping and geomechanical classification CMRR - Coal Mine Roof Rating. This engineering tool describes qualitatively the geological and geomechanical conditions of the roof rock mass for coal mines to select appropriate roof support. Different classes of CMRR were identified at Barro Branco coal mine. Therefore, based on the CMRR classification it was considered the maximum roof span at crossings, bolt length and bolt load capacity. The methodology is adequate to reduce roof falls and predicting the adequate support for different roof typologies for different zones in the mining.