Applicability of geomechanical classifications for estimation of strength properties in Brazilian rock masses.

Many authors have been proposed several correlation equations between geomechanical classifications and strength parameters. However, these correlation equations have been based in rock masses with different characteristics when compared to Brazilian rock masses. This paper aims to study the applicability of the geomechanical classifications to obtain strength parameters of three Brazilian rock masses. Four classification systems have been used; the Rock Mass Rating (RMR), the Rock Mass Quality (Q), the Geological Strength Index (GSI) and the Rock Mass Index (RMi). A strong rock mass and two soft rock masses with different degrees of weathering located in the cities of Ouro Preto and Mariana, Brazil; were selected for the study. Correlation equations were used to estimate the strength properties of these rock masses. However, such correlations do not always provide compatible results with the rock mass behavior. For the calibration of the strength values obtained through the use of classification systems, stability analyses of failures in these rock masses have been done. After calibration of these parameters, the applicability of the various correlation equations found in the literature have been discussed. According to the results presented in this paper, some of these equations are not suitable for the studied rock masses

Geotechnical behavior of weak rocks in Córrego do Sítio mine, Santa Bárbara (MG)

Nesse trabalho, é discutida a caracterização de filitos na Mina Córrego do Sítio, em Santa Bárbara-MG, bem como determinados fatores que influenciam a resistência ao cisalhamento da xistosidade. São apresentados e discutidos resultados de caracterização mineralógica, índices físicos e de ensaios de cisalhamento direto em três áreas dessa Mina. O intemperismo atua na variação de determinadas propriedades, tais como porosidade, índice de vazios, peso específico seco e, também, na resistência mecânica, com queda acentuada de coesão após ensaio de alterabilidade. Por outro lado, a acentuada rugosidade presente na superfície de xistosidade das rochas da Cava Crista influencia no comportamento do maciço rochoso, conferindo-lhe maior resistência ao cisalhamento.In this work the characterization of phyllites in the Córrego do Sítio Mine, Santa Bárbara, MG, is discussed, as well as some factors that influence the shear strength along the foliation surface. Results of mineralogic description, physical indices and direct shear tests of three areas in this mine are presented. Weathering acts on the variability of some properties, such as the porosity, void ratio, dry specific gravity, and also on the shear strength, with considerable reduction of the cohesion after alterability tests. In addition, the roughness observed on the foliation surface in the rocks of Crista pit influences the rock mass behaviour, increasing its shear strength

Evaluation of rock slope stability conditions through discriminant analysis.

A methodology to predict the stability status of mine rock slopes is proposed. Two techniques of multivariate statistics are used: principal component analysis and discriminant analysis. Firstly, principal component analysis was applied in order to change the original qualitative variables into quantitative ones, as well as to reduce data dimensionality. Then, a boosting procedure was used to optimize the resulting function by the application of discriminant analysis in the principal components. In this research two analyses were performed. In the first analysis two conditions of slope stability were considered: stable and unstable. In the second analysis three conditions of slope stability were considered: stable, overall failure and failure in set of benches. A comprehensive geotechnical database consisting of 18 variables measured in 84 pit-walls all over the world was used to validate the methodology. The discriminant function was validated by two different procedures, internal and external validations. Internal validation presented an overall probability of success of 94.73% in the first analysis and 68.42% in the second analysis. In the second analysis the main source of errors was due to failure in set of benches. In external validation, the discriminant function was able to classify all slopes correctly, in analysis with two conditions of slope stability. In the external validation in the analysis with three conditions of slope stability, the discriminant function was able to classify six slopes correctly of a total of nine slopes. The proposed methodology provides a powerful tool for rock slope hazard assessment in open-pit mines

Numerical modeling of failure mechanisms in phyllite mine slopes in Brazil

This paper presents three case studies comprising failure mechanisms in phyllite mine slopes at Quadrilátero Ferrífero, State of Minas Gerais, Brazil. Numerical modeling techniques were used in this study. Failure mechanisms involving discontinuities sub parallel to the main foliation are very common in these mines. Besides, failure through the rock material has also been observed due to the low strength of phyllites in this site. Results of this work permitted to establish unknown geotechnical parameters which have significant influence in failure processes, like the in situ stress field and the discontinuity stiffness. Keywords: Soft rocks, Mine slopes, Numerical modeling, Failure mechanism

Fuzzy Algorithm of discontinuity sets

The clustering of discontinuity sets is not always a trivial task, especially when only the pole density diagram is used, the classical method. This process is extremely subjective since the size of the counting circle, the pole overlapping, and the presence of outliers between families make it difficult to define their characteristics. In these cases, it is useful to apply numerical and classical methods together. For that, this article proposes an algorithm based on the Fuzzy K-means method that allows the clustering of the discontinuities without the influence of these factors. The algorithm had its results compared to two fracture sets studied in literature and it has proved its efficiency

Modelagem numérica da ruptura por flambagem em um talude de mina.

Nesse trabalho, o mecanismo de ruptura por flambagem flexural ocorrido na Mina Pau Branco, do Grupo Vallourec & Mannesman, em 2002, foi estudado a partir da aplicação de modelagem numérica por elementos finitos. O programa Phase2, da Rocscience Inc., foi empregado nas análises. Apesar de ser um método aplicável a maciços contínuos, as descontinuidades podem ser incluídas no modelo, utilizando-se elementos de juntas. A modelagem é mais simples, evitando-se as dificuldades de utilização de um modelo de elementos distintos, devido à relativa complexidade geológica da seção analisada, onde foi necessário representar a variabilidade da atitude da foliação, devido ao padrão de dobramentos observado. A consideração dessa variabilidade de atitude foi essencial para reproduzir a ruptura por flambagem. Foram feitas retroanálises do mecanismo de ruptura, obtendo-se valores representativos do estado de tensões in situ e dos módulos de rigidez normal e cisalhante das descontinuidades de foliação. Os parâmetros gerados pelas análises são extremamente úteis para futuras análises de estabilidade nos taludes de filito da Mina Pau Branco. Além disso, o trabalho também serve de referência para estudos futuros de flambagem flexural, já que são muito raros os trabalhos envolvendo a modelagem numérica desse mecanismo.In this work the failure mechanism of flexural buckling that occurred in Pau Branco Mine, of Vallourec & Mannesman Group, in 2002, was studied through the finite element method. The software Phase2, from Rocscience Inc. was employed in the analyses. Despite being a method applicable to continuous rock masses, discontinuities may be included in the model using joint elements. Modeling is simpler, avoiding the difficulties of using distinct element methods, due to the relative complexity of the geological section examined, where it was necessary to represent the variability of the foliation attitude, due to the pattern of the observed folds. The consideration of this variability was essential to reproduce buckling failure. Back analyses of failure mechanism were done, leading to representative values of the in situ stress state and the normal and shear stiffness modulus of the foliation discontinuities. The parameters generated by the analyses are extremely useful for further stability analyses in the phyllite slopes of Pau Branco Mine

Analytical method for calculating the volume of rock blocks using available mapping data field.

The occurrence of potentially unstable rock blocks in rock masses is very common. These blocks are delimited by discontinuity planes and can have various shapes, depending on the orientation of the discontinuities which form them. Discontinuity planes are breakouts of the rock material continuity created due to geological events which took place during the rock mass formation throughout geologic periods. These planes normally present a geometrical pattern. They form families of discontinuity which are defined as plane sets that have similar orientation in tridimensional space. The behavior of rock masses is extremely dependent on block size. As discussed by Hoek (1983) rock masses may behave as a pseudo-continuum in case of block size is very small in relation to the excavation size. Palmstr?m (1995) also discussed the influence of block size in geomechanical behavior of rock masses. Estimates of block volume were used in the geomechanical index RMi, proposed by Palmstr?n, as a measure of the degree of jointing of a rock mass. Palmstr?n proposed empirical methods for volume calculation based on geomechanical parameters, such as the volumetric count jointing index and the RQD. This author explained that block volume is also an expression of the overall geometry of the rock mass, as it is a three-dimensional measure. The calculation of the volume of rock blocks is also essential for stability analysis and support design. Equilibrium limit methods have been used to analyze stability of rigid blocks in excavations in rock masses. Body forces must be considered in this analysis and the most important is the block weight which is primarily the function of the block dimension. Calculation of rock block volume is not trivial because the block geometry is three dimensional in most of the situations. Although blocks are considered regular solids they might assume many different shapes, depending on the number of discontinuity planes which form them. Tetrahedral and complex polyhedral blocks are examples of these shapes. In this paper, an analytical solution to calculate the volume of rock blocks is presented. It was developed for tabular, prismatic and tetrahedral blocks and further extended to apply in polyhedral blocks. In addition, statistical distributions for rock block volumes are also presented. They can provide a model of rock mass structure (pseudo-continuum or an array of discrete blocks) by the knowledge of typical block sizes

Influence of slake durability on geotechnical behavior of phyllites of Pau Branco Mine.

Propõe-se, nesse trabalho, o estudo da influência da durabilidade no comportamento geotécnico das rochas do Talude Oeste da Mina Pau Branco, de propriedade da Vallourec & Mannesman (V&M) Mineração, localizada na serra da Moeda, no município de Brumadinho, Minas Gerais. No local, a ocorrência de fraturamento acentuado e de dobramentos, aliados ao clima regional, levou à formação de rochas com baixa resistência e elevados graus de alteração, fato que favorece mecanismos de instabilização. Foi avaliada a influência da durabilidade nas propriedades físicas dos filitos com graus distintos de alteração e resistência, através da realização de ensaios de durabilidade. Buscou-se, também, a partir do referido ensaio, identificar a existência de correlação entre a durabilidade e o modo de ruptura nos taludes em filitos, verificando, assim, se o referido índice pode ser utilizado como um indicador expedito do tipo de mecanismo de ruptura e da resistência do material. Os estudos incluíram, ainda, análises cinemáticas, para identificar condições para ocorrência de rupturas, segundo descontinuidades existentes no talude, e comparar tal ocorrência a rupturas observadas em campo. Para tal foi utilizado o software Dips5.0, da Rocsciense, Canadá.This paper discusses the durability's influence on the geotechnical behavior of the West Slope's rocks of Pau Branco Mine, this being the mining site on Vallourec & Mannesman (V&M) property, located at Serra da Moeda, Brumadinho, Minas Gerais. At the site, the occurrence of acute fracturing and folding of the rocks, plus the effects of the local weather conditions, were responsible for forming low mechanical strength and high alteration grade rocks, favoring destabilization mechanisms. The influence that durability has on the phyllite’s physical properties was evaluated through slake durability tests with different alteration grade and strength phyllites. Through the same tests, an attempt was made to identify the existence of an association between the durability and the failure mode in phyllite’s slopes. This correlation was important to be found because it could be used as an expeditious indicative of the failure mechanism and the strength of the lithologies that exist in the slope studied. Furthermore, these studies included kinematic analysis to identify conditions for the occurrence of failures caused by discontinuities and to compare these failures to the ones observed at the mining site. For this purpose, Rocsciense's software Dips 5.0 was used