Summary

Geomechanical classifications in a schist rock mass, Ouro Preto, Minas Gerais were applied in a slope located in Inconfidentes highway, close to urban neighborhood of Ouro Preto. The Rock Mass Rating (RMR), the Rock Mass Quality (Q) and the Rock Mass Index (RMi) were obtained. Two different sectors were identified along the slope. Sector 1 is highly weathered, presenting failures through rock material and a large quantity of rock debris below the slope. Sector 2, which is very close to Sector 1, but much less weathered than it, has presented rock block failures, typical of discontinuous rock masses. Due to high degree of weathering, especially in Sector 1, the geotechnical survey has presented many difficulties. Values of unconfined rock strength in Sector 1, measured by Schmidt Hammer, for instance, were influenced by the degree of weathering, as only less weathered portions could be used for measuring rebound hardness values. RQD values, which are directly related to the discontinuity frequency and spacing, were more representative of the difference between sectors than rock strength. Discontinuity spacing was directly measured in unstable rock blocks, as it shows a strong relationship with rock mass behavior in both sectors. Discontinuity persistence has to be estimated, especially in Sector 1, due to the conditions of the outcrop, covered by failures and vegetation. According to RMi the rock mass in both sectors is classified as moderately strong. However these values are influenced by the unconfined rock strength, measured with Schmidt Hammer, bringing considerable uncertainties to this classification, especially in Sector 1. Estimation of strength and deformability properties of the rock mass using geomechanical classifications were done but some values exhibited important discrepancies, depending on the classification and correlations assumed.

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