Abstract

A high mine slope in the disused Santa Barbara open pit mine (Tuscany, Italy) is cut in the Shale-Limestone Chaotic Complex (SLCC), which is a typical bimrock made up of a scaly-fabric clayey matrix including heterometric calcareous blocks. The slope shows evidence of instability phenomena: mainly rotational landslides and toppling countercracks. In order to characterize the mechanical behaviour of the SLCC bimrock, in situ large size non conventional shear tests were performed. The aim of in situ tests is to overcome the size limitation of laboratory specimens and namely to take into account the influence of calcareous blocks on the strength of the bimrock. Bimrock’s strength parameters under natural conditions can be evaluated by means of a limit-equilibrium analysis taking into account shear test data and the geometry of the sliding surface.

1. INTRODUCTION

The Shale-Limestone Chaotic Complex (SLCC) is a mélange formation of the Northern Apennines mountain belt, which, from a geomechanical point of view, represents a typical block-in-matrix rock (bimrock) [1, 2, 3, 4].

The SLCC forms a wide mine slope (with a maximum height of 200 m and a planar extension of about 1300 m x 400 m) in the disused Santa Barbara lignite open pit mine (Tuscany, Italy) (Fig. 1). Since the beginning of its excavation, the slope suffered from a diffuse instability phenomena characterized by rotational landslides and toppling countercracks. Gravitational movements must be considered as still active: rotational landslides show evidence of reactivation due to intense precipitation while toppling countercracks has displayed a net slip up to a meter, over the last two years.

In the past, during mine activity, several stability analyses were carried out, assuming the SLCC as an homogeneous material governed by the only mechanical properties of the clayey matrix, without taking into account the influence of calcareous blocks on the mechanical behaviour of the bimrock [5, 6, 7]. On the other hand, recent advances in the understanding of bimrock’s mechanical properties have pointed towards a strong influence of the blocks on the strength of the bimrock [8, 9, 10, 11, 12].

For this reason, a research study on the geomechanical characterization of the SLCC according to modern bimrock theories is being carried out.

(Figure in full paper)

The research is conducted by the Department of Chemical, Mining and Environmental Engineering (DICMA), University of Bologna. In the first step of the research program the geometrical properties and spatial variability of calcareous blocks were investigated by means of advanced image and geostatistical analyses [13, 14].

The activity is now focusing on defining the mechanical behaviour of the SLCC. In order to investigate the actual strength of the bimrock, non conventional in situ shear tests are being carried out on large-sized material specimens (80 cm x 80 cm x 50 cm). The tests are performed with the financial and logistical support of ENEL Santa Barbara Department.

2. GEOTECHNICAL CHARACTERIZATION
OF THE CLAYEY MATRIX

In order to characterize the SLCC clayey matrix from a geotechnical point of view.

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