ABSTRACT: The Northern Apennine chain in Italy is characterised by scattered outcrops of many oligo-miocenic clay-limestone melanges. The Shale-Limestone Chaotic Complex (SLCC) is one of these melanges and it outcrops in the Valdarno Basin, eastern part of Tuscany. From a geotechnical point of view the SLCC represents a typical bimrock, made up of a highly sheared and tectonized, scalyfabric, clayey matrix containing marly-calcareous blocks from a few millimetres to tens of meters in size, randomly distributed. In this paper focus is given to high artificial slopes (a few hundred meters) of a dismissed pit-mine cut in the SLCC bimrock. An accurate characterization of the block size distribution in the matrix was carried out by means of photographic surveys on natural and artificial outcrops, that allowed a high number of georeferenced images to be collected. Images were analyzed with an image processing technique, providing important indications about the block-size distribution in the studied bimrock.
1. INTRODUCTION
The Shale-Limestone Chaotic Complex (hereafter called as SLCC) is a typical melange widely outcropping in the Northern Appennine chain in Italy. It is made up of a highly tectonized clayey matrix containing heterometric, randomly distributed, marly-calcareous blocks in a typical block-in-matrix fabric. The SLCC forms wide mined slopes in the Santa Barbara dismissed pit-mine (Fig. 1), located near San Giovanni Valdarno, Tuscany.
Fig. 1. Panoramic view of the SLCC slopes in the Santa Barbara dismissed pit-mine. A1 and A2 are two of the investigated outcrops discussed later in this paper.(available in full paper)
The slopes, with a total length of 1.5 km and a total height of 180 m, underwent since the beginning of their excavation large mass movements and rotational landslides. The Santa Barbara mine complex is a property of ENEL S.p.A., the largest power company in Italy, which is promoting a research study on the geological and geomechanical characterization of the SLCC in order to properly characterize the mechanical behavior and to assess the stability of the slopes.
This paper presents the first results of this research study, conducted at the Department of Chemical, Mining and Environmental Engineering (DICMA), University of Bologna. A detailed geological and geomechanical characterization of the SLCC was carried out by means of field surveys and bibliographic research. The activity is now focusing on the marlycalcareous block-size distribution in the SLCC, according to the bimrocks theory [1, 2, 3, 4, 5, 6]. The investigation was carried out by means of photographic surveys and an image analysis technique.
2. GEOLOGICAL CHARACTERIZATION OF THE SLCC
The SLCC represents one of the many melanges extensively mappable in Tuscany. Tuscan melanges are interbedded in the oligo-miocenic turbiditic formations, like the Macigno of the Tuscan Nappe, and their origin is still under debate in the geological community [7, 8, 9, 10]. Some authors attribute the origin of Tuscan melanges to sedimentary submarine mass movements, like debris-flows and mud-flows [7, 8]. These phenomena generate huge chaotic accumulation of hheterogeneous and heterometric material (i.e. olistostromes) deriving from preexisting formations. The scaly fabric of the clayey matrix shows that the deposition and diagenesis of the sedimentary body took place in a context of tectonic activity during the formation of the Northern Appennine chain [7, 8, 11].