ABSTRACT:

The paper synthesizes the results of a study that was aimed at the rock engineering estimation of the present static condition of excavated rock slopes at a large limestone quarry site. The quarry site was divided into six lythostructural homogeneous sectors. On the basis of field geo-structural surveys, laboratory mechanical characterization of the rock and joints, rock mass classification procedures and kinematical analysis, including the block theory, a judgement has been made on the quality of the rock slope behaviour along the entire quarry face. A rockfall analysis has also been performed. In all the quarry sectors, the present geometry of the quarry rock slopes is prone to localized, cortical failures although with different intensities. This kind of evidence has been qualitatively verified using a idealized DEM model of the slope and of the blocky rock mass structure. The computation evaluations of the rock block trajectories have shown that blocks could reach the quarry plant area and, in spite of a protection embankment, could even reach the state road that runs along the side of the quarry. Definitive remediation, consisting of reexcavation to flatten the rock slopes, has been suggested to prevent the rockfall.

1. INTRODUCTION

Excavated rock slopes, such as those that result from massive exploitation operations in surface mining, can undergo instability events. These events can involve local instabilities, ranging from minor ones, such as rock blocks fall, which are risky above all for quarry operators and equipment, to large scale slope failures which, apart from evident safety problems, can hinder further exploitation of the ore body sectors or impact the neighbouring environment, e.g.: [1, 2, 3]. This paper summarizes the main steps that were conducted for a geomechanical study of a large limestone quarry, where the geostructural condition sometimes induced local failures in a sector with an over-steepened face profile. This sector has been reexcavated. The rock face, could however, be at risk to rock blocks falling towards the quarry plant and the “Flaminia” state road below (figure 1). The study was therefore aimed at: 1) evaluating the rock mass condition along the excavated quarry face, which has been divided into different zones of structural lithological homogeneity, 2) identifying unfavourable features that could promote block kinematics, 3) making stability evaluations of the rock slope, 4) suggesting slope remediation measures in order to prevent rock block detachment and runout.

2. SITE AND GEOLOGY SETTING

The limestone quarry is located on the orographic right side of the Burano river, in the Northern Marchean Appennine mountains (Italy). The quarry rock slope, which develops for about 600 m along state road no. 3 known as “Flaminia”, and the river itself, shows an approximate North - South trend. The quarry floor is located at the bottom of the Burano valley, near the river level, and the quarry crest is about 130m high. The average dip of the global quarry slopes is about 60°W, while the natural mountain rock slopes show an average dip of about 40°.

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