Evoluzione di scarpata in roccia per distacco di blocchi: caso di studio su un versante in calcari cristallini

Abstract

The paper reports on a case of a slope in crystalline limestone in the southwestern Alps, quarried for the production of high quality CaCO₃. The limestone, of Jurassic age, is in tectonic contact with a phylladic schist, and is densely jointed and fractured. The excavation front - about 120 m high - is subdivided into benches; each bench is 10 m high and has a face dipping 70°. Quarrying is carried out under unfavourable conditions of interaction between excavation faces and natural discontinuities: some benches near the contact zone between limestone and schist, were severely damaged by failure processes (mainly sliding) with the collapse of large (up to 200 m³) rock blocks. A 3D statistical model is presented as obtained on the basis of geological-structural data (scan-line sampling of joints) of the quarry site integrated by high resolution photographs. Geometric parameters of discontinuities at the outcrop scale characterizing the slope are: a) attitude of pseudo-schistosity planes conformable to the slope with dip angle equal to 55°, which is steeper than the mountain side dipping 38° b) 4 families of fractures are present dipping from 0° to 82° on the average and having an average trace length of 0.7 to 1.5 m; fracturing degree is from 0.1 to 1.4 m^-1. Actual failure features as compared to the simulated ones show that in-situ failures are characterized by means of spherical (equal area and/or equal angle) projections of possible combinations of the discontinuity planes delimiting rock blocks. Single blocks are delimited - depending on free surfaces - by 2-3 discontinuity planes; aggregation of two or more single blocks, which is favoured by a pervasive jointing, gives rise to composite blocks of great size and very complex shape, which are strictly defined by the combination of fractures satisfying kinematic conditions. The unfavourable attitude of pseudo-schistosity planes controls the sliding of failed blocks. In particular, elaborated methods give information on the type of instability which may occur and on volumes of-rock involved.