Previsione della presenza di massi nello scavo di gallerie in depositi glaciali: approccio geostatistico mediante T-PROGS (Transition Probability Geostatistics)

The aim of this study is the prediction of boulder (erratic blocks) frequency encountered during the construction of a tunnel in a morainic amphitheatre of the northern Italy. This area is characterized by the presence of glacial and fluvio-glacial deposits of Pleistocene age. The ability to predict the size and frequency of boulders during construction is essential for assessing the type of machines to be used for tunnelling. A geostatistical method is proposed herein that utilizes a transition probability-based geostatistical approach (T-PROGS) to model spatial variability of boulders by 3-D Markov Chains. T-PROGS has several advantages over traditional geostatistical methods because has a conceptual framework for incorporating many geological information into the development of simulations. This is accomplished by linking fundamental observable attributes mean lengths, material proportions, anisotropy, and juxtapositioning with Markov chain model parameters. The geological data base used in this study to compute the geostatistical simulations consist of: borehole data, geological survey in the excavation, measurement on outcrop of boulder characteristics (larger, smaller and intermediate axis, respectively L1, L2 and L3) and an interpretative geological cross-section across the study area. The result of this study allows us to predict the size and frequency distribution of boulders during the construction of the tunnel. The higher value of boulders is in correspondence of glacial deposits, where a concentration of 27 boulders/10000 m3 of sediments has been simulated. The variability between different simulations that have been computed allows us to quantify the result uncertainty.