GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 3.2 647 The Malta-Gozo tunnel project: preliminary geophysical offshore investigation L. Petronio 1 , L. Facchin 1 , F. Zgur 1 , E. Gordini 1 , L. Baradello 1 , D. Cotterle 1 , R. Romeo 1 , E. Forlin 1 , G. Visnovic 1 , M. Deponte 1 , A. Affatato 1 , F. Coren 1 , P. Galea 2 , S. Pace 3 , A. Micallef 2 , D. Spatola 2 1 Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS, Italy 2 University of Malta 3 Transport Malta Introduction. In the autumn of 2016 OGS and the University of Malta carried out a series of geophysical offshore investigations in the Gozo Channel (Malta). These surveys are part of the study commissioned by the Ministry for Transport and Infrastructure of Malta to evaluate the feasibility of a sub-sea tunnel between the islands of Malta and Gozo in the Mediterranean Sea. Objectives. The general aim of this study is to provide new contributions and improvements to the geological model of the area, so as to refine the conceptual design of the tunnel and to better drive its proposed route. Specific objectives of the project are as follows: to characterise the nature, thickness and spatial variability of geological formations below the seafloor; to characterise the geologic, geotechnical and hydraulic properties of these formations and their spatial variability; to detect features of geologic and geomorphic relevance within these formations (e.g. faults, karstic formations); to develop a 3D geologic model of the study area. Geological setting of the study area and its implications for tunneling. The islands are composed almost entirely of marine sedimentary rocks, mainly limestone of Oligo-Miocene age. Some minor quaternary deposits of terrestrial origin are also present. The rock sequence comprises five main distinct formations which, although slightly disturbed by almost vertical faults displacements, lie almost horizontally across the islands with a minor generalised NE dip.The five main rock types are (in order of decreasing age): - Lower Coralline Limestone: this is the oldest exposed rock type in the Maltese Islands and started being laid down during Oligene age. It is a 140 m thick, hard limestone unit that forms sheer cliffs that may be from ten to over a hundred meters high; - Globigerina Limestone: this formation is subdivided into three units (Lower, Middle and Upper Globigerina Limestone) by two pebble beds. It is a softer finegrained limestone unit that forms irregular slopes, and that start being deposited during Miocene (Langhian to Aquitanian). It thickness varies from 23 m to 207 m; - Blue Clay: this is a very soft unit that within the islands normally forms rolling low slopes that are mostly covered by carbonate raw soil or scattered rubble. The age is Langhian to Tortonian, in the Miocene. Blue Clay is generally impermeable and holds water, with a thickness up to 65 m; - Greensand: even if not sufficient thick (12 m), this formation proved distinctive enough to have deserved a separate name. It consists of massive, friable, intensely burrowed marly limestone, deposited during the Tortonian age in the Miocene; - Upper Coralline Limestone: this formation is a complex association of limestone. The deposition of these marine sediments stopped around 10 million years ago when the seabed rose above sea level. This formation constitutes the coralline plateaux that top most of the hills of Gozo and Malta. It overlies the Blue Clay in an irregular pattern and its thickness can reach 162 m. A system of horst and graben structures of E-NE trend characterizes the area interested by the project. These structures are indicated by prominent ridges and valleys, with sub-horizontal strata dominating. Rock faulting and displacements are widely present in north Malta and south Gozo and in the channel between them, where the geology is not clearly understood yet. There are a series of geological implications for tunneling in this area. The most relevant are: 1) the presence of faults, that degrade the rock quality and its mechanic characteristics.