A hydraulic bypass tunnel of about 850m in length and 9m in diameter is planned at the La Penna dam, along the Arno river (Tuscany, Italy). The tunnel has the purpose to laminate the floods of the Arno river. In this note the preliminary study for the Geotechnical characterization of the rock masses involved in the tunnel excavation is presented. The geological and Geotechnical settings of the area were investigated by means of field surveys and the rock-mass behaviour was investigated both in terms of wedge-fall hazard and stress-strain behaviour. The study outlined the presence of two alternating homogeneous rock masses with different strength properties which affect in different ways the excavation of the tunnel. More detailed in-situ investigations and laboratory testing are required in order to minimise the uncertainty in the design and execution stages.
The La Penna hydroelectric dam has also the function to laminate the flow of the Arno river upstream Florence (Italy). The dam is a concrete arc-dam realized in the 1954–57 (Fig.1). An hydraulic by-pass tunnel of about 850m in length and 9m in diameter is planned for excavation in order to improve the discharge capacity of the dam from 630m3/s to 1350m3/s In this note the geological and Geotechnical studies performed in order to verify the feasibility of the La Penna by-pass tunnel are presented.
The La Penna basin is located on the Mt. Faltering Formation (Fig.2) which is constituted by arenaceous and arenaceous -politic turbidities with inter-layers of TNT (Thin Bedded Turbidities), the Mt. Faltering Formations has a maximum thickness of about 900m and it is dated to the Late Oligocene-Early Miocene [1, 2, 3, 4, 5]. Figure 2 – Geological map of the area around the La Penna dam. Key: FALL = Mt. Faltering Formation; L = overlying lacustrine plio-pleistocene deposits; Q = quaternary deposit; a = alluvial; Gk = geostructural ego mechanics station S = prognostic drill hole; T = planned by-pass tunnel. The lower portion of the Mt. Faltering Formation is constituted by thick beds of arenaceous turbidities with Ta, Ta-e and Ta/c-e Bowman sequences with massive sandstones (S3 interval of Lowe [6]). The upper portion of the Formation is mainly a turbidity siltstone, characterized by Tc-e and TD Bowman sequences. The petrographic composition is mainly due to quartz, feldspar and micas. The general geostructural setting is a monoclinic with dipping towards ENE/ESE of about 15°–25°. A geological cross-section across the by-pass tunnel is shown in Fig. 3.
The Mt. Faltering Formation is characterized by the principal bedding and by three main joint sets (Fig.4). The bedding dips towards S0: 95°/16, and the three sets of joint towards J1: 183°/85°, J2: 244°/86°, J3: 313°/83°. The joint sets J1 and J2 are referable to a hk0 ^b system, while the J3 set are referable to the ac system (sense Hancock [7]).This structural setting is coherent with the general tectonic setting of this portion of the Northern Apennines chain, and indicates the absence of local tectonic disturbances.
