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ndagini geofisiche e geomeccaniche hanno consentito di caratterizzare una Deformazione Gravitativa Profonda (DGPV) profonda 250 metri, che interessa circa 900 Mm3 di gneiss quasi completamente destrutturati per alterazione e metasedimenti (anidriti, gessi, marmi e dolomie) fortemente alterati e localmente ridotti a spessi livelli di sabbie dolomitiche. I log stratigrafici e geomeccanici di tre sondaggi profondi (300-400m) hanno consentito di tarare indagini geofisiche di superficie e di eseguire profili sismici verticali VSP, registrazioni FWS e BHTV. L’elaborazione delle indagini sismiche ha evidenziato inversioni di velocità associate a livelli di rocce particolarmente destrutturate, mentre la correlazione tra caratteristiche di fratturazione ed alterazione delle carote e logs sonici eseguiti nei sondaggi ha permesso la caratterizzazione delle proprietà fisico-meccaniche dei litotipi. La modellazione numerica suggerisce come l’innesco della DGPV sia da ricondurre alla strutturazione tettonostratigrafica della valle ed a intensi processi di alterazione, favoriti dalla decompressione dei versanti legata a deglaciazione e da un rilevante deflusso idrico sotterraneo.

A reliable assessment of the strength, deformability and hydraulic properties of rock masses at depth is crucial for a number of applications, including geological and structural investigations, deep tunnels and underground openings. Nevertheless, when dealing with complex geological conditions (lithologically/structurally complex or damaged rock masses), a cost-effective, reliable characterisation of rock mass conditions by direct investigation is often unfeasible due to extreme variability of lithology, fracturing, and damage state of rocks. In this context, geophysical investigation techniques can be particularly useful to establish correlations between the elastic properties of rock masses and suitable, quantitative albeit empirical descriptors of rock mass quality and performance commonly used in rock engineering. Nevertheless, most empirical relationships available in the literature apply to the near-surface (upper few tens of meters of earth's crust) and use rock mass descriptors (e.g. Barton "Q") poorly suitable for complex rock masses (including strongly deformed or folded metasediments or weathered fault rocks). We analysed the relationships between sonic wave velocity measured in boreholes drilled up to 400 m in depth and rock mass quality described using the Geological Strength Index (GSI). This is a descriptor of rock mass structure and weathering, suitable to be applied to nearly all kind of rock types and structural conditions. Data were gathered from a detailed geological investigation carried out for a 9.2 km long headrace tunnel in a crystalline core complex of the north-western italian Alps, composed of two gneiss units (i.e. Antigorio and Verampio units) interlayered with meta-sedimentary rocks. The meta-sedimentary sequence is composed by gypsum-anhydrite, fractured marbles and calcschists, and at least two layers of uniform sand derived by decomposed carbonates. Continuous, high quality coring (HQ diameter) was obtained in 300 to 400 meters deep boreholes by both conventional and wireline techniques. Geomechanical core logging allowed to perform a continuous, detailed description and characterisation of drill cores according to GSI. 600 m of geophysical logging were performed with a centimetre-scale acquisition pace using a "full-wave" sonic logger (transmitter operating at 27 kHz, receivers recording up to 100 kHz, transmitter-receiver distance 1 m). The dataset shows a wide range of Vp (1700 to 5300 m/s), testifying high lithological variability and structural complexity of the investigated domain. We performed a statistical analysis of the collected data (>500 Vp-GSI couples), whose scattering required accurate outlier identification and removal. Non-linear regression analysis allowed to identify a best-fitting empirical relationship integrating different lithologies, weathering and rock mass damage conditions over a wide depth range.

The project of interest involving a hydroelectrical diversion tunnel through a crystalline rock massif in the Alps required a detailed hydrogeological study to forecast the magnitude of water inflows within the tunnel and possible effects on groundwater flow. The tunnel exhibits a length of 9.5 km and is located on the right side of the Toce River in Crevoladossola (Verbania Province, Piedmont region, northern Italy). Under the geological framework of the Alps, the tunnel is located within the Lower Penninic Nappes in the footwall of the Simplon Normal Fault, and the geological succession is mostly represented by Antigorio gneiss (meta-granites) and Baceno metasediments (metacarbonates). Due to the presence of important mineralized springs for commercial mineral water purposes, the abovementioned hydrogeological study focused on both quantity and quality aspects via rainfall data analysis, monitoring of major spring flow rates, monitoring of hydraulic heads and pumping rates of existing wells/boreholes, hydrochemical and isotopic analysis of springs and boreholes and hydraulic tests (Lefranc and Lugeon). The resulting conceptual model indicated dominant low-permeability (aquitard) behavior of the gneissic rock masses, except under conditions of intense fracturing due to tectonization, and aquifer behavior of the metasedimentary rocks, particularly when interested by dissolution. Groundwater flow systems are mainly controlled by gravity. The springs located near the Toce River were characterized by high mineralization and isotopic ratios, indicating long groundwater flow paths. Based on all the data collected and analyzed, two parametric methods were applied: 1) the Dematteis method, slightly adapted to the case study and the available data, which allows assessment of both potential inflows within the tunnel and potential impacts on springs (codified as the drawdown hazard index; DHI); 2) the Cesano method, which only allows assessment of potential inflows within the tunnel, thereby discriminating between major and minor inflows. Contemporarily, a groundwater flow model was implemented with the equivalent porous medium (EPM) approach in MODFLOW-2000. This model was calibrated under steady-state conditions against the available data (groundwater levels inside wells/piezometers and elevation and flow rate of springs). The Dematteis method was demonstrated to be more reliable and suitable for the site than was the Cesano method. This method was validated considering a tunnel through gneissic rock masses, and this approach considered intrinsic parameters of rock masses more notably than morphological and geomorphological factors were considered. The Cesano method relatively overestimated tunnel inflows, considering variations in the topography and overburden above the tunnel. Sensitivity analysis revealed a low sensitivity of these parametric methods to parameter values, except for the rock quality designation (RQD) employed to represent the fracturing degree. The numerical model was calibrated under ante-operam conditions, and sensitivity analysis evaluated the influence of uncertainties in the hydraulic conductivity (K) values of the different hydrogeological units. The hydraulic head distribution after tunnel excavation was forecasted considering three scenarios, namely, a draining tunnel, tunnel as a water loss source, and tunnel sealed along its aquifer sectors, considering 3 levels of K reduction. Tunnel impermeabilization was very effective, thus lowering the drainage rate and impact on springs. The model quantitatively defined tunnel inflows and the effects on spring flow at the surface in terms of flow rate decrease. The Dematteis method and numerical model were combined to obtain a final risk of impact on the springs. This study likely overestimated the risk because all the values assigned to the parameters were chosen in a conservative way, and the steady-state numerical simulations were also very conservative (the transient state in this hydrogeological setting supposedly lasts 1–3 years). Monitoring of the tunnel and springs during tunnel boring could facilitate the feedback process.

We investigate the relationships between the in situ P-wave velocity (Vp) of rock masses, measured by borehole acoustic logging, and their Geological Strength Index (GSI), to support a reliable assessment of equivalent continuum rock mass properties at depth. We quantified both Vp and GSI in three deep boreholes drilled in a crystalline core complex of the central Italian Alps. The boreholes were driven up to 400 m in depth and provided high-quality drill cores in gneiss, schist and metasedimentary rocks with variable lithology. Geological and geomechanical logging was carried out for over 800 m of cores, and acoustic logging was performed for more than 600 m of borehole length. High-resolution core logging in terms of GSI was obtained using an original quantitative approach. Candidate empirical correlation functions linking Vp and GSI were tested by a two-step statistical analysis of the experimental dataset, including outlier removal and nonlinear regression analysis. We propose a sigmoid Vp–GSI equation valid over a depth range between 100 and 400 m. This accounts for extremely variable lithological, weathering and rock mass damage conditions, complementing existing shallow-depth approaches and showing potential for practical applications in different engineering settings.