Road connection between S.S. 225 “Valfontanabuona” highway and A12 motorway (Rapallo junction). Caravaggio and Fontanabuona tunnels. Firedamp risk in tunnelling through gassy rock masses. Technical – scientific advisory support during design phase

Client: Tecne S.p.A.
Project: Road connection between S.S. 225 “Valfontanabuona” highway and A12 motorway (Rapallo junction). Caravaggio and Fontanabuona tunnels
Date: June- September 2023

The Construction Design (CD) of the road connection between the State Road 225 “Valfontanabuona” and the A12 Motorway (Figure 1) involves the construction, with full section method and traditional technique, of 2 natural tunnels (Caravaggio and Fontanabuona) for a total length of approximately 4700 m and of niches and service tunnels.
SERENGEO provided in technical – scientific assistance, during CD, on safety against air – methane mixtures detonation / deflagration during tunnels’ construction works.

Figure 1. Route of the road connection between the 225 “Valfontanabuona” Highway and the A12 Motorway. Caravaggio and Fontanabuona tunnels are represented in yellow dotted line.

Objectives

  • Evaluation of the hazard level associated with potential methane emissions during tunnels’ construction and technical classification of the 2 natural tunnels, niches and service tunnels;
  • Technical – scientific guidance for safe design and construction of tunnels (with traditional excavation technique) in potentially gassy rock masses.

Activities performed

  1. Bibliographic and hystorical documentation’s analysis on:
    a. geo-structural framework in the area surrounding Caravaggio and Fontanabuona tunnel route;
    b. surface methane emissions and evidences of methane presence in geological formations similar to those that will be involved in Caravaggio and Fontanabuona tunnels excavation;
    c. mining licenses, issued by the Ministry of the Environment and Energy Security (MASE), for the exploration and extraction of hydrocarbons (oil and/or gas) and stratigraphy of exploratory and production wells closer to the route of the two tunnels (Figure 2);
  2. Analysis and interpretation of the results of geognostic and methane investigations, carried out along the two tunnels route (Figures 3 and 4);
  3. Critical analysis of excavation method and technique, section and technical data of the two tunnels and relative niches and service tunnels;
  4. Definition of the hypothetical methane emission model and technical classification of the 2 tunnels in relation to methane risk, according to “Grisù” Guideline;
  5. Development of the safety measures and procedures to be adopted during tunnelling works to ensure the highest workers safety level against air – methane mixtures’ detonation / explosion hazard.

Figure 2. Location of Bettola, Berceto, Torrente Luretta, and Caldirola exploration permits, Rallio exploitation license, “S. Sebastiano 001” well (UNMIG, VIDEPI project, https://www.videpi.com/videpi/videpi.asp) with and tunnels route (in the red box).

Figure 3. Boreholes along the Caravaggio and Fontanabuona tunnels ruote where methane concentration measurements were performed on drilling mud samples (FB7, FB8, and FB9quater, marked with blue circles). Boreholes where no methane concentration measurements were performed are indicated by red circles.

Figure 4. Geo-structural profile of Fontanabuona tunnel. Representation of the boreholes (blue diamonds) where drilling mud samples were taken and subjected to gaschromatographic analysis. The red diamonds identify boreholes where no sampling was done.